Ramjet Problems

guest blogger john hare

Ramjets come up just about every time someone takes a fresh look at space launch. It seems such an obvious approach that very few of us have skipped looking into them. The trade studies that have been done repeatedly show that there is no benefit to their use in acceleration missions, which space launch is. The problem is that an intelligent person taking a fresh look at the problem is usually unaware that most of us have looked, and that many of us have been in the same argument dozens of times over the years. I spent time and money (small amounts by industry standards) checking it out for myself. Something that looks that good is not something you drop because somebody you never met says so.

I thought I would draw one of my little cartoons to illustrate the way some of us see the problem. Sometimes a visual image to hang the discussion on helps get the point across in both directions. It is also possible that the new player will see something that everybody else has missed.

ramjet

The rocket on the left is straightforward two stages to orbit with no frills. In the center is the turborocket that I play with which still requires two stages above the launch assist platform. On the right is the ramjet system with a rocket booster to get the vehicle up to ramjet speeds, then ramjet, then still two stages to reach orbit due to the mass penalty of the thermal protection during the ramjet phase. In the launch paths, the rocket is out of the thicker atmosphere by mach three, while the turborocket is there a little longer, while the ramjet has to stay in the air for minutes more with stagnation temperatures well over 1,000 degrees. In addition to the thermal protection, the ramjet vehicle must also have an airframe that provides lift and is controlable from mach 2 through 6. Turning the first two stages into an ejector ramjet doesn’t substantially change the ratios.

The references I have found by experienced investigators that still advocate ramjets for launch are virtually all written by people with a financial interest in keeping the discussion open, read grants.

If you believe you see a way for the ramjets to improve space launch, by all means investigate them. You might be the one that finds a good path. While you are at it though, remember that we don’t take your arguments for any more seriously than you take our arguments against. We can have good productive discussions so long as the parties concerned are respectful of others positions and courteous in making their points. The productivity of the discussion goes away when winning the argument becomes more important than seeking the truth. It is somewhat more difficult in this case because it has been going on for so long.

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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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39 Responses to Ramjet Problems

  1. jstults says:

    Succinct description of the challenge (from wikipedia’s LACE page):

    Also, the higher inlet and airframe drag losses of a lifting, air-breathing vehicle launch trajectory as compared to a pure rocket on a ballistic launch trajectory introduces an additional penalty term 1 / (1 + 1 / ((L / D)(a / g))) into the rocket equation known as the “Air-Breather’s Burden.”[3] The “air-breather’s burden” term implies that unless the lift-to-drag ratio (L / D) and the acceleration of the vehicle as compared to gravity (a / g) are both implausibly large for a hypersonic air-breathing vehicle, the advantages of the higher Isp of the air-breathing engine and the savings in LOX mass are largely lost.

  2. Mike Lorrey says:

    copying from my last cagejet comment, with some additional commentary at the end:

    Well I understand the issues with staying in the atmosphere. There have been a number of TPS improvements since Shuttle, which I’m sure you are aware of: X-33’s metallic stuff, SHARP, etc. I do understand the air breathers problems of greater aerodynamic losses for a winged vehicle, etc.

    However, even if we stick to a two staged kerolox ballistic rocket, I fail to see how using an air augmented ejector cowling around the rocket engine would not significantly improve the rockets performance and orbital payload. The proof of concept at least suborbitally is the GNOM missile, the development program tested its PR-90 rocket: http://www.astronautix.com/lvs/gnom.htm

    “The concept was evidently been proved on a subscale tactical missile, the PR-90, now on display at MAI’s museum at Orevo, north of Moscow. The PR-90, with a launch mass of only 1500 kg, of which 550 kg was payload, could reach an altitude of 40 km and a range of 100 km. The booster unit used 200 kg of RAM-10 ballistite with a specific impulse of 180 seconds to get the missile up to ram-air ignition speed. Then the air-augmented unit, with a specific impulse of 550 seconds, cut in and used 300 kg of propellant to boost the vehicle to its 1 km/s cut-off speed. An equivalent liquid propellant missile (such as the American Lance) weighed over twice as much. A solid propellant equivalent (such as the French Pluton) would weigh three times more. ”

    When it comes down to the nuts and bolts, other than Gnom and the French Onera program, as well as retirees from Marquardt, I don’t see anybody testing hardware today. The examples that have been flown IMHO disprove the unsupported claims of detractors.

    One commenter on a wikipedia discussion over air augmented rockets stated that the main disadvantage is that hardware costs you $1000/kg of dry weight while fuel only costs you a buck, but this is an incredibly simplistic analysis. If the gross weight of an unaugmented launcher is 2-3 times that of an augmented launcher, then obviously the augmentation system is going to have to be a LOT heavier per lb of thrust than just a rocket engine. From our prior comparisons of T/W we see that this simply isn’t true. More fuel also means larger and more massive fuel tanks, and larger and more powerful rocket engines, etc etc.

    As for the costs specifically, everybody knows that the cost of fuel is nothing compared to the overall cost of launch, and that it is gross weight that determines overall the vehicle cost.

    As for atmospheric losses, they may be several times greater with a winged vehicle (which I wasn’t discussing immediately above here) than with a ballistic rocket, but they are still significantly less than the gravity losses of lofting twice the gross mass.

  3. Anom says:

    Mike and John,

    Both of you seem to make excellent points that are supported by past parametric analysis.

    It looks to me like most arguments boil down to whether or not you want an air-augmented rocket or ramjet to help you to reach Mach 6 on the earlier stages of a multi-stage vehicle. This appears to be an economic argument that is maybe unfair to the ramjet and air-augmented crowd, because they have not received a Trillion dollars in engineering investment since the 1950’s like “normal” staged chemical rockets have. Maybe they should not receive this investment for staged rockets, if it is true that ramjets are not economical when compared to conventional chemical rockets, but we don’t know because haven’t really tried.

    For reusable single-stage to orbit vehicles (SSTO), air-augmentation may be attractive.

    I would guess that an SSTO using LOX/RP-1 and air-augmentation would have similiar performance in terms of orbital payload, mass fraction, weight, and engine Isp to an SSTO using LOX/H2 without the ramjets, except the LOX/RP-1 vehicle would have half the structural size and half the empty weight for reentry than the LOX/H2 SSTO would have.

    It would be interesting to see if the SpaceX Falcon-1 first stage (at 3,000 lbs) and Merlin engine could be converted into a cheap air-augmentation demo to see if engine Isp could be increased and if the 1st-stage could have the mass fraction to make it to orbit (i.e. an SSTO but not reusable) with this air-augmented Merlin engine.

  4. johnhare john hare says:

    When you get past all the analysis, it comes down to money. You wake up this morning as Elon Musk with a problem that you need more performance from the Falcon 1 first stage.
    Do you initiate a development program for ramjet, TPS, supersonic lifting airframe, or do you put two production engines on a larger first stage tank?
    Which will cost more?
    Which will take longer?
    Which is the safest route financially in the long term?
    Which is most likely to run into problems unforseen due to lack of experience?
    Think about it as if it were your money, it’s hard to make payroll with cool.

  5. Anom says:

    John,

    I agree with you, except my comments were not directed towards using air-augmented rockets as something that is “cool”. I think that we are saying the same things about the economics of the decision making for ramjets on rockets.

    My comments were more focused on the economics of a single-stage-to-orbit (SSTO) version of existing rocket stages. It is possible that adding ramjets or air-augmentation to an existing LOX/RP-1 1st stage like the Falcon-1 could be enough to have it reach orbit in one stage, and eventually be economical.

    If you are Elon Musk, and you find out that adding ramjets to your Falcon-1E 1st stage could allow it to place 450-kg into orbit without a 2nd stage, then you might approach DARPA or AFRL for the $10 million to $20 million that it might take to do this SSTO experiment. Both DARPA and AFRL have created and funded multiple (at least 4)programs using Falcon-1’s or Merlin engines in unique configurations, so this is maybe something reasonable that goes beyond doing something “cool”.

    The Gnom/P-90 air-augmented rocket from the 1960’s, and the Falcon-1 1st stage appear to have similiar performace goals. The AFRL Fully-Reusable Access to Space Technologies (FAST) program originally wanted to buy 4 Merlin engines from SpaceX to place on a reusable X-37-type booster for sub-orbital hops to ~ Mach 20, but it looks like they are now working with Lockheed on a more expensive and less capable demonstrator.

    It may not be as expensive or as far-fetched as you think for DARPA/AFRL to spend over $100 Million on an air-augmented SpaceX vehicle that could become a re-usable SSTO. DARPA, AFRL, and others probably consistently approach SpaceX for these 1-off rocket launches, because their existing production capabilities makes these DARPA/AFRL programs affordable for their purposes.

    As the reusable Masten and Armadillo vehicles continue to grow in size and capability, and as their base of DARPA and AFRL contracts continue to grow, it is possible that DARPA/AFRL’s desire for cheap responsive spacelift of ~ 400-kg payloads to orbit could push them to experiment in this direction. Integrating a low weight ramjet/scramjet into Masten’s next generation vehicle maybe a lot cheaper and more reasonable than you think.

  6. archibald says:

    Any source on the GNOM outside astronautix ? sounds an interesting concept…

  7. johnhare john hare says:

    Anom,
    In the event that you are 100% correct abvout the utility of ramjets, an SSTO would still have to carry the dead mass of the extra engine through another 18-19 mach numbers to orbit. Adding air breathing to an SSTO likely decreases it’s ability to reach orbit.

    I wouldn’t be surprised if DARPA/AFRL funding was to materialize as you say. While the air breathing options haven’t had a Trillion, they have had many Billions over the last several decades with far less hardware to show. I have a reference somewhere in storage that is a hardback book of conference papers on super and hypersonic ABEs from the late fifties. Also there is an International Society Of Air Breathing Engines and I bought the papers on disk from one of their conferences.

    I’m trying to make the point that this is not a new problem. I didn’t buy into the arguments against either until I spent some time and money checking for myself. Check on ISOABE through AIAA.

  8. Kelly Starks says:

    Sorry, can’t follow your logic even with the cartoon.

    So a ramjet/turboram/rocket-ram would need to stay in the air to work longer (up to mach 6 of 7, rather then a little over 2) and get hotter. We have high temp TPS and materials, which you would need for a RLV anyway.

    Given you burn half a rockets LOx/RP-1 to get to mach 6, the ramjets you carry to orbit are likely no heavier then the big LOx tanks your Ramjet powered craft doesn’t need. Given a smaller ship with Ramjets is likely to be easier to build and service then a bigger bulkier craft with big flimsy LOx tanks, I still don’t see the downside.

    NASA projects a Rocket/ramjet combined cycle would double the average ISP from surface to orbit for a given fuel. For RP that means your mass ratio for a SSTO goes from 10 to 3.2, which makes a SSTO a much easier concept to mass budget.

    So what’s the big “obvious” downside? You get a smaller, lighter, more serviceable, and hence potentially a lower op cost craft.

  9. Doug says:

    “So what’s the big “obvious” downside? You get a smaller, lighter, more serviceable, and hence potentially a lower op cost craft”.

    My sentiments exactly! I’ve just begun my hobby based experiments into air augmented rockets. I’m seeing some CG/CP relationship enhancements (moving the rocket motor up into the airframe closer to the CG) in- favor of air augmented design especially for the reusable re-entry shapes. For pure launch to orbit I would still consider air augmented first stage such as the augmented nozzle. I converse occasionally with Professor Paul Czysz and I’m quite sure he would offer some arguments and enhancements to your illustration. When you fly through the “air-breather corridor” one must design the vehicle to utilize the heat and drag and convert it back into useful energy. That includes the energy that is stored in the fuel during the cooling process as in triple point hydrogen. If the vehicle simply absorbs and stores it as in TPS then it’s a waste. You counter the inlet drag with the lift from compression “compression lift” and the ISP of super heated hydrogen injection, including base burning and then there is the concept of using the shock wave to compress and burn the fuel which requires no motor at all merely a double diamond stabulator. Reduce the inlet wetted area with inward compression inlets. Simply strapping fifties style air breathers on the side of a rocket certainly won’t cut it the entire vehicle must become the propulsion unit. Dual fuel certainly offers some sizing variables that can achieve convergence when applied correctly to RBCC propulsion. Ejector based augmented rockets have simple parts counts which simplify the maintenance process. John you may have given up but I assure you others have not. The politics of the past may not have not favored air-breathers however that may soon change. The passion still exist and the argument is not over. I agree rockets alone are easier not sure the end result is always better. Depends on the mission and the end game.

  10. johnhare john hare says:

    Kelly,
    Pure rocket SSTO mass ratio for LOX/RP is 16. Mass ratio just from mach 6 is on the close order of 6. That ratio of 6 is for all the hardware that you carried up to mach 6 if you insist on SSTO. Mass ratio for the rocket to ramjet speeds and the ramjet portion will be 2 at best. You have dropped your mass ratio from 16 to 12 if everything works out as planned. That leaves 8.33% of the take off mass for hardware and payload for the ramjet case instead of 6.25% payload and hardware for the all rocket case. There is no way you are going to get the ramjet, airframe, TPS, and all the accessory hardware in that 2.08% of GLOW, much less beat the all rocket case. At least if you stage you can leave all that metal behind.

    Doug,
    When the numbers don’t close, it’s time to move on. As I said in the post, don’t expect me to take your arguments for seriously unless you can show me something new and relevant. I’m a businessman, and chasing illusions is a fast way to go bankrupt. I’ve watched it happen.

  11. Kelly Starks says:

    >.. Pure rocket SSTO mass ratio for LOX/RP is 16. Mass ratio
    > just from mach 6 is on the close order of 6.==
    >
    I heard 10-12 for Lox/rp. Get about 12 runing the equations?

    Nit though – bigger is…

    > ..Mass ratio for the rocket to ramjet speeds and the ramjet portion will be 2 at best. .

    Its 2 for all rocket configurations to mach 6! You list 16/6 a touch over 2.6. Also with a Rocket ramjet combined cycle your ISP at take-off is about 700, and more then 2-3 times that at high supersonic pure ramjet. Might want to check your equatinosd – or read up on RBCC..

    See “reference.kfupm.edu.sa/content/s/c/sccream_(simulated_combined_cycle_rocket_125726.pdf”

  12. Kelly Starks says:

    FYI
    Concept assessment of a hydrocarbo fueled RBCC-powered milspaceplane.

    sei.aero/eng/papers/uploads/archive/SEI_JANNAF_Sentinel_2007.pdf

  13. johnhare john hare says:

    I couldn’t figure out how you got your numbers for a few minutes. Figure in gravity and aerodynamic losses to get my numbers.

  14. A_M_Swallow says:

    A White Knight TSTO mother aircraft and rocket based proposal. Start the ramjet by pulling the mother aircraft along the runway at say 100 mph. Fly the aircraft until the limit of the ramjet and then release the rocket.

  15. johnhare john hare says:

    A_M_,

    Ramjets don’t produce usefull thrust at 100 mph. Until you get up to high subsonic, they are nearly useless. Until you reach mach 1.5 or so, they are a really poor engines. SSTO is not going to work carrying the dead weight of a ramjet system all the way to orbit after it becomes ineffective. Given honest numbers, including vehicle development costs, and ramjets just can’t compete financially.

  16. Martijn Meijering says:

    Ramjets don’t produce usefull thrust at 100 mph.

    But ejector ramjets do.

    Given honest numbers, including vehicle development costs, and ramjets just can’t compete financially.

    I’d like to hear more about that. Earlier you were talking about mass ratios, but those numbers do not seem to establish there is no place for airbreathers as long as you don’t take cost/kg or cost/seat into account.

    What if anything makes you think there is no place for airbreathers? Let’s restrict ourselves to ejector ramjets and variants operating with the ejectors always on during ascent. Can we say without further analysis that that would require very expensive TPS development? Is there anything else that would be particularly expensive? The ramjet itself seems to be simple enough.

  17. Mike Lorrey says:

    To correct John Hare’s assumptions, in the notional Falcon 1e first stage, the air augmentation ejector would not be an additional engine, it would be a special cowling around the first stage Merlin engine to guide air and help contain combustion of that air with excess fuel in the exhaust, since the engine is designed to run fuel rich anyways, running a little richer and using a cowling/nozzle to derive thrust from that air combustion will boost both thrust and Isp.

    The cowling may be discarded by the vehicle at 130k ft as dead weight. By that time it’s going to be exceeding mach 8 anyways.

    I’ve thought of using the F1e first stage as an SSTO, but also using the second stage as a propulsion unit on a single seat manned capsule/lifting body vehicle to send astronauts to ISS or the future space hotel. The Russians are charging NASA $57 million per seat to fly to ISS on Soyuz. How much could we underbid that with Falcon1e and a small one man vehicle? Mercury was some 1500 kg, a large chunk of which was heavy electronics, heavy old heat shielding, and a metallic airframe. I can’t believe we couldn’t build something less than 1000kg to fly on Falcon 1e. Putting an air augmentation nozzle on the first stage and you’ve got hella dV.

  18. johnhare john hare says:

    The problem with ramjet class airbreathers of any type is that you have to design the entire vehicle around that engine. The mass, cost, and complexity of all the required systems kills the supposed advantages of the ‘simple ramjet’. You cannot simply add a cowling to boost Isp. If you are not taking cost per whatever into account, then what is the point of trying to find a better way?

  19. Martijn Meijering says:

    If you are not taking cost per whatever into account, then what is the point of trying to find a better way?

    As far as I can tell there wouldn’t be one. But you haven’t really been discussing costs. Why would you have to design the whole vehicle around a partially airbreathing engine and why would that automatically be very expensive, either in development cost or in operational cost? I’m not saying it wouldn’t be, just trying to understand.

  20. Kelly Starks says:

    > john hare
    >
    > Ramjets don’t produce usefull thrust at 100 mph. ===

    There have been ramjet powered craft that start of at lower speeds then that. Now they generally have a lower ISP then rockets bellow 350 mph or so, but thats a seperate question.

    Also if you integrate a ramjet and rocket they both can operate at full effeciency at zero craft velocity.

    >== SSTO is not going to work carrying the dead weight of
    > a ramjet system all the way to orbit after it becomes ineffective.

    Actually the ramjet might not weigh any more then the fuel and LOx tanks it replaces (They have very high T/W ratios.) and you can use smaller rocket engines since you can roughly halve the take off weight ( burn half your weight getting to Mach 6). And making the SSTO smaller and lower drag would cut the power needed to boost it to orbit even more.

  21. Kelly Starks says:

    > john hare
    >
    > The problem with ramjet class airbreathers of any type is
    > that you have to design the entire vehicle around that engine. ..
    >
    How? Or more importantly how would that drive the costs up? You need to build you craft to have hypersonic flight capacity either way. Granted a full rocket craft can leave the air sooner in its accent – but unless your throwing it away – its going to need to come down at orbital speeds. I assume your not talking about staging just as you leave the air at Mach 3 or so, or airbreathing would be even more advantageous.

    Say you take a Pyramid shaped DC-X craft and add ram rockets. The sides of the craft are going to work adequately for ram intakes at the base. L/M adn others have even designed ramjets that work past M6 hung out on pylons.

  22. Kelly Starks says:

    > Mike Lorrey
    >
    > == in the notional Falcon 1e first stage, the air
    > augmentation ejector would not be an additional engine,
    > it would be a special cowling around the first stage Merlin
    > engine to guide air and help contain combustion of that
    > air with excess fuel in the exhaust,==

    You might want have a more adjustable air intake to adjust for speed – but yes this sounds like a completly practicle way to start.

    > ==
    > The cowling may be discarded by the vehicle at 130k ft
    > as dead weight. By that time it’s going to be exceeding
    > mach 8 anyways. ==

    I don’t like droping bits, but that would certainly be cheaper and lighter then the rocket stage it replaces.

  23. Mike Lorrey says:

    Kelly Starks

    “I don’t like droping bits, but that would certainly be cheaper and lighter then the rocket stage it replaces.”

    I dont like dropping bits either, but after a lot of thought and analysis, I realized that obsessing about that was causing the issue of the perfect being the enemy of the good. A ramjet can be mass produced like soda cans, its not complex or heavy, and if you design it for a single use its going to be much lighter than designing it to be reusable. Once the aerodynamics and combustion engineers finish designing the prototype, give it over to some industrial engineers to figure out how to mass produce it cheaply, something which generally isn’t done in the rocket business, they all get hand built like Lotus’.

    I’d love to develop a pure SSTO RLV, but you can’t just do that without a lot of development of systems that are part way there. Baby steps. Remember the power law. The F1 burns 80% of its fuel getting to 30% of orbital velocity. Every pound of first stage fuel mass you dont have to carry and burn adds directly onto useful payload. I think it would be a shame not to try.

    And yeah, you can’t say its impossible unless you give air breathers at least a decent level of funding to prove otherwise. The few examples we do have of air breathing ramjet propulsion totally disprove the “can’t close” argument.

  24. johnhare john hare says:

    19Martijn Meijering

    If you are not taking cost per whatever into account, then what is the point of trying to find a better way?

    As far as I can tell there wouldn’t be one. But you haven’t really been discussing costs. Why would you have to design the whole vehicle around a partially airbreathing engine and why would that automatically be very expensive, either in development cost or in operational cost? I’m not saying it wouldn’t be, just trying to understand.

    I must have misunderstood your intent on your comment before this one to mean that you didn’t see where cost per kg or per seat were important. To me, cost is the whole ball game, apparently we agreed while seeming to disagree.

    The costs of development a vehicle around a ramjet would be quite high. You have to do a lifting wing or body that has low drag at high supersonic speeds. A rocket launch vehicle is an engine and tanks optimised for low mass and doesn’t lift or have low drag at mach several. The required lifting and control surfaces along with the aerodynamic body shape required is so much different that the rocket that it requires a comprehensive development effort in it’s own right. The normal rocket vehicle has far too much drag at mach several to operate effectively with ramjets. Look at any realistic study involving ramjets to see the subtle and the obvious differences.

    The heating comes in a manner and duration that launch and reentry vehicles never see. Several minutes of acceleration at ramjet rates heat up the entire vehicle. Notice that the Concord had skin temps of a few hundred degrees at mach 2, and the SR71 was reported as red hot at just over mach 3. The ramjet vehicle will be operating at multiples of those speeds for an extended time. Rocket reentry vehicles have high drag and rounded surfaces to disapate heat. The ramjet vehicle will have to have low drag and sharp edges which are temperature concentrators. The heat can be handled, but it is another development effort that many wave off. Reentry vehicles have a heat shield on one side that doesn’t have to provide much lift or be low drag.

    The simple ramjet has great performance at the design mach number, which makes it a good high speed cruising engine. At other speeds the variable inlet and nozzle become heavier, more complex, and therefore more expensive. The thrust to weight quoted of 30-40 might happen at one mach number, with the high quoted Isp at a very different number, with sub-optimum performance at all others.

    As I’ve said before ramjets look really good until you look deeper into the problem. The problem is complicated by the people that won’t study the entire picture. The claim by some that the ramjet would be lighter and cheaper than the tanks they think it would replace is one. The rocket engines and tanks for an RP/LOX launcher look to be about 3% of GLOW. At best they could cut the GLOW by a third if everything went perfect and they staged the vehicles. The ramjet, aero surfaces, and TPS would have to mass less than 1.5% of the new GLOW to be lighter. The tanks are also simpler to build, and far simpler to operate than the ramjet and it’s required subsystems.

    The list goes on but you got my point many lines back.

  25. Martijn Meijering says:

    I must have misunderstood your intent on your comment before this one to mean that you didn’t see where cost per kg or per seat were important. To me, cost is the whole ball game, apparently we agreed while seeming to disagree.

    Yeah, I worded it badly. We agree cost is what matters.

    You have to do a lifting wing or body that has low drag at high supersonic speeds.

    Why is that? It sounds plausible for a scramjet or for a ramjet that has a phase during ascent where it runs purely in airbreathing mode. But couldn’t an ejector ramjet vehicle with its ejectors always on fly a mostly rocket-like trajectory?

    A rocket launch vehicle is an engine and tanks optimised for low mass and doesn’t lift or have low drag at mach several.

    I’m intrigued by the possibility of combining this with minimum-cost expendable pressure-fed launchers. Those would be heavy and cheap.

  26. johnhare john hare says:

    The rocket vehicles are out of the atmosphere before the ramjets could make much contribution. They are not designed to handle the brutal flight conditions a ramjet necessarily flies through. Testing is also missed by many advocates. Look at the rocket companies building ‘simple’ pressure fed engines. They spend years developing and testing systems that are supposed to be two tanks, two valves, and a thrust chamber. That turns out not to be the case for engines that can be tested bolted to the ground. Testing an engine that doesn’t even start working properly until it goes supersonic is going to be a major cost issue.

  27. Kelly Starks says:

    Mike Lorrey

    I agree that at least its a start. My concern over droping bits is that it really limits where you can lauinch from, adn how often you can launch – as well as seting a high floor to your launch costs. If you drop stuff – they need to clear out a huge downrange area to keep folks out of your way. Something folks rae going to be reluctant to do – and curently costs a lot.

    I suppose you could drop it in some controled or flyback way – or just consign yourself to only launch in extreamly remote areas though.

  28. Kelly Starks says:

    > john hare
    >
    > The costs of development a vehicle around a ramjet would
    > be quite high. You have to do a lifting wing or body that has
    > low drag at high supersonic speeds. ==

    Have you looked at actual hypersonic craft? I mean a X-15 was basically a cylinder with chine’s and stubby wings. Upgrades were planed with delta wings and integrated ramjets, but if your not optimizing for cruise performance its not that complex a hull design. Really if you launch vertically and are going to throw the LV away, it doesn’t really need the wings. As for the assent – if you accelerate faster you gain the speed you need as you ascend into thinner air. (It would probably be to convenient if you could go into higher thinner air just as you boost fast enough that the faster speed compensates for thinner air in the same sized intake.)

    If your not going to throw the LV or stage away, its going to need to be able to fly through the air at hypersonic speeds anyway. Which means you need to deal with heating etc. Not nessisarily the heat soak rates of a hypersonic craft, but modern TPS should deal with that easy enough adn without a lot of weight penalty.

    >== and the SR71 was reported as red hot at just over mach 3. —

    Off hand it was 1100 to == The claim by some that the ramjet would be lighter and
    > cheaper than the tanks they think it would replace is one.
    > The rocket engines and tanks for an RP/LOX launcher look
    > to be about 3% of GLOW. ==

    Kinda heavy for Rockets. A good rocket can get over 100/1 T/W. Your eiather assuming a high thrust to weight for the vehicle, or assuming heavy rockets — heavier then the Ramjets T/W you weer assuming.

    Given a ejector ramjet could replace some of teh rocket thrust with ramjet thrust – and not need to worry as much about variable geometery, etc – you’ld replace the Rockets with a similar weight of ramjet.

    Given the ramjet doesn’t need to provide as much enough power to assend vertically, they don’t need to provide nearly as much thrust as the weight of the craft. So if you assume 50/1 t/w for the engine, but a ramjet thrust to vehicle weight of 1/2. The ramjets weight under 1% GLOW. LOX or RP tanks are usually about 1%-2% of the weight of what they hold (LH tanks are over 10 times that at least), so if the rams replace half the fuel/LOx weight at glow, thats .5-1% savings in tanks (more for the skins and supports now that I think of it). Which is similar to the ramjets.

    Also I’ve seen projections that with modern materials you could cut ramjets weights by a factor of 4 – but this is all getting very detailed. Were close to needing a trade study of designs.

    Also issues like lower thrust adn delta-v loads with the smaller craft that plows a smaller hole in the sky, vrs the drag from the longer in atmosphere flight.

  29. johnhare john hare says:

    Yes I’ve looked at hypersonic craft, the X-15 was a quite sophisticated vehicle due to the flight regime it experienced.The time they tried a ramjet test on one it burned off.

    If you launch vertically you run out of air well before a ramjet does any good at all. You have to have lifting surfaces to stay down in the air.

    A launch vehicle does not need to fly through hypersonic air. By the time it is hypersonic it is well out of the atmospere, and in reentry it is in a high drag mode that is the opposite of hypersonic flight.
    Adding TPS is one of the issues that screws up ramjets as desirable.

    I’m being generous when I say 3% for rocket engine and tanks combined, advanced designs are getting below that. You are trying to suggest that you can save mass and money out of that 3% with ramjets.

    A ramjet is possibly able to supply thrust though a mach range of five from transonic to mach six. That is about 1,500 m/s. If you run the rocket equation on RP/LOX, you will see that the mass ratio for that amount of acceleration is about 1.5. That means that a ramjet that worked perfectly and burned no fuel at all could only improve mass ratio to about 11 instead of 16 in the all rocket case. By the time you add the real fuel required and the required systems to make it work, the ramjet will cost payload per unit of dry mass, not gain as you are suggesting.

  30. Kelly Starks says:

    > == You have to have lifting surfaces to stay down in the air.

    No (ask any air-to-air missle), though flying on hull lift alone does add a lot of drag.

    > == A launch vehicle does not need to fly through hypersonic air. ==

    It does if you want to land it and reuse it?

    > ==Adding TPS is one of the issues that screws up ramjets as desirable.

    Again, you need that anyway if you want to reuse it.

    > == You are trying to suggest that you can save mass and
    > money out of that 3% with ramjets. ==

    Actually I have been talking about rocket ramjet hybrides, which can dramatically increase systems ISP from take off. Others were talking about pure ramjets. Given NASA own projections are for a doubling of average ISP from ground to orbit, a 3% dry weight savings – as well as making a smaller more rugged craft taking less delta-V to orbit does not seem unreasonable.

  31. Blaise says:

    If on ground with the Ramjet stationary, I could inject air at 250 knots from a source bolted on to the device, and there is fuel + ignition, why would the ramjet not operate? If I can maintain this airflow for 10-20 secs, that should suffice to get the device to it’s operating speed of 300-400 miles per hour, assuming I provision enough fuel. Could
    I have your comments. Thanks

  32. johnhare johnhare says:

    You’ve just added a turbofan to the ramjet with the mass of both. Ramjets are only efficient in a narrow velocity range, outside of which they are useless. I had to go back over the original post to remember what I had said on the subject. I believe your question was answered then.

  33. Demi says:

    Do ejector ramjets have the same drawbacks?

  34. johnhare johnhare says:

    Same problems, we went over that extensively in comments years ago.

  35. Chris Stelter says:

    How about a linear motor to accelerate the rocket to ramjet speeds (at least Mach 0.5)? I realize this wouldn’t work well with very large rockets, but should work pretty well if you wanted a HTHL vehicle for some reason.

  36. Chris Stelter says:

    A linear motor would remove the need for that entire first little rocket booster, which is super annoying. Also, could be the ramjet stage could have small rockets to push up out of the atmosphere and encapsulate the entire rest of the rocket, which could be a single stage.

  37. David Luther says:

    Who wants to contribute to BUILDING hardware? I’m too old to talk about it. Our team of volunteers is working to get funding and includes a few guys who know a little more than I do. OK, a lot more. Hope we find a guy who knows how to sign a paycheck!

    https://exospace.wordpress.com/
    http://www.exodusaerospace.com/

  38. john ashcroft says:

    I think you put the problem well, but I think there are ways round it.
    Sled launch for boost, but in a tunnel … http://www.g2mil.com/tunnel-launch.htm.
    But the tunnel doesn’t need to be as long with higher accelerations.
    Inclined tunnels are easier to make than you may think with raise bore machines, 6-8m diameter, and up to 2000m on a single bore now quite possible. https://www.herrenknecht.com/en/products/core-products/mining/raise-boring-rig-rbr.html. Launch 500-600m/sec.
    Then nozzleless, or semi-nozzleless air breathing rocket ( nozzleless initially and a low expansion ratio nozzle with an expansion ratio of 2-3x with an ablating nozzle made from the casing of the rocket. The airbreathing to mach 5, and rocket to to 6, and 60,000m
    Rocket from there, but you would need to discard the thermal protection, but then this is done anyway.
    The advantage is that the tunnel launch is reusable. Nozzleless rocket stage could also be very cheap. Nozzleless rocket is little more than a pipe with propellant, and the nozzleless phase could be under simple aerodynamic guidance, probably by grid fins, that work well supersonically (aka falcon rocket recovery) .
    Most of the mass fraction, and cost of rockets are in this boost phase. If you can lower the cost of this phase then it looks more attractive to use lower cost, lower isp 2nd stage.

  39. johnhare john hare says:

    With Falcon9 boosters landing and being reused now as a matter of course, this concept has become far less interesting.

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