Turborocket Upgrades

guest blogger john hare

John Bossard over at Plasma Wind http://plasmawind.typepad.com/ introduced me to air turborockets after a propulsion conference in 2000. My hard references on the subject were from his recommendations. He has built operating engines on this cycle. Never being willing to let well enough alone, I suggest a few upgrades. He may take the ideas apart if he has time and interest, and it doesn’t conflict with his business.

 A normal turborocket is an air breathing engine with a rocket gas generator driving a turbine which drives the air compressor. The fuel rich turbine exhaust mixes with the compressed air to burn in the afterburner. The result is an engine with over twice the thrust to weight of a jet engine with immunity to flameouts. It can reach higher altitudes and higher airspeeds than any turbojet, and is simpler to operate. The down side is that it sucks down far more fuel than a jet, and won’t operate in vacuum like a rocket.

The turborocket is a niche engine for air breathing acceleration from mach zero to mach five or so. It can actually do the things people try to claim a ramjet can do. It is also good for relatively short duration cruise when engine mass is very critical and the good thrust to weight ratio outweighs the high fuel consumption.  It also uses far less fuel than a rocket when in its’ proper working environment.

A rocket may have an Isp (Dense Fuels) of 300 with a T/W of 100. A turbojet may have an Isp of 3,000 with a T/W of 10.  A turborocket might have an Isp (dense fuels) of 750 with a T/W of 20. Clearly the turborocket needs either a better Isp, or better T/W to make it a clear win over the more traditional engine cycles. For this particular cycle, hydrogen makes a lot of sense.

Hydrogen gives a 30-40% Isp boost to rockets, at the expense of tripling propellant tankage per unit of propellant mass, when H2/O2 is compared to Kero/O2. It is an ongoing argument  whether LH2 is good for launch vehicles. Upper stages benefit far more from the extra performance. Hydrogen on air breathing engines though, exhibit a 300% increase in Isp.

Hydrogen has about 3 times kerosenes energy per pound when burned with atmospheric air, so even being 10 times as bulky per pound, it only has tanks a bit over 3 times the size of the kerosene per BTU. Since acceleration engines have to lift the take off mass, cutting your fuel load by two thirds has to have an effect on engine mass, along with wing and landing gear mass of course.

Hydrogen has another valuable trait in its’ specific heat characteristics. It is the best possible coolant with about 16 times the cooling capacity of air. 16 pounds of air introduced to 1 pound of colder hydrogen will meet it about halfway on temperature. Hydrogen can cool enormous quantities of air either through a physical heat exchanger, or with Mass Injection Pre Cooling (MIPC). Many studies have used precooling in some form, though most of them just get too complicated for the simple functions we need for a launch system.

I suspect a straight switch to hydrogen from dense fuels will less than double Isp, considering the oxydizer mass to be carried. I suggest using the cooling properties of liquid hydrogen to precool a core flow for a turborocket. This potentially increases mass flow through the gas generator while much decreasing the on board oxydizer requirements.

 

ejector turborocket

After the initial compression of air, about a sixth of the total flow is sucked into the core for gas generator use. A pound of LH2 as injection precooling for each sixteen pounds of air per second will almost double the density compared to uncooled air that has been through the same compressor. True gas generators operating in ejector mode compress the air/hydrogen mixture and burn it in the middle chamber that drives the turbine. The combination of precooling and ejector compression should give a pressure in the middle chamber of about four times that of the main chamber. With a turbine pressure ratio of four, and a core mass flow of 20% of total compression, a fairly high main compressor ratio (for turborockets)  should be achievable.

The turbine can be cooled by the remaining hydrogen required to reach stoichiometric burn in the main chamber. I suspect an Isp in the 2,000 range can be reached during acceleration periods, with cruise phase considerably higher. This while retaining most of the strengths of the turborocket system.

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johnhare

johnhare

I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.
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68 Responses to Turborocket Upgrades

  1. Randy Campbell says:

    Roderick:
    We’d not be looking at a test-bed of a first stage, rather like the EZ-Rocket the idea would be to flight test the ATR engines in an aircraft and gather flight data. The Facetmobile “shape” elongated and made more super/hypersonic acceptable would be a choice for “down-the-road” testing at this point I think easy and inexpensive to build would be the more direct criteria.

    Though I’ll note here that the FMX has a long Mach “shadow” behind the upper fuselage high-point and along that rear triangle that would suit sheilding an upper stage.

    John H:
    I was thinking about lenticular also, but most of them seem to do better at higher speeds rather than lower so for an initial flight item I’m thinking a lifting body shape would be better. Speaking of the toroid tanks though, somewhere on one of these hard drives I’ve got a copy of a Boeing patent for a toroid tank concept for a Blended-Wing-Body aircraft. Due to the size of the tank the patent actually shows that the tank structure IS part of the overall aircraft structure so the idea has merit.

    Randy

  2. Robert Clark says:

    I was interested to read of the ATR concept. However, it still uses the heavy compressors/turbines of turbojets and it also needs oxidizer in addition to the burnable fuel to run.
    On another forum I was thinking of ways to get the high compression required for high thrust for jets without using compressors/turbines and not using oxidizer other than that in the surrounding air. I copied this below.

    Bob Clark

    =======================================
    The reluctance to use airbreathing engines for part of the time to reach orbit is due in large part to the fact that jet engines are heavy compared to the thrust they can produce. See the list of thrust-to-weight ratios for some engines here:

    Thrust-to-weight ratio.
    http://en.wikipedia.org/wiki/Thrust-to-weight_ratio#Engines

    The thrust-to-weight ratio for turbojets might be only 5 to 6, where as for rocket engines such as the space shuttle main engines might be 73 or above. A big part of this poor thrust-to-weight ratio for jets is the complexity and weight of the compressors and turbines jet engines have to carry:

    Jet engine.
    http://en.wikipedia.org/wiki/Jet_engine

    However, the thrust-to-weight ratio for ramjets because of their simplicity can be quite high:

    Ramjet Performance Primer.
    “There are no physical limits to the minimum weight of a ramjet other than design and materials. The 1950’s Marquardt RJ43-MA-7 had a thrust/weight (T/W) ratio of about 40. With today’s engineering and materials that could probably be brought up to 150-200 without too much effort. Such T/W ratios would make ramjet powered vehicles excellent accelerators.”
    http://www.alt-accel.com/ramjet2.htm

    Airbreathing engines need compression of the air to create high thrust. Turbojets use compressors. Ramjets are able to get high compression from the high velocity of the incoming air alone, dispensing with the compressors and accompanying turbines. Then the suggestion is to replace the compressors/turbines in turbojets with other means to achieve this high compression. One method that has been tested is the ejector ramjet, where rocket exhaust is used to accelerate air into the intake of a ramjet, thus allowing the ramjet to operate even at zero speed.
    This method still needs to use onboard oxidizer, for the rocket, to operate. An ideal method would only use the burnable fuel to operate, as do ramjets and turbojets. For a turbojet/ramjet intended as the first phase of a SSTO vehicle that uses rockets at the end stage, what might work is to use the very high pressure turbopumps that high performance rocket engines such as the space shuttle main engines use. Since these high pressure turbopumps are needed to be carried along for the rocket phase anyway perhaps they can be used as well during the airbreathing portion of the trip.
    There are several ways this might be accomplished. The shuttle liquid hydrogen turbopumps can produce 500 bars of pressure of the liquid hydrogen with a through put of 73 kg/sec each. I’m imagining this high pressure liquid hydrogen be directed into the intake of the jet engine. You want to do this in a way to compress the air. One way might be that the turbopump outlet into the jet engine be in the form of an annular (ring) opening all around the inlet, some distance into the inlet. This would tend to compress the air together as the liquid comes out directed inward to the center. You also want the air to be forced back to the rear of the engine so the liquid hydrogen would need to be angled somewhat also backwards towards the rear.
    The liquid would tend to spread out however, and for a large intake say a meter across or more for the large supersonic turbojet inlets, it’s not certain how far the liquid would go to penetrate into the middle portion of the air to achieve the high compression needed here as well, not just the outer air. You want most of the air to be compressed at least to the 20 bar range commonly seen with turbojets in order to achieve the high thrust achieved by the means of compressors.
    Then another possibility would be to use an analogue of the ejector ramjet compression method. This works by using supersonic exhaust from a rocket to force the air into the intake, thus being compressed as is the case with ramjets flying at supersonic speeds. Then what we could do with the turbopump’s output, is to use the Bernoulli principle to convert the very high pressure into a supersonic velocity:

    Bernoulli’s principle.
    Incompressible flow equation.
    http://en.wikipedia.org/wiki/Bernoulli%27s_principle#Incompressible_flow_equation

    For a streamline at constant height, (1/2)(velocity)^2 + pressure/density = constant. With pipelines leading out of the liquid hydrogen turbopumps of about 30 cm wide, a density of liquid hydrogen of 72 kg/m^3, and mass flow rate of 73 kg/sec, I calculate the flow speed as 33 m/s. Then if we want to convert the pressure of 500 bar = 50,000,000 pascals to high velocity we would get a speed of 1180 m/s, about Mach 3. Then this supersonic flow could be directed into the intakes to accelerate and thereby compress the air as is down with ejector ramjets.
    Still another possibility to get the air to flow at high speed to induce similar compression as with a ramjet might be to ionize the air and accelerate it by electromagnetic fields. The turbopumps use a turbine which is a key means by which electric power is generated. They operate at 70,000 horsepower while weighing only about 700 pounds. There is pretty high efficiency conversion of turbine mechanical power to electrical power. However, we would need a lightweight means of ionizing and electromagnetically accelerating the air. A couple of possibilities for the ionization might be by using a microwave generator or electrically charged wires running throughout the inner volume of the intakes.
    In any case some of the exhaust from the jet would have to be bled off to run the turbopump. This might seem to reduce the performance of the jet engine but actually quite a large proportion of the power generated in usual jet engines is used just to run the compressors:

    What is a Gas Turbine Engine and How Does it Work?
    “The cycle that governs the operation of a gas turbine engine is referred to as the Brayton constant pressure cycle. The engine compressor typically requires about 2/3 (!) of the usable heat energy produced in the burner to turn at maximum speed; the remaining energy can then be used to produce thrust or mechanical power, or a combination of the two.”
    http://www.turbokart.com/gasturbine.htm

    To get an idea of the power we need, we’ll use as a model the J58 engine which powered the SR-71 to Mach 3+. I haven’t seen any numbers on the horsepower generated by the J58 but I’ll estimated it from the 1 horsepower per 2.5 pounds thrust common for turbojets:

    Turbojet.
    Thrust to power ratio.
    http://en.wikipedia.org/wiki/Turbojet#Thrust_to_power_ratio

    The J58 generated about 25,000 lbs thrust in usual turbojet mode, so a horsepower of 10,000 hp. Note though that fuel needed to run the J58 is much less than the 73 kg/sec liquid hydrogen put out by the shuttle turbopump. This page gives its fuel use in the usual turbojet mode as 0.9 lb/(lbf-h), i.e., .9 lbs/hour for each pound of thrust:

    Pratt & Whitney J58.
    Specification of J58-P4.
    http://en.wikipedia.org/wiki/J58#Specification_of_J58-P4

    This is 22,500 lbs/hr of fuel at 25,000 lbs thrust, or 6.25 lbs/sec, 2.8 kg/sec. This is in jet fuel. Hydrogen would give higher thrust and indeed will use about half the fuel for the same thrust as shown in the attached diagram of turbojet/ramjet/scramjet Isp’s. So this would be 1.4 kg/sec of hydrogen. This is 1/52nd the usual mass flow rate of the turbopump of 73 kg/sec. The power used by a turbopump is proportional to the mass flow rate, so the power needed would be 70,000 hp/52 = 1,346 hp
    This about 1/7th the power output of the J58 engine. A problem though is whether this would supply sufficient compression for the high air inflow of the jet. We might need to flow more fuel through the turbopumps than is burned by the engines. But this would mean we are running the jet engine fuel rich. However, the Isp for jet engines is so high we could afford to run fuel rich and still have a significantly better Isp than rockets.

    ============================================

  3. John Bossard says:

    Bob,
    you pose some very interesting suggestions, and I appreciate the time and effort it took to write your comment. It’s especially helpful when someone takes the time to provide references.
    I believe your comment deserves a good reply comment in return, and as such, it will me take a bit more time than usual to prepare. But I will try to have a response to you by tomorrow (7/22) if possible.
    In the meantime, if you’d like to learn more about some of the details ATR operation and characteristics, I suggest you read the ATR-related blog posts at Plasma Wind. The Introduction describes the general operation of ATR cycles, and the performance attributes and applications posts go into more detail on these characteristics, and how they can be used.

  4. John Bossard says:

    Bob,
    You present some interesting ideas. I will comment on them, based on my knowledge base, and perhaps my own opinion. But I would like to preface these comments by mentioning that my comments, or any other person’s for that matter, are not necessarily the final word on the matter. Many, perhaps most, of the great technical achievements of history were made almost precisely because they ran counter to prevailing wisdom of the time. So, listen to some other opinions and use them to help you, but don’t let them stop you from further developing our ideas and concepts if you think you’re on to something.

    First I would say, the notion of “heavy” turbomachinery has to be understood in the context of its application. Turbopump-fed rocket engines, for example, may have a lower T/W than an equivalent thrust pressure-fed rocket motor, but their higher Isp more than makes up for their lower T/W. I think the same is true regarding the use of airbreathing engines as an propulsion element in a suborbital or orbital launch vehicle.

    Thrust/Weight ratio is quite important in sizing a vertical takeoff launch vehicle. Obviously, you need a T/W greater than 1 (where W is the gross liftoff weight). Typical launch vehicles have liftoff T/W around 2-4. Lower than around 2 and there can be stability problems, higher than 3 or 4 means that you can sustain very high g-forces near you first stage burnout.

    Its not simply low T/W performance that is the issue, but the T/W in association with delivered specific impulse (Isp). As an example, consider a “magic” propulsion box that uses no expended propellant, but is able to produce a T/W of, say, 1.1, so it can lift itself, and just a bit more. Although the T/W is low, the Isp is infinite. Such a propulsion system could take you orbit, although the vast percentage of our on-orbit mass would be propulsion system mass. Is that bad?

    Regarding the complexity and weight of the jet engines vs. rocket engines, the SSME is quite a complicated engine, and also possesses turbines, and pumps. Perhaps more significantly however, is the fact that, to get its high performance, it operates with very small performance margins, in terms of stresses, temperature limits, etc. That means that the engine is terribly unforgiving for any off-nominal conditions. Small defects in fabrication or assembly can result it the catastrophic failure of the engine. This is why it must be taken off its shuttle, inspected, and rebuild after most flights. This is one of the reasons why its expensive to use. I’ll be getting back to the advantages of margin later.

    In terms of actual T/W numbers, the SSME masses about 7004 lbm. At launch, the engine makes about 375,000 lbf (at 100% power-level), giving it a launch T/W of about 53.

    Turbojet turbines and rocket turbopump turbines have comparable power densities. It’s really the mass of the compressor versus the mass of the pump/impeller where a good portion of the mass differences arise. Compressors have to do work on gases, which are highly compressible, and take a great deal more energy per unit mass to raise their (total) pressure, whereas liquids are relatively uncompressible , and the specific work to increase their pressure is much lower. Air is bulky, so moving it around and using it to make thrust makes the engine bulky. If you look at turbojets, they take up a lot of volume, and that’s because they are massive air-handling machines. But bulky does not necessarily mean heavy.

    Ramjets:
    As one of my colleagues used to say “The problem with ramjets is that when they’re sitting at the end of runway, any fuel you put in them just runs out the end”. This is just a throw-away line, but the point is that a fundamental problem with ramjets is that they don’t make static thrust. Contrary to common opinion, ramjets can in fact make thrust at subsonic flight speeds, sometimes as low as Mach 0.7 (no reference), however they have terrible specific impulses, subsonically, and they need to get up to around mach 2 or 3 before they begin to operate efficiently. This inability to make static thrust is a fundamental liability of ramjets. They always require some type of booster to get up to speed, and this represents a significant logistical impediment to their use. Thus, some other propulsion system has to take the ramjet from zero, through mach 1, and up to mach 2 or 3 before they begin to function. Ironically, rockets are usually used to boost ramjets (and scramjet, too). Rockets have their worst performance making thrust at sea-level/static conditions.
    There are some other, more subtle issues with ramjets, as well. Ramjets rely on their supersonic inlet to convert flight speed into total pressure. Air that is shock-compressed in the ramjet’s inlet is then fed directly into the ramjet’s combustor. This has the consequence that ramjets are very sensitive to any disturbances to the incoming airstream. Changes in angle of attack, yaw, and/or roll, as well as acceleration, and even atmospheric conditions, can all deleteriously effect the incoming airflow. These disturbances can easily be sufficient to cause an inlet unstart, in which the shock structure on the inlet is disgorged, resulting in a flameout in the combustor. Even without an unstart, flow disturbances can induce flameouts. And while the basic elements of a ramjet are simple and lightweight, the fuel contoller for a ramjet is anything but. Fuel input into the combustor is a careful balancing act between the resultant combustor pressure and the incoming air stream. Too much fuel, you get a flameout, too little fuel, you get a flameout. The net effect is that ramjets are quite finicky, and require a careful start-up process, and cannot accommodate rapid changes in flight conditions. Is this a limitation for launch vehicles? Maybe your flight trajectory can be designed to be “mellow”, but you certainly will have to put up with much-reduced performance margins.

    For sake of comparison, I think its worth asking the question: Why aren’t ramjets in use today?”. We’ve had a number of ramjet-powered missile systems: BOMARC, terrier, Taos, to name a few. The basic answer is that ramjets required a boost function, which is usually accomplished with some sort of rocket motor. Since you needed rocket propulsion anyway, and rocket powered missiles could launch themselves, they were thus logistically easier and less expensive to deal with, even though they have significantly lower Isps, and had to be bigger to make a given range. Solid rocket motors are also a lot more forgiving in terms of logistics, and this gets back to performance margins.

    I must disagree with the statement that the T/W of ramjets “could probably be brought up to 150-200 without too much effort”. First, I doubt that these T/W’s are achievable (no reference), and second, anytime you’re dealing with airbreathers, its always a lot of effort. The testing alone is a huge effort: supersonic wind tunnels, supersonic flight tests, etc. I don’t know if you ever saw Marquart’s facilities, which were located right at the Van Nuys Airport in LA, but they were very impressive, in terms of the compressors, steam ejectors, air vitiaters, and the general air flow handling equipment.

    Ejector ramjets, in which rockets are embedded in the combustor section of the ramjet, are in fact, a viable approach, and a number of different programs have been and are investigating these concepts over the years. Aerojet, for example, has been working on and off for nearly two decades on such an ejector rocket concept, which they called the “strutjet”. MSFC also supported this related work under a program called “ISTAR”, I believe. Note however, that it is the rockets that are providing the static thrust, not the ramjet. An element of the ejector ramjet is that the rockets can also induce or “eject” an airflow into the ramjet, thus effectively improving the thrust output of the system, and increasing its Isp. References that I have seen indicate that with a good ejector design, additional airflows of around 10% of the rocket flowrate should be possible. So this represents a usable improvement.

    I believe that you are also suggesting that an injected spray of liquid (a liquid fuel and/or an oxidizer) may also be used as a compression mechanism for airflows. Anyone who has turned on the shower and felt the breeze created by the shower spray knows that the movement of sprays can certainly induce airflows. (My doctoral work was focused on liquid and fuel sprays, so I like sprays).

    The possibility of pumping or compressing air using sprays gets down to the basic process of how one can actually pump or compress a gas. A compressor compresses a gas by using pressure body forces to mechanical push the air molecules together. A spray process “compresses” a gas, or more specifically raises the total pressure, by using shear forces between a moving droplet of liquid and the adjacent air to accelerate this air. Once the air has some speed, you can convert its speed into (total) pressure by allowing it to deaccelerate, or diffuse. Using shear forces to accelerate a gas is not particularly efficient. The greater the velocity difference, the more energy is lost or wasted, so the compression process becomes asymptotic. The net effect of all this is that spray “pumping” can never approach the compression ratios that you can achieve by using pressure body forces, such as those exploited in compressors.

    In my opinion (and I could be wrong), it will not be possible to use high pressure liquid injection to “shear-pump” air to high pressures. In fact, if you inject the spray into the inlet with a high enough velocity (supersonic droplet velocities, for example), you start to become less of a “spray-compressor” and more of a liquid-jet cutting tool. The droplets can easily have enough energy to knock a hole through your inlet walls. I have never seen any data that suggests that you can achieve 20 bar pressure rise using spray pumping. But I could be wrong, and maybe there’s some data out there on it. If you find some, I would sincerely be interested in knowing about it.

    Now that having been said, it IS theoretically possible to use spray-cooling to increase the total pressure of an incoming air stream. This principle was first identified by Dr. A.H. Shapiro in his seminal paper “The Aerotheropressor- A Device for Improving the Performance of a Gas-Turbine Power Plant”, Transactions of the ASME, April, 1956. This concept exploits the cooling induced by an evaporating spray to increase the total pressure of a gas flow, effectively moving down the Rayliegh line. This concept was later exploited as part of a new engine concept, known as a “transition engine”, by myself in 1995 (“The Transition Engine: A Combined-Cycle Engine Concept for SSTO/Trans-atmospheric Vehicle Applications”, AIAA 95-2480).

    The air ionization approach is also interesting and novel. I’ve done a little work using high voltage DC sources to create “ion winds”, and these do indeed make a bit of thrust. Its hard to induce a lot of air movement with this approach, but maybe there’s some clever geometries that can make this work. The power supply system and HV charging equipment is by no means a trivial problem with electric propulsion. I think this area could bear a lot of fruit in terms of mass reduction for power supply and conditioning systems, but that will take some serious and sustained development efforts to realize usuable gains there. FYI, for the electrical supply, the highest power density systems come from, yes that right, turboelectric generators. You burn some fuel, expand it through a turbine, spin a generator. These systems sometimes have an order of magnitude higher power (and energy) density than batteries, supercapcitors, RTGs, fuel cells, etc. ATRs already have the turbomachinery….hmm…..

    A couple of points regarding the J58 and the SSME turbopumps. First, I believe the J58 turbine actually outputs around 160,000 shaft hp (http://www.hill.af.mil/library/factsheets/factsheet.asp?id=5786 ). As you probably already know, that the Thrust Specific Fuel Consumption (TSFC, and sometimes reduced to just SFC) is the flowrate in lbm per hr divided by the thrust produced (also, the inverse of the TSFC multiplied by 3600 gives the Specific Impulse. The above ref quotes a max fuel flow of 8000 gallons per hour, or about 14 lbm/sec, somewhat different that what you quoted at around 6.25 lbm/sec, but I don’t think this changes the argument.

    The power required by a turbopump is proportional to both the fuel flow rate and the delta P that the pump creates. The required fuel pressure in the J58 is far lower than the 3500+ delta P required in the SSME. You could estimate the J58 fuel pump power required by multiplying the fuel flow rate by the delta P, then dividing by rho and 2. I show it being more like around 25 or 30 hp for an assumed delta P of about 300 psi. (I believe the J58 uses a fuel pump driven from an accessory gear off the main turbomachinery shaft. At least, that is a conventional arrangement).

    Well, I hope I provided some useful feedback regarding your comments and ideas. In summary, I believe that Turbocompressors have evolved because they provide the trade between efficiency, pressure rise, delivered flowrate, and mass. And that’s why we use them. Despite being heavier, they offer some powerful logistical advantages. I guess that’s why I think the ATR offers some general advantages, but advantages that can only be truly appreciated when one looks at the overall system, where there are multiple, completing system requirements.

    Nevertheless, other, novel approaches are well worth pursuing. It may be from some long-shot, high-risk approach that a viable and game-changing advantage comes from. And its guys like us who pursue these long shots, alone and ridiculed in our garages .
    For an example of this, check the picture gallery on the plasma wind blog site for “other novel concepts”.

    Good luck!

  5. Robert Clark says:

    Thanks for the very informative response, Mr. Bossard. I do have an idea why the rated shaft horsepower for the J58 is wildly out of whack with the 1 horsepower per 2.5 pounds thrust estimate, even when you use the higher 32,500 lbs. thrust rating.
    The 1 hp per 2.5 lb. of thrust estimate is rather close for the examples given on this page:

    Convert Thrust to Horsepower.

    “How much power does the 747’s Pratt & Whitney engine produce? As we discussed earlier, a static engine does no work no matter how much thrust it produces because it results in no motion. We must instead focus our attention on a plane that is in motion. For example, our 747 typically cruises around 600 mph (970 km/h). However, we are faced with a new problem because the plane does not necessarily need every bit of its static thrust to fly at that speed. In fact, static thrust is really an ideal maximum amount of thrust that an engine can produce in a test environment. As discussed in a previous question about thrust ratings, any jet engine will produce less thrust in actual use than the static value.
    Furthermore, aircraft are equipped with throttles that allow a pilot to adjust the amount of thrust an engine produces. A good example is the SR-71 Blackbird equipped with Pratt & Whitney J58 turboramjets that produced a combined static thrust of 65,000 lb (289 kN). Even though the Blackbird could reach speeds in excess of Mach 3, however, it actually needed very little of this thrust in cruise flight. Most of the thrust was required to accelerate through the speed of sound, but once at Mach 3, the SR-71 engines were throttled back to only 30% or so.
    “The conclusion of this explanation is that in order to determine the power a jet creates in flight, we need to know the exact amount of thrust necessary to fly at a particular speed. We typically know the static thrust rating of an engine or the airspeed of a plane during flight, but the problem is that we usually don’t know the amount of thrust that corresponds to a particular speed at a specific point in time. It is because of this disconnect that it is so difficult to calculate the power generated by the engines on a particular plane.
    “Luckily, we do have access to data from a NASA report that does provide all the data we need to illustrate a sample case. The data is provided for a Boeing 747-200 cruising at Mach 0.9 at 40,000 ft (12,190 m). In this example, the aircraft’s engines produce 55,145 lb (245,295 N) of thrust, only a quarter of its rated static thrust, to cruise at a velocity of 871 ft/s (265 m/s). Using the equations provided above, we calculate the power generated by the 747 to be 87,325 hp (65,100 kW).
    “The NASA data also includes a few other planes, so let’s compare the power generated by the subsonic 747 airliner to a supersonic fighter like the F-4 Phantom II. In this example, the F-4 cruises at Mach 1.8 at 55,000 ft (16,765 m). The aircraft’s two turbojet engines produce 11,560 lb (51,430 N) of thrust at its cruise speed of 1,742 ft/s (531 m/s). This combination of force and speed equates to a power of 36,620 hp (27,310 kW).”
    http://www.aerospaceweb.org/question/propulsion/q0195.shtml

    Note that these numbers are close to the 2.5 to 1 estimate. However, note also that these examples are basing it on how much speed the *entire aircraft* achieves. But clearly this would be dependent on a lot of factors of the general aircraft aside from the engine such as drag, weight, etc. And the article even suggests the horsepower would be zero in a static test environment. But clearly a static jet engine is still putting out alot of power in this case.
    Another means of calculating the power of a jet engine would be by using the equation power = (thrust)x(velocity) mentioned on this page, but using for the velocity the exhaust velocity of the engine, not the aircraft’s speed.
    Since the exhaust velocity of a turbojet is so high this probably explains the higher power rating for the J58.
    This different method for calculating the jet engine power is probably closer to the one we need for calculating how much power we can bleed off for running a separate high performance, high pressure turbopump.

    In regards to the ejector-ramjet analogue for creating high pressure through high velocity (supersonic) fuel injection, I’m inclined to agree it wouldn’t create enough pressure for the high volume of air we need.
    However, I’m still optimistic about the possibility of achieving the compression of the large volume of air by using a wall of liquid propellant at very pressure, such as the 500 bar of the SSME liquid hydrogen turbopumps. I suggested before an annular ring opening for the liquid propellant inside the air intake. This may or may not be optimal. But for analyzing its feasibility let’s first just imagine this high pressure liquid being directed directly inwards into the intake from the front. It would be like a high pressure water hose being directed into the intake.
    To insure the liquid and air just didn’t flow out the back you could constrict the cross-section so that the flow becomes somewhat choked. This would allow the pressure to build up before entering the combustion chamber.
    You would need to allow the air also to enter the intake so you don’t want the hose to take up the entire intake opening. The idea however is to create a wall of liquid to compress the air. Perhaps a relative small gap between the hose outer diameter and the intake walls would allow most of the air to be so compressed.
    As the air directly in front of the hose opening is moved inward, the incoming air from the sides *might* tend to fill in this space. However, the liquid is at such high pressure while the incoming air is at sea level pressure or less, I don’t know if this will actually take place. What might actually happen, and would actually be just as effective, is that the incoming air would be pushed against the sides of the intake wall by the liquid and be also thereby compressed.
    The fuel output of the SSME turbopumps is much higher than the fuel need for the J58, as expected for the SSME producing so much more thrust than the J58. You might be able to create a much smaller version of the SSME turbopump for a J58 sized engine. (BTW, I’m aware the current fuel pumps for the J58 are so much less power than the SSME turbopumps. This undoubtedly is because the pressure in the combustion chamber in the J58 is so much less than in the SSME, and the required fuel flow rate is also so much smaller. However in my scenario I’m guessing you’ll need quite high pressure to create the “wall of liquid” to create the compression of such large amounts of incoming air. It would be so much the better if it could be accomplished using the relatively low pressure J58 fuel pumps!)
    Creating the SSME turbopumps was a major technological challenge, however. I don’t know if they can simply be scaled down. Possibly you could instead use a larger version of the J58 engine and use the same SSME turbopump. Note that since you are dispensing with the compressors and turbines of the J58, just keeping the intake and combustion chamber, scaling this up might not be that big a challenge.
    Alternatively, you could use several copies of the J58. Again because you made the engine lightweight by dispensing with compressors and turbines, this would still have quite good thrust to weight ratio, under the assumption the very high pressure fuel flow method can induce the usual compression ratio of the J58.
    The feasibility of this method could be easily tested without using combustible fuels as the liquid. You could use water at the pressures seen with the SSME turbopumps of 500 bar. There are commercial water pumps that can put out these pressures. You could see if such high pressure water injected into a pipe with an increasingly constricted diameter caused the air to be compressed to pressures required for a turbojet engine, about 10 to 20 bar.
    If this worked you might then try cryogenic liquid nitrogen to also see if the cooling effect you mentioned could improve the pressure increase even further.
    Another key question that would need to be addressed is the mass/volume of liquid propellant being used to compress the large amounts of incoming air. From the the Wikipedia page on the J58 engine it uses 450 lbs/sec of air, 205 kg/sec. At an incoming sea level density of 1.2 kg/m^3, this is 170 m^3/sec of air as it first enters the intake.
    Since as you say the fuel usage of the J58 is greater than I estimated, I’ll say the liquid hydrogen fuel usage might be larger as well, call it 4 kg/sec. Could this 4 kg of flowing liquid be used to compress 205 kg of gas (air)?
    Said another way, the volume of this liquid hydrogen at a density of 72 kg/m^2 would be 1/18 m^3 every second. Could this be used to compress 170 m^3 of air?

    Bob Clark

  6. Robert Clark says:

    In regards to the method of converting the high pressure liquid hydrogen (500 bar) to a high velocity stream at ca. 1,100 m/s via the Bernoulli principle and using it in an analogous way to an ejector ramjet, your argument that this would be like a waterjet and would just cut through the air without compressing it (and possibly also through the intake walls) may indeed be correct.
    Perhaps we could get it to be further like an ejector ramjet and therefore get the compression ejector ramjets get by vaporizing the liquid hydrogen at this high speed. We might be able to do this by heating just the outside of the pipe but more likely at this high speed we would need heated wires or concentrically arranged heated pipes within the larger pipe.

    Bob Clark

  7. Robert Clark says:

    I’m still wondering about that small amount of fuel at high pressure or high velocity being used to compress that large amount of incoming air.
    Another possibility: use only a portion of the fuel in this way to compress its corresponding amount of air, then use the combusted fuel/air as a further ejector ramjet to compress the rest of the air and fuel. This would be like a two stage ejector ramjet. You could even extend this idea to several stages.
    (BTW, in looking at some of the literature on “ejector ramjets” a.k.a “rocket ejectors” it appears the “ejector” is regarded as the part of the engine that draws in the air, not the rocket part as I was thinking. Puzzling, but I’ll try to be consistent with that usage.)
    Say you wanted to use only half the fuel at the first stage. Let’s say we’re using liquid hydrogen with an air to hydrogen mass ratio of 40 to 1, about what you would get at stoich taking into account oxygen is about 1/5th of the total air.
    Likely at this first stage you wouldn’t get much compression with this small amount of LH compressing the much larger amount of air. Then the thrust wouldn’t be especially good. But nevertheless after this first stage combusts, what you would get out would be high velocity exhaust at 40 times larger mass than before. Indeed it would now be at the same mass as the rest of the air that has to be combusted with the 2nd half of the fuel.
    It wouldn’t necessarily have to be half the fuel you used at the first stage; it could be smaller or larger. Larger amounts though would mean for this part of the fuel you’re getting worse performance. So I’m inclined to believe, though am in no way certain, that smaller amounts would be more optimal for the first stage. You would need more than one combustion chamber using this method but since each combustion chamber is smaller hopefully the total mass would be the same.
    If you used 3 stages, you could use 1/4th the fuel and air at the first stage. This would be combusted, at low performance, to compress the same amount of air and fuel, that is 1/4th, getting better performance, which combustion would be used to compress another 1/2 the air and fuel, getting even better performance. Again these proportions could be varied to see which gave the optimal thrust and Isp.
    IF it turns out that you can get good final performance by using a significantly smaller amount at the first stage, say 1/4th, 1/8th, etc., then what you might do instead is just use a quite small turbine engine at this first stage. This would improve the performance at the first stage and the mass would still be significantly less than a normal turbojet since only the much smaller jet at the first stage has compressors and turbines.
    Here are a couple examples of small turbojet engines used for manned aircraft:

    MICROTURBO TRS 18-1
    ENGINE SPECIFICATIONS.
    http://www.bd-micro.com/FLS5J.HTM#ENGINE

    Turbojet engine TJ 100.
    http://www.pbsvb.cz/dlt_motor_tj100.php?lang=en

    This last might also be something amateurs could test out. There are tiny jet engines meant for remote controlled model airplanes. Amateurs have also built examples of small ramjet engines using air from compressed air cylinders at the front to provide the compression. Then you could instead use one of these model airplane jet engines at the front of the ramjet to act analogously to an ejector ramjet.
    Model airplane jet engines:

    Advanced Micro Turbines.
    http://www.amtjets.com/specs.html

    Example of an amateur-made, compressed-gas ramjet:

    Coffee Mug Ram Jet Engine.
    http://www.youtube.com/watch?v=UUJP5Jh9_Bg

    Bob Clark

  8. Randy Campbell says:

    John B., Bob, etc…

    I’m not sure if anyone else knew this but PART of the reason the fuel numbers of the SR never seem ‘exactly’ right is because once it leaves the fuel tanks the mix was used (and along the way some inevitably gets lost I suppose) everyhere else in the fuselage/engine system FIRST! Directly from the tanks it was the hydralic fluid for the engine systems and flight controls, then would be used for cooling life support, the airframe, etc, and only at the very end of its journey would it be injected into the engines to burn!

    John B. if you can look for an archive copy of or site-mirrors that saved some of the alt-acc.com information from when it was up and running. Before Plasmawind came along it was pretty much THE best place for air-breathing information in general and ramjet engines in particular on the web and a lot of the information was gathered no where else on the web.

    Bob:
    The Gluhareff Pressure jet engine, see:
    http://www.tipjet.com/
    http://www.tipjet.com/tech_data.htm
    … uses high speed (supersonic flow) propane that has been super-heated to entrain air through a three-stage sonically tuned intake system with no moving parts with enough static thrust to get itself moving and once ram-air begins thrust increases to optimum levels with a smooth throttling control and fairly good SFC. The idea of using hydrogen for this same purpose (which is basically what’s being discussed as I understand it) is probably do-able though more likely with the fuel injected as a high speed gas after traveling through a series of heat exchangers, (hull, intake, combustion chamber, nozzles, the to injection) rather than being injected as a liquid.

    Randy

  9. John Bossard says:

    Randy:
    thanks for the reference link, always good to know more sources. You’re quite right. Because the fuel was required to perform multiple tasks, it was highly specialized, had lots of funky additives, etc. This contributed to it being hard to ignite, and why they went for TEB injection for ignition. While reliable, it was logistically complicated. (I’ve used TEA/TEB for igniting rocket engines. It works well, just don’t uncap the supply bottle. The green color comes for the boron compound).

  10. Robert Clark says:

    Thanks for the info on the Gluhareff Pressure jet engine, Randy.
    This is indeed quite analogous to what I was proposing for using high velocity injected fuel to induce pressure in the air. This method doesn’t even need high performance turbopumps for the fuel but gets it to high pressure by heating it from the heat produced by the engine.
    (I still would like to see tried though the method of just using very high pressure fuel, not at high velocity, to compress the air by a “wall of liquid”.)
    I was interested to see in the description of the Gluhareff jet that pressure was raised in several stages. I don’t believe though they are also using separate combustion burners at each stage.
    You might be able to get even higher thrust by using more than one combustion chamber in the stages. The tech info on that Gluhareff engine only gave it a thrust/weight of about 4 – 5 to 1.

    Bob Clark

  11. Robert Clark says:

    Here are some other proposals different from the ejector ramjet idea, based rather on the idea of rotary rockets, that I sent to an amateur rocketry list.

    Bob Clark

    — On Sat, 8/1/09, Robert Clark wrote:
    > From: Robert Clark
    > Subject: Re: [AR] Another prize suggestion – Re: Hypersonic ‘WaveRider’ poised for test flight.
    > To: “arocket”
    > Date: Saturday, August 1, 2009, 6:59 AM
    >
    > There seem to be a million dozen different ways of doing
    > the turbine-free air compression for jet engines. But it
    > seems like all the jet engine and aircraft manufacturers are
    > only focused on improving turbine engines. Why?
    > Perhaps because turbine engines in their view are already
    > good enough, at least on the measure we’re trying to improve
    > on: thrust/weight. For while a rocket engine might have a
    > T/W of 70:1 and a jet engine might only be 6:1, because of
    > lift, where a typical jet aircraft might have a lift-drag
    > ratio of 12:1, the T/W for the jet becomes effectively 72 to
    > 1.
    > What the jet engine professionals are very interested in
    > is improving fuel efficiency since fuel costs are a big part
    > of airlines profit margins. The methods suggested of doing
    > away with compressors/turbines look like they will actually
    > make fuel efficiency worse, such as the ejector ramjet for
    > example, the exact opposite of what the jet engines pro’s
    > are majorly focused on.
    > But how about the rocket guys? They’re focused on getting
    > to orbit and what happens at quite high Mach numbers. What
    > happens at subsonic speed to low supersonic speed holds very
    > little interest.
    > So if this is going to be done, it looks like we’re going
    > to have to be the guys to do it.
    >
    > And here’s another one of the million dozen different
    > ways:
    >
    > I just saw this on Selenian Boondocks:
    >
    > Rotary PDE
    > Nov 23rd, 2008 by johnhare
    > guest blogger john hare
    > http://selenianboondocks.com/2008/11/rotary-pde/
    >
    > Hare suggests using multiple combustion chambers within a
    > rotating torus. He suggests this for rocket engines using
    > pulse detonation propulsion. But the idea would also work
    > for jet engines and you don’t need to use detonation
    > for the method. Regular combustion would work.
    > Detonation propulsion still is not well understood. Hare
    > was suggesting the detonation wave to provide the
    > compression, but you could just have the rotating torus
    > generate the compression required for a jet engine by
    > centrifugal force. The force to rotate the torus would be
    > provided when each combustion chamber released its exhaust
    > in turn.
    > As Hare mentions, this idea for the rocket case is
    > analogous to Gary Hudson’s rotary rocket, at least in the
    > second incarnation where Hudson made the rotating
    > engines be internal rather than on external rotors.
    > Hudson then might be a good one to ask about its
    > feasibility for the jet engine case.
    >

    John Hare discusses a variant of the Gary Hudson Rotary Rocket to get high thrust for the thrust provided by the rockets at the rotor tips here:

    Roton Revisit.
    Nov 5th, 2008 by johnhare
    http://selenianboondocks.com/2008/11/roton-revisit/

    In the comments section on this page, Wayne Gramlich also pointed to a page on an internal type of rotary rocket engine by Roger Gregory:

    halfwaytoanywhere.com rotary rockets.
    http://www.halfwaytoanywhere.com/

    These are rocket engines. I wanted to investigate the case for airbreathing propulsion.
    John Hare in responding about the Gregory rocket engine noted it might be dangerous to have the engine be rotating at ca. 600 m/s rim speed while at high pressure, 10,000 psi chamber pressure, and at the high temperature of combustion chambers.
    This would be a problem in the airbreathing case as well. You might want to use the high velocity rotating torus just for compression and not have the combustion there. Firstly, how fast would you need to rotate it to get the ca. 10 to 20 bar compression typical of turbojets? Perhaps someone can give the equations for centrifugal compression of a gas given rotation speed and radius.
    As a first guess I’ll estimate it by what happens with ramjets. Ramjets get good compression and thrust and Isp when the aircraft is flying in the range of Mach 2 to Mach 3, about 650 m/s to 1,000 m/s. I mentioned before a case of a ramjet able to get a thrust/weight ratio of 40 to 1 using hydrocarbon fuel. So we could get likewise good compression, thrust, and Isp if our rotating chamber encounters the air, when still as at start or when moving as when flying subsonically, at a rotating rim speed of 650 – 1,000 m/s. There are flywheel rotors that can rotate at these speeds, though I believe 1,000 m/s is near the edge of what is currently being done.
    But this would be a problem if we made the high temperature combustion chamber be rotating at this speed. The highest rim speed flywheels use for example carbon fiber or synthetic fibers such as Kevlar to withstand the tensile stresses at these speeds. The chamber pressure when using air would not be nearly as high as the Gregory rocket case of 10,000 psi using liquid fuel and LOX. But still the materials used for fast flywheels could not withstand the temperatures of combustion.
    Perhaps regenerative cooling could be used as for rockets. But the allowed temperature would be significantly lower for the flywheel materials compared to the metals used for rocket engines.
    A few other possibilities. Use two torii, one for the combustion chamber, one for the compression chamber. The compression chamber torus may compress the air by encountering the incoming air at supersonic rim speed, as happens with ramjet compression, or just by using centrifugal compression. Using centrifugal compression, with the air introduced at the hub, might be preferred since you don’t have to deal with the shock waves or the heating from the low altitude, high density air suddenly encountering an inlet at supersonic relative speed. In any case, once the air is sufficiently compressed it would then be presented to the combustion chamber.
    The combustion chamber would be rotating as I suggested before due to exhaust vented from nozzles around the toroidal chamber, as with Hero’s engine (like a lawn sprinkler.) Note in this scenario, unlike what I first proposed in a prior message, the compression is NOT provided by the rotation here. So you don’t need high rotational speed and can use higher temperature resistant metals. You convert this low speed rotation of the combustion chamber into high speed rotation of the compression chamber by principles of mechanical advantage, such as with gears, levers, etc.
    Note you also have some flexibility with this combustion chamber. You might be able to get better efficiency and have lesser technical complexity if the combustion chamber does not rotate. Instead it could be a standard fixed combustion chamber but you could have some of the exhaust bled off to connect to a Hero’s engine, for lack of a better term. Or you could have instead use a heat eschanger to heat hydrogen using the heat only from the combustion chamber to thereby drive the Hero’e engine. Since hydrogen is of low molecular weight you wouldn’t need to get very high temperatures here to get high velocity exhaust from the nozzles of the Hero’s engine. As before the rotation of the Hero’s engine would be used to drive the rotation of the compressor torus.
    There are several other variations. For instance instead of a toroidal compression chamber you could have a piston and cylinder arrangement driven by the Hero’s engine, as the piston is seen here driven by a flywheel:

    Energy Storage II.
    http://zebu.uoregon.edu/2001/ph162/l10.html

    The jet engine would likely have to be pulsed in this case but we could emulate continous operationn by having two or more pressure cylinders operating sequentially.
    As a variation on this idea we could use fan blades, cupped or flat, that drive the air for compression as suggested by John Hare here:

    Cagejet Turborocket
    Oct 17th, 2008 by johnhare
    http://selenianboondocks.com/2008/10/cagejet-turborocket/

    This would look analogous to a paddlewheel on a steamboat. The advantage of using this or of the piston and cylinder is that you wouldn’t need very high rotation or lateral speeds, just sufficient force to compress the large volume of air. Again this could be done by using methods of mechanical advantage, driven by the Hero’s engine.

    Bob Clark

  12. some slob says:

    So, everyone, I hope these comments haven’t closed, because I just stumbled across them…

    After wading through most of this stuff I figured I could just cut to brass tacks as I didn’t see my idea here already.

    It’s pretty simple.

    “TSTO on the cheap and easy!”:

    Step 1 – Strip out interior of a common 747-8 transonic aircraft, including JP4. Inflate in-wing fuel bladders with helium to maintain wing structural integrity. Integrate interior of fuselage with LH2 cryotankage running the length of the aircraft.

    Step 2 – Add ramscoops to outside of 747. Connect to a suitable heat exchanger “powered” by the onboard LH2. Add mechanical arrangements to fraction and separate the incoming air (we will use it in step 6).

    Step 3 – Replace the 747’s 4 turbofans with 6 ramrockets optimised for subsonic flight – ramrockets to be powered by LH2 warmed by the onboard air liquefaction equipment.

    Step 4 – Attach some kind of orbiter craft to top of 747, with orbiter to be powered by onboard aerospike engine(s) fueled by carrier-aircraft-made LOX and preloaded LCH4.

    Step 5 – “Hop” from runway to air with orbiter (full of LCH4 only) attached to 747 (loaded full with LH2), proceed to cruising speed (Mach 0.8).

    Step 6 – “Skip” oxidizer loading by cruising at speed with the air-liquifier on; the LOX is diverted to the orbiter tankage (loading it for an insertion burn with the LCH4) while the remaining liquid air fractions (nitrogen, etc.) are used as cold working mass to be injected into the ramrocket engines (I envision little aerospikes in them – the still-liquid air fraction(s) would be injected where the virtual “air spike” is supposed to sit in the inside of an annular rocket-nozzle/hot gas generator, rapidly expanding and improving outbound mass flow – incoming air should keep the “spike” tapered and make a good combustion zone).

    Step 7 – “Jump” when the orbiter is full of LOX (and the astronauts of bagels – tee hee) and the 747 reaches a nice altitude like 40,000 ft; separate vehicles at subsonic speed, drop the 747 for the return journey to the runway and light the orbiter’s rockets for a burn to Mach 25 and Low Earth Orbit.

    Inspiration: Shuttle Carrier Aircraft + NASA GTX + SpaceShipOne + LACE + Skylon + Black Horse + Pegasus(rocket)

    Advantages: “Free lunch” (ha ha) ride for orbiter to 40,000ft and ~500mph, in-flight oxidizer loading utilizing LH2 fuel that will be burned anyway, subsonic vehicle separation (to conserve stability), kickass dead-simple (and relatively cheap) ramrocket engines on the carrier craft that will provide static thrust AND collect both working mass and oxidizer in-flight to improve performance, aerospike (overall most efficient!) rocket engine(s) on orbiter, everything is 100% re-usable.

    Disadvantages: Heavy LH2 tankage, heavy orbiter (certainly!), heavy heat exchanger – all things that can be engineered lighter.
    Oh, and a jumbo jet full of explosives… like the Shuttle External Tank, but with wings. Don’t be scared.

    Am I missing anything?

    Oh yeah – I like your turborocket ideas but seriously turbomachinery is heavy, expensive and has zillions more points of failure than a rocket (e.g. thin vanes pulling multiple G at ridiculous temperatures). A rocket with some duct wrapped around it (ramrocket!) is hardly more complex than a rocket alone and can provide static thrust also, plus this particular design will have several metric f*cktons of LH2 available to burn so using fuel-gulping engines means almost nothing. As for the orbiter – if all you can fit on it for cargo (minus pilots) is 1,000kg but you can reach LEO or even geosync each flight (with a fully reusable launch system) you return your bilion+ $ investment in the first system’s lifetime – anyone with something to get into space will buy your services.

    BTW the 747 was a suggestion – you could also try an AN-225 or something else with sufficient lift capacity (ha!).

    _d

    P.S. – Criticize me! I am an ignorant youth. Also I want feedback.

  13. johnhare johnhare says:

    JH: I found this in the filter and decided to paste them here and comment on them. It was posted by “Some Slob” I hope that’s just a screen name.:-)

    So, everyone, I hope these comments haven’t closed, because I just stumbled across them…

    After wading through most of this stuff I figured I could just cut to brass tacks as I didn’t see my idea here already.

    It’s pretty simple.

    JH: not really

    “TSTO on the cheap and easy!”:

    Step 1 – Strip out interior of a common 747-8 transonic aircraft, including JP4. Inflate in-wing fuel bladders with helium to maintain wing structural integrity. Integrate interior of fuselage with LH2 cryotankage running the length of the aircraft.

    JH: You’ve just redesigned the wing and fusilage, that’s going to cost. A new vehicle might be cheaper.

    Step 2 – Add ramscoops to outside of 747. Connect to a suitable heat exchanger “powered” by the onboard LH2. Add mechanical arrangements to fraction and separate the incoming air (we will use it in step 6).

    JH: That’s a lot of modification and machinery thrown out there.

    Step 3 – Replace the 747’s 4 turbofans with 6 ramrockets optimised for subsonic flight – ramrockets to be powered by LH2 warmed by the onboard air liquefaction equipment.

    JH: Ramrockets are terrible for subsonic flight. Fuel consumption approaches the rocket alone. Plus you would have to develop them from scratch. Also, ramrockets need oxidizer in the rocket side.

    Step 4 – Attach some kind of orbiter craft to top of 747, with orbiter to be powered by onboard aerospike engine(s) fueled by carrier-aircraft-made LOX and preloaded LCH4.

    JH: See the airlaunch home page for the difficulties of top mounted orbiters.

    Step 5 – “Hop” from runway to air with orbiter (full of LCH4 only) attached to 747 (loaded full with LH2), proceed to cruising speed (Mach 0.8).

    JH: By hop, do you mean take off?

    Step 6 – “Skip” oxidizer loading by cruising at speed with the air-liquifier on; the LOX is diverted to the orbiter tankage (loading it for an insertion burn with the LCH4) while the remaining liquid air fractions (nitrogen, etc.) are used as cold working mass to be injected into the ramrocket engines (I envision little aerospikes in them – the still-liquid air fraction(s) would be injected where the virtual “air spike” is supposed to sit in the inside of an annular rocket-nozzle/hot gas generator, rapidly expanding and improving outbound mass flow – incoming air should keep the “spike” tapered and make a good combustion zone).

    JH:You have added a very complex subsonic engine cycle to a cutting edge air liquifier with no visible justification. This is turbofan country at cruise.

    Step 7 – “Jump” when the orbiter is full of LOX (and the astronauts of bagels – tee hee) and the 747 reaches a nice altitude like 40,000 ft; separate vehicles at subsonic speed, drop the 747 for the return journey to the runway and light the orbiter’s rockets for a burn to Mach 25 and Low Earth Orbit.

    JH: Air launches at 40,000 feet are available with almost unmodified aircraft.

    Inspiration: Shuttle Carrier Aircraft + NASA GTX + SpaceShipOne + LACE + Skylon + Black Horse + Pegasus(rocket)

    Advantages: “Free lunch” (ha ha) ride for orbiter to 40,000ft and ~500mph, in-flight oxidizer loading utilizing LH2 fuel that will be burned anyway, subsonic vehicle separation (to conserve stability), kickass dead-simple (and relatively cheap) ramrocket engines on the carrier craft that will provide static thrust AND collect both working mass and oxidizer in-flight to improve performance, aerospike (overall most efficient!) rocket engine(s) on orbiter, everything is 100% re-usable.

    JH: See above on ramrocket engines.

    Disadvantages: Heavy LH2 tankage, heavy orbiter (certainly!), heavy heat exchanger – all things that can be engineered lighter.
    Oh, and a jumbo jet full of explosives… like the Shuttle External Tank, but with wings. Don’t be scared.

    Am I missing anything?

    JH: Simplicity. Calling a complex method simple doesn’t make it so. Complexity is only desirable in direct relationship to it’s advantages.

    Oh yeah – I like your turborocket ideas but seriously turbomachinery is heavy, expensive and has zillions more points of failure than a rocket (e.g. thin vanes pulling multiple G at ridiculous temperatures). A rocket with some duct wrapped around it (ramrocket!) is hardly more complex than a rocket alone and can provide static thrust also, plus this particular design will have several metric f*cktons of LH2 available to burn so using fuel-gulping engines means almost nothing. As for the orbiter – if all you can fit on it for cargo (minus pilots) is 1,000kg but you can reach LEO or even geosync each flight (with a fully reusable launch system) you return your bilion+ $ investment in the first system’s lifetime – anyone with something to get into space will buy your services.

    JH: Read up on turbines a bit. Fear of turbines seems to be a bad meme in rocket circles.

    BTW the 747 was a suggestion – you could also try an AN-225 or something else with sufficient lift capacity (ha!).

    _d

    P.S. – Criticize me! I am an ignorant youth. Also I want feedback.

    JH: Part of being an innovator is the requirment for a thick skin AND open ears. You will need to be able to hear critisizm without taking it personally, and the ability to filter the wheat from the chaff. Innovators without the thick skin give up too quickly. Innovators that don’t listen get killfiled as it’s a waste of time explaining to them. The middle of the road is a difficult path, but the most rewarding if you can hack it. Watch people shred my ideas to see what I mean. The ones ready for immediate use would be going to my friends in the industry direct instead of free on a blog.

    So, everyone, I hope these comments haven’t closed, because I just stumbled across them…

    After wading through most of this stuff I figured I could just cut to brass tacks as I didn’t see my idea here already.

    JH: All these ideas have been tossed out individually from time to time. What kills most of them is the cost, risk, and complexity. If it is difficult to get one of them at a time funded, how hard do you think it wil be to get all of them at once done?

    It’s pretty simple.

    “TSTO on the cheap and easy!”:

  14. Martijn Meijering says:

    John, have you looked at the ejector ramjet and supercharged ejector ramjet concepts studied by Marquardt in the late sixties? They too are concepts capable of operation between Mach 0 and Mach 5. The supercharged ejector ramjet can also be used in ducted fan mode, acting as a very efficient low speed engine during descent and landing. This gives you crossrange, which is nice.

    Another random idea: what about peroxide and subcooled methanol? Methanol is a good coolant and so is peroxide. Peroxide is very dense and methanol is fairly dense. The melting point of methanol is -97C. During airbreathing such an oxygen containing fuel might benefit less from atmospheric oxygen, but because of the more balanced O/F ratio it could allow for better thrust.

  15. johnhare john hare says:

    Martin,

    It is my opinion that the turborocket based concepts are far superior to the ejector series below mach 2. Above that, you really want to be out of the atmosphere soon enough that the advantage range is quite small. Turbine based systems are actually easier in this field than iaircraft use since we can trade efficiency for simplicity and thrust/weight. From a standing start, I believe that a turbine based system could be fielded faster and cheaper than the ejector series if you factor in testing and vehicle integration.

  16. some slob says:

    @ j hare –

    I suppose my post makes more sense if you consider the question to have been “how would *I* get to space” rather than “how would *we* get to space”.

    Let’s say you’re a country who’s just gotten out of 3rd-world debt and wants to move toward space. Ejector ramjets offer shorter and less expensive development/testing/manufacturing lifecycles than turbojets considering you don’t own any turbojet IP or have any commercial giants (Boeing, Ariane, etc.) clamoring to set up shop in your borders. The fact that turborockets also have yet to ever fly will make them even more costly than ramrockets from scratch – if you’re a brand-new space explorer with zero experience in advanced avionic technology trying to integrate someone else’s proprietary engine without melting it OR violating patents… you see what I mean? Growing your own IP fruit tree will be easier than trying to embed someone else’s turbofan design in it as it grows. As for your reply:

    1 – The whole point of modding a 747 is that they’re everywhere – you only have to ADD to them w/out redesigning the airframe to make them available for air launch – brand-new spacepower nations like Mexico would be able to afford something that easy.

    2 – As above – all the “modification and machinery” added is something that has been done for years. The Shuttle Carrier has been extensively modified but remains un-redesigned – it’s still a 747. Simply bolting equipment onto an existing structure that is never expected to pass Mach 1 is a time-honored tradition in avionics; see Boeing 737 AEW&C, etc.

    3 – You got me – I forgot about the ramrocket oxidizer. You could siphon it in-flight from the LOX-maker or carry another internal tank. The point I was trying to get across is that while ramrockets would SUCK for subsonic flight you’d have massive amounts of fuel (a 747’s-fuselage-worth) just waiting to be burned anyway; the subsonic portion of the flight with the carrier craft DOESN’T MATTER when considering efficiency – it’s not GOING ANYWHERE important, just to the landing runway to be refueled. If you’re Guiana and you’ve just built one of these carrier/orbiters France will start paying through the nose for flights – it doesn’t matter how efficient they are if you can leave the gravity well repeatably and reusably. Once the technology starts rolling then you can kaizen it – it’ll get more efficient over time when you have the money to improve it, plus you’ll OWN it (specs & all) outright.

    4 – The video of Shuttle Enterprise holding stable and level while the SCA drops off (check wikipedia) sounds a lot louder than all your naysaying about top-mounted air launch. I believe what I see and what I see is a heavy, chunky b*tch of an aircraft with horrible subsonic aerodynamics and poor fuel efficiency just hanging there in glide while the carrier drops away. If NASA can do it… everyone else can do it cheaper.

    5&6 – It’s cutting edge and complex and added to the subsonic engine cycle because carrying all that hydrogen onboard would be a waste if it wasn’t used to fly the plane. Carrying JP4 (and you’d have to with turbojets/turborockets – H2 embrittlement would kill a turbofan’s alloys, necessitating replacement) would defeat the purpose of stuffing the plane fuselage w/ fuel, which would negate the air liquefier, which would kill in-flight oxidizer loading – which, you have to admit, is a pretty sharp idea.

    7 – Air launches are available with completely unmodified aircraft… if you jump from them (ha ha). Seriously though the in-flight oxidizer loading is why I say modify this stuff – simply carrying the orbiter on a (mostly) unmodified jet and tanking it up in-flight with a third aircraft would be troublesome with cryoliquids. Also you’d have another big-ass subsonic craft wanging around your airspace while you try to get something done. In retrospect it’s probably even cheaper overall than in-house development of anything but I am a slave to aesthetics… 1+1 = orbit, no remainder. Also the ramrocket-powered-carrier-craft would be MY next logical step in development after a 3-craft stage anyway, because I am young and have plenty of time for that sort of thing, plus I could live off the IP when I get old.

    So you poked really nice holes in my theory, which I wanted. I hope you can get over your own close-mindedness about turbofans – just because everyone is circumcised (i.e. using turbofans) does not make it the most comfortable thing in the world. I was trying to consider what might present the (currently) most easily available AND most easily upgradeable solution to get to orbit for countries “just starting out” in the space race; e.g. what would *I* do if I suddenly had a nation-sized budget for spaceflight and wanted to keep doing it all the time. Developing ramrockets early on would make for savings and simplicity down the road, plus IP you could sell to other nations emerging from terran isolation. I admit I don’t like turbojets but I’d totally consider using them if they weren’t so damn complex mechanically AND intellectually – I could certainly buy an efficient, well-specced P&W turbofan on the cheap but I’d ultimately have to go back to P&W for spare parts, repairs, upgrades, etc. and they’d never tell me exactly how to build my own – that bothers me.

    Overall I’m sure we can agree that nobody is trying hard enough to get us off this rock…

    _d

  17. johnhare john hare says:

    I suppose my post makes more sense if you consider the question to have been “how would *I* get to space” rather than “how would *we* get to space”.

    Let’s say you’re a country who’s just gotten out of 3rd-world debt and wants to move toward space. Ejector ramjets offer shorter and less expensive development/testing/manufacturing lifecycles than turbojets considering you don’t own any turbojet IP or have any commercial giants (Boeing, Ariane, etc.) clamoring to set up shop in your borders. The fact that turborockets also have yet to ever fly will make them even more costly than ramrockets from scratch – if you’re a brand-new space explorer with zero experience in advanced avionic technology trying to integrate someone else’s proprietary engine without melting it OR violating patents… you see what I mean? Growing your own IP fruit tree will be easier than trying to embed someone else’s turbofan design in it as it grows. As for your reply:

    JH: Ramrockets sound like dead easy solutions on the first pass. On the second pass some of the problems start showing up. They would have to be developed from scratch in your scenerio. Straight rockets would be faster and cheaper. You really don’t want to fly them subsonic for any length of time, they use over ten times the fuel of a turbofan in that regime. For a straight climb to 50,000 feet from the runway with a massively overweight bird, maybe. For cruise while playing LOXmaker, no way as you will run out of aircraft fuel well before you get your spacecraft LOXed. In your scenerio, the Microcosm or Beal route would make more sense on the KISS principle.

    1 – The whole point of modding a 747 is that they’re everywhere – you only have to ADD to them w/out redesigning the airframe to make them available for air launch – brand-new spacepower nations like Mexico would be able to afford something that easy.

    JH: In your original, you suggested massive renovations.

    2 – As above – all the “modification and machinery” added is something that has been done for years. The Shuttle Carrier has been extensively modified but remains un-redesigned – it’s still a 747. Simply bolting equipment onto an existing structure that is never expected to pass Mach 1 is a time-honored tradition in avionics; see Boeing 737 AEW&C, etc.

    JH: The machinery you suggest has never been demonstrated in flightweight hardware.

    3 – You got me – I forgot about the ramrocket oxidizer. You could siphon it in-flight from the LOX-maker or carry another internal tank. The point I was trying to get across is that while ramrockets would SUCK for subsonic flight you’d have massive amounts of fuel (a 747’s-fuselage-worth) just waiting to be burned anyway; the subsonic portion of the flight with the carrier craft DOESN’T MATTER when considering efficiency – it’s not GOING ANYWHERE important, just to the landing runway to be refueled. If you’re Guiana and you’ve just built one of these carrier/orbiters France will start paying through the nose for flights – it doesn’t matter how efficient they are if you can leave the gravity well repeatably and reusably. Once the technology starts rolling then you can kaizen it – it’ll get more efficient over time when you have the money to improve it, plus you’ll OWN it (specs & all) outright.

    JH: The fuel use of the carrier aircraft does matter when there is so much of it required that it eats away the payload capacity.

    4 – The video of Shuttle Enterprise holding stable and level while the SCA drops off (check wikipedia) sounds a lot louder than all your naysaying about top-mounted air launch. I believe what I see and what I see is a heavy, chunky b*tch of an aircraft with horrible subsonic aerodynamics and poor fuel efficiency just hanging there in glide while the carrier drops away. If NASA can do it… everyone else can do it cheaper.

    JH: It can be done yes, but you can’t handwave away the issues involved.

    5&6 – It’s cutting edge and complex and added to the subsonic engine cycle because carrying all that hydrogen onboard would be a waste if it wasn’t used to fly the plane. Carrying JP4 (and you’d have to with turbojets/turborockets – H2 embrittlement would kill a turbofan’s alloys, necessitating replacement) would defeat the purpose of stuffing the plane fuselage w/ fuel, which would negate the air liquefier, which would kill in-flight oxidizer loading – which, you have to admit, is a pretty sharp idea.

    JH: The carrier aircraft will have to carry the whole mass of the orbiter oxydizer by the time of launch. IMO, it would be more efficient to launch with a full load of LOX as opposed to making it in flight unless there is a long cruise time built into the flight plan. That way your high altitude launch would be with a fully fueled orbiter from an aircraft with just enough fuel to get home on empty and no massive liquification machinery to lift.

    7 – Air launches are available with completely unmodified aircraft… if you jump from them (ha ha). Seriously though the in-flight oxidizer loading is why I say modify this stuff – simply carrying the orbiter on a (mostly) unmodified jet and tanking it up in-flight with a third aircraft would be troublesome with cryoliquids. Also you’d have another big-ass subsonic craft wanging around your airspace while you try to get something done. In retrospect it’s probably even cheaper overall than in-house development of anything but I am a slave to aesthetics… 1+1 = orbit, no remainder. Also the ramrocket-powered-carrier-craft would be MY next logical step in development after a 3-craft stage anyway, because I am young and have plenty of time for that sort of thing, plus I could live off the IP when I get old.

    So you poked really nice holes in my theory, which I wanted. I hope you can get over your own close-mindedness about turbofans – just because everyone is circumcised (i.e. using turbofans) does not make it the most comfortable thing in the world. I was trying to consider what might present the (currently) most easily available AND most easily upgradeable solution to get to orbit for countries “just starting out” in the space race; e.g. what would *I* do if I suddenly had a nation-sized budget for spaceflight and wanted to keep doing it all the time. Developing ramrockets early on would make for savings and simplicity down the road, plus IP you could sell to other nations emerging from terran isolation. I admit I don’t like turbojets but I’d totally consider using them if they weren’t so damn complex mechanically AND intellectually – I could certainly buy an efficient, well-specced P&W turbofan on the cheap but I’d ultimately have to go back to P&W for spare parts, repairs, upgrades, etc. and they’d never tell me exactly how to build my own – that bothers me.

    JH: Don’t make the mistake of believing that anyone that disagrees with you has not researched the problem. Used turbofans will be cheaper than ramrockets for the forseeable future. You can’t sell IP (whatever you mean by that) if it is worthless to the potential customers.

    Overall I’m sure we can agree that nobody is trying hard enough to get us off this rock…

    Some people are trying hard enough, it’s just that the ones that are haven’t been in business for very long, and don’t have the bottomless pockets of the public purse to draw from. The previous government efforts have been like hitting someone with a stick, it hurts but doesn’t stop them, and the bruise will heal. The new space profit based efforts are like hitting them with a virus, they don’t notice until it spreads all over and stops them for the count.

    _d

  18. This makes it a good choice as an endo-atomospheric intereceptor engine, and as an airbreathing component to a space launch system. Interestingly enough and almost as an aside, I would note that, in principle the GG-cycle ATR will in fact be able to produce thrust even in a vacuum, although at a far-off-design operating condition.

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