Some considerations here, in no particular order.
Pressurizing cryo liquid with the same material's gas phase in the short
term suffers not so much from re-liquefaction of the gas, as from
extreme chilling of the gas, thus loss of gas pressure, thus increase in
mass and volume-flow of gas required to keep up pressure.
Someone else already mentioned that running the cryo on the edge of
boiling to provide self-press can be tricky, and also reduces fluid
quality through your injector.
One solution I've seen mentioned is floating an insulating layer of some
sort on the liquid - think the cryo equivalent of ping pong balls - but
that has its complications also. Engine ingestion, not working so well
anyway when there's significant slosh, not working at all when the
tank's in free fall.
Materials for flexible accordion bladders that stay flexible at cryo
temps are scarce. There are no suitable plastics that I'm aware of (not
that I've made a recent study of what's out there.) There are thin
stainless steel bellows commercially available that are rated for cryo
temps, but they're not cheap, they require careful handling, and they
may mass more than you'd like.
Another factor for LOX (not so much for LH2, I'd guess) is that you'll
need significantly larger-diameter (heavier) gas-feed plumbing, valves,
and if you use them regulators, than you would for helium. A given
volume of the smaller He molecules will flow through significantly
smaller plumbing than O2.
Using a heat exchanger and/or tank heater for like-material press gas
seems to me potentially the lightest alternative, albeit not the
simplest. One downside: At the end of your burn, you'll have tanks full
of relatively high pressure propellant gas - the mass involved may
surprise you. The use-eveything-but-the-squeal option of course is to
figure out a way to burn those gases for propulsion (or on-orbit power)
also, as ULA's IVF does.
Henry
On 12/11/2015 1:51 PM, Brian Feeney wrote:
Ah, Noooo balloons! I frustrated myself to no end with them more than a
decade ago. They are expensive to make, fill, logistics bla bla.
Thanks for the suggestion though. :)
Flexible insulated accordian bladder concepts are on the brain at the
moment. Running the gasses through a heat exchanger as suggested is also
a possibility.
Cheers
Brian Feeney
On 2015-12-11 3:39 PM, "George Herbert" <george.herbert@xxxxxxxxx
<mailto:george.herbert@xxxxxxxxx>> wrote:
Ah, balloon?
You might consider one tank and an offboard, separate
reused/recovered high pressure pressurant tank loaded into the main
tank a minute before launch or something.
George William Herbert
Sent from my iPhone
On Dec 11, 2015, at 12:11 PM, Brian Feeney <alaiadesign@xxxxxxxxx
<mailto:alaiadesign@xxxxxxxxx>> wrote:
The reason for 2 separate tanks is ignition at altitude. There
will be about 1 hour more or less from completed LOX and Fuel
filling to the engine firing. Any gas in a single tank unless
there is an insulated pistion as suggested, will be mostly cooled
to liquid.
On 2015-12-11 3:05 PM, "George Herbert" <george.herbert@xxxxxxxxx
<mailto:george.herbert@xxxxxxxxx>> wrote:
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 <mailto: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
<mailto: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 <mailto: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