Author Topic: An Alternative Lunar Architecture  (Read 597144 times)

Offline Chris Bergin

RE: An Alternative Lunar Architecture
« Reply #20 on: 01/31/2006 11:58 pm »
Here's some interesting information:

I have acquired the document NASA-TN-D-6365 "The Utilization of Halo Orbits in Advanced Lunar Operations" written by Dr. Robert Farquhar in July 1971, and have made it available for download from the FTP server at:

CLICK HERE FOR PDF DOWNLOAD
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Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #21 on: 02/10/2006 01:18 am »
This is a great document!  Looking through it, I've been wondering how this architectural concept could be adapted for the CEV/LSAM architecture in the most efficient way.  Here's the best I have come up with:

The CEV and LSAM would launch, dock, and inject to the Moon just as in the current ESAS LOR scenario.  But about a day or so before lunar arrival, the crew would transfer to the LSAM and the CEV would separate from the LSAM, unmanned.  Then they would both execute little correction maneuvers to configure for different lunar passages.  The LSAM would go directly into a lunar orbit (polar, equatorial, or otherwise) designed to have its landing site in the orbital plane.  The CEV would burn at perilune to slow itself down slightly and then coast out to L2 and burn again for L2 insertion.

An advantage here would be that the LSAM descent stage would not be burning to slow both the LSAM and CEV into low lunar orbit, but only itself.  So the 900 m/s LOI maneuver would now be just for the LSAM, much like a direct descent mission.  The CEV, on the other hand, only expends ~200 m/s at the lunar swingby and ~150 m/s for L2 insertion.

After the surface mission was complete, the LSAM ascent stage would launch to lunar orbit and then inject to L2.  This requires more DV on the ascent stage than is currently planned, because now the ascent stage is going to L2 rather than just to low lunar orbit.  This will require another 670 m/s of DV on the ascent stage.  But there is no plane change burn, because all locations on the Moon have defineable injection conditions to L2 at any time.

As the LSAM ascent stage approaches L2, it can either do an insertion burn of 150 m/s, and then be semi-permanently captured at L2, or alternatively, if we want to throw away the ascent stage (as is the current plan) we could nudge the CEV back out of L2 into a trans-lunar orbit, where it would meet up with the LSAM ascent stage and get the crew.  Then the CEV flies by the Moon again, and does another burn for trans-Earth injection, arriving back at Earth five days later.

(I like the idea of capturing the LSAM ascent stage at L2, because then the LSAM ascent stage could be used as a "poor-man's" L2 gateway, or perhaps even a com relay for future missions.  But it's another 150 m/s of DV, so that would have to be considered.)

So tallying up all the DVs you can see the pros and the cons.  For the CEV, they are pretty significant.  The CEV in this scenario only needs 700 m/s of DV capability (350 m/s in and out of L2) vs. 1724 m/s of DV for the ESAS LOR scenario.  The reason the DV numbers are so different is that in the LOR scenario the CEV had to carry both the plane change penalty and the TEI burn, which is 900 m/s.  So if we went with L2, the DV requirement on the CEV would be cut by 60%.  As you might imagine this would have a big impact on the gross mass of the vehicle, probably cutting it by a third over the current design.

The reduction in DV capability for the CEV also brings the CEV for the lunar mission more in line with the other missions for the CEV, which don't require nearly so much DV.  For ISS, 700 m/s would be MORE than enough, and for Mars missions the CEV sort of just hangs onto a larger vehicle and doesn't really contribute anything until it's time for the crew to reenter.  If the Mars mission was staged out of L2, then the DV requirement for the CEV would again be the same as the lunar mission.  This would fit really well with the artificial-gravity Mars NEP vehicle I like so much.  But it would also fit well with a high-thrust mission too, like chemical injection (which could take advantage of lunar propellants).

For the LSAM, there would be benefits and penalties.  The big benefit is now the LSAM wouldn't have to brake itself AND the CEV into low lunar orbit, through a DV of 900 m/s.  This should be quite beneficial.  The LSAM also doesn't need to carry the plane change penalty of 200 m/s.  But the LSAM ascent stage needs more DV to get (and possibly stay) in L2.

There's also the time penalty to account for and understand.  It will take an additional three days for the LSAM ascent stage to get to L2 from the lunar surface, on a minimum energy trajectory.  For a little more DV, you can speed that up to two days, but beyond that the DV penalty gets really bad.  Then it will take another three days for the CEV to "fall" back down towards the Moon for the powered lunar swingby maneuver.  So five-six days extra over the standard trajectory.  BUT!  And this should be carefully considered in a fair comparison--the ESAS study is showing all kinds of "phasing" and "loitering" maneuvers in lunar orbit to try to reduce that plane change penalty.  While I can imagine that there will be some favorable cases, my read on this (and I'm a fairly good astrodynamicist) is that in general, the loitering time needed to reduce plane change penalties will outweigh the transit time penalties of the L2 architechure.  Same with DV--sometimes LOR will be better, but in general, due to the ESAS constraint of "global access, anytime return", I think it will be worse, and if we design to the worst-case DV scenario, then what does it really matter if sometimes it gets better?

For the EDS and the cargo launcher things get better too.  Because now you're pushing a much lighter CEV through the TLI burn, and since 1 kg of injected mass roughly translates into 1 kg of injection propellant mass, then a 1/3 reduction in CEV mass translates is reflected again in the mass of the EDS.

There appear to be some real benefits to using L2 as a staging location using the ESAS vehicles, as well as future reusable vehicles.  With a reusable LSAM staged out of L2 and refueled by lunar propellants, only the CEV needs to be sent out each time.  Hopefully at some point it will be thrown to the Moon by a MXER tether, further reducing costs!

It also sets up the whole lunar architecture for a much cleaner segue to Mars operations that the LOR architecture we're carrying today.  Mars vehicle pieces could be sent by EDSs out to L2, aggregated, and then sent back down to a highly-elliptical orbit for high-thrust injection, or if they're low-thrust, directly out to Mars from a location on the very edge of the Earth's gravity well.  From L2 your spiral-out time would just be a few days.

Thoughts, comments?

Offline Jim

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RE: An Alternative Lunar Architecture
« Reply #22 on: 02/10/2006 01:38 am »
LSAM ascent stage only has batteries for 3 hrs and not much more O2.  It receives most of its consumables (power, O2, water, etc) and heat rejection from the descent stage.  This would be a big weight increase for the ascent stage and would ripple back to the descent stage.

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #23 on: 02/10/2006 01:52 am »
Quote
Jim - 9/2/2006  8:38 PM

LSAM ascent stage only has batteries for 3 hrs and not much more O2.  It receives most of its consumables (power, O2, water, etc) and heat rejection from the descent stage.  This would be a big weight increase for the ascent stage and would ripple back to the descent stage.

Good points...we're talking an LSAM ascent stage redesign then.  Maybe move some of the functions off the descent stage to the ascent stage, especially if it is intended to operate autonomously at L2 for an extended period of time.  Downside is all that mass gets moved through the ascent DV as well.  But the CEV gets a lot smaller and so does the CLV.

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #24 on: 02/10/2006 03:59 am »
From page 184 of the ESAS report.  Note that anytime return is only possible in some conditions, whereas in the L2 architecture, the launch window from the surface is always open.

"The requirement to return anytime from the surface of the Moon to Earth was the design driver of the SM propulsion system. The CEV SM is common across all of its missions and is sized by the lunar mission application. The lunar mission requires a total of 1,450 m/sec of delta-V, combining a 900 m/sec TEI maneuver and a worst-case 90-deg nodal plane change. This capability enables “anytime return” if the lander is able to perform a coplanar ascent to the CEV. For sortie duration missions of 7 days or less, the CEV’s orbital inclination and node will be chosen to enable “anytime return” from any location on the lunar surface. Outpost missions will also have the ability to return anytime the outpost is located at a polar or equatorial site. For other sites, loitering on the surface at the outpost for up to 14 days may be required to enable a coplanar ascent to the orbiting CEV."

The 90-degree nodal change mentioned in here also consumes one day.

Offline Manel

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RE: An Alternative Lunar Architecture
« Reply #25 on: 02/11/2006 06:56 pm »

For the last days I am following this forum. Quite very interesting

First a greeting to all. Later I would like to mention that I find very useful the ideas on the use of the point L2 in a future lunar architecture

Some time ago that I´m thinking about it,   and about the possibility of using the spent ascent stages  of the LSAM to do a small station in this point. Studying also the possibility of sending an unmanned CEV in one of the first expeditions together with a not crewed LSAM,   that they would serve like backups for later expeditions, one at L2  the other as a shelter on the lunar surface

My question to VANILA is,  if it has been considered the possible advantages of using the  L1 point in the same conditions ?   Perhaps the mission would need less days to complete ?

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #26 on: 02/11/2006 07:09 pm »
Quote
Manel - 11/2/2006  1:56 PM
My question to VANILA is,  if it has been considered the possible advantages of using the  L1 point in the same conditions ?   Perhaps the mission would need less days to complete ?
Hi Manel, welcome to the forum.  Yes, L1 has been considered, much more extensively than L2 is past trades.  There is a time advantage to using L1--a direct trajectory to L1 takes only four days, whereas the lunar-swingby trajectory to L2 takes eight days.  From L1 or L2 the design of the architecture to and from the lunar surface, as well as the advantages, would be very similar.

The salient difference between the two architectures is that the direct L1 trajectory takes at least 710 m/s of delta-V to get into and out of L1, for a total DV penalty of at least 1420 m/s.  I say at least, because if you go faster the DV penalty increases.  The direct to L2 DV penalty is 1100 m/s, one way, BUT what is really exciting about what Farquhar found is that with a powered lunar swingby, you can get the DV penalty down to only 330 m/s, each way.

So a direct L1 mission would require 1420 m/s of DV, whereas a lunar-swingby-L2 mission would require only 660 m/s.  That's a big difference for the propulsion system.  There is a lunar swingby trajectory to L1, and it also only requires 330 m/s, but it takes 3 weeks to fly the trajectory.  It looks like this:

Offline Manel

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RE: An Alternative Lunar Architecture
« Reply #27 on: 02/11/2006 07:26 pm »



Vanilla, very well explained. Thank you very much

Offline Manel

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RE: An Alternative Lunar Architecture
« Reply #28 on: 02/12/2006 07:59 am »
One more question
What are the numbers, delta V and time, for reaching the L1 point through an "type 8 trajectory" ?
First braking at perilune at the far side and a final braking at L1
Thank you in advance

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #29 on: 02/12/2006 02:27 pm »
Quote
Manel - 12/2/2006  2:59 AM
What are the numbers, delta V and time, for reaching the L1 point through an "type 8 trajectory" ?
First braking at perilune at the far side and a final braking at L1
Thank you in advance
The previous trajectory was a prograde powered lunar swingby, with a 225 m/s burn at perilune and a 105 m/s burn at L1.  The flight time from TLI to perilune was 6.6 days, and then another 16.5 days of phasing orbits before the burn at L1.

I had initially thought that a retrograde lunar swingby would be both faster and less DV to L1, and I was correct on the former, but wrong on the latter.  The retrograde lunar swingby takes 3.5 days from TLI, then a 218 m/s burn at perilune on the far side, followed by an 8.7 day coast out to L1 and a 316 m/s burn into L1.  So the total time and DV for the retrograde lunar swingby to L1 is 12.2 days and 535 m/s.

A lunar swingby trajectory to L2, at 8.8 days and 330 m/s still beats them both.

Offline Manel

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RE: An Alternative Lunar Architecture
« Reply #30 on: 02/12/2006 02:56 pm »



Clearly the use of the L2 point is the practical method for the next phase of lunar exploration

Only that needs an important enlargement in the crewed capacity of the LSAM´s ascent stage

Offline Jim

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RE: An Alternative Lunar Architecture
« Reply #31 on: 02/12/2006 07:49 pm »
Quote
Manel - 12/2/2006  9:56 AM


Clearly the use of the L2 point is the practical method for the next phase of lunar exploration

Only that needs an important enlargement in the crewed capacity of the LSAM´s ascent stage

This would change the ESAS.  Larger LSAM ascent stage means larger descent stage, which means more prop for landing, which means more prop for LOI.  This all flows back into a larger EDS and therefore bigger CaLV.  We should do the ESAS as proposed and try not to preturb it  as little as possible as not to mess up the funding.  Changes, upgrades, and enhancements can come later

Offline Manel

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RE: An Alternative Lunar Architecture
« Reply #32 on: 02/12/2006 09:09 pm »

Not necessarily. The CEV needs only a delta V  of  700 m/s  vs  1.720  in the ESAS scenario  (for TEI and possible plane change).   Vanilla dixit
The LSAM ascent stage gets heavier and this translates in more fuel for the descent stage
But also this descent stage does not have to brake the CEV at LOI
I believe that the final result must be a lighter  CEV+LSAM,   than translates in a smaller EDS
Therefore,  go for the moon,  changes can wait

Offline Chris SF

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RE: An Alternative Lunar Architecture
« Reply #33 on: 02/15/2006 11:36 am »
When alternative is noted, does that mean there's a lack of confidence in the current future plans?

Offline Jamie Young

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RE: An Alternative Lunar Architecture
« Reply #34 on: 02/15/2006 04:50 pm »
Quote
Chris SF - 15/2/2006  6:36 AM

When alternative is noted, does that mean there's a lack of confidence in the current future plans?

That's a good question and was this a potential plan back in the Apollo days?

Offline Jim

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RE: An Alternative Lunar Architecture
« Reply #35 on: 02/15/2006 05:06 pm »
Quote
Jamie Young - 15/2/2006  11:50 AM
Quote
Chris SF - 15/2/2006  6:36 AMWhen alternative is noted, does that mean there's a lack of confidence in the current future plans?
That's a good question and was this a potential plan back in the Apollo days?

Apollo was always for a quick return and also had only equatorial landing sites.

Offline Martin FL

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RE: An Alternative Lunar Architecture
« Reply #36 on: 02/15/2006 10:20 pm »
Which we don't need now, so would this (in everyone opinions) be a better option, or just an option?

Offline mkirk

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RE: An Alternative Lunar Architecture
« Reply #37 on: 02/15/2006 10:32 pm »
Quote
Manel - 12/2/2006  4:09 PM


Not necessarily. The CEV needs only a delta V  of  700 m/s  vs  1.720  in the ESAS scenario  (for TEI and possible plane change).  


I here what you're saying but if I were the one riding in it I would like to have all that extra delta V for expanded abort options as well as additional in flight operations and mission objectives...such as rendezvous/docking, orbit changes, etc...

The additional delta V is what is assumed in the abort options for the current reference trajectories including such things as a retro-grade TAL...that gives me a warm fuzzy...even if it is a more expensive one.

Mark
Mark Kirkman

Offline Jamie Young

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RE: An Alternative Lunar Architecture
« Reply #38 on: 02/16/2006 06:33 pm »
Could someone explain what Delta V is? Thanks.

Offline Jim

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RE: An Alternative Lunar Architecture
« Reply #39 on: 02/16/2006 06:59 pm »
The Greek symbol Delta represents change in a value.  V stands for Velocity


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