Propellant pressurization

I'm reading "Modern Engineering for Liquid-Propellant Rocket Engines".
The section on fuel tank pressurization talks about using helium from
a room-temperature high-pressure tank to pressurize the propellant
tanks.

Surely not.

The mass of pressurized tanks holding gases scales with the moles and
temperature of the gas held. The same rule applies unchanged to both
propellant and pressurant tanks. So if the gas temperature isn't
changed, and it doesn't undergo a chemical reaction changing the
number of moles, then the pressurant tank has to weigh as much as the
propellant tank. A bit more, actually, because of (a) pressure drop
and (b) left-over pressurant gas in the source tank and lines.

The book does talk about changing the temperature of the gas. But the
primary focus is on storable rockets, so they change the gas
temperature from 300 K to 500 K or so (can't remember the final
temperature, but it wasn't hot enough to heat the propellant much).
This 66% temperature increase doesn't help the pressurant tank mass
enough.

The book doesn't appear to discuss the mass of the pressurant tank.
It does talk about storing liquid pressurant, but says this hasn't
been done. It talks about heating the pressurant in the tank, but
does not talk about keeping the pressurant tank cold before launch
(except for the special case of liquid hydrogen pressurant).

It seems like cold pressurant tanks are a fundamentally necessary
rocket technology. The only alternatives are boiling your propellant,
which is only interesting if your propellant is cyrogenic, or burning
your propellant and reinjecting that.

Here is a comparison I did:

Room-temp Cold Liquid Liquid
Helium Helium Neon Helium
Propellant tank 1000 l 1000 l 1000 l 1000 l
Prop tank pressure 1 MPa 1 MPa 1 MPa 1 MPa
Prop tank mass 37.8 kg 37.8 kg 37.8 kg 37.8 kg
Pressurant temp 500 K 500 K 500 K 500 K
Pres mass 1.1 kg 1.1 kg 5.4 kg 1.1 kg
Pres tank pressure 60 MPa 10 MPa 3 MPa 3 MPa
Pres tank temp 300 K 45 K 45 K 4 K
Pres vol 16.6 l 9 l 4.5 l 4.5? l
Pres tank vol 18.2 l 10 l 9 l 9 l
Pres tank mass 27 kg 3.7 kg 1.1 kg 1.1 kg
Dewar vol -- 12 l 11 l 11 l
Dewar mass -- 100 g 100 g 100 g
Pres System mass 28 kg 4.9 kg 6.6 kg 2.3 kg

For reference, in 2001 liquid helium was $3.25 per litre. For tiny
batches like this, I'm sure it's much more. Still, helium doesn't
seem to cost very much.

All tanks are spherical 6063-T6 aluminum. Valves, heat exchangers,
and lines are extra (and significant). I have the pressurant tank
initial pressure about linear with the tank mass. This isn't
necessary -- it comes from trying to get the pressurant tank volume
reasonably small. The pressurant tank initial pressure can be varied
without changing the other numbers much at all; lower initial
pressures actually increase the pressurant tank mass slightly, as you
need more residual pressurant.

These dewars are unrealistically lightweight because I didn't take
into account the minimum practical thickness for an aluminum tank.
These numbers work better when you scale up.

Surely not.

The mass of pressurized tanks holding gases scales with the moles and
temperature of the gas held. The same rule applies unchanged to both
propellant and pressurant tanks. So if the gas temperature isn't
changed, and it doesn't undergo a chemical reaction changing the
number of moles, then the pressurant tank has to weigh as much as the
propellant tank.

I see what your getting at. You can have a blow down tank and the
weights would be the same less plumbing. But size matters and we want
things small when we travel through a atmosphere. Blowdown system have
the problem that the pressure drops off at lot and this can be a issue
for a pump feed rocket. Also we usally need the gas for other things
as well.

so they change the gas
temperature from 300 K to 500 K or so (can't remember the final
temperature, but it wasn't hot enough to heat the propellant much).

At 600K half the amount of He is needed compared to 300K.. well not
quite. And you don't really want to heat the propellant.

Prop tank pressure 1 MPa 1 MPa 1 MPa 1 MPa
Prop tank mass 37.8 kg 37.8 kg 37.8 kg 37.8 kg

thats really high pressure, is this for a pressure fed rocket?. The
Shuttle's are about 30 psi or 0.2 MPa IIRC. How big are your tanks.
Cus thats real light for that kind of pressure if thay are sizeable
tanks. If it is a pressure feed engine, tank pressurization weight is
something you are going to have to live with. In that case your best
bet is probably gas generators. Liquid He is *very* hard to keep a
liquid (4K bp IIRC) compared even to LH2.


All tanks are spherical 6063-T6 aluminum. Valves, heat exchangers,
and lines are extra (and significant).

All these high pressure tanks are almost always carbon fiber and are
now common enough that thay are not too expensive.

In article ,
Iain McClatchie wrote:
The mass of pressurized tanks holding gases scales with the moles and
temperature of the gas held. The same rule applies unchanged to both
propellant and pressurant tanks. So if the gas temperature isn't
changed, and it doesn't undergo a chemical reaction changing the
number of moles, then the pressurant tank has to weigh as much as the
propellant tank...

This is mitigated somewhat by being able to use more aggressive tank
technologies (e.g. fiber-wound tanks) for pressurant without running into
minimum-gauge considerations. But it's not an accident that big rockets
show a distinct trend toward using propellant-boiling pressurization
systems whenever possible, and even when they don't, they do tend to store
their pressurants cold and heat them before use.

The book doesn't appear to discuss the mass of the pressurant tank.
It does talk about storing liquid pressurant, but says this hasn't
been done.

It's been done a little bit (notably by Goddard!). Do bear in mind that
Huzel&Huang is very definitely Rocketdyne's take on engine design, *not*
an industry-wide survey.

For pressurization in particular, NASA SP-8112, "Pressurization systems
for liquid rockets", is worth reading. It *is* more of a survey. It's
available (as a scanned PDF) on the net. (As are almost all of the
NASA 8000-series SPs, many of which are fascinating reading for rocket
designers.)

It talks about heating the pressurant in the tank, but
does not talk about keeping the pressurant tank cold before launch
(except for the special case of liquid hydrogen pressurant).

That's actually an outright omission, perhaps the result of having the
book written by engine designers rather than vehicle designers. Chilled
pressurant tanks are common, exploiting both the higher density of the gas
and the greater strength of tank materials at low temperatures. The
Saturn V tended to locate pressurant tanks inside cryogenic propellant
tanks, for that reason, and if memory serves, Atlas's helium tanks had LN2
jackets.

...The only alternatives are boiling your propellant,
which is only interesting if your propellant is cyrogenic...

It can be done even for non-cryo propellants. An extreme case is the
Titan II second-stage oxidizer tank, which has no in-flight pressurization
system! It gets some helium (I think) from ground-support equipment, and
any pressure makeup that happens after that is from the oxidizer boiling
in the tank as pressure drops off. Similarly, Centaur's tanks are mostly
self-pressurizing.

or burning your propellant and reinjecting that.

Or variations thereon. The original Arianes did some of their tank
pressurization using turbine exhaust gas, cooled with water injection.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

HS For pressurization in particular, NASA SP-8112, "Pressurization systems
HS for liquid rockets", is worth reading.

Thank you.

BTW, has anyone ever done a liquid neon system, or (more likely) liquid
neon refrigerated helium? Neon refrigerated helium sounds like a cheap
and lightweight intermediate between liquid nitrogen (heavy in the tank
at 28 g/mol) and liquid helium.

Also, do LN2-jacketed helium tanks dump the LN2 just before launch, or is
the valve to do that heavier than the LN2 dumped? I like the idea of
putting the pressurant tank in the fuel/oxy tank, but it does mean another
pipe going through the big tank's wall.

HS Or variations thereon. The original Arianes did some of their tank
HS pressurization using turbine exhaust gas, cooled with water injection.

Isn't there a major problem with water in the pressurant condensing to
ice, and that ice scratching up the interior of the tanks and valves and
orifices? I read that NASA has to be very careful about particulates in
the LOX in the shuttle, but also that they pressurize with turbine exhaust,
which has H2O. I'm astonished. Does the SSME not use aluminum tubing?

Greg You can have a blow down tank and the weights would be the same less
Greg plumbing.

I actually didn't think of that. If understand correctly that a blow
down tank is a tank with a perhaps larger initial ullage space, that
simply depressurizes as it drains, then if we compare that tank to a
tank pressurized from a seperate same-temperature supply (regulated to
the same terminal or even mean pressure), the blow down tank is still
a bit heavier. The problem is that the entire volume of the blowdown
tank has to contain the initial pressure, whereas only the pressurant
tank has to contain the initial higher pressure in the seperated system.

Greg Also we usally need the gas for other things as well.

Yes, I had not yet considered that.

Iain Prop tank pressure 1 MPa 1 MPa 1 MPa 1 MPa
Iain Prop tank mass 37.8 kg 37.8 kg 37.8 kg 37.8 kg

Greg thats really high pressure, is this for a pressure fed rocket?

Yep.

Greg How big are your tanks.

Propellant tank 1000 l 1000 l 1000 l 1000 l

l == litre.

Greg If it is a pressure feed engine, tank pressurization weight is
Greg something you are going to have to live with.

I can live with tank weight + 13%. +74% I can't.

I agree that liquid helium is not very reasonable, but chilled helium
gas looks good.

Greg All these high pressure tanks are almost always carbon fiber and
Greg are now common enough that thay are not too expensive.

Good to hear.

On Thu, 15 Jan 2004 03:52:00 GMT, (Henry Spencer)
wrote:


or burning your propellant and reinjecting that.

Or variations thereon. The original Arianes did some of their tank
pressurization using turbine exhaust gas, cooled with water injection.

It was also proposed in the later Wasserfall designs, to eliminate the
stored gass pressurization. Of course it did have turbines, ore
turban exhaust, nor the water cooling.

Alan

(Iain McClatchie) wrote in message

If understand correctly that a blow
down tank is a tank with a perhaps larger initial ullage space, that
simply depressurizes as it drains.

Yep you undersand correctly
the blow down tank is still
a bit heavier. The problem is that the entire volume of the blowdown
tank has to contain the initial pressure, whereas only the pressurant
tank has to contain the initial higher pressure in the seperated system.

Very true..

My H2O2/propane engine is so small (2l) that tank weight isn't
something i think about much. I just make it 3x thicker than I need
and hydro test it to 2x the pressure i will finaly use..(about 2MPa)

Greg

(Iain McClatchie) wrote in message

If understand correctly that a blow
down tank is a tank with a perhaps larger initial ullage space, that
simply depressurizes as it drains.

Yep you undersand correctly
the blow down tank is still
a bit heavier. The problem is that the entire volume of the blowdown
tank has to contain the initial pressure, whereas only the pressurant
tank has to contain the initial higher pressure in the seperated system.

Very true..

My H2O2/propane engine is so small (2l) that tank weight isn't
something i think about much. I just make it 3x thicker than I need
and hydro test it to 2x the pressure i will finaly use..(about 2MPa)

Greg

In article ,
Iain McClatchie wrote:
BTW, has anyone ever done a liquid neon system, or (more likely) liquid
neon refrigerated helium?

Not that I've heard of.

Also, do LN2-jacketed helium tanks dump the LN2 just before launch, or is
the valve to do that heavier than the LN2 dumped?

On Atlas, I *think* the LN2 is dumped before launch, with a modest amount
of insulation around the tanks preventing too much warming up.

Incidentally, one reason why the classic Atlas doesn't put the pressurant
in the LOX tank is that the entire pressurization system is jettisoned at
staging -- it's mounted on the booster-engine ring. Atlas is blowdown
during the sustainer-only burn, with the hydrostatic head from increasing
acceleration compensating somewhat for gas expansion in the tank.

HS Or variations thereon. The original Arianes did some of their tank
HS pressurization using turbine exhaust gas, cooled with water injection.

Isn't there a major problem with water in the pressurant condensing to
ice...

Not on Ariane, which uses storable propellants. With cryo fluids, yes,
that could get a bit problematic.

...I read that NASA has to be very careful about particulates in
the LOX in the shuttle, but also that they pressurize with turbine exhaust,
which has H2O. I'm astonished.

As well you should be. SSME turbine exhaust goes into the combustion
chamber; it's not used for pressurization. Shuttle tank pressurization
is by propellant boiling.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

On Fri, 16 Jan 2004 00:35:33 GMT, Alan Jones
wrote:

On Thu, 15 Jan 2004 03:52:00 GMT, (Henry Spencer)
wrote:


or burning your propellant and reinjecting that.

Or variations thereon. The original Arianes did some of their tank
pressurization using turbine exhaust gas, cooled with water injection.

It was also proposed in the later Wasserfall designs, to eliminate the
stored gass pressurization. Of course it did have turbines, ore
turban exhaust, nor the water cooling.

Alan

That should read: Of course it did not have turbines, or turban
exhaust, nor the water cooling.