The important thing is, figure out the major cooling factors (adiabatic,
pressure work expelling propellant, cooling by propellant, cooling from chilled
tank walls, etc). That inflates the required expulsion gas volume, just make
it match...
You can do without separate tanks if you want. One big one with enough
required ullage head. Usually you would not fully charge the ullage pressure
until right before flight to minimize precooling of the gas. Or you can heat
the gas, on the ground.
George William Herbert
Sent from my iPhone
On Dec 11, 2015, at 11:55 AM, Brian Feeney <alaiadesign@xxxxxxxxx> wrote:
Thanks for the feedback! Will hydrogen gas on LH2 work or will it have the
same problem as O2 gas on LOX?
I understand the hydrogen challenge - lots of prior discussion over the years
on the list. The larger volume pressure tanks are structurally a part of the
preliminary design and at the relatively low pressures add little to the
weight.
Cheers
Brian Feeney
On 2015-12-11 2:23 PM, "Ben Brockert" <wikkit@xxxxxxxxx> wrote:
Unless there is an insulated piston in the LOX tank, the oxygen tank will
not pressurize as you expect. As the LOX is pressurized it effectively
becomes sub-cooled relative to the pressurant, and the pressurant gas will
condense into the liquid.
If there is an insulated piston or diaphragm it can work. If there is some
helium gas in the tank and the pressurant is diffused well so that it
stratifies, it can possibly act as a gas piston between the liquid and gas.
Or you can just use vastly more oxygen gas so that the LOX at the top of the
tank heats up to the point that it is at equilibrium with the pressurant.
Slosh will then cause drops in tank pressure though.
Hydrogen on fuel works fine, as long as you pay attention to the unique
hazards of hydrogen gas. Like its very wide flammable range and long-term
incompatibility with some materials.
Ben
On Friday, December 11, 2015, Brian Feeney <alaiadesign@xxxxxxxxx> wrote:
Is it safe to use Hydrogen and separately oxygen as pressurizing gasses?
Set up:
- Gaseous Oxygen held in a separate low pressure vessel; about 300 psi.
- Prior to ignition a valve opens allowing O2 gas release into the LOX tank
to pressurize.
- LOX tank and pressure tank are equal volume with small ullage space
allocated in the LOX tank.
- pressure will decline by 50% through the burn based on the set up, that
is mission desirable to reduce thrust proportionately.
Hydrogen gas - same set up as above pressurizing the fuel supply feed side.
Dry nitrogen available to purge tanks, feed lines etc.
If repeat firings then only LOX and fuel lines would be purged up to the
main inlet valves, not the tanks which would be re-filled - some venting
may be necessary for refill even at these relatively low pressures?
Helium is the usual choice however, cost is high and given the description
above, I don't see the added safety benefits?
All comments, suggestions welcome.
Thank you,
Brian Feeney