Subject: Re: Tank Mass for Pressure Fed Vehicles
From: gchudson@aol.com (GCHudson)
Date: Apr 23 1996
Newsgroups: sci.space.policy

Bruce Dunn wrote:

> Absolutely true.  The pressurization system for a "pump fed"
booster is actually no less complicated than that of a "pressure fed"
booster, and sometime more complicated. It is simply smaller than its
counterpart in a pressure fed booster.  Since it is complexity and 
parts count rather than size which governs reliability, a pump fed
booster has just as much opportunity to have a fatal pressurization
failure as does a pressure fed booster.  Historically however,
pressurization failures in pump fed boosters are rare or non-
existent (I don't know of an example - does anybody know of one?).
This suggests that in pressure fed boosters, pressurization failures
should be rare.<

It is true that the pressurization systems for p-fed booster can get
pretty complex; I can attest to this from personal experience.  On our
Liberty 1A pressure fed, which had a 75K thrust engine (LOX-JP4),
we spent two dollars on pressurization for every dollar spent on the
engine itself, including main prop valves.  That didn't even include
the helium storage bottles.  The problem was trying to control tank
pressure very accurately, to within plus or minus a few psi out of 400.
This was done in part to eliminate POGO, and to allow the main tank to
be fabricated with very thin safety margins (using forgiving 5000 series
aluminum).  The money we spent on that pressurization system was the
reason we ultimately chose to go with a pump-fed cheap engine for the
Liberty-X whch was meant to be an expendable SSTO.  In the end, it
priced out a lot cheaper than the pressure fed vehicles and would have
delivered three times the payload for the same development cost.