Lunar lander 2.0 | Lunar lander design | Forum

I feel like now might be a good time to start working on a new lunar lander render in SketchUp to replace the current one, which is a little dated and a little sparse compared to our latest ideas.

We should spend perhaps a few days, a week at most, discussing changes we'd like to make before letting Rocket (or anyone else who can/wants to!) get to rendering when he next has the time.

Some ideas:

• Replacing the pointed apex of the 4 main leg beams with a small square shape at the top so that the whole structure looks more like a square-based pyramid with the top cut off.  This gives us a 50-75 cm square frame at the top to which we can attach additional structures.
• Maybe we can simplify the leg design to involve less moving parts?
• The current stairs lack hand rails which may be a problem and also present sharp edges which a suit might catch on (although we could always put a rubber liner around it).  Maybe some kind of ladder would solve both problems at once?
• We should try to brainstorm a list of required subsystems and think about where they will go.  We should try to get rough estimates of shapes/sizes for an IMU, a laser or radar altimeter, a communications unit, a power system (we've given this little thought so far, for the lander!), a computer system, etc.
• We should think about our RCS system – how the 4 thrusters will be mounted and where the fuel tanks for them will go.
• We should give some more thought to the means by which the lander docks with the rest of the stack.

Can anybody think of anything else?

Rocket-To-The-Moon said:

I'm not precisely sure why I initially designed the legs to go through two motions. It would definitely be more simple if they just swing downward through ~140º. The foot pads and steps would then be on the opposite side of the leg while in the stowed position. This would be much simpler to build.

Sounds good.  The fewer moving parts that are involved the better, from a simplicity and reliability point of view.  Once we have some better hard numbers, i.e. we know the diameter of the CM (and hence the space that the lander has to fit into) and some tank sizes we may even be able to investigate the possibility of having the legs in the correct position from the start.

Rocket-To-The-Moon said:

By reducing the apex of the structure it will cause it to be somewhat less rigid and it also might reduce the unimpeded space that is necessary for the fuel tanks. It probably won't use that much less material either because instead of going up the extra material will be used as the cross members. My thoughts were that it wouldn't be too difficult to attach stuff to them in their current position, but I am totally open for discussion on this.

I am open for discussion on it too.  I take your point about the reduced strength of the structure, and I never expected the square top approach to reduce material (if I said that anywhere I wasn't thinking straight at the time).  I am having a hard time visualising how we would go about attaching things to the current frame and I worry about how strongly we can attach the seat to a pointed top (if the astronaut is a little off balance on the seat then there will be a torque about the apex).  But if you can produced more detailed renders showing how things could work out then I am happy to consider keeping the apex.

Rocket-To-The-Moon said:

A hand rail is definitely something that I have intended all along but I deemed it too difficult to implement for the small amount of improvement. I never intended for the astronaut to walk upright as if they were stairs. I will try to model the hand rail in this 2.0 version so that we have a better product.

Ah, I had been envisioning their use as stairs the entire time.  I'll look forward to seeing a more accurate render of your original thoughts.

Rocket-To-The-Moon said:

I also think that triangular panels of solar cells should be placed in between the fuel tanks. It may not be efficient to do this on all three sides since only two sides can face the sun at once. Putting the panels on all four sides would mean that the orientation isn't critical during the landing.

This is something I hadn't even considered before, I'll have to digest it a bit before making comment.  At first glance it seems like an okay idea, though.

Rocket-To-The-Moon said:

So you know; I'm moving house this week which will take away a little of my time. I'll be doing it mostly during my normal work hours so it shouldn't have too much of an impact.

No problem!  Take your time, there is no rush on the new render, I just thought it might be a nice thing to have soonish (largely based on the reactions from the Welding Web folks to our current render).  Hope your move goes smoothly.

Rocket-To-The-Moon said:

I'll get to working on a simple RCS model and they I may add them to the old lander. Expect new renders within a few hours.

Are you thinking of something along the lines of the Apollo RCS blocks?  Just a square box with four orthogonal nozzles coming out of it?  I think 4 of those held away from the centre of the lander on poles would be a fine approach.

I like the actual look of the thruster units.  Even if we do opt for cold gas I think a similar kind of nozzle may be a good idea to ensure that as much of the gas as possible goes in the right direction (rather than immediately dispersing in all directions into a vacuum), but I could be wrong about that.

I'm not sure I'm sold on attaching the thrusters to the fuel tanks, although it does save on the mass of the poles I was envisioning (although they may save their own mass in propellant mass since the further the thrusters are out from the centre the more efficiently they'll work).  If we did use chemical rockets I would worry about heating of the tanks by the jets, especially since at least two of the tanks (LOX) will be cryogenic and hence have some kind of insulation on the outside which may melt/burn.

I suppose I should point this out here: I've been working and was planning today to post in the forums a Wiki page called CLLARE by numbers, which tries to put all the basic calculations related to the feasibility of CLLARE in one single place (to help convince people, again like the Welding Web folks) that we are not half-assed about this.  The page isn't finished yet, but noticably it is the first time in the project's history that we have considered fuel masses.  It turns out fuel will be a lot more of our total mass than I thought (and all the calculations there are using LOX/H2O – I am worried that LOX/RP-1 would be totally infeasible!).  If our lander mass is 500kg all up without fuel and with the pilot on it, the total mass of it with the required amount of LOX/H2O will be, by my calculations, 1172.72 kg.  I have a fair amount of confidence in this figure because it matches quite well with the expected 1200 kg total mass of the Langley Lightest Lander, which used cryogenic propellants and had a structural mass of 400 and something kg (not counting astronaut mass which brings the whole thing up to about 500).

As a result of this, I'm feeling less confident about our ability to use cold gas to spin the thing around – 1200 kg is a fairly heavy thing.  But then, I don't have any real experience with high pressure nitrogen jets, maybe they can produce more thrust than I am giving them credit for.

The figures above will also help us figure out what sort of tank sizes we will need.  I want to figure out how the fuel mass is split between LOX and H2O.  I have read that a LOX:H2O ratio of 6:1 is optimal, but I don't know if that is by mass, volume, molecules, what (I made a post asking about this here).  Once I know that I should be able to give you some rough tank sizes which will help to refine our lander's sense of scale and our mass estimate.

Rocket-To-The-Moon said:

Here is the lander with docking poles. The magnets at the top are not yet represented.

I'm not exactly happy with the asymmetric design, but I can't easily make it symmetric since the stairs are there. This is probably fine for axial loads (using the lander's motor to accelerate the whole stack), but using the RCS will put substantial side loads on the poles. It might not be unmanageable, but it is somewhat discomforting.

In all honesty, I have never really felt comfortable with the entire docking poles concept, it feels very structurally flimsy to me, I just can't for the life of me think of another way to dock the whole thing to the bottom of the stack without impeding access to the chair during the transfer space walk.

This reminds me that I have been meaning to email Gary Schneider from OLF to get his opinion on the idea of using the LM's engine for the entire mission, I may do that today.

i suppose it should have been obvious that it wasn't by volume, thanks for catching that.

Assuming it is indeed by mass, a 500kg total mass lander (including astronaut) calls for 96kg of liquid hydrogen and 576.6kg of liquid oxygen.

Wikipedia says "The density of liquid hydrogen is only 70.99 g/L (at 20 K)", so 96kg would be 1353.7 L, but presumably that density is for completely depressurised liquid hydrogen just sitting in Earth's atmosphere.  We would want to store it under pressure (both to save space and to avoid having to pump it into the engine). We'll have to look into what sort of pressures are safe/typical for liquid hydrogen storage.

Wikipedia gives a density of liquid oxygen of 1.141 kg/L, so 576.6 kg would be 505.4 L.  Again, this is probably a depressurised density that we can improve on significantly.

At this point, somebody who has some experience in the storage and plumbing of cryogenic liquids would probably be a huge help.