Yet another lander concept | Lunar lander design | Forum

Anticipated disadvantages:

• Balancing everything could be a little tricky
• No possibility of discarding spent tanks.
• Likely to be less resistant to micro meteorites and radiation, without the addition of some kind of shell.
• Higher center of gravity.

Over all, I think this design or something that shares some design features with it might be a good way to go if Luke's spent tank dropping plan turns out to be unfeasible or undesirable. What does everyone think?

Luke Maurits said:

Excellent to see more lander designs being proposed!

The biggest concern I have with a single, central tank (and the reason I haven't proposed any designs along these lines yet) is balance.  Your diagram has the avionics, helium etc "counter balancing" the seat.  While these things could certainly counter balance an empty seat, once you put a 70-80 kg human in that seat wearing an EVA suit (maybe 20 kg if we can get a very light weight elasticated suit like Webb's ideas, a lot more for a traditional pressurised suit) there is going to be a definite "heavy side" and "light side".

One solution to that would be to put the stuff on that side farther from the center of mass. Also, in addition to the electronics we would probably want to put the RCS tanks on that side, as well as any extra life support gases, and so on.

That said, the quality of electronics and software we are going to be able to fit on the lander mean that maybe we can just deal with this by careful control of thrusting?  This may require extra RCS propellant, but the extra mass of that feels unlikely to me to exceed the mass that we can save with such a minimalist design, so it's probably still a good way to reduce mass overall.  The increased plumbing simplicity is also very attractive to me.

The RCS idea strikes me as workable, but perhaps a little worrisome. How bad it would be depends very much on how off center the center of mass was.

If we were really keen to be able to drop empty tanks after landing, maybe we could just have two cylinders stacked atop on another with some very light weight support keeping them up.

Perhaps. That decreases some of the simplicity and mass advantages, of course, but it could end up being very much a good thing, depending on how much the extra fuel tank material would weigh. Once again, we'll want to run the math on this at some point.

The micrometeoroid issue is worth discussing.  The avionics box can be armoured without adding a lot of extra mass, but I'm not sure about the tank.  Maybe if we built this tank out of titanium or something it would be tough enough to just handle impacts, without a shell around it?  Don't forget there will be insulating foam or some such around it, and probably then some reflective foil, and these outer layers would absorb some of the impact.

I suspect that a good, strong tank material and insulation with reasonable ballistic characteristics ought to be just fine, though I might be horribly mistaken. We may also want to think a bit about how we will do temperature regulation.

Do you have any ideas for how to dock it to the CM nose?  Mounting a nose-diameter ring to the top of the tank and having latches on the CM nose grab onto this ring?  Or some kind of external structure that attaches to the legs maybe and braces against the OSM?

Either of those could work. If we went wit the first route, the CM would have to be fairly mechanically strong for launch. IF we went with the second one, the external frame would add a little mass. My gut feeling is that it is a bit more feasible than the first one, but I am no engineer.

Admittedly it would be nice if the natural orientation of the lander under gravity had the astronaut upright but really, even if that were the case, an RCS failure bad enough that we could no longer correct for the imbalance would probably be a very severe problem anyway.  I suppose we could also have the tank mounted at a slight angle, leaning away from the chair?

Leaning it away could work, but it would complicate flying the thing. As fuel got used up, the center of mass would change.

I suspect this would turn out to be a good trade off on the whole.  The extra mass is two additional hemispheres, but the mass we get to drop is two hemispheres plus some vertical walling.

When you put it that way, I am inclined to agree.

What precisely do you mean by temperature regulation?  Just keeping the cryogen tank as cold as possible?

The cryogenic gasses need to stay cold, the seat should stay not-too-cold to prevent freezing the astronaut or wasting a lot of suit power, and the electronics and power systems will have their own temperature ranges they need to stay inside of. We can probably regulate the internal flow of heat using thermal insulators. We'll want to have a reflective layer over the tanks to keep them cool. Not sure what we'll want to do for the rest, but it shouldn't be a big problem.

As I think about this, it increasingly looks fairly easy.

In my mind, the structure that holds the latches that grab the ring on the lander (or whatever attachment scheme we come up with) are connected to the rest of the CM nose by heavy springs / shock absorbers, etc.  But I am also no engineer and I don't know if this would work.

That would deal with sudden accelerations, but it wouldn't help a bit with sustained force. Think of it this way: No amount of springs and shock absorbers will let you put a car on a plastic lawn table without it breaking.

Luke Maurits said:Quick question: the "base plate" that the legs attach to and which everything else sits atop: what shape is it?  The small arial view in your rough diagram makes it look like a narrow rectangle, but I didn't know if that was due to it being a quick diagram or was your actual intent.

That was just because the diagram was quick and crappy. I was thinking that it would probably be shaped like a diamond or a stretched hexagon, to better accommodate the increased width of the tank relative to the rest of the lander.

Very interesting concept. I think I like it.

Just one quick comment. Is having a high center of mass not a good thing? It is just like balancing a broom on your finger. When the mass is up high it is quite easy to balance. I am obviously referring to the powered descent and ascent phases. Having a high CG is less than ideal for the actual landing and it would be very critical that there is essentially zero lateral velocity.

Along the same line of though as the last sentence. I strongly favor a four legged lander because it would be much less likely to tip over if there is any lateral velocity upon touchdown. A three legged lander is just an accident waiting to happen.

Luke Maurits said:

Perhaps we could have impact sensors of some kind on each of the feet, and as soon as one touches the ground the RCS system automatically begins thrusting downward with all thrusters, continuously until all 3 feet are on the ground.  This shouldn't waste too much RCS propellant (a few seconds worth) and should make a roll pretty much impossible, right?

That's a disaster waiting to happen. If we do something like that, we'll want to make sure the radar altimeter is reading something reasonable OR some switch has been manually flipped to enable it. Otherwise, a random hardware or software glitch could doom the mission by wasting too much of the RCS fuel.

Luke Maurits said:

Given concerns about landing stability, do we want to consider some minimal protection of the astronaut in the form of roll bars? 

This would add some extra mass admittedly, but would also at least prevent an astronaut being crushed in a bad landing. I guess it will come down to how much spare mass we have.

I hate to say this since it could easily be taken the wrong way, but if our lander were to tip over, there is a decent chance the mission would be doomed anyway. Even in lunar gravity, it is likely to be too heavy for a single astronaut to flip it back upright, and it might sustain damage when flipped. If, however, this wasn't the case, a set of roll bars might be apropriete.

With regards to the misbalance of the lander: Rather than wasting a lot of engine propellant generating torque and a lot of RCS propellant cancelling it out, is there any reason we couldn't have the engine mounted on a set of rails so that it could slide along an axis, aligned with the seat-avionics line?  The engine could be moved along the rails by a pneumatic cylinder or some such.  This way, in all situations (braking while in descent orbit and during powere descent), we could have the engine positioned such that it didn't create any torque.

That's doable, but I think it might be easier to put either the seat or the package of stuff on the other side on the sliders to adjust the balance. Those parts wouldn't need as complex of mechanical connections, just electric ones for the most part. Also, the sliders wouldn't have to be nearly as sturdy.