Lunar Orbits and their potential uses

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The downside: during closest approach it would be screeching at blurring speeds over the Moon, making observations or deploying a lander difficult.

It probably makes sense to quantify that. In a circular orbit 100 km above the lunar surface a spacecraft is moving 1,634 m/s. The polar NRO Whitley describes in the presentation linked above is 2000 x 75000 km. Thus at its closest approach to the Moon a spacecraft in that orbit is moving at only 1,582 m/s! (Someone might want to double check that calculation.)

But of course the task of landing is more difficult. Whitley says it takes 730 m/s and half a day to get from an NRO to a LLO.

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The downside: during closest approach it would be screeching at blurring speeds over the Moon, making observations or deploying a lander difficult.

It probably makes sense to quantify that. In a circular orbit 100 km above the lunar surface a spacecraft is moving 1,634 m/s. The polar NRO Whitley describes in the presentation linked above is 2000 x 75000 km. Thus at its closest approach to the Moon a spacecraft in that orbit is moving at only 1,582 m/s! (Someone might want to double check that calculation.)

But of course the task of landing is more difficult. Whitley says it takes 730 m/s and half a day to get from an NRO to a LLO.

That's pleasantly informative. 

It still sounds like this orbit would be troublesome for landing, although not impossible.  Reviewing that figure of 730 m/s at least looks consistent with most delta-v charts of capturing and transfering into LLO.  NRO is definitely at the borderline of orbiting the Moon; good for orbiters like Orion (or perhaps Russia's Federation ship) but lousy for landers.  On top of that 730 m/s an additional 1700+ m/s is needed to go from LLO to the surface one-way, totally roughly 2,500 m/s. 

The Apollo's LEM only had to tackle landing from LLO, with a round-trip effort of ~3.5 km/s whereas a lander descending from NRO would require 5 km/s for a round-trip.  It will have to be a more robust vehicle, although if reused the advantage NRO could bring would be the ability to bring in the required fuel load from Earth to supply the landers more easily.

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The Apollo's LEM only had to tackle landing from LLO, with a round-trip effort of ~3.5 km/s whereas a lander descending from NRO would require 5 km/s for a round-trip.

Yes the Apollo solution is a great place to start an analysis!

Since then NASA has really only had one major human lunar lander design that was part of a "plan of record:" Altair. The Altair design received a lot of criticism, some of which was quite informative. In particular some critics came up with alternatives to Altair's Apollo on Steroids approach. One that would be particularly good to take off the shelf and re-examine was called a lunar descent "uncrasher" stage.

It might be that a big stage could handle quite a number of the NRO round-trip propulsion maneuvers, including some of the major descent, carrying a smaller single-stage lander/ascender. Note this ends up being fully reusable....

[redliox]

The Apollo's LEM only had to tackle landing from LLO, with a round-trip effort of ~3.5 km/s whereas a lander descending from NRO would require 5 km/s for a round-trip.

Yes the Apollo solution is a great place to start an analysis!

Well obviously it's the only comparison to use for actual crewed vehicles in lunar orbit.  Also you get an idea how simplistic everything was.  The LEM did a good job but it wouldn't meet the current desires to reach the poles or descend/ascend from beyond LLO.  Not to mention both it and the CM would be too cramped, especially if you hoped for a 4th astronaut.  Apollo served a purpose, but we need to do better the second time around.

Since then NASA has really only had one major human lunar lander design that was part of a "plan of record:" Altair. The Altair design received a lot of criticism, some of which was quite informative. In particular some critics came up with alternatives to Altair's Apollo on Steroids approach. One that would be particularly good to take off the shelf and re-examine was called a lunar descent "uncrasher" stage.

I think Altair was asked to do too much too soon and on very little funding.  It was supposed to provide essentially 3/4 of the mission's delta-v post TLI with Orion chiefly for the return home (which sadly is how Orion led to becoming a large but under-powered orbiter).  I'd like to compare the Altair's needs as well.  Unlike Altair, a hypothetical (crew) lander shouldn't be tasked to handle the entire mission or else it gets too bloated; that should be the lesson learned from Constellation.

It might be that a big stage could handle quite a number of the NRO round-trip propulsion maneuvers, including some of the major descent, carrying a smaller single-stage lander/ascender. Note this ends up being fully reusable....

Regarding a crasher/uncrasher stage, especially if the lunar lander is going to be stowed and reused at a station in orbit (NRO, DRO or otherwise), an option to include a disposable stage on it should be considered.  The lander would still need to ascend back to orbit, but the descent first to LLO could be aided by a small stage exactly like modern upper stages, many of which are either solids or hypergolics...and more to the point easy to order and handle versus an all-cryogenic setup.  Naturally this is assuming the lander can't be nominally entirely reusable.

It would be good to see a lunar lander maintained as part of a lunar station; funding it as well as anticipating the orbital descent needs would be the trouble though.

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Did a little number crunching with the help of this delta-v calculator: http://www.strout.net/info/science/delta-v/

I was curious if a lunar lander, using a somewhat simple expendable stage, could travel from high lunar orbit down to the surface and then back again.  I made the follow assumptions:

-Transfer from High-to-Low Lunar orbit 0.68 km/s
-Transfer from Low Lunar orbit to surface 1.73 km/s
-Total transfer d-v from High orbit to surface 2.5 km/s one-way, 5 km/s round-trip
-Lander is 30 metric tons (i.e. twice size of LEM) with a dry mass of 8.8 metric tons
-Descent stage based on a Castor 30 2nd stage
-Lander and Descent stage start from High Lunar orbit already prepared

The first challenge for a lunar lander is getting from either NRO or DRO (which are at the upper limits of the lunar gravity well [hence the favoritism of them lately]) down to LLO.  Instead of fretting over an advanced cryogenic engine...it occurred to me we already use small, disposable engines to change satellite orbits; the Descent stage would be doing this same function with a crewed lander.  In the case of a Castor 30, it would be able to brake the 30 mt lander (44 counting the attached stage) by a bit over a full 1 km/s. 

Depending on orbital mechanics, this could either put the lander in an elliptical low orbit or a crash course with the surface.  In either case, the lunar lander would brake the remaining ~1.5 km/s with its own propellants.  The good news is a 30 mt one-way lander can easily achieve this even with hypergolics using 14 mt of propellants with some reserve delta-v as well.

The not-so-good news is the return to high orbit.  With hypergolics (and conserving fuel as best as possible), the lunar lander could lift off with 1.8 km/s; sufficient for LLO but not much else.  LOX/Methane does better with 2.1 km/s, and LOX/Hydrogen succeeds with just over 2.6 km/s.  It's just on the cusp of possible, but even with a disposable stage it's not easy.

A one-way lander could probably descend from high orbit down to the surface without any disposable descent staging; although with a stage assisting more payload can be incorporated into the lander itself.  A crew vehicle, certainly if meant to be reusable, may require assistance even if you have the best cryogenics.  That is the tradeoff with staging from NRO/DRO/ect.