[AR] pressurization (was Re: Portland State Aerospace Society)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Wed, 20 Apr 2016 16:18:37 -0400 (EDT)
On Wed, 20 Apr 2016, Paul Mueller wrote:
LOX is not significantly below nitrogen's boiling point (-297 F for LOX and
-320 F for nitrogen at atmospheric pressure).
Somewhat above it, in fact... but nitrogen dissolves in / condenses into
LOX. You can't avoid having some of this happen; what you can do is
minimize it,
The most important thing is a diffuser -- baffles or porous materials or
both in the pressurant flow path, so it enters the tank at low velocity,
as spread out as possible. If you just have a pressurant pipe entering
the top end of the tank, that's likely to give a fast gas jet impinging
directly on the liquid surface, which is exactly what you don't want.
Using a diffuser avoids agitating the liquid, and also encourages the gas
to stratify, with a cold layer above the liquid surface and warmer layers
above that, which reduces average gas density.
In flight, reducing liquid sloshing helps with this even if it's not
necessary for control.
Low thermal conductivity in the walls also helps. Thin stainless is good.
Thick aluminum with an internal waffle-grid pattern is bad, as the Atlas V
guys discovered to their sorrow.
In a well-stratified system, it can help a lot to have a bit of
non-condensible gas in the tank at the start -- you get a mostly
non-condensible layer above the liquid that tends to act as a barrier.
(This is probably an accidental side benefit of the Western practice of
using helium for ground pressurization even in systems that use boiled
propellant in flight.)
You can provide a more explicit barrier, in the form of a float. One of
the late Charles Pooley's more interesting ideas was to squirt a bit of
water into the LOX through a small orifice, during loading: the resulting
small ice grains float on LOX, covering the surface and reducing
gas-liquid contact.
Even at the very best, the last little bit of LOX into the engine -- the
topmost layer -- will be warmer and will have some nitrogen in it.
I believe the German V-2s used compressed air to pressurize the LOX.
Kind of, sort of. Just before launch, the LOX tank was pressurized
slightly with compressed air from ground equipment. In flight, LOX
pressurization was maintained by boiling LOX in a little heat exchanger in
the turbine-exhaust stream. The alcohol tank was unpressurized at
startup, and there was a little ram-air tube that pressurized it slightly
during ascent to prevent the alcohol boiling as atmospheric pressure
dropped.
What *looks* like a tank pressurization system in a V-2 diagram, with
compressed-air bottles up in the nose, actually wasn't used during ascent
at all! That system raises the pressure in the fuel tank before reentry,
so the tank doesn't collapse as atmospheric pressure starts to rise.
(The V-2 guys hadn't thought of the idea of separating the warhead before
reentry, so how the rocket hardware behaved during reentry mattered.)
(Ref: Sutton, "Rocket Propulsion Elements", 1st edition, 1949.)
I believe the Russian rockets use nitrogen.
Correct. And at least the older ones carry it as LN2, boiled in heat
exchangers. Soviet rockets couldn't use helium because the US controls
most of the world's helium supply, and Russian rockets have inherited the
tradition. :-)
(All natural gas contains a trace of helium, but there are a couple of gas
fields in the US which have quite significant helium content, 10-15% if I
recall correctly. So that is basically where the world's helium comes
from. Separating it from ordinary natural gas, or from air, is possible
but prohibitively expensive for most purposes.) (This is also why the
Hindenburg was filled with hydrogen, not helium -- at a time when people
hadn't quite given up on military uses of airships, the US was reluctant
to sell bulk helium to Nazi Germany.)
I think the main reason helium is used for ground ops on Western rockets is
the "cost is no object" and "that's the way it's always been done" mentality
left over from Apollo (and predecessors).
Some of that, but also, Western practice pressurizes the tanks long before
launch. So if you try to use GOX for ground pressurization of LOX, it
will start condensing into the LOX, running up the GOX requirements and
warming up the LOX, both of which are undesirable. For *lengthy*
pressurization of LOX, helium is simpler. (This is also why the S-IVB,
the restartable top stage of the Saturn V, pressurized its LH2 tank with
GH2 during burns but with helium during coast, and its LOX tank entirely
with helium.)
I believe the Russians used nitrogen because helium was very hard to get
over there in those days, and that may have been a reason why it took
them a while to develop liquid hydrogen capability (helium *IS*
mandatory for purging/pressurizing LH2 systems)...
As noted above, yes, helium was impractical for them because they lacked a
good quantity source of it. This may have hampered their LH2 work, but
also, they were never as sold on LH2 as the US was, and still aren't. In
hindsight, the US's initial enthusiasm for LH2 was excessive, the result
of too much focus on Isp and not enough on dry mass and practical
problems.
(You can make a case for LH2 when there are gross-mass constraints, e.g.
an improved upper stage for an existing rocket or air launch from an
existing carrier aircraft. For clean-sheet designs, it's questionable.
Falcon 9 can get you to GTO or a Mars trajectory with two LOX/kerosene
stages; that could have been done many years earlier if people hadn't been
so hypnotized by hydrogen that they never considered the idea.)
Henry
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