Proposal for a simplified mission stack | Mission Planning Workgroup | Forum

Here's a simpler lunar landing stack based on a new idea of mine and brmj's idea of launching with the LL docked to the nose of the CM.

The new idea of mine involves merging the previous Retro Module and Mission Extension Module into a single unit, which I'll call an Orbital Support Module (OSM) for the purposes of this thread.

The OSM would attach to the rear of the CM and be shaped like a truncated cone, with the rear diameter a little larger than the front diameter (such that the slope of the OSM matches that of the CM and when the two are mated the pair looks like a single large truncated cone).  It would be at most a foot long.

In the centre of the OSM would be the deorbit rockets that were part of the old RM.

Around these rockets, on the inside of the OSM's wall structure, would be a number of brackets to which oxygen, nitrogen (or another inert gas), methanol and water tanks *could* be attached for extending the CM's endurance.  However, for short missions these brackets could simply be left empty, reducing the OSM mass.

With the brackets empty and no tanks, the OSM would be functionally equivalent to just the old RM.

With tanks on the brackets, the OSM would be functionally equivalent to the old RM-MEM combination, combined into a single structure.

The OSM would also feature simple RCS units around its outside.  These would solve the previously mentioned problem of not being able to rotate without translation.  This would remove the need for RCS units on larger modules that go behind the CM for lunar missions.

I don't think there would be complicated electronics inside the OSM at all so radiation shielding would not be a big problem.  The OSM would also not need to be able to reenter the atmosphere.  It's walls could be made entirely of a composite material.  With a titanium frame this should make it a pretty light unit.

On circumlunar flights, a PM consisting of just a big cylindrical tank and an engine would attach directly behind the OSM.  Because the OSM is an expanding cone shape, the diameter of the PM can be larger than that of the CM, allowing the tanks to be shorter and helping us fit within the height restrictions of the Falcon 9 fairing.

On lunar landing flights, a "PM heavy" would attach directly behind the OSM.  This would be identical to the regular PM only longer to accommodate more propellant.  All other aspects would be the same.  The Lunar Lander would be attached to the nose of the CM – this introduces no aerodynamic problems due to the the fairing and it makes transfer shorter and easier.  The biggest problem is that the LL is then quite exposed during the trip into lunar orbit.  We will need to think of a way to deal with this, but the advantages of having the　LL docked there are so great that I think this extra challenge is worth it.  We can minimise the amount of challenge involved by making the lander as simple as possible, with minimal vulnerable areas.  This further encourages minimalism which helps to get our mass down.

Thoughts?

brmj said:

It would introduce a little extra mass into the minimal orbital configuration, but it otherwise looks quite favorable.

It's true that it would do this, but I reckon we could make it such a small amount of mass as to be worth while.  The suborbital and orbital configurations will be launched (most likely) from our own hybrid booster anyway, we should be able to design that booster to lift whatever mass we require (as long as it's sensible for the mission role).

I like your sketch, it conveys the general idea and it's a good one.  I have been thinking about a super light weight lander option that I may post sketches or details of either later tonight or tomorrow.

We may even be able to hold a competition for lander design eventually.  If we have the mass and size of everything else worked out roughly we could ask people to submit ideas which have a mass under our limit (determined by Falcon 9 capacity) and are able to dock in some secure manner with a CM whose shape, size and rough structural details we can give them.

Rocket-To-The-Moon said:

My thoughts on the simplification of the mission stack involved switching the LMM tanks and the lander so that the tanks are on the bottom of the stack and the lander is in the middle. This would help to keep the center of mass in the middle even after the lander has been used. Also the tank(s) could attach directly to the engine mounting plate and the thrust would be transferred directly to the tank(s). The LMM frame would then run from the top of the tank(s) to the bottom of the CM.

This could work, too.  It would retain the feature of being able to use two PMs of different size but otherwise identical.  On circumular trips, a small PM would attach behind the CM-OSM combo, and on lunar trips a larger PM would attach behind a "lander hanger module", which would be itself attached behind the CM-OSM.

I think we should give the CM-LM docking idea more thought (your concerns about forces on the CM during launch are valid, but not necessarily unfixable), but if we do decide to abandon it then we should definitely fall back to your new arrangement rather than the earlier one, since it simplifies manufacturing significantly.

I've just realised that, of course, the retro rocket pack in the middle of the OSM would need to be removable since it is not necessary for missions where there is a large propulsion module on the back that can do the deorbit burn (although we may like to keep it in there on these missions if we can afford the mass, as a back up).  This shouldn't be hard to arrange at all, though.  I may upload a concept diagram later.

Another thing in favour of docking the LL to the CM if we can: it provides a means of emergency propulsion for the CM.  Suppose something goes wrong with the propulsion module.  If we're still in the pre-landing part of the mission we could simply detach the CM-OSM-LL combo from the PM and use the LL to push it around.

Not a huge advantage worth inconveniencing ourselves for if there are too many other problems with this plan, but something else to consider when weighing up pros and cons.

brmj said:

That's a really good thought, actually. We might want to run the math and see if the OSM-CM-LL stack can return to earth from lunar orbit pre-landing.

If the LL has an unfuelled and manned mass of 500 kg, it'll need about 950 kg of propellant to match the Apollo LM's delta-v budget of 4960 m/s.  If the OSM-CM combo has an unmanned total mass of 1250 kg then the 950 kg of propellant in the LL will be able to give the OSM-CC-LL stack (mass 2700 kg) a delta-v of 1912 m/s, which is absolutely enough for a lunar escape burn, in fact more than twice what is needed (which means we could also do a big slow-down burn before reentry if we wanted).

Worst case scenario: we come up with a lander that has an unfuelled and manned mass of 200 kg (very light!), then we'll only have 380 kg of propellant on it.  If the OSM-CC-LL stack has an unmanned total mass of 1750 kg (very heavy!), then we'll be able to get a delta-v of 785 m/s which is still enough to get home from lunar orbit.

It seems like no matter what the exact numbers for our vehicle masses turn out to be, it's highly likely that we'll be able to use the LL to do an emergency TEI burn from lunar obit if we have it docked to the CC.  This is a pretty nice safety feature to have.  It would definitely be worth our while to try our hardest to come up with a means to dock the LL to the CC such that the forces during launch are endurable.  If any aeronautical engineers who have recently joined the project happened to feel like advising us on how feasible that is, they should feel free. :p

Alternative concept, with extra RCS propellant: