Author Topic: Uncrasher stages?  (Read 7271 times)

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 28616
  • Minnesota
  • Liked: 8585
  • Likes Given: 5602
Uncrasher stages?
« on: 08/26/2013 06:05 PM »
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?
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline jongoff

  • Recovering Rocket Plumber/Space Entrepreneur
  • Senior Member
  • *****
  • Posts: 6214
  • Lafayette/Broomfield, CO
  • Liked: 2356
  • Likes Given: 790
Re: Uncrasher stages?
« Reply #1 on: 09/05/2013 03:43 AM »
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?

I have no idea if I was the first to come up with it (unlikely in aerospace--most good ideas have been independently invented a few times). But here's a blog post I just put together today, with a spreadsheet and some charts:

http://selenianboondocks.com/2013/09/centaur-uncrasher-stages-for-simplified-lunar-landings/

Some interesting takeaways:

1- UnCrasher Stages (and Crasher stages too most likely) enable amazingly light landers for heavy payloads--if my simplifications aren't too far off and my math not heinously wrong, if you can do the staging point 200m/s from landing, you can land an Altair class payload with the lander itself being not much bigger than an Armadillo SuperMod.
2- It turns out that there is a lower limit on marginal initial Mass in L2 (mIML2) where an UnCrasher does no good compared to a lander that just goes straight from EML-2 by itself.
3- The payload improvement for a given amount of mIML2 compared to just using a lander without an UnCrasher is decent (~20-36%) but not blow your mind awesome.
4- By enabling really small, weight-insensitive, high-capacity cargo landers using an UnCrasher that is a lightly modified Centaur stage, this is a low-development cost route to at least the cargo part of a lunar program. The low sensitivity of the landers makes it seem like less traditional players might be able to develop useful landers this way.

I'll be crunching some more numbers now that I've figured out how to get Solver running on my latest version of Excel.

If anyone wants to pull the graphs from my blog post and put it here, feel free--I'm too lazy/tired/busy.

~Jon

Offline jongoff

  • Recovering Rocket Plumber/Space Entrepreneur
  • Senior Member
  • *****
  • Posts: 6214
  • Lafayette/Broomfield, CO
  • Liked: 2356
  • Likes Given: 790
Re: Uncrasher stages?
« Reply #2 on: 09/05/2013 04:05 AM »
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

Offline sdsds

  • Senior Member
  • *****
  • Posts: 5479
  • "With peace and hope for all mankind."
  • Seattle
  • Liked: 581
  • Likes Given: 677
Re: Uncrasher stages?
« Reply #3 on: 09/05/2013 04:10 AM »
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."
« Last Edit: 09/05/2013 04:35 AM by sdsds »
-- sdsds --

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 28616
  • Minnesota
  • Liked: 8585
  • Likes Given: 5602
Re: Uncrasher stages?
« Reply #4 on: 09/05/2013 05:06 AM »
Wonderful, Jon!

I'll have to take some time to do some analysis.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline jongoff

  • Recovering Rocket Plumber/Space Entrepreneur
  • Senior Member
  • *****
  • Posts: 6214
  • Lafayette/Broomfield, CO
  • Liked: 2356
  • Likes Given: 790
Re: Uncrasher stages?
« Reply #5 on: 09/05/2013 05:30 AM »
Great stuff! Minor nit: Joseph-Louis Lagrange. Not LaGrange.

D'oh! Will fix that.

Quote
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.)

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. After that, the DV from there to EML-2 may vary a bit based on speed, but I don't know that you'll squeeze out much extra performance. By that point you only are lugging around the Centaur's dry mass and almost no propellant, so even if you could knock say 200m/s off the DV it would make only a modest difference in the lander payload.

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". No analysis was done to figure out what the max assist DV really should be. Is 200m/s too close to the ground? Do we need to add in some extra DV margin for hovering during separation? 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.  How much lander mass is needed based just on supporting and landing the payload itself (ie that is payload-driven not propellant driven)? How is the best way to model that? Etc.

All of those various details will shift around the specific numbers a bit, but I think that the qualitative conclusions are solid--this is a way to enable small landers to put big cargo on the surface.

Wish I could get a tiny bit of study money tossed our way to refine this a bit. Only so much I can do in my "lack of sleep" time for free, and I know a great CU grad student that I wish I could bring on as an intern for stuff like this.

Quote
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."

Yeah. I still need to run the numbers for a manned lander. Since it has to go back to orbit from the lunar surface in addition to whatever tiny landing dV it needs, that may end up being a much bigger, more capable vehicle. The Cargo lander was just the easiest to analyze quickly.

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

Offline jongoff

  • Recovering Rocket Plumber/Space Entrepreneur
  • Senior Member
  • *****
  • Posts: 6214
  • Lafayette/Broomfield, CO
  • Liked: 2356
  • Likes Given: 790
Re: Uncrasher stages?
« Reply #6 on: 09/05/2013 05:32 AM »
Wonderful, Jon!

I'll have to take some time to do some analysis.

Thanks Robotbeat! I'm almost positive I've made at least one or two mistakes, and would really appreciate any feedback I can get from the NSF crowd. I think that approaches like this may be the key to an affordable lunar architecture, but since we're not making any money on this (currently), this is mostly just a hobby effort by myself at the moment.

~Jon

Offline R7

  • Propulsophile
  • Senior Member
  • *****
  • Posts: 2738
    • Don't worry.. we can still be fans of OSC and SNC
  • Liked: 954
  • Likes Given: 662
Re: Uncrasher stages?
« Reply #7 on: 09/05/2013 06:25 AM »
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 ;)
AD·ASTRA·ASTRORVM·GRATIA

Offline A_M_Swallow

  • Elite Veteran
  • Senior Member
  • *****
  • Posts: 8543
  • South coast of England
  • Liked: 362
  • Likes Given: 158
Re: Uncrasher stages?
« Reply #8 on: 09/05/2013 03:37 PM »
One of the simplest manned landers is to put a 5 tonne MMSEV on a cargo lander.  The astronauts then drive to the hab.  Optionally refuel the lander to get back to the spacestation.

Offline jongoff

  • Recovering Rocket Plumber/Space Entrepreneur
  • Senior Member
  • *****
  • Posts: 6214
  • Lafayette/Broomfield, CO
  • Liked: 2356
  • Likes Given: 790
Re: Uncrasher stages?
« Reply #9 on: 09/05/2013 04:18 PM »
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 ;)

Interesting idea. Might be possible, but you'd need to making Centaur into a hovering platform...not sure if it would save you much at that point, as you'd have to add a lot of extra propulsion, and your payload would still need some sort of landing gear. But there may be situations where that would make sense.

~Jon

Offline simonbp

Re: Uncrasher stages?
« Reply #10 on: 09/05/2013 04:32 PM »
Well, maybe not that extreme, but a minimal terminal descent system like Surveyor (basically an overgrown RCS) would allow you to shift as much delta v as possible to the uncrasher/retrograde/braking stage.

Also, since the LH2 tanks are empty, the stage could do a real slow-boat trajectory back out to L2. That would take less than 100 m/s (maybe closer to 50 m/s) of delta v from low lunar orbit.
« Last Edit: 09/05/2013 04:34 PM by simonbp »

Offline Proponent

  • Senior Member
  • *****
  • Posts: 5598
  • Liked: 1143
  • Likes Given: 684
Re: Uncrasher stages?
« Reply #11 on: 09/05/2013 04:44 PM »
100 m/s from LLO to L2?  How?

Offline simonbp

Re: Uncrasher stages?
« Reply #12 on: 09/05/2013 04:56 PM »

Offline gbaikie

  • Full Member
  • ****
  • Posts: 1628
  • Liked: 49
  • Likes Given: 5
Re: Uncrasher stages?
« Reply #13 on: 09/05/2013 05:05 PM »
100 m/s from LLO to L2?  How?

Not possible. But if uncrasher does less of delta-v for landing on the Moon- making the lander's delta-v less than 1000 m/s. Then one significantly increases the landers' payload, and lander could be used gain orbit [or descent] with smaller payloads [or for sub orbital lunar hops].
So uncrasher's function is increase the usability of one type of lander.
Though uncrasher still needs more than 100 m/s. Perhaps uncrasher could use SEP, which allow it to dock, and travel in environment above LLO- and using chemical propulsion when need higher thrust.
Of course, uncrasher could separate module than SEP tug.

Offline sdsds

  • Senior Member
  • *****
  • Posts: 5479
  • "With peace and hope for all mankind."
  • Seattle
  • Liked: 581
  • Likes Given: 677
Re: Uncrasher stages?
« Reply #14 on: 09/05/2013 10:39 PM »
[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.

Ah, right! Put that way it becomes pretty clear the bulk of the return delta-v is determined by two-body dynamics deep in the lunar gravity well.

Quote
No analysis was done to figure out what the max assist DV really should be. Is 200m/s too close to the ground?

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?
-- sdsds --

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 28616
  • Minnesota
  • Liked: 8585
  • Likes Given: 5602
Re: Uncrasher stages?
« Reply #15 on: 09/05/2013 11:05 PM »
...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?
It depends on exactly the trajectory. If you assume it's brought to a standstill (and the rest of the delta-v can be done impulsively, i.e. infinite thrust), the problem is fairly easy.

The specific energy of 200m/s is equivalent to being about 12.3km above the lunar surface. http://www.google.com/search?q=.5*(200m/s)^2/(1.622m/s^2)
.5*v^2/g=h
And the time to fall 12.3km in lunar gravity is:
t=sqrt(2*h/g)
t=sqrt(2*.5*v^2/g^2)=v/g
https://www.google.com/search?q=(200m/s)/(1.622m/s^2) = 2 minutes.
(and, here we see how I did some steps unnecessarily... Could've gone straight from given velocity to how long it takes to accelerate to that velocity in lunar gravity. Oh well.)

Two minutes should be plenty of time for both a reboost of the centaur stage into orbit (its T/W ratio will be incredibly high since it'll have no payload but itself and be nearly empty) and for the terminal descent stage to ignite give enough impulse to land properly, perhaps even do some steering.

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?
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline jongoff

  • Recovering Rocket Plumber/Space Entrepreneur
  • Senior Member
  • *****
  • Posts: 6214
  • Lafayette/Broomfield, CO
  • Liked: 2356
  • Likes Given: 790
Re: Uncrasher stages?
« Reply #16 on: 09/05/2013 11:11 PM »
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?

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

Offline jongoff

  • Recovering Rocket Plumber/Space Entrepreneur
  • Senior Member
  • *****
  • Posts: 6214
  • Lafayette/Broomfield, CO
  • Liked: 2356
  • Likes Given: 790
Re: Uncrasher stages?
« Reply #17 on: 09/05/2013 11:13 PM »
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?

That's why it pays to do an actual detailed simulation instead of just using the "cookbook" answers. I'd still want some margin, but you want to make sure you're not sandbagging it with margin on top of margin.

~Jon

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 28616
  • Minnesota
  • Liked: 8585
  • Likes Given: 5602
Re: Uncrasher stages?
« Reply #18 on: 09/05/2013 11:45 PM »
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?

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
Do you have Matlab? It's too expensive for me.

I've been using Scilab, which is a very high-quality, open-source (and free) clone with a good IDE. Well-maintained but with a slightly different syntax (really easy, though). They use it more in Europe. I'd give it a shot next time you want to do something that'd be awkward to do in Excel.

There's always Python, which is sort of becoming a de-facto standard in high-level languages used in the sciences and in engineering. (SAGE is a good MATLAB-like Python platform.)

But really, it's scary how often Excel is used. It's the /real/ de facto standard nowadays. My internship advisor (Physics PhD, does lots of NIAC proposals and the like...) at NASA who once may have bothered with Mathematica or Matlab now just uses Excel because, well, it's available. A little sad, I know, but don't feel like you're an odd-ball for using Excel!
« Last Edit: 09/05/2013 11:45 PM by Robotbeat »
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline DGH

  • Full Member
  • *
  • Posts: 168
  • Liked: 7
  • Likes Given: 4
Re: Uncrasher stages?
« Reply #19 on: 09/06/2013 01:00 AM »
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

If I am reading this correctly some of these numbers are thought provoking especially for the crasher.
From C3 to direct lunar descent have similar Delta V to L2-lunar descent.
A Delta IV Heavy which can deliver over 10 MT to C3 which would be 3.5 to 4.5 metric tons to the lunar surface.
The base SLS almost 25 mt to C3 could deliver 10 to 12 mt to the surface.
Apollo could with LESA do 15+ mt tons to the Lunar surface.
With this over 20+ MT to the surface.
I would be intrigued to see the numbers from TLI to Lunar orbit.

Offline muomega0

  • Full Member
  • ****
  • Posts: 861
  • Liked: 65
  • Likes Given: 1
Re: Uncrasher stages?
« Reply #20 on: 09/06/2013 12:49 PM »
Quote
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.'

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.
The boiloff will vary by lunar location, the highest rate at the equator, and lowest at the poles, where ISRU would likely be most productive.
Many concepts include a reverse sunshield of varying mass to reduce the rate only for long duration stays.

If the uncrasher stage was powered by methane, one would likely loose 15 to 20% of lander payload mass.

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
"Better approaches" depends on whether you want a mission in the 10 years after SLS development/ISS splashdown or dozens over 25 to 30 with a 7 year delay. 

Use fresh engines for the expensive equipment, and reuse for the cheaper supplies.  Quite a bit a work to do in this trade  :)

From: http://selenianboondocks.com/2013/09/centaur-uncrasher-stages-for-simplified-lunar-landings/

Quote from: jgoffblog
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:
Using LH2 rather than methane in the uncrasher gives a 15 to 20% boost in payload, and if the goal is ISRU, or less production line costs, then it has not been shown that it is cost effective to include methane or lower ISP in the crasher.  At 0.1%/day, that's a 150 day to break even.   Perhaps land a few reusable, moveable solar arrays and cryocooler equipment and make it zero boiloff. 

Again, its the explore sooner versus sustainability debate.

Comment from blog:
" 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."

The goal is to demonstrate equipment needed for deep space exploration without entering the gravity well to save costs.  The proper sequence is to deploy a LEO ZBO depot and a L2 Gateway to demonstrate hardware and crew can achieve the trip times to Mars.  Depending on your POV on GCR, this may mean one trip or multiple over many years.  Regardless, it is only crew tended in the latter.

Lower cost infrastructure is the key to enable the multiple destination and missions forward.

Quote from: jgoffblog
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).
It is very difficult to shift from old technology and architectures to focus on enabling sustainable exploration, which includes common components to reach the destinations. 

As you point out, any lunar effort should be focused on this major cost driver:  reducing the lander mass to decrease IMLEO launch costs--quite the impact...err well you know what i mean.
« Last Edit: 09/06/2013 12:54 PM by muomega0 »

Offline kkattula

  • Member
  • Senior Member
  • *****
  • Posts: 2508
  • Melbourne, Australia
  • Liked: 10
  • Likes Given: 3
Re: Uncrasher stages?
« Reply #21 on: 09/07/2013 07:15 AM »
At first I was a bit nonplussed by this concept, why would you take the main propulsion stage that close to the surface and not bother landing it, ala DTAL?

However, any extended human presence on the Moon is going to require a lot of cargo, with the occasional crew. So disposable, low delta-v, 'landing-kits' for cargo make a lot of sense, and keeps the re-usable space-tug well away from dust and errant rocks.

For the crew lander, it really doesn't need to be much more than a dedicated ascent (to L1/2) stage. For the nominal 14.7t landed payload, 2600 m/s dv, 311 Isp storable propellants, that gives a dry mass of around 6.2t. That's a decent amount of mass to work with.

Tags: