Centaur UnCrasher Stages for Simplified Lunar Landings [Updated 11:35pm MDT 9/4/13]

[Editor’s Note: I was recently pinged by a friend on NASASpaceFlight.com about an “UnCrasher” lunar lander architecture concept I had written about a few years ago. I went to point him to a blog post on the idea, when I found that I never actually got around to writing it up officially. So, I found some time over the weekend to start some spreadsheets to refresh my memory on the concept (and to explore it a bit more rigorously), and decided now was as good a time as any to write it up.]

What In the World is an “UnCrasher” Stage?
What I’m calling an “UnCrasher” architecture is a semi-reusable two-stage approach for delivering payloads to the lunar surface from a lunar orbital rendezvous point or a facility/depot, either in lunar orbit or at one of the Earth-Moon Lagrange points. The system consists of three parts: the lunar surface payload, the lander, and the reusable “UnCrasher Stage”. The UnCrasher Stage would be a lightly modified high-performance upper stage (basically a Centaur or DCSS upper stage with some kits added to them) that would propel the lander and payload from the starting assembly point to lunar orbit, and then would perform a significant fraction of the descent burns to slow the lander and payload down from lunar orbital velocities. Staging could occur with as little as say 200m/s of delta-V left for the landing. After staging, the lander would continue on to the lunar surface, and the UnCrasher would relight, and perform a boostback maneuver to accelerate itself first back into lunar orbit, and then maneuver from there back to the starting point in preparation for future missions.

The name “UnCrasher” is basically a play on “Crasher” stages, which have been proposed in the past, but which expend their deorbit stage immediately after separation, letting it crash safely into the lunar surface at some significant distance downrange of the landing target. One way of thinking about it is that the UnCrasher concept is basically a lunar version of what Elon’s trying to do with the F9R first stage, but in many ways it’s even simpler, and should be even more amenable to low-maintenance gas-and-go reuse. Unlike a reusable earth-to-orbit launcher first stage, the UnCrasher never has to fly through an atmosphere supersonically, and doesn’t get exposed to dust or debris kicked up from landing plumes. Plus, in-space stages can use much more benign rocket combustion cycles like the Expander Cycle used on the RL-10. In many ways when you think about all the architectural elements you’d likely have in a Cis-Lunar transportation network, the UnCrasher stage will probably be the easiest one to reuse.

Genesis of the UnCrasher Architecture Concept
To be honest, I don’t have great documentation on the circumstances in which I invented this concept, and like most good concepts in aerospace, I’m probably not the only one to independently invent the idea. But I think the genesis came from debates with DIRECT supporters who at the time were claiming that you needed a launcher with a much bigger Payload Fairing than the ones flying on EELVs in order to do significant lunar landing missions. ULA had already proposed their Dual Thrust-Axis Lander concept (which Masten is now working with them to prototype, under the name “XEUS”), which would have placed a series of landing thrusters and legs mounted on the side of a Centaur stage, enabling it to land horizontally after the Centaur main propulsion system had done most of the braking from Lunar Orbit. I had also seen several concepts for “Crasher” architectures over the years. This was also right around the time that Masten and XCOR signed an MOU to jointly market a LOX/Methane lunar lander. I don’t think they ever found any serious customers, but at the time I was investigating a wide range of different ideas. I wanted something that could enable reuse of in-space elements, and that could minimize the development costs of unique elements in the architecture–the more you could repurpose a flight-proven system like Centaur or DCSS, the less development cost it would take to get you to an actual lander capability. Some day I may dig up some old notes that provide a better backstory, but that should do for now.

UnCrasher Performance Analysis/Methodology
As I said in the Editor’s Note up front, I decided to do some spreadsheets to get a better idea of how such an UnCrasher would perform. I’m pretty sure I ran the numbers on this idea on a less detailed basis a few years ago when I first came up with the idea, but I couldn’t find the files anymore, so I started from scratch. In my second, and more useful, spreadsheet, I did an analysis of an IML2-constrained UnCrasher architecture. If you’re morbidly curious enough to want to follow-along, here’s a link to a copy of the spreadsheet.

Here’s a quick overview of my methodology:

  1. I started with some high-level stats on the Centaur-based UnCrasher stage. In addition to the actual dry mass of the Single-Engine Centaur, I added about 450kg of “kits” to cover things like an Integral Vehicle Fluids system, long-duration passive insulation, a multi-use resettable separation system, a few small Sticky Boomsâ„¢ to bring things together, long-range comms, etc. I wasn’t trying to go for super-high fidelity, but more to gain a decent first-pass understanding of how the system behaves. This analysis can be used as a starting point for more detailed analyses later–I just wanted to see if the idea was worth anything.
  2. I also created some high-level parameters for the lander. I assumed a LOX/CH4 propulsion system with a 360s Isp, and a 80% pmf for the lander–ie of the non-payload part of the landing system, 80% of the remaining mass was propellant and 20% was structure. This is a tiny bit better than the Apollo LEM Descent Stage, but a number I figured was realistic.
  3. I assumed that the total required Delta-V from L2 to the lunar surface was 2600m/s. This is only an approximation though, which depends a ton on how fast of a transit you want to do, what your T/W ratio is at various points, etc. To do this analysis for real you’d likely want to use at least a 3DOF simulator, but for this time around I decided to use the “cookbook” values.
  4. For the actual analysis section, I created several rows, each with a given “marginal Initial Mass in L2” (mIML2) which included all the marginal mass you would need to do a given cargo mission, including the UnCrasher propellant, the lander and its propellant, and the payload. I didn’t include the dry mass of the UnCrasher, because it would be reused several times.
  5. I then guessed an optimal “assist Delta-V” for the UnCrasher stage for each mIML2 row. This assist Delta-V is the portion of the ~2600m/s from EML-2 to the lunar surface that would be provided by the UnCrasher. This also happens to be the amount of Delta-V the UnCrasher needs to provide post-staging to boost itself back to EML-2. This term was the variable which would be adjusted to optimize the payload performance to the lunar surface.
  6. I then calculated for each row how much UnCrasher propellant would be needed for each of those two trip-legs. The descent leg is a function of the UnCrasher dry mass, the mIML2, and the UnCrasher assist Delta-V and Isp. The ascent leg is a function of the UnCrasher dry mass, and the assist Delta-V and Isp.
  7. Once I knew how much of the mIML2 for each row was tied-up in the UnCrasher propellant, I subtracted that from the mIML2 to calculate how much mass was left for the lander and payload.
  8. I took the total lander/payload gross mass, and the remaining Delta-V required for the lander, and used them to calculate how much lander propellant was needed to perform the landing maneuver. I then used the pmf to calculate the lander dry mass.
  9. Payload was what was left after accounting for the UnCrasher propellant, and the lander propellant and dry mass.
  10. I then iterated on the assist Delta-V for each row until I had maximized the lander payload. [Note (9:37MDT 9/4/2013): After publishing this, I finally figured out how to activate the Solver function in Excel–much easier, and should enable multi-variable optimizations.]

As you can see, this was a pretty simplistic analysis, but good for getting a high-level overview of system behavior. One of these days I’ll need to relearn Matlab well enough to use it for analyses like these, as the manual iterations on the assist Delta-V column was a royal pain in the neck. Or, I could have done all the math algebraically to come up with the landed payload with the assist Delta-V as the variable, and then taken the first derivative of that equation and solved for the zero points. But I didn’t have a whiteboard handy, and the spreadsheet was quicker for me than trying to set something fancier up.

So, what did I learn? Quite a bit actually.

Surprising Result #1: Optimal Assist Delta-V vs. mIML2 Regimes
I figured a priori that there would probably be three regimes once I ran this and charted the results. I figured the first regime (starting with the smallest mIML2) would start out with the optimal assist Delta-V at some relatively low but non-zero number, which would ramp up gradually with mIML2. The second regime would be when the optimal assist Delta-V hit some arbitrary maximum value–I picked 2400m/s, which would leave only 200m/s for the lander itself. Then I figured there would be a third regime that would kick in once a full UnCrasher propellant load could no longer deliver 2400m/s of assist Delta-V to the lander and payload.

It turns out there were four regimes, as can be seen from this chart:

In addition to the three regimes I predicted, there was a fourth regime that existed below approximately 4700kg mIML2 (for this set of UnCrasher and lander parameters). Below this threshold, it turned out that the optimal assist Delta-V was zero–ie using a Centaur-sized UnCrasher to provide any Delta-V actually decreased the payload to the lunar surface. I’m not 100% sure I understand why this is, but my guess is it has something to do with the fact that when you have say 2000kg of mIML2, spending any of it to accelerate and decelerate the ~2700kg Centaur stage just didn’t provide any benefit.

Two other things to point out–the transition between regime 2 and 3 was at ~32500kg mIML2, and the transition from regime 3 to regime 4 (the point where assist delta-V was Centaur prop-load limited) was at about 42500kg.

Surprising Result #2: Payload Benefit of an UnCrasher
The second analysis I did was to analyze for each mIML2 point what the payload of a single stage LOX/CH4 lander would be if no UnCrasher stage was used. This allows us to compare how useful the UnCrasher stage is compared to just having a lander fly from EML-2.

The results were non-trivial, but more modest than I would’ve thought–the maximum benefit was around 36%, and over most of the range it was closer to 30%. Nothing to sneeze at, mind you! But not some big multiplier:

The biggest benefits were in the regime where the UnCrasher was providing the max 2400m/s of assist delta-V.

Surprising Result #3: Lander Dry Mass Benefit of the UnCrasher
With the previous result being positive, but not particularly overwhelming, I was curious if the UnCrasher gave much of a benefit in lander dry mass compared to the previously mentioned single-stage lander without an UnCrasher. Dry mass is often a good figure of merit for both the cost of developing a system, and the cost of producing one. Also, since I used the same pmf for all the landers, the propellant also scaled linearly with the dry mass.

The result in this case was a lot more eye-popping:

A couple things to notice:

  • For the single stage lander, the dry mass is a linear function of mIML2. This isn’t particularly surprising.
  • In all three regimes where the UnCrasher is net-useful, the dry mass of the lander is drastically lower than that of the single-stage lander. The most extreme case being in regime 3, where the UnCrasher-assisted lander masses only 5% (!!!) of the mass of the single-stage lander.
  • The lander mass actually starts decreasing in regime 2 and 3 compared to the high end of the first regime (where the UnCrasher didn’t improve the landed payload).
  • At the transition point between regime 2 and 3 (where the assist Delta-V was “saturated” at 2400m/s), the lander dry mass for a 14.7 tonne payload is only 220kg. (!!!)
  • At the transition point between regime 3 and 4 (the point where the UnCrasher stage is now fully-loaded), the lander dry mass for a 20 tonne payload is only about 300kg.
  • My guess is that in these most extreme cases, the lander pmf model probably breaks down, and may need a payload-mass driven term to account for structure. But even then this is pretty impressive.

The reason for this behavior is pretty clear–as the UnCrasher stage kicks in, it lowers the delta-V the lander needs to provide to as little as 200m/s. That’s just not a lot of propulsion system that you need at that point. In fact, the right approach may be to take the payload, and just have attachment points for landing-gear/propulsion modules at four corners (sort of like DTAL, but with the payload instead of the Centaur becoming the backbone). Would definitely make it a lot easier to get cargo unloaded without having a multi-story lander parked underneath the payload… It’s also worth noting that the landed payload is likely not very sensitive to the lander pmf at this point. Even if the lander pmf dropped to 20% instead of 80%, that would only cut into the delivered payload by about 20%ish.

Basically using an UnCrasher (or Crasher) approach enables the actual lunar lander to be a lot lighter and smaller than would be possible using other approaches. 220kg dry mass (with 880kg prop) is actually really close to the dry and wet masses of the LOX/CH4 SuperMods that Armadillo Aerospace was flying a few years back. Because the payload isn’t very dry-mass sensitive, this indicates that at least for one-way cargo landers, an UnCrasher approach may enable low-cost firms like the Masten/XCOR teams of the world to deliver useable lunar landers. I’d love to see a Masten/XCOR XEUS derivative that could land an ISS module on the lunar surface…

And yes, before someone asks, you could do just about as well with storables. Heck, the UnCrasher does enough of the lifting that you could probably do monoprop peroxide for the lander. This was a really interesting finding for me, though I’d love to have someone check my math.

One Last Observation: Altair LSAM Comparison
One other fun observation I noticed around 1am last night while finishing up this analysis. At the point of minimum lander dry mass (32500kg of mIML2), the payload on the surface is 14.75 tonnes. This is almost identical to the planned cargo landing capacity of the Altair Lunar Surface Access Module that was part of the cancelled Constellation program. Out of curiosity, I backed out an estimate for the net TLI injection mass for that much mIML2 (I feared it might be a lot more than cargo LSAM’s ~49tonnes). My BOTE gave me around 37 tonnes, or about 75% of the TLI injection requirement for LSAM. So not only does it lower the cargo lander size down to something that a small company could realistically build for non billions of dollars, but it’s also significantly more efficient in terms of TLI injection mass (and hence Initial Mass in LEO–IMLEO). Admittedly, the error bars on this calculation are bigger. You’d really need to do a more detailed architectural analysis than I want to do for free to know for sure. But this approach looks really promising.

Next Steps
I can see three fruitful next steps for analysis:

  • Running an analysis for an UnCrasher dropping off a lander that goes down to the surface, and then returns itself to EML-2 (ie something like what a manned lander would do). It’d be interesting to see if the regimes shift around with that, and how payloads compare vs. say a two-stage LOX/CH4 lander that stages on the lunar surface like the old LEM did.
  • Running an analysis that combined an UnCrasher drop-off, and an UnCrasher pickup on the way back to EML-2 (possibly even “suborbitally”). My gut says that an UnCrasher pickup *might* help. Might be worth running two analyses–one that has the UnCrasher carry all of the propellant for all four of its legs from the start. And another where it leaves the pickup legs’ propellant at the EML-2 depot until it gets back from the dropoff mission. This one will be tricky as it’s likely a multi-variable optimization. Might require relearning Matlab a bit.
  • Running a more detailed analysis on the transportation network between LEO and EML-2, including boiloff losses, more accurate masses for the various “kits” needed, etc. This would provide a more end-to-end architecture analysis that would allow you to show how badly this would thrash traditional approaches both on development and operations costs.

[Update 11:35pm MDT 9/4/13: Here’s an updated spreadsheet now that I figured out how to activate this version of Excel’s solver function. I’ve added a new worksheet (here) for Crasher cargo landers, and updated all three charts to add a comparison between Crashers and UnCrashers. Big takeaways are that Crashers have a wider “sweet spot” of maximum available assist Delta-V, and have slightly better performance than UnCrashers, but only about 5-10%ish. I could definitely see using them in certain circumstances like when you have an UnCrasher that’s ready for retirement, or when you really need that last little bit of payload margin. One other takeaway is that I really need to come up with a good way of capturing the non-propellants-driven portion of a lander’s dry mass, because I’m pretty sure that’s going to dominate during the “regime 3” area of maximum UnCrasher assist Delta-V.]

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
Jonathan Goff

About Jonathan Goff

Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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20 Responses to Centaur UnCrasher Stages for Simplified Lunar Landings [Updated 11:35pm MDT 9/4/13]

  1. Stellvia says:

    “In fact, the right approach may be to take the payload, and just have attachment points for landing-gear/propulsion modules at four corners (sort of like DTAL, but with the payload instead of the Centaur becoming the backbone).”

    Bonus points if it looks like Space 1999’s Eagle lander ;-D

    Great work. Lots to think about and digest here.

  2. A_M_Swallow says:

    Part of the requirements of a reusable stage are the number of restarts between maintenance and also major maintenance. A long service life between engine maintenance will also be important.

  3. Stellvia,

    Agreed. Even with a depot at EML-2 with some robotic inspection capabilities, the ability to maintain an RL-10 is going to be pretty limited. I don’t know how many restarts they’re qualified to, but if you use the RL-10 for all major maneuvers, you would need about 6 lights per mission. I think they’re rated for like 100ish relights, though I don’t know if that’s without any touch labor. But definitely a topic for research. I didn’t want to get into all those details though in this blog post–was more just focused on what we could learn from the physics/orbital dynamics of the situation.


  4. Robert Clark says:

    Thanks for that. It’s quite important that proposals for lunar landers are done that don’t have to be the size of the Altair. The Altair’s 45 mT size was a big part of the Constellations large size, and therefore large cost.
    I like also in your proposal you included your stage departing either from LEO or from a Lagrange point. Some other proposal’s I’ve seen did develop landers smaller than Altair but then they specifically sized them to only act as landers to the Moon from L1 or L2.
    It’s only a small increase in delta-V to act as a lander that also goes round trip from low lunar orbit to the lunar surface, as for a departure from LEO, compared to one that goes roundtrip from L1/L2 to the lunar surface only. So it’s only a proportionally small increase in the size of such a lander required.
    Such a lander would have greater flexibility to do several types of missions instead of only being able to leave from a Lagrange point station. For one thing it could be used to set up for instance propellant mining operations on the Moon before the Lagrange point station is set up.

    Bob Clark

  5. AMS says:

    Just as a point of interest, one TEU (Twenty-foot Equivalent Unit, a standard shipping container) is 24 tonne gross mass, usually around 3 tonne tare mass (87% mass fraction). The standard 40 ft container is just over 30t gross with a similar mass fraction. Strap a thruster module on each end and suddenly you have a cargo lander where you can drop off a good deal of the structure (the container itself) and just reboost the thrusters inside a container themselves.

    Also fun to note that the container itself (only half filled due to mass constraints) fits perfectly inside a Falcon 9 payload fairing.

  6. A_M_Swallow says:

    Fortunately RL-10 are not the only engines.

  7. Chas Becht says:

    So who wants to edit the spreadsheet to calculate all this for ACES?

  8. Chas,
    That’s why I gave the links to the spreadsheets! Have fun. 🙂

    Seriously though, the reason I picked Centaur was that the payload numbers I was getting as feasible were already plenty respectable, and since Centaur is a flight-proven stage, you don’t have to sink precious development dollars into that, and can instead focus them on the lander and payloads.


  9. DougSpace says:

    I think what you are describing could be an efficient cis-lunar transportation system. However, from a business standpoint, might there be greater value in initially designing and testing fewer vehicles? Specifically, could a craft be launched from Earth which could complete all phases of a cis-lunar transportation system? In this way, the initial investment could be recouped quickly and then, as the operations become profitable, additional, more efficient craft could be designed, tested, and deployed to improve the efficiency. I try to describe this in my CisLunarOne.com concept. Teleoperated harvesting of lunar polar ice for propellant is critical to completing L1/2 and Cis-lunar Circuits for revenue from potentially the first launch.

    Also, can you guys comment as to the trade-offs of lunar tail landing versus belly landing? I prefer belly landing but someone pointed out that, if you have four engines (one at each corner) then, if just one fails then you lose the craft whereas if you land on just one main engine then it would be about 1/4 as likely to fail.

  10. Doug,
    The whole point of the UnCrasher is that you’re taking an existing, flight-proven stage like Centaur or DCSS, and only making some light mission-specific mods to it, not designing a new stage from scratch. The only vehicle you’re actually designing is the lander itself. Trying to do a single stage vehicle that goes from LEO to the Moon and Back would be much, much more expensive (and huge!).

    As for horizontal versus vertical landing, there are ways to handle engine-out with more than one engine. It’s something to be considered, but not a show-stopper per se.


  11. DougSpace says:

    Thanks Jon.
    To what uses would you see this cis-lunar transportation system being used? Would your lander be large enough to deliver ice harvesting equipment? Would it be large enough to later be man-rated? Also, if propellant were available to the system, could all components of this cis-lunar transportation system be reusable? Can you elaborate a bit on the engine-out options? Thanks.

  12. Peter says:

    It would be interesting to see a lunar landing architecture that combined an UnCrasher stage and spin-stabilized landers:



  13. Robert Clark says:

    DougSpace, your web site won’t open.

    Bob Clark

  14. Luis Reyna says:

    Hi Jon,

    Excellent post, I got referred here by the latest article from The Space Review on Golden Spike, by Jim Lovell, specifically on reply to a comment where they pointed the Boeing Reusable Lander concept relying on a “crasher” stage, which until now I have to admit I had totally missed.

    I think your uncrasher idea is the way to go, pursuing reusability to the highest extent. It clearly relies heavily though, unless I misunderstood the concept, on cryogenic propellant depots, which despite being so obviously useful, aren’t being pursued seriously enough, or that’s at least my perception, surely not by NASA. Furthermore I don’t see anything serious being developed on that regard by them at least for the rest of this administration.

    I’m not involved on the aerospace industry being just an aficionado and a space enthusiast, and I have recently being educating myself a bit more on these stuff, and realize that orbital propellant depots are not the panacea, probably it would be very useful to have it in coexistence with for instance SEP and a host of other technologies. However I do believe it should be pursued more seriously than it is today. One way I think it could be incentivized is by means of a prize. A while ago I read the X Prize Foundation was looking for the next possible X Prize, and I think a Cryogenic Propellant Depot would be perfect for it, though I got to admit I don’t know the best way to implement a prize to pursue this. Thought I’d just live the idea around.

  15. Jonathan Goff Jonathan Goff says:


    It’s true that the uncrasher stage only really makes sense if you have either cryogenic propellant depots or at least cryogenic propellant transfer. It’s a technology we’ll need at some point one way or another, because SEP isn’t going to get us up and down into gravity wells. But I agree with you that there’s nowhere near enough serious work being performed on the concept at this point in time. It’s still too politically volatile–as much glad talk as there is about how depots and HLVs can coexist, HLV supporters are mortally terrified that a depot means you don’t *need* HLVs anymore, and if they aren’t strictly required they’re a lot more likely to have their gravy train shut down.

    A depot related prize would be neat, but Congress has so far gutted funding for the Centennial Challenges, and without a decent sized purse, it’s unclear if you’ll get anyone to take it seriously. For instance, look at the old NanoSat launch prize that got yanked a while ago. They only had $2M for what should’ve been a $10M prize, and almost all of the serious competitors shied away from it, leaving just the long-shots. For a depot prize, I also think you’re likely going to need at least a $5-10M prize, even if you set a goal that could be met with something in the high-end-cubesat range of demo hardware.


  16. George Turner says:

    I had an idea for a light weight two-stage lunar lander and an UnCrasher stage might work well with it. The concept was to reduce the number of hatches in the ascent module to one (located in the bottom center of the floor), reusing all the engines (if desired), and keeping the ascent stage as light as possible by turning it into a pop-up Bigelow tent, with a front face for the control console and windows.

    The floor of the ascent module is shaped like a hexagon with a standard docking hatch in the middle, around which (mostly under the floor) are the ascent propulsion tanks, along with batteries and guidance electronics. The front face of the ascent module containing the windows and control consoles is hinged at one edge of the hexagon, and it folds backwards over the docking hatch to shrink the ascent module’s total height for launch. Attached to the upper lip of the front face and all around the edges of the hexagon is Bigelow fabric, which makes the ascent module an inflatable tent with a solid front, or “face mask”.

    Folding over the top of all that are six triangular petals, each also hinged to an edge of the hexagonal floor. These cover and enclose the Bigelow fabric (and the folded control console and windows) during launch, and then they open up like a flower. Each of the six petals would permanently latch in place when deployed, and at the end of each would be three or more vertical thrusters of roughly 500 lbsf, or a larger number of Dracos or Aerojet 445 100-lb thrusters. The petals serve as the RCS thruster posts on the Apollo LM ascent stage, but also replace the center ascent engine by moving that thrust to the perimeter of the vehicle (though this isn’t actually required for the pop-up tent concept).

    The upper side of each petal (which was the inner side when folded) is covered in solar panels. You could mount a pair of fuel tanks under each petal (on top when folded) to give each petal an independent pressure-fed fuel supply, but I’m not sure if that location is any better than putting the tanks in the floor, sides, or rear of the ascent module, and certainly adds complexities regarding pressurization and plumbing, especially since the ascent module tanks need to be refillable. If you pushed the idea further, you could take a page from the old pop-up trailer tents and make the Bigelow section a big bigger, expanding out partially over some of the petals to make more room for beds or internal storage.

    So after launch, the petals fold out 180 degrees like a blooming flower, and then the habitable area inflates like a Jiffy Pop popcorn bag, with the face-mask with the control console and windows forming one side of the bag. The ascent stage gets re-used multiple times, and could use the solar-cells on the petals to power ion thrusters for orbit maintenance in between missions.

    The disposable descent stage mates to the bottom of the ascent stage using a standard docking interface, along with fuel transfer that can also feed the ascent stage engines. The center of the descent stage forms an airlock section with another standard docking interface (and hatch) on its own bottom. So all docking is done from the bottom, whether to the descent stage airlock or the bare ascent stage. With modern cameras and flat screen displays, windows won’t be needed for docking (but could possibly be placed in the floor of the ascent stage anyway).

    This configuration avoids the structural penalties of docking at the top of the ascent stage, because that arrangement requires the ascent stage structure to handle all the potential compressive forces generated from being sandwiched between a command and service module – and almost the entire mass of the lander. By mating from the already beefy bottom-end, the top of the ascent stage can be a pop-up tent. This also allows the docking radar to be on the bottom of one of the fold-out petals on the ascent stage.

    The petals fold out to roughly twice the width that was available inside the launcher’s payload shroud, and of course the ascent thrusters would be on the ends of the petals. Since the system will probably be using the same launch system for the ascent and descent stages, the tips of the petals would be far wider than the descent stage, so they could provide all the descent thrust without their exhaust impinging on anything below.

    That means that with a little extra engine mass on the ascent stage, the descent stage doesn’t need its own engines. Even using Aerojet 100-lb thrusters would give you 7200 lbs of thrust with only 300 pounds of engine mass, so the weight penalty isn’t very high. This makes the expendable descent stage much cheaper, since it’s just legs, tanks, an airlock, disposable items (such as a rover) and any required support systems for fuel transfer and the like. It also puts more distance between the engine bells and the lunar surface, and spreads out the exhaust footprint.

    All the ascent engines would be burning during at least part of the descent phase, so they’re already checked out before the crucial ascent or abort phases. Unlike Apollo, the ascent engine wouldn’t be an unfired-unknown while on the surface. With six posts (petals), any post could completely fail and the vehicle could still fly by shutting down the opposite post and flying on the remaining four posts as a quad. Of the four posts still burning, if either or both of the two posts on the far side of the dead post failed, you could fire up the post opposite the original dead post and still form a stable triangle. Since the engines already had sufficient thrust to handle the descent phase (if no UnCrasher is used), even half the posts should provide more than enough thrust for the ascent.

    So in normal operations, an ascent stage is launched (either with or without a descent module) and it unfolds its petals and deploys its “tent top” in LEO. Since it uses a standard docking interface it could mate up with the ISS (either with or without a descent module), and it could be checked out and flown in LEO because it can be refueled (which wasn’t true for Apollo). Then it gets sent to lunar orbit, where it awaits the arrival of a crew (and in later operations awaits new descent modules and crews). But since the descent modules don’t have their own propulsion system (to save costs), the UnCrasher stage could serve as both the delivery system to low lunar orbit and reduce the size and weight of the descent stage tanks and the ascent stage engines by providing most of the delta V for descent.

    This all gives the vehicle shorter landing legs, no long ladder, no extra hatch in the ascent module, no crawling in and out, an airlock for surface operations so the ascent module never has to be depressurized, and should produce a much lighter ascent module. If the ascent module has more than enough thrusters for basic missions (whose mass penalty is quite small), you could keep upgrading and changing the unpowered descent modules, moving through different configurations while relying on the same ascent stage parked in lunar orbit and re-used time and time again.

    The wide-splayed thruster arrangement on the petals would allow the ascent stage to compensate for a rather wide range of CG locations and cargo loadings, and since the descent modules are made up mostly of standard docking arrangements, tanks, pumps, and landing legs, with no high-risk engine development, it would be easy for Japan, the ESA, the Russians, and American commercial firms to build their own specialized descent stages on a mission by mission basis. If a mission requires more total thrust than the ascent module alone can provide, then that descent module would have to provide a share of the total thrust. The descent modules could naturally evolve from tanks with legs and an airlock into very small habitats, long before a bigger ascent stage is ever developed and launched, or even a second ascent module. And of course attempts would be made to leave all the lunar dust down in the descent stage.

    Without an UnCrasher, after the first mission a new descent module would have to be launched from Earth to lunar orbit, possibly using SEP and taking a slower, more energy efficient path. Then it would rendezvous and dock with the ascent module and the combined assembly would await the arrival of a crew capsule from Earth. The crew capsule would dock with the bottom of the descent module, the crew would pass through the descent module and enter the ascent module, and then they’d begin their descent to the surface. Upon returning to lunar orbit, the capsule would dock with the bottom of the ascent module, which would be left in lunar orbit.

    With an UnCrasher that mates to the bottom of the descent module, the crew would have to transfer from the Earth departure capsule to the LM first, and then undock from the capsule and rendezvous with the UnCrasher stage, which would mate up like the capsule had done. The UnCrasher would provide most of the delta V for landing, then it would undock and be cut loose. The lander would accelerate away from it under the power of the ascent module until there’s enough separation for the UnCrasher to turn around and accelerate back up to orbit. That’s workable but cumbersome, and like Apollo, it means the ascent module has to make a 180 during an abort.

    You could try putting the UnCrasher stage on top of the ascent module, with struts that connect directly to three of the ascent stage’s petals (which is structurally efficient), and just use it as another fuel tank during descent. But that would mean the UnCrasher would have to use the same fuel mix as the ascent engines instead of more efficient LH2/LOX. It would leave the UnCrasher already oriented for re-acceleration into lunar orbit after separation, but it also puts the UnCrasher right in the way of the ascent module during an abort, so that configuration is out.

    A third option is to have the UnCrasher connect to the back side of the lander, similar to the ULA dual thrust-axis lander concept based on an ACES stage, but without actually trying to land the horizontal section (and adding landing gear to it, etc). Since the ascent and descent modules will not be burning fuel for anything other than minor RCS corrections during the UnCrasher powered flight phase, their CG won’t be shifting. Since the ascent module always starts the descent fully fueled, it’s mass is a constant from mission to mission. Since each descent module can vary one from another, and since all other parameters are known, each descent module design can properly position the mating point for the UnCrasher stage.

    The rearward location would allow a capsule to dock with the lander and perform a crew transfer regardless of the presence of the UnCrasher, and it would let the UnCrasher mate up regardless of the presence of a capsule. If the UnCrasher mates only to the descent module, the ascent stage could perform an abort without worrying about the status of the rest of the assembly. As an added bonus, the ascent module would be aimed upwards during the deceleration phase, so an abort would only require a separation from the descent stage, and upward thrust to clear the assembly, and then a 90 degree pitch back to re-accelerate to orbit. If there is no abort, the assembly pirouettes 180 degrees around local lunar vertical, and instead of the UnCrasher providing braking and a slight upward thrust, it’s aligned for a slightly upward horizontal departure vector, while the lander is aligned for vertical flight with a slight braking. After the rotation in the horizontal plane, the lander separates and thrusts either upward or downward, clearing the flight path of the UnCrasher stage.

    In carrying most of the fuel load for lunar deceleration, the UnCrasher frees up volume in the descent stage, and since the size of the descent stage is limited by available launcher shroud sizes, this leaves more internal habitable volume in the descent stage, which can expand beyond a mere airlock section to include lots of extra equipment and usable space. But the architecture works even without an UnCrasher, and since the descent module designs are largely decoupled from the ascent module (which is a known constant), they can be designed for use without an UnCrasher or be optimized for a particular type of UnCrasher. The concept should allow the near lowest cost lunar return, provide lower costs through re-usability, and plenty of room for optimization and growth, without requiring the use of massive launchers like the Ares V.

  17. Wes says:

    Jon & Luis,
    NASA is doing work on the technologies for cryogenic propellant depots. Once we get back from this shutdown and the NASA webpages go back up, look up CPST, the Cryogenic Propellant Storage and Transfer mission. I suppose this still may not be “enough”, but there are other portions of cryo prop tech development in the works that people (i.e. funding sources) are really interested in and actively looking for the money.

  18. Wes,
    I was familiar with the CPST mission, but wasn’t clear if it was actually making any progress. I thought it had stalled out a bit because the NASA Glenn approach was simultaneously underwhelming, and more costly than they had budget for.


  19. Luis Reyna says:

    Regarding my suggestion of pursuing cryo depot development with a prize scheme I wasn’t putting my hopes on NASA through the Centennial Challenges or something like that for the same reason you point out. Actually I was thinking more something like this. I don’t know what would be the right size for such a prize, or what the right approach would be for implementing it, but I agree with you that at least at least a $5-10M would be desirable. If someone has any ideas or would like to spread the word among the community, maybe presenting a more formal and well thought out proposal to the X Prize Foundation or some other organization (I can’t think of another right now, maybe Darpa but I’m even more clueless how to approach that road) could have some results hopefully.

    On another hand, I realize SEP isn’t suited for a descent/ascent stage, but it will probably be useful for other purposes. I think we’ll need them all, or as much as we can get. Anyway, I think SEP is being more thoroughly developed than depots, regardless of whether or not the asteroid relocation mission moves forward or not. More specifically, I was thinking an SEP tug could be used to retrieve the uncrasher stage from a lunar orbit after dropping the Lander, if it were necessary/desirable, but this suggestion will probably reveal my amateur condition on these topics :-s.

    Wes and Jon,
    I had the same impression as Jon about the slow progress of the CPST program.

  20. steven rappolee says:

    other possibility’s;

    (A)(1) Uncrasher stage designed to land first and fill up with IRSU
    more fuel for a heaver lander?
    (A)(2) uncrasher stage (at end of life?) lands and becomes hopper(IRSU)
    (A)(3) Uncrasher as surface hab, storage ext
    (A)(4) uncrasher is parked behind a fuel depot sunshade to extend fuel depot( also a retired uncrasher)

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