Wouldn't using a pressure fed system with hydrogen on an upper stage be
optimizing for the worst characteristics of each system? You have the least
dense fuel possible making the largest tanks, in a system where your tanks
have to be rated for high pressures and very heavy, on the stage where
every pound counts.
If you are considering hydrogen for a fuel, pressure fed is pretty much off
the table. As your tanks have to be around 9 times larger than kerosene for
the same energy.
On Sun, Dec 13, 2015 at 2:10 PM, Henry Vanderbilt <
hvanderbilt@xxxxxxxxxxxxxx> wrote:
It seems clear from Brian's comment about using leftover propellant gas
for RCS and perhaps other things afterwards that he's talking about an
upper stage.
Reduced Isp losses against atmospheric back-pressure with higher chamber
pressure is a large part of why pumps tend to win for lower stages. In
vacuum, pressure-fed trades a lot better because you lose much less Isp
from not going to higher-pressure (heavier) tanks. Pumps trade even worse
for a smaller upper stage where minimum-gauge means you may not save much
tank mass via pumps anyway.
OTOH, he also said something about the possibility of recovering helium,
which implies the stage itself might be recovered, at which point the extra
cost of pumps might get spread over multiple missions.
Were I looking at a clean-sheet-of-paper upper stage with low
manufacturing cost a major criteria, I'd look long and hard at some sort of
autogeneous-pressurization pressure-fed.
Henry
On 12/13/2015 2:46 PM, Ben Brockert wrote:
On Fri, Dec 11, 2015 at 7:02 PM, William Claybaugh
<wclaybaugh2@xxxxxxxxx> wrote:
Simple systems tend to be lower cost and higher reliability....
Which is one of those classic phrases that sound obvious and true at
first read but isn't actually true at all. Compare the reliability and
life cycle cost for 200,000 miles of a car from 1965 and a car from
2015. The newer car is also vastly more complex, with entire systems
that don't exist on the older car, yet the newer one is cheaper and
vastly more reliable.
If you argue that it's because the technology has improved, then take
a modern car and strip a lot of stuff out of it to make it simpler,
and show me a cheaper and more reliable version of itself, and explain
why the manufacturer doesn't do that already.
The reason almost every orbital rocket has pumps is because even a
really shit pump makes for a more effective system than the most
clever and simple pressure fed system. And I say that as someone who
has worked on a dozen different pressure fed vehicles.
Ben