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Could we do a moon mission today?
In article ,
Doug... wrote: The one plausible reason for braking into orbit would be to make the system fully reusable. Which is not a ridiculous idea, just really hard to do... Correct me if I'm wrong, here, but it seems to me that you could construct a "lunar taxi" that would perform the functions of braking into lunar orbit, entering a transearth trajectory and aerobraking into a stable LEO "parking orbit" from which it can be resupplied. You then add a return capsule to this lunar taxi, which separates from the taxi as it approaches Earth... That's a plausible approach, although having two separate vehicles does complicate things somewhat, and (as you note) the taxi needs seriously rad-hard electronics. Another option is to do one-pass aerobraking instead of the gradual kind. That avoids the radiation problems, but now you need a reusable heatshield, and there are some worries about whether you have enough aerodynamic control to cope with a somewhat-variable atmosphere. Since the taxi module isn't manned during its repeated trips through the Van Allen belts while aerobraking, this process becomes feasible. Yes, you'll have to really harden all the taxi's systems so that it doesn't incur undue radiation damage itself... but I'm under the impression that we have that technology. It's feasible in principle, but somewhat of a headache -- costly, limiting, and not necessarily 100% perfect. ...And remember, you could design the dock so that it wouldn't have to be manned unless you needed a crew up there to perform an on-orbit resupply and mating with new lander and TLI modules. That saves a lot of weight and system complexity. Actually, designing it for lengthy periods unmanned may be more complex than assuming it will have a crew continuously. The tradeoffs would need looking at. I have to believe that you could put together such a system using the present launch capabilities -- especially if you add the maxed-out configuration of the Delta IV-Heavy that certainly seems to be within a few years' reach... Certainly. In fact, it ought to be possible with just Atlas V 402 (the high-end no-strapons medium configuration), given money, patience, and good design. -- MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer first ground-station pass 1651, all nominal! | |
#23
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Could we do a moon mission today?
In article ,
Louis Scheffer wrote: It's true you need a heatshield, because of the radiation problem, but it's not obvious to me that capture into orbit is more massive. The Apollo heat shield was quite heavy. It wasn't as heavy as the fuel you'd need for 3-4km/s of braking. (The fuel for the equivalent maneuver, TLI, was over half of the mass of an Apollo in parking orbit.) Perhaps you could use a lighter heatshield (perhaps derived from shuttle technology, so it's non-ablative) and do some cross between aerobraking and aero-capture, where you shed your energy in just a few orbits. I've never seen an analysis of this, but it seems like it might be even lighter than direct entry. Probably not much. You have somewhat less heat to deal with, but getting adequate maneuverability is tricky (such a braking maneuver is much fussier than a reentry which ends in a landing -- there is no such thing as shedding too much velocity in the latter case), and the spacecraft would have to be beefed up for longer-duration operations. (The Apollo CM by itself was good for only a few hours.) I think you end up being pushed to do the braking pretty much in one pass, a la aerocapture. You have to shed most of the excess velocity on the first pass if you don't want to wait days for the second pass. By the way, the shuttle tiles are not particularly lightweight, especially after you finish beefing them up to handle a sharper reentry. (Assuming you could -- a lunar reentry is very different from LEO reentry.) -- MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer first ground-station pass 1651, all nominal! | |
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