Semi-Staged Combustion

It is interesting comparing the two best known first stages in the US that use kerosene and LOX. The Atlas 5 and the Falcon 9 use a similar fuel in their first stages and then diverge in the technical aspects. The Atlas 5 with the RD 180 engine has about 10% higher Isp at sea level while the Falcon 9 Merlin has nearly twice the thrust/weight ratio. The over all Falcon 9 first stage seems to have a much lower dry mass ratio which makes up the difference in engine performance and then some.

There are going to be new vehicles designed by the various companies eventually that would like to benefit from the competitive advantages of both vehicles. A high thrust/weight ratio engine with high Isp that also has low dry mass is a desirable target. The more these features can be designed in, the more mass is available for payload, reusability, or both.

One of the engine cycles that is discussed from time to time is the dual chamber concept. It is more or less a gas generator cycle with an exhaust pressure high enough to inject into a lower pressure thrust chamber to burn with fuel or oxidizer to get useful thrust. I suggest it might be possible to get very near RD 180 Isp with very near Merlin thrust/weight with a variation of the concept. A low stage dry mass being part of the goal, I add in a few features that may be unique.

Semi-Staged CombustionIn the cartoon I have two high pressure chambers on the outside with a lower pressure chamber in the middle with an altitude compensating nozzle.

The black boxes in the tanks are the electric inducer pumps from the previous post.  They are to keep the propellants at high enough pressure to the main pumps to suppress cavitation as well as keeping required tank pressurization to a minimum.

The small blue tanks in the inter tank area are for the liquid hydrogen that serves multiple purposes. First the hydrogen feed hits a heat exchanger in the LOX  tank to keep it cold enough to stay liquid and suppress cavitation even as tank pressure drops. Then it hits a heat exchanger in the RP tank for the same purpose. Then it is used to cool the turbine blades the same way that jet engines use air cooling. Finally it burns with the excess LOX from the gas generator to produce thrust.

With the pumps providing pressures to the main engine similar to that of the RD 180, the Isp of them should be similar. About 10% of the propellant goes to the gas generator driving the pumps with a residual pressure of 300 psi after the turbine. If the 300 psi engine was a normal kerosene engine, one would expect an Isp in the 250s from that portion of the thrust system. With the lean (LOX rich) gas generator driving a hydrogen cooled turbine at much higher than normal turbine inlet temperatures, the warm hydrogen mixes with the hot oxygen as it is used for film cooling of the blades and burns in the secondary chamber above the throat. The hydrogen/kerosene/LOX engine at 300 psi could approach the ISP of the main engines due to the higher performance of hydrogen. Hydrogen usage will be a fraction of a percent of the total propellant load.

The compensating nozzle of the low pressure engine in the center would allow reasonable Isp of that portion at sea level, especially with the hydrogen component. The higher expansion ratio made possible would allow much higher Isp at altitude, which, with the hydrogen component, could give vacuum Isp higher than the RD 180. I think the potential result is low hardware mass combined with high first stage performance.

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Electric Inducer Pump

It takes pressure in the tank to suppress cavitation in the pumps for rocket engines. It is customary to use helium for most pressurization due to its’ low molecular weight. Unfortunately, it can take a lot of helium in some cases. Some propellants can self pressurize under the right conditions, though here is where the molecular weigh becomes important. O2 with a molecular weight of 32 is eight times the mass of helium at 4.

Heating up the pressurant gas helps considerably, though helium can be heated up as well. Sub-cooling the propellant helps suppress cavitation and allows a lower pressure in the tank to be effectively pumped. I am going to suggest an inducer pump in the tank instead.

If the required pressure in the tank can be reduced from 30 psi or more to 5 or so as the vehicle reaches altitude, the pressurant gas quantity can be reduced by a factor of 6.  An electric inducer pump in the tank might make this possible. A pump that is bypassed early in the flight is gradually brought online with increasing power as the tank level drops and with it the head pressure.

header pumpThe waste heat from the pump can be used on the pressurant gas to reduce the required mass. The pump power can be gradually increased to keep a constant 100 psi  or whatever the spec requires to the turbo pumps.

The objections I have had in the past to electric pumps have mostly to do with the mass penalties of the electric motors, and to a much greater extent the battery mass to reach engine pressure. A relatively low pressure pump used as an inducer gets around some of this. If pressurant gas is reduced along with the elimination of their tanks, that should compensate for the all of the motor weight and some of the battery weight. If the batteries are those of the satellite payload, then there might even be a mass savings. Many of the satellite payloads have a considerable amount of their mass in batteries which might as well help haul the freight during launch when they are probably bored anyway.

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Notchbell Nozzle

Several years back I suggested a type of compensating nozzle that should be inexpensive to build and test. Unfortunately the ones I made for demonstration with compressed air were hit and miss as I didn’t have the theory quite right. Hit and miss is not good enough for serious companies, so I mostly dropped the idea for a while. I thought a few of my acquaintances in the business might do something with the idea for a while. Now I think the idea has  been mostly forgotten as unworkable.

A few years back I did finally find the missing part of the concept and did nothing about it as I thought at the time that others had picked it up and moved on. Since I don’t think that has happened, I am going to repost the concept.


On the left is the engine with the notch showing on the right side. The notch allows the atmosphere to enter the bell to compensate for over expansion at lower altitudes. At higher altitudes and in vacuum the exhaust gradually uses the whole bell with some losses through the notch. This will allow a nozzle to be optimum at sea level when most are over expanded. It will also be nearly as good as a full diameter high expansion nozzle in regimes with the exhaust under expanded.

The missing ingredient in the prior concept was appreciation for the momentum of the exhaust at the notch site. The momentum, especially with the rounded notches that I was advocating before, would prevent the atmosphere from entering the notch in a controlled manner. The addition of a sharp edge at the notch to assure a clean break and a slight reverse on the notch edge to direct the exhaust inwards controls the momentum of the exhaust in a manner that allows the atmosphere to interact and provide pc/pa compensation at a range of back pressures.

A compensating nozzle allows lower pressure engines to operate more efficiently in a launch vehicle. They should allow a payload increase of 1-5% depending on the vehicle and the assumptions going in. For a VTVL that wants to operate at very low pressures in the landing phase, a compensating nozzle would be a very important upgrade, though the successes of Blue and SpaceX take some of the edge off that argument.

This is a public domain concept as I described it here years ago. So anyone that wants to see what I am talking about can build a quick and dirty nozzle to use with shop air. The ones have done were an air chuck and fiberglass. About $10.00 in materials. I know it works at 135 psi. Then you can try a higher pressure gas if it might be useful to you or someone you know.

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Old Engines

Some before, and many after the accident criticized the use of old engines for the Antares. Many references were made to the long storage time with some mention of them as antiques. The refurbishment of these engines didn’t seem to slow down the slings, arrows and insults much.

My question is, at what point do the old Shuttle engines for the SLS reach the same category as the  failed ones on Antares? After all, at the probable flight rate, the used SLS engines will be in the same general age range at some point as the failed ones at Wallops.

Eventually there might be new engines, but how far down the road?

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Working full time on the Martian surface is within US Radiation Worker limits

The US Radiation Worker annual radiation limit is 5 rem, or 50mSv/year. Divided into the 2000 annual working hours, that’s 0.025mSv/hour.

The Mars Curiosity rover measured an average dosage on Mars of 0.67mSv/day at about -4km altitude. That’s 0.028mSv/hour.

If you worked somewhere lower altitude, like Hellas Basin (-7km or even possibly -8km) or a place like Valles Marineris at -5km and valley walls nearby, you should be able to get that down to the 0.025mSv/hour of US Radiation Worker limits.

Or work French hours. 😉

Or limit it to 1000 hours per year like commercial airline pilots, the rest indoors.

So really, at least during solar max (when GCR is at a nadir), the surface radiation levels of Mars don’t seem like any insurmountable barrier at all, provided you can adequately shield everywhere else and provided you’re okay with US radiation worker limits. For instance, 3m of polyethylene at -5km altitude (Valles Marineris) gives you 22mSv/year (although the model I use for that calculation is questionable to me… it seems there are too many low-energy neutrons). 1m of water/PE is 48mSv/year. Of course, you can also bury below a bunch of regolith or burrow to achieve arbitrarily low radiation levels.

Long-term, I suspect we’ll find drugs that are effective. Or we’ll find out if the Linear no-threshold (LNT) model is correct or not. In any case, when there are millions of people on Mars, it’ll be much easier to produce high-quality data about relative risks for low-dose radiation and also easier to develop enough statistical power to show whether or not drugs like Amifostine can protect against low-dose chronic radiation as well as the acute radiation we know it’s effective for.

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Homo Cosmicus: Vestibular Implants

EDIT: David Birchler mentioned in the comments that an implant is not required. The technology is called galvanic vestibular stimulation and it can stimulate the sensation of pitch, roll, and yaw. Since surgery is not required, it sounds like this really IS doable as a countermeasure for dizziness on landing (perhaps combined with training) and the sensation of coriolis in a short arm centrifuge. In fact, for the former, it looks like this is already being tested as a training tool for astronauts:
and here, it shows that GVS training can allow quicker adaption to different vestibular environments:
Here’s a company which is developing the tech (along with Mayo Clinic) synced to Virtual Reality: VMocion

For a few years, now, I’ve been convinced that one of the best ways of making human spaceflight affordable and even competitive with robotic spaceflight in some areas is by engineering as close to the human as possible.

For instance, to solve radiation issues, you could shield the entire spacecraft. But this requires a lot of mass. Mass-wise, better is to shield just the habitable areas. Better still is to focus on areas the crew spends the most time in, like the sleeping quarters. Better still (though weird and awkward) might be a garment with radiation shielding. And yet we can do better still: radiation countermeasures in drug form, which protects at the cellular level. Drugs like Amifostine. (But we need better ones.) Or even closer, the nuclear (as in, cell nucleus) level: You could propose either genetically modifying people themselves to produce radioprotectants (or perhaps engineering our microbiome to produce radioprotectants without requiring engineering of actual humans). In each case, the closer to the human you engineer, the lower the mass and ultimately the cheaper it is (at scale).

Another example of this would be microgravity. There have been drugs that have been used to help maintain bone density, such as those used for osteoporosis, particularly Bisphosphonates, which have already been tried on ISS (although there are more powerful drugs available which haven’t been tried, such as Forteo). But exercise seems largely sufficient for the usual durations. And on a larger scale, you could try internal short-arm centrifuges. And on a larger scale, tethers for artificial gravity. But at each point, the mass overhead becomes greater. So I prefer the drug-based countermeasures if possible.

Another effect is a sense of dizziness after astronauts return from long stints in microgravity. The dizziness doesn’t last too long, but it’s feared to prevent rapid escape from the vehicle after landing (say, on Mars) if there’s a problem. I think this is a corner-case-of-a-corner-case, i.e. you have to have a survivable landing but still have to have a reason to immediately exit the vehicle AND be close enough to other help while also not being too injured too move AND you have to be so dizzy that you can’t exit the vehicle.

But let’s say that’s the only showstopper to microgravity. (and it’s not a showstopper, but let’s say it is) Another possibility is vestibular implants. Some people actually have damage to their vestibular system from disease or injury, and so they can be given an artificial vestibular implant, using external MEMS gyros attached to their head, to restore a sense of balance:

First Successful Installations of Vestibular Implants in Humans

But because the signal is now synthesized, you can now modify it. You can impose the feeling of gravity on an astronaut in microgravity, prepping the astronaut for landing. Or perhaps smoothing out the strong Coriolis effect from short-arm centrifuges. You should be able to reduce the dizziness an astronaut feels after returning to gravity.

Also, it’d allow for some crazily-immersive VR.

Now, I personally think that the brain is already sophisticated enough that we can actually train ourselves to tolerate the Coriolis forces (and this is borne out of a study from MIT), and probably also learn how to combat the dizziness that is felt upon landing (perhaps by regularly spinning around just in the air while in microgravity). So I don’t think this is purely necessary. But I do think that long-term, we need to start thinking in this direction in order to make mass human spaceflight more feasible. Human spaceflight seems intrinsically expensive because of all the overhead required for humans in space versus robots. But we can engineer ourselves, much like we developed clothing to enable living in colder climates.

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The Slings and Arrows of Outrageous Lunar Transportation Schemes: Part 4–Propellantless Horizontal Soft-Landing Methods

As mentioned previously in Part 2 of this series, one of the key elements in establishing a lunar beachhead is developing ways to safely and affordably land payloads and people on the lunar surface. In Part 3, we discussed a way of hard-landing bulk raw materials on the surface, but in this post, I’ll focus on two related methods for horizontally soft-landing equipment and people on the Moon, while significantly reducing the amount of propellant required compared to traditional rocket-based soft-landing.

While the idea of horizontally landing on a planet with negligible atmosphere may sound odd at first, there is a method to my madness.

Horizontal Soft Lithobraking
I’ve mentioned this concept in the introduction a previous blog post1, but to recap for those who haven’t read the previous post, back around the time of the Apollo Program, Krafft Ehricke2 invented a concept for horizontally landing payloads and people on the lunar surface.

Lithobraking Slide Landers

Lithobraking Slide Landers

Ehricke’s concept involved having a lander with ski-like skids land on a very long (10s of km long) bulldozed regolith track. The lander skids would drag along the surface exchanging momentum with the lunar regolith. In this way the rocket delta-V associated with landing could be significantly diminished. Ehricke even invented a new field of engineering called “harenodynamics” to study the fluid-dynamics-like properties of regolith particles in that situation.

While probably doable, and while the concept would likely significantly increase the payload deliverable from Earth, it’s not without its share of drawbacks and challenges. Obviously, prepping such a landing strip would be no small feat. You would have to clear all boulders, level the ground as smoothly as possible, probably brake up and rake the regolith, and then you’d probably have to redo the strip after each landing, as the landers spray regolith in every direction. You’d probably also want a really, really accurate gravimetric map of the approach trajectory, and some good navigation aids, to enable the vehicle to hit the track, and hit it with the right velocity vector.  This concept would likely result in a lot of wear and tear to both the track and the lander. Not to mention getting dust on pretty much everything, and being on the “that scares me, and I’m fearless” side of sketchy. But there’s also been a decent amount of research put into it, so it might be feasible.

That’s all I’ll say about his concept, but you can read more about it here on page 27. For the rest of the post, I’d like to focus on a newer variation on the theme that I think has real potential.

MFA™ Hover-Brake Horizontal Landing
This second concept is a combination of a concept I heard from a friend at XCOR about a decade ago3 combined with some newer technology from a cool company in the Bay Area called Arx Pax. Arx Pax is probably most famous for their “Hendo Hoverboard”, which can levitate while carrying a full-weight human about an inch over a copper sheet, without using superconductors.

The Hendo Hoverboard

The Hendo Hoverboard

Their Magnetic Field Architecture (MFA™) technology works by using four “Starms” or “Hover Engines” which are rotating discs with magnets arranged on them in a way that projects most of the magnetic field down from the disc into the conductive medium you’re trying to hover over.

Arx Pax Hover Engine 3.0

Arx Pax Hover Engine 3.0

As the Hover Engines rotors rotate, they induce a circular eddy current in the conductive medium, which creates a magnetic field that pushes back on the Hover Engines. Using two pairs of counter-rotating Hover Engines at the four corners of their hoverboard, they can push against the conductive medium with enough force to lift both the hoverboard and a human rider. They can also induce lateral forces by rotating the Hover Engines relative to the surface of the conductive medium. They think there may also be ways to modify their Hover Engines to induce push, shear, and pull forces on a conductive target.4 For those of you interested in learning more, Arx Pax just announced this last week that they’re now selling both a pair of the Hover Engine 3.0 modules shown above (capable of levitating 60kg at a height of 6mm over a 12.5mm thick sheet of 6101-T6 aluminum, with a system weight of <7kg each, as per this spec sheet)5 for $9,999 for the pair, as well as a MFA™ bundle kit with smaller hover engines and all the pieces you need to make and control your own hovering device for $1,589, both of which you can now order online here.

For this lunar landing application, you would need three elements: a long conductive track, multiple Hover Engines on the horizontal lander in order to levitate the lander over the track, and a magnet array on the bottom of the vehicle to decelerate the vehicle using eddy current braking with the long conductive track. The Hover Engines can help both keep the vehicle from touching the track, or from bouncing away from it, while also keeping it centered on the track, absorbing a lot of the shocks associated with a lunar landing, and providing additional deceleration force as the vehicle slows6. As the Hover Engines approach the conductive track, the repulsive force should increase, and as the Hover Engines get too far above the track, the repulsive force should drop off, thus providing at least some natural feedback The amount of power needed to keep the Hover Engines rotating is probably modest enough to be powered by an APU like ULA is proposing for their Integrated Vehicle Fluids system7. While there still are rather demanding requirements for hitting the landing track within the right cone of velocity vectors, the non-contact nature of this landing makes it somewhat less scary. And depending on the design of the eddy current braking magnets, the braking drag may be able to be applied gradually after the vehicle has established a clean hover on the track, if so desired.

One important question is still how much up-front infrastructure this may require. You’ll still need the bulldozer to remove boulders, and level the road. But you also no need a conductive layer of some thickness. The thickness is going to be driven by the float height, gravity, number and size of Hover Engines, and the conductivity of the conductive track. For earth hovering, they used a copper track of decent thickness8. If we needed that thick of a track, that could add up relatively quickly, even if we go with lightweight aluminum instead of copper9, to get the thickness down to much more reasonable levels.

Residual Resistance Ratio
Most people are familiar with superconductors, but did you know that if you chill high purity metal conductors to cryogenic temperatures, that their resistivity also drops off dramatically? In most cases not all the way to zero, but enough to make a real difference. You need a very pure alloy (typically 99.99% or more of the base metal), fully annealed, with minimal impurities. But if you can get the metal in that condition, the conductivity can go up by 50-100x or more compared to their room temperature values. For instance, the figure below (from page 1135 of this paper) shows the conductivity vs temperature curve for high-purity aluminum. As you can see, the resistivity at room temperature is ~2.7×10^-8 Ohm*meters, but at LN2 temperatures it’s 10x lower, and LH2 temps it is ~3400x lower.

High Purity Aluminum Resistivity vs. Temperature

High Purity Aluminum Resistivity vs. Temperature

If you could chill pure Aluminum cold enough to reach ~125x the room temperature conductivity of 6101-T6 sheet (ie resistivity = .025 Ohm*m), you would need ~100µm of aluminum to conduct electricity as well as the 12.5mm thick sheet of aluminum at room temperature used for the Hover Engine 3.0 modules. From the chart that’s somewhere around 45-50K. 100µm is a lot more workable than 12.5mm. The 100µm thick layer could be thermally deposited onto a microwave fused regolith track to provide a strong mechanical backing, while keeping the initially imported aluminum mass quite low10.

Running Some Initial Numbers
If you say had a 7.5m wide track with 100µm thick aluminum, the aluminum would mass approximately 2.03mT/km of track11. So if you could hard-land a 70mT sample of such high-purity aluminum 12, and could recover approximately 2/3 of the aluminum, that would let you lay about 23km of conductive track from a single hard-landing flight. If you were using that 23km track to decelerate something from lunar orbital velocity (~1800m/s), and assumed that you were landing on ~80% of the track length13, you could get the landing Gs down to ~9Gs (over approximately 20s). That’s only a little bit more intense than a Gemini launch on a Titan II missile and a bit gentler than a ballistic reentry in a Soyuz capsule, but probably within what a healthy human could handle safely14. With two hard landings worth of aluminum under the above assumptions, you’d be able to get the G rate down to a totally survivable 4.5G deceleration (with a 46km long track).

But how much would all the Hover Engines mass to do this? Not as much as you’d think. So, if you assume that you’ve chilled the aluminum enough that 100µm of pure aluminum can conduct as much as 12.5mm of 6101-T6 aluminum can at room temperature (40-50K as mentioned above), so it behaves similarly to the nominal track thickness, that means that for landing a 20mT payload attached to a 6mT ACES/Xeus/HoverEngine lander, you’d need ~64 of the Hover Engine 3.0 modules (say 8 pods of 8 Hover Engines each), which without further weight optimization would mass around ~450kg. You could probably cut that down substantially with clever design15, but that’s already in a similar order of magnitude to the mass of the landing kit for a rocket-powered Xeus lander.

As an aside, can you see why I was so interested in having a way to land large amounts of bulk raw materials on the lunar surface? Even though there’s plenty of aluminum in the regolith, and you could eventually setup facilities that could produce many tonnes of aluminum per year, being able to land that early-on dramatically lowers the cost of landing those facilities in the first place.

How to Chill
Getting back to the topic, can we really get aluminum cold enough to enable that level of conductivity? I’m honestly not sure. It may be possible to have the bulldozer that makes the path intentionally sink the track into the ground with berms around it in a way to keep the aluminum in shadow as much of the time as possible. If you combine that with some sort of “cryogenic selective thermal coating”16 that could help keep the temperature of the track passively cool for most of the day. Will it be enough to get the track down to the desired ~40-50K? I’m not sure. According to this chart from a site with data from the LRO Diviner mission, polar sites get down to around ~50K at night, so I don’t think it’s entirely crazy at least for polar locations. At more equatorial positions, even with a properly dug trench, you’ll likely need a thicker aluminum track to make things work.

Lunar Surface Temperatures over Time at Various Latitudes (from LRO Diviner Instrument)

Lunar Surface Temperatures over Time at Various Latitudes (from LRO Diviner Instrument)

One additional knob to turn is that apparently if you have a two layer track, with a highly conductive non-ferromagnetic upper layer, and a lower ferromagnetic layer, that the ferromagnetic sheet modifies the induced magnetic field in a way that actually increases the force pushing back on the Hover Engine. So theoretically, if you did a three layer design (first the microwave sintered regolith underneath, then a layer of melted NiFe material magnetically extracted from the lunar dust, and finally a layer of thermally deposited aluminum), you might be able to get a bit more bang for the imported aluminum buck.

Multi-Use Track
One other nice thing about a track like this is that it can provide two other uses other than just propellantless landing of people and payloads. First, the track is actually a pretty impressive conductor. A 100µm thick, 7.5m wide conductor has about the cross sectional area of a 1.25in (30mm) diameter rod. That’s a lot of conductor. And if chilled to 125x the room temperature conductivity of aluminum, you’re talking about a conductor equivalent to a 14in (350mm) diameter bar of aluminum at room temperature. A conductor that big could likely conduct megawatts of power over non-trivial distances. This means that you could have solar farms and installations located along the track, which all use the track as the high-power backbone to connect themselves to the main beachhead facility. If you had a polar facility with four tracks spaced 90 degrees from each other (say earthward, anti-earthward, and along and against the moon’s orbital velocity vector), you could locate solar farms along each track in a way that half of them are always in sunlight, so you wouldn’t need a lot of power storage capacity.

Second, the tracks also would serve as ideal highways/railroads for moving stuff between the main facility and other facilities down the line. Without air, and without rolling friction, these tracks could basically function as maglev railways linking installations up and down the track. The same Hover Engine kits used for landing could form the backbone of a hover vehicle that could rapidly move very heavy payloads up and down the track at very little energy cost.

And so long as you locate a track along a “great circle” route (ie a surface track that is in a plane that intersects with the center of the Moon), the tracks could be used for all three applications (landing, transportation, and power). While the first one or two could be built using Earth resources to accelerate their availability, once you have access to lunar aluminum, you can start putting tracks like this down wherever they are convenient, and as long and wide and thick as is convenient. I even have an idea 17 for how you could use tracks like this for propellantless launching of payloads, but that’s a post for another day.

It’s probably more complicated than this. These ideas are very conceptual, and could use a significant amount of further baking. In fact, if I get a chance to flesh this out further, I’ll probably do follow on posts as Part 4.1, 4.2, etc. when I get the time. But they at least suggest that there may be ways to eventually cut the cost of delivering payloads to the Moon in-half, by getting rid of most of the propulsive landing delta-V. And for setting up heavy infrastructure on the Moon, the sooner you can get that big of a landing-cost savings, the more your overall system cost goes down.

Next Up: Slings

[Update: I still need to dig into this more, but it may be necessary to put more thought/analysis into the eddy current damping part of this idea. All of the kinetic energy effectively has to be dissipated as thermal energy (by resistance losses by the eddy currents flowing through the metal). The problem is that as the metal heats up, it becomes less conductive. So there’s probably a mass limit that a given track thickness/length can realistically handle. The math is currently above my hand-calc levels, but I’ll do a follow-on Part 4.x post if I get a chance to dig into this more.

One other interesting idea I’d like to look at is using pure Beryllium as the conductor instead of pure aluminum. Beryllium has 2x the heat capacity of aluminum (~1800J/kg*K vs ~900J/kg*K), and has a much higher melting point (~1550K vs ~933K), so it is much better as a heat sink. It isn’t as good of an electrical conductor though, with 3/2 the room temperature resistivity of aluminum, but it is only 2/3 the density, so for a given mass/length, you get approximately the same resistance, just with 3/2 the cross-sectional area for the Beryllium. It’s RRR (ratio of resistivity near absolute zero compared to room temperature resistivity) of ~2000, which while not as high as the highest grade coppers or aluminums, may still be good enough to be quite interesting. Needs more analysis though.]

Posted in Lunar Commerce, Lunar Exploration and Development, Slings and Arrows of Outrageous Lunar Transportation Schemes, Space Development, Space Settlement, Space Transportation, Technology | 29 Comments

Perfectly energy-efficient rocket vehicle

So I was watching a video of a talk by Geoffrey Landis (my old mentor when I was an intern at Glenn), and he made a very interesting point.

If you’re trying to maximize energy efficiency for a rocket and you have the ability to change Isp, you should set the exhaust velocity equal to the rocket’s current velocity. Think about it for a little bit, and you’ll see that it’s true.

We make some simplifying assumptions here, such as assuming arbitrarily good mass fraction, and thrust occurring outside any gravity wells (or, equivalently, thrust that is arbitrarily higher than acceleration due to gravity at that point). But with these assumptions, this finding is true.

One thing that’s a little surprising is that you’d have PERFECT efficiency under this scenario. That means you’re wasting exactly no energy by accelerating all that propellant, which goes a little counter to the intuition. A perfectly efficient rail gun (with an arbitrarily low-mass acceleration cart) would not be any more energy efficient.

…the downside, here is that your initial exhaust velocity is zero, which implies an infinite amount of fuel. This isn’t so bad because we can just truncate the initial portion of the flight by picking a certain minimum Isp to keep mass fraction to a reasonable number. We lose some efficiency this way, but it’d small. Alternatively, you could use rail-launch for the low-speed portion of the flight and maintain arbitrarily-good efficiency. The other side of this is that it implies 700-900s Isp for the last portion of the flight if going all the way to Earth orbit. That implies use of non-chemical thermal rockets (such as NTR), but you’d be no better off energy-wise since the only practical propellant for those (while still reaching those Isps) is hydrogen, and producing hydrogen requires a lot of energy (energy which is not at all utilized in the rocket! Hydrogen is considered basically inert… you could also use helium). Besides, terrible mass fraction.

Anyway, there is also an energy-ideal exhaust velocity if you can’t adjust the Isp. I believe it’s about 5/8th the final delta-v, if I remember geoffrey’s talk. I may try to recreate this calculation.

Another point: It makes things like Aerojet’s thrust-augmented rocket look pretty interesting from an energy standpoint.

I have a bunch of thoughts related to this that I may blog later on. Here’s Landis’s talk. (He argues for NTR here. I disagree with him, but very interesting talk, and he definitely makes a convincing case that NTR is not some unobtainable technology.)

Overall, I appreciate the change in focus to energy. I think we have an (understandable) obsession with mass in the rocketry world, but that doesn’t tell the whole picture and can somewhat bias our intuition.

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The Slings and Arrows of Outrageous Lunar Transportation Schemes: Part 0–An Elevator Pitch for the Moon

[Note: After getting a few posts into this series on the “how” of alternative lunar transportation concepts, I realized that some commenters were asking about why we should even be going to the Moon, or what we’d be doing there. So even though this series focused more on the “how” of lunar transportation development, I figured a short up-front post about why we should care might be useful.

Also, I should add an up-front caveat that I’m not doing this post or series in an effort to bad-mouth other destinations such as NEOs, Mars, Venus, etc, or to propose that NASA should focus all of its money an effort on the Moon, or anything like that. I think all of those destinations are potentially interesting, and that humanity will be better-off if it can figure out how to pull all of them into its economic sphere. The point of this post and series is to explain the ways the Moon could be interesting, and to explore what technologies or approaches might enable that.]

While trying to exactly predict future markets and economic trends is often an exercise in futility, I think we can still make a broad brush-strokes “elevator pitch” for why the Moon is potentially interesting as a destination for space development. As the only pre-existing destination in space that can be accessed both quickly and frequently1, the moon is interesting for at least three or four application areas: resources, adventure, science, and possibly settlement:

  • Resources: There are many resource-intensive things that can be done in space once the cost of resources in the required orbits come down far enough. These include orbital space settlements in LEO or the Lagrange points, massive communications or power-beaming platforms in high-LEO, MEO, or GEO, space tourism destinations, and all sorts of travel to destinations beyond Low Earth Orbit. If the cost of lunar-derived materials and/or manufactured goods delivered to these destinations can be competitive with Earth-launched or NEO-derived materials, then lunar mining and resource processing could be a very important reason to go to the moon.
  • Adventure: In addition to raw materials, the Moon is a potentially very interesting destination in itself. Both for scientific exploration, but also for plain adventure. People visit all sorts of dangerous places on earth just for the shear thrill and beauty of it. If the cost of travel to/from the Moon can come down, the Moon is a potentially very interesting destination for adventure tourism and public or private exploration.
  • Science: We’ve barely scratched the surface of understanding our nearest celestial neighbor, and on its far side there is a radio-pristine area for locating various types of observatories. Once again, If the cost of transportation to/from the Moon came down rapidly there are many international agencies and private societies who might be interested in studying the Moon and using it as a platform to explore the heavens.
  • Settlement: This one has more “if’s” involved, but If it turns out that 1/6g is sufficient for long-term human health2, and If the cost of travel to/from the lunar surface can come down significantly, it very well could be a good destination for Earth’s first off-world settlements. It’s probably not as ideal as Mars or Venus, but if the other three activities are going on, and humans can adapt to the lunar environment, you will see settlement of the Moon on at least some scale.

I could probably go on, but to me those provide ample justification for being interested in going to the Moon, but as you can see, the success of all those applications depend strongly on driving down the cost of going to and returning from the Moon, both in absolute terms, and also relative to other competing alternatives such as Earth launch using RLVs and NEOs.

How to actually do that is the focus of the rest of this series.

Posted in ISRU, Lunar Commerce, Lunar Exploration and Development, Slings and Arrows of Outrageous Lunar Transportation Schemes, Space Development, Space Exploration, Space Settlement | 15 Comments

The Slings and Arrows of Outrageous Lunar Transportation Schemes: Part 3–Intentional Hard Landings

The first advanced lunar transportation concept is the idea of intentional hard lithobraking landings of bulk raw materials. The idea was recently suggested by frequent Selenian Boondocks commenter Paul Dietz in the comments to this previous post. Basically you would launch a rocket with one or more large chunks of raw material on a trajectory that grazes the surface of the Moon, using the lunar regolith to decelerate the raw material to the point where it can be subsequently collected. Even if the payload is coming in on a hyperbolic trajectory from Earth without stopping in low lunar orbit, it’s theoretically possible for a very shallow impact to not vaporize the payload, leaving it largely intact.

Artificial Shallow-Impact Meteorites
In Dennis Wingo’s excellent book MoonRush, he makes the argument that due to the Moon’s shallow gravity well, it’s possible for a subset of lunar nickel/iron meteorite impactors to arrive at low enough relative velocity and a shallow enough angle for the majority of the material to survive intact in a large chunk.

This concept though would be basically trying to create that phenomenon artificially, with an earth-launched chunk of raw material, such as a big ball of some alloy that may be hard to produce on the Moon (copper conductor alloys? high strength aluminum alloys?). Impact conditions could be carefully controlled using the upper stage as a shepherding vehicle, similar to what was done with the LCROSS mission1, but potentially with the upper stage performing a last minute maneuver to avoid intercepting the lunar surface, allowing it to return to earth for recovery/reuse.

Why would you want to do something like this?
Primarily because it would dramatically reduce the cost of delivering some bulk raw material to a lunar settelement site. The delta-V from LEO to a lunar soft-landing is approximately 6km/s. With high performance LOX/LH2 propulsion, that means that only 1/4 of your LEO mass will be dry mass on the lunar surface. For instance, an ACES stage with 70mT of prop loaded in LEO could deliver approximately 24mT to the lunar surface, with the stage and Xeus landing hardware taking up at least 6mT of that (leaving a net ~18mT of payload). On the other hand, the same stage could hard land over 65mT of bulk material while recovering the stage back to LEO. In the case of a big chunk of raw material, unless it is something really exotic, most of the cost is going to be in the launch of the material and the propellant to get it to the moon. If you can increase the payload to the moon for the same propellant by a factor of 3.6x, that’s likely going to decrease the cost on the lunar surface by a somewhat similar margin.

What would you use this for?
As mentioned above, the most likely use of this would be to hard land bulk raw materials. Specifically you’d want something somewhat tough so it could survive the impact intact, in spite of impact forces probably in the 1000s of Gs, most likely a large chunk or ball of metal. Here are some specific applications I can think of:

  • Copper conductor materials — copper is relatively rare on the Moon, but is used a lot as a conductor for power transmission, and could be a key element in manufacturing electromagnets or conductors for other propellantless launch/landing systems. Landing equipment that could metal and process high-purity copper or copper alloys into useable conductors may be a lot easier than landing big spools of conductors, if this approach allows you to cut the cost of the copper by almost a factor of 3.6x.
  • High-Strength Aluminum Alloys — while aluminum itself is pretty common on the Moon, most high-strength alloys, particularly ones you’d want to use for structural components or pressure vessels, are ones that require alloying elements that may be hard to source locally, and which frankly will likely take a long time to build up the smelting capabilities on the Moon even if the alloying elements are available. These alloys could be alloys that can be cast using locally produced sintered regolith molds. Or they could be alloys designed to be converted into 3D printing powder. Or it could be wrought alloys that are designed to be melted and converted into sheet or bar forms. Once again, processing equipment to convert the bulk raw material into usable forms is likely going to be massive enough that you’ll be somewhat limited, but with proper thought the 3.6x cheaper aluminum alloy raw material may still be useful in some applications.
  • Toughness-Enhancing Alloys for Ni/Fe Asteroidal Material — in the ongoing discussion on the previously mentioned ESIL-8 post, it was mentioned that many examples of meteoric steel, while beautiful and stainless steel-like are actually rather brittle. It might be possible to send along the right mixture of alloying materials to mix with magnetically recovered Ni/Fe asteroid fragments on the Moon to turn it into a much better and more useful grade of stainless steel, for 3d printing, and/or production of pressure vessels for habitation or material storage.

Implementation Details
Some potential implementation issues that may need to be addressed for this concept include:

  1. What materials can actually survive an impact at the velocities associated with a hyperbolic trajectory from Earth?
  2. What is the optimal impact trajectory for controlled landing in a recoverable manner? Does too shallow result in large dispersions? Does too steep result in vaporization or the material being buried below the surface? How much of this can be analyzed up-front versus needing to be experimented with?
  3. How big of an impact crater are you going to form? How far will regolith fly?
  4. What is the optimal shape for the material? A solid sphere? A hollow sphere?
  5. From an earth TLI, where on the Moon can you realistically target an impact point for that has the right impact flight path angle?
  6. Where would you want material to be targeted relative to a settlement, to minimize risk to the settlement while simultaneously maximizing the odds of being able to recover the material easily?
  7. Can you design a trajectory that allows the shepherding upper stage to avoid lunar collision and return to Earth for recovery and refueling?
  8. Do you need to “prepare the ground” any by having a robot remove large boulders from the target area, and/or “rake” the regolith?

There are probably other questions, but the potential of being able to cut the cost of raw materials delivered to a lunar site by 3.6x makes this intriguing, even if a little on the messy side.

Next Up: Propellantless Soft-Landing Options

Posted in ISRU, Lunar Commerce, Lunar Exploration and Development, Slings and Arrows of Outrageous Lunar Transportation Schemes, Space Development, Space Settlement | 50 Comments