At 10:02 PM 2/2/2003 +0000, Ian Woollard wrote:
Rotary of course proposed water cooled transpiration panels. And that may be stunningly lightweight.
It could well work. I think others have looked at it; and it seemed to them to be heavier, mainly due to pumps. My feeling is they'd missed something, like why on earth do they think they need pumps? I am also a bit worried about the steam disassociating. Still, none of the ICBMs used it AFAIK, and one would have expected them to look into it.
Well, how much pressure would you need behind the transpiration panels in order to ensure proper coolant flow? It might be lighter to use a pressurized system to drive to flow. Not sure about disassociation issues, but my intuition tells me that they will occur at the far side of the boundary layer from the surface, where the temperatures will be higher. As for ICBM re-entry vehicles, they are one use vehicles, so an ordinary ablative shield makes more sense for them.
Do you have any references to these issues anywhere; I've heard this a few times, but never fully understood the problem. I mean I know about some of the Saturn issues.
Personally, I don't think that the issues are nearly as bad as Randall does. Furthermore, I think the fact that they are obvious and in your face throughout the design process makes them easier to solve correctly than the subtle problems likely to be introduced by the extreme optimizations required to make an SSTO useful.
Religious issues aside, most of the problems center around atmospheric interference during separation and flight in close proximity, failure modes relating to refused staging, and what to do if prior to the staging event some failure requires you to abort and land immediately. The first can be solved by resolving not to stage in the atmosphere. Since the ideal stage point for many TSTO trajectories is beyond the atmosphere anyways, this is relatively easy to achieve if you have a way to abort the intact stack to landing at any point while it remains in the atmosphere. I don't believe that this is particularly difficult to achieve if both stages are designed as full VTVL vehicles. The greater mass allowances of a TSTO allow you to design landing gear to handle landings with a large fraction of the propellant still on board, such as might be required if an abort is called early in the flight and there is little time to dump propellant. That takes care of the first and third categories of problems.
Refused staging is the really unpleasant one, IMO. I see two ways to solve it. First, equip the staging latches with explosive bolts. If the latches don't release on command, blow the bolts. It requires pyros, but I think pyros are acceptable for emergency gear. A solenoid pin-puller might replace the explosive bolts if you really don't want pyros. Second, if your vehicle is set up to return the first stage to the launch site (RTLS), life becomes easier, since there will be enough propellant on the first stage to land the stack from the suborbital trajectory to an emergency down-range landing site.
These are just my ideas for what the issues are and how to address them -- I've probably left something out (which Randall will remind me of), and there are certainly other ways to deal with those scenarios that I haven't thought of.
-p
Mars or Bust!
www.marssociety.com
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