I'm looking for some material on "uncrasher stages." Does anyone have info on this? Who thought up this idea first? I know Jon Goff has talked about it, but was he the first to think of it?
Great stuff! Minor nit: Joseph-Louis Lagrange. Not LaGrange.
Edit to add meaningful content:Jon, could you elaborate on the thinking behind the idea that the, '“assist Delta-V” for the UnCrasher stage [...] also happens to be the amount of Delta-V the UnCrasher needs to provide post-staging to boost itself back to EML-2.' This would be trivially true if the two trajectories were orbits around a single gravitational mass, but that isn't necessarily the case since the Earth is out there too.... I think the assist delta-v is possibly an upper bound on the boost-back delta-v. But it seems like if you could tolerate a longer time-of-flight you might be able to find a three-body trajectory back to the starting point that required considerably less delta-v. (This relates to step 5 in your methodology, in case that wasn't clear.)
Your bottom line about this approach is really exciting: "it lowers the cargo lander size down to something that a small company could realistically build for non billions of dollars."
Wonderful, Jon!I'll have to take some time to do some analysis.
Neat concept. Higher energy propellant = payload increase. Centaur boost back almost free, offset by saving the stage inert mass.But if you get that close to the surface why not "land" the payload from hovering Centaur with Sticky Boom and omit the lander. Sticky Crane
100 m/s from LLO to L2? How?
[When] you're leaving just 200m/s for the lander [then] you're going to need to spend at least 1800m/s just to get back to LLO (the nearest "stop and catch your breath location") or you'll crash.
No analysis was done to figure out what the max assist DV really should be. Is 200m/s too close to the ground?
...After a lander separation 200 m/s short of a soft touch-down, how many seconds does the uncrasher have for relight before it would otherwise crash into the lunar surface?
Yes there are certainly concerns there. Perhaps one way to get insight into at least one concern would be to transform the problem into the time domain. After a lander separation 200 m/s short of a soft touch-down, how many seconds does the uncrasher have for relight before it would otherwise crash into the lunar surface?
BTW, I believe the Apollo LM had a significantly greater margin in delta-v margin than would be required if the descent were entirely automatic. How much "hovering" time was it given, again?
Quote from: sdsds on 09/05/2013 10:39 pmYes there are certainly concerns there. Perhaps one way to get insight into at least one concern would be to transform the problem into the time domain. After a lander separation 200 m/s short of a soft touch-down, how many seconds does the uncrasher have for relight before it would otherwise crash into the lunar surface?Exactly. I could do a really crappy 3DOF analysis in Excel, but using a 3DOF simulator done in Matlab/Simulink would be much better. The analysis is totally doable, just involved enough that for now I went the simpler route for first-pass. Even if it turns out the number is 300 or 400m/s, the behavior is still qualitatively the same, even if it isn't quantitatively the same.~Jon
Here's a spreadsheet that now includes both UnCrashers and Crashers. Clearly the actual dry mass will need to include some sort of payload-driven mass component, but it shows theoretically the advantages. It looks like Crashers have a wider range of mIML2 values where they're operating at their maximum assist DV, and they have a small performance advantage over UnCrashers (to be expected), but the impact is a lot less than you would think for what is basically the lunar equivalent of an F9R stage one style "boostback" maneuver.~Jon
Quote from: sdsds on 09/05/2013 04:10 amQuoteEdit to add meaningful content:Jon, could you elaborate on the thinking behind the idea that the, '“assist Delta-V” for the UnCrasher stage [...] also happens to be the amount of Delta-V the UnCrasher needs to provide post-staging to boost itself back to EML-2.'Well, the "assist Delta-V" is the delta-V from EML-2 to the point where the lander stages off. Say for instance, 600m/s to get from EML-2 to LLO, and then 1800m/s of the 2000m/s for a landing (ie you're leaving just 200m/s for the lander). In that case, you're going to need to spend at least 1800m/s just to get back to LLO (the nearest "stop and catch your breath location") or you'll crash. All that said, this is a first-pass analysis. Lots of the mass numbers were pulled out of me ... assumptions... The Delta-V numbers were "cookbook". Is 2600m/s right for an EML-2 to lunar surface run? How much if any boiloff can we expect on the UnCrasher during this mission? Can we between subcooling and passive insulation keep it from boiling off at all? Etc.
QuoteEdit to add meaningful content:Jon, could you elaborate on the thinking behind the idea that the, '“assist Delta-V” for the UnCrasher stage [...] also happens to be the amount of Delta-V the UnCrasher needs to provide post-staging to boost itself back to EML-2.'Well, the "assist Delta-V" is the delta-V from EML-2 to the point where the lander stages off. Say for instance, 600m/s to get from EML-2 to LLO, and then 1800m/s of the 2000m/s for a landing (ie you're leaving just 200m/s for the lander). In that case, you're going to need to spend at least 1800m/s just to get back to LLO (the nearest "stop and catch your breath location") or you'll crash. All that said, this is a first-pass analysis. Lots of the mass numbers were pulled out of me ... assumptions... The Delta-V numbers were "cookbook". Is 2600m/s right for an EML-2 to lunar surface run? How much if any boiloff can we expect on the UnCrasher during this mission? Can we between subcooling and passive insulation keep it from boiling off at all? Etc.
Edit to add meaningful content:Jon, could you elaborate on the thinking behind the idea that the, '“assist Delta-V” for the UnCrasher stage [...] also happens to be the amount of Delta-V the UnCrasher needs to provide post-staging to boost itself back to EML-2.'
Though come to think of it, the right way to do a lunar landing mission (even a sortie) might be to land a hab module/lab first, and then send a slimmed-down manned lander that is still small enough to not be obscenely expensive.I think the whole "a Lander will cost Billions to develop and hundreds of millions per flight" is more applicable if you insist on doing a lander like Altair. But when there's so many better approaches, I'm baffled why you would.~Jon
Surprising Result #2: Payload Benefit of an UnCrasherThe 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:
One Last Observation: Altair LSAM ComparisonOne 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).