> So what sort of propulsion are we thinking about here? Hybrid?  
 > Liquid? Both schemes require liquid oxygen in an insulated tank, plus  
 > a pressurization system of some sort.

For myself at least, i'm not far enough along in the decision process
to say what motor is best. However, if the pumping scheme was
reasonable, it seems that liquids have big advantages in terms of mass
ratio. Would the cost reductions associated with a solid or hybrid
system be large enough to justify a possibly much larger motor? I
don't know.

 > In order to get decent performance from your fuel, you need to have a  
 > chamber pressure in the megapascals. This means that the LOX (and  
 > liquid fuel, if applicable) must be forced into the combustion  
 > chamber at great pressure - atmospheric is about 0.1MPa, and you need  
 > about twenty times that, bare minimum. There are several ways to  
 > accomplish this. NASA uses huge expensive turbopumps, drawing  
 > propellant from tanks at low pressure and compressing it to hundreds  
 > of megapascals.
 > At the other end of the spectrum, you could just build really strong  
 > tanks and pressurize them before launch. The problem here is that the  
 > ratio of tank weight to tank volume gets to be pretty bad as you go  
 > up in pressure, making it impossible to achieve the desired dry/full  
 > mass ratio for the rocket stage.
 > There have been some interesting developments lately with pistonless  
 > pumps - I think this bears some serious consideration. You basically  
 > carry a small, very high pressure tank of helium, and use two more  
 > small chambers that are alternately being filled from the tanks at  
 > tank pressure, compressed with helium, and emptied into the  
 > combustion chamber at much higher pressure, in a continuous fashion.  
 > Here's a good PDF about it: http://www-rohan.sdsu.edu/~sharring/ 
 > JPCpistonless2.pdf

The pistonless pump is really interesting. We have discussed
alternative pumping schemes from time to time.

One of my favorite alternatives, which hasn't been shot down yet AFAIK
is to use motor heat to vaporize some of the cryogenic liquid, thus
maintaining the pressure.

 > For a liquid engine, you just build two of these systems, and only  
 > one tank has to be insulated. For a hybrid engine, you build one, and  
 > feed it into the top of a cylinder containing machined paraffin wax.
 > Combustion efficiency is related to the ratio of the pressure in the  
 > chamber to the ambient pressure outside. Obviously this means you  
 > want as high a chamber pressure as possible when you're at sea level,  
 > but once you're out of the atmosphere it doesn't matter anymore,  
 > since the ambient pressure drops to zero. This means higher stages  
 > can have much simpler pressurization schemes.

After this time i'll stop saying this for awhile, because this will be
i think the third time, anyway, here it goes. We could have a write up
on the Wiki that would cover the rocket equation, the effects of
staging, delta-V estimates for an orbital mission, comparisons between
the different technologies in terms of mass fraction, and cost.

Ideally the write up would allow a math-literate to follow where our
conclusions were coming from and to ask their own what-if questions.

I think that would be valuable, and unusually accessible
information. So whoever has time, feel free to go ahead and write that
up ;)

Oh, and BTW, going through calculations of this sort is a prerequisite
to answering Richard's 1st question, "What sort of propulsion system
are we thinking about here?"

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