brmj said:
Sorry I've been conspicuously absent from this. The laptop failure and a new quarter starting have made it difficult, but I should have tried harder. Anyway, on to my assessment of the plan.
Welcome back! It's great to have you around again.
brmj said:
I think a lot of really good work has been happening here. I think I like the spherical capsule and ballute combo, though the manufacturing details could be a problem.
Do you refer here to manufacturing the spherical CM or the ballute? Or both?
brmj said: The problem of lander component positioning and the resulting center of mass difficulties worry me a bit. If you want a really crazy idea on that topic no one has thrown out yet, we could perhaps have the whole legs, tank and engine assembly pivot, with an explosive bolt or similar holding it in front of the capsule for launch and the trip to the moon, and then having it swing into position below the astronaut's feet with a little spring-loaded catch or something once we are ready.
I had thought of that general idea, i.e. having the way the landing gear is attached to the outside of the CM facilitate "swivelling" it around, but I was worried about whether or not it would be possible to do this and still make the attachment sturdy enough to survive landing forces. It is something to consider, although I suspect making the internal configuration (seating, etc) either similarly rotatable or just otherwise more orientation-neutral could be simpler overall.
brmj said:
Beyond that particular lander orientation concern, another one is how it impacts the plan to use "D Blocks" for the landing. In the position they are currently depicted in, the maneuver needed to make that work could be at the very least a bit disconcerting and provides an additional failure point in the case of RCS difficulties, for example.
I have been meaning to draw a diagram detailing the entire flight plan, showing the maneuvers involved in using the individual PMs, to facilitate discussion of this. Is it really so awkward currently? After firing the TLI PMs in LEO, we need to rotate the entire stack around 180 so the PM engines are correctly aimed for LOI. This is a large and heavy thing to turn around, especially with just the CM RCS, but we also have 3 days to do it in under normal circumstances, so it can be quite gentle. If we need to turn around very quickly on the way to the moon in some sort of emergency we can just separate the CM from the entire stack and use the landing engine. After LOI, the CM and one PM can separate and are already facing in the correct direction for the PM burn to start dropping the lander's altitude. Admittedly, the PM thrust does not point right through the landing configuration's centre of mass.
brmj said:
Yet another idea, this one dealing with both problems: Mount the lander components in line with the D Blocks, mating the capsule to the D Blocks with a frame component distinct from the things that would be jettisoned. Lean the seat a little, so that it is more like sitting in a low recliner or lounge than lieing flat on one's back. Land with the seat still in that configuration, requiring the astronaut to just deal with it. The lower gravity should prevent working in that position from being so very uncomfortable or difficult as to render this infeasible.
I quite like this approach, in that it represents the optimal position of everything from the point of view of keeping things symmetric and with thrusts pointing through centres of mass. I am not as worried now as I was at first about the seating orientation. The idea that the CM needs to have a definite "up" and "down" in its layout is distinctly "Earthly" thinking. In the microgravity of space and the greatly reduced gravity of the moon, things are more flexible, and we should embrace that as providing an opportunity to think outside the box, and come up with an innovative solution that solves our problems. If anyone here is a fan of the Ender's Saga series of books, I cannot help but think at this point about "the enemy's gate is down". :)
brmj said:
The stacked D Blocks worry me a little, and I think your mass fraction estamites are too optimistic. As you split it up into more tanks, you get more tank wall and the mass fraction gets worse? If not, you may want to redo the math taking that into account. Also, I think the numbers you say are for a launch vehicle with considerably larger tanks than ours. Mass fraction improves with tank size, since tank mass scales aproximately with surface area.
You have some good points here. I obviously didn't think about this too much. Mass fraction depends, as far as I can reason, on two things. Size, and shape. By using the same mass fraction to simulate both the stacked blocks and one small lunar descent block plus one big everything-else block approaches, I've ignored the dependence on size. So this needs to be looked at again. One thing to keep in mind is that the stacked blocks approach gives us more flexibility with shape. With the one big tank design, a sphere is awfully impractical, so we use a long cylinder. With smaller blocks, perhaps spheres could work. Whether or not the improvement in mass fraction due to the change in shape would cancel out the decrease due to change in size would require more detailed knowledge.
I think perhaps the most sensible thing we can do is try to delay thinking about this too much until such time as we find someone with more knowledge of cryogenic tanks, find some better data more applicable to tanks of our size (perhaps from NASA's Altair lander?), or one of us finds the time and motivation to study this a bit. Instead, we should focus on getting better estimates of the total CM mass. Instead of making up values for the tank mass fraction and seeing how large of a CM we can use, let's find a decent figure for the CM mass and then find out what the minimum tank mass fraction has to be to fit on a Falcon 9. This is the easiest way to tell if this approach is feasible. If we need a higher fraction than 0.9, we're probably screwed, but if we can get it lower than that then we'll know that at least there is a chance of finding a solution once we get better tank data.
This means we should focus now on solving the engineering problems associated with the CM, it's interior seating solution and it's lunar landing attachments.