Propellant pressurization

(Iain McClatchie) wrote in message om...
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.

A slick way around this is to use a lightweight tank of liquid
nitrogen and evaporate the N2 with a heat exchanger or heater. The
Soyuz type launchers do this for the core and 4 boosters. There is a
toroidal LN2 tank, and one of 4 pumps is for the LN2, which is heated
to pressurize the propellant tanks. Dates back to '50's.

A version I was going to use for a rocket designed in '93 was to have
a tank of saturated LN2 warmed to about 120 K so the vapor pressure
was about 400 psia, and the liquid would be evaporated and heated to
200 K (100 C lower than the fuel temp) by a heat exchanger through the
room temp fuel was passed.
This would cool kerosine about 17 C.

The mass of the hardware to contain enough pressurizing gas will be
much lighter
than a high pressure gas tank.

(Greg) wrote in message m...
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

You have a working H2O2/propane engine? Can you share details / experience?

H2O2 concentration?

Cat pack construction or direct injection?

Chamber / nozzle cooling method?

L*?

Tank pressure / chamber pressure?

Delivered Isp / run times?

John Carmack
www.armadilloaerospace.com

Sorry -- my last post may have been missleading. The fuel tank is 2l
in size and its a 30-60N thrust engine. Like the whole engine and fuel
system will fit into one of your engines. And i'm afraid that I don't
have much in the way of firing time either. About 1 min total and very
poor perfomance with a 316SS engine and silver "wool" as the catalyst.
The H202 is 70% concentrated from 50% ie lots of inhibitors which is
why we only get 1 min firing time.

I read your web pages to get ideas

so h2o2+propane (lpg) is next but a bit down the track. But i have a
tank somewhere rated for 3MPa.

Greg

(Greg) wrote in message m...
Sorry -- my last post may have been missleading. The fuel tank is 2l
in size and its a 30-60N thrust engine. Like the whole engine and fuel
system will fit into one of your engines. And i'm afraid that I don't
have much in the way of firing time either. About 1 min total and very
poor perfomance with a 316SS engine and silver "wool" as the catalyst.
The H202 is 70% concentrated from 50% ie lots of inhibitors which is
why we only get 1 min firing time.

I read your web pages to get ideas

so h2o2+propane (lpg) is next but a bit down the track. But i have a
tank somewhere rated for 3MPa.

Greg


I STRONGLY suggest that you buy unstabilized semiconductor grade
peroxide for your 50% feedstock. Solvay "ultra pure" or equivalent
(you don't need "ultra high pure" or "pico pure", that is overkill).
This is so pure that even if you concentrated it way up, it is still
going to be at least as pure as normal propulsion grade.

Yes, it is more expensive than food grade or technical grade, but
propellant cost is really not a dominant factor in rocket
experimentation, and it makes a lot of issues just go away. We are
paying $1/lb from Air Liquide in multiple drum quantities, but I have
heard that it is more like $2/lb in smaller quantities. For the
uncertainty that it removes, it is still a bargain.

Note that a stainless engine isn't going to last very long when you
light up a hydrocarbon in it. For simplicity, you would be better off
with a big hunk of copper.

We had a hard time getting industrial ethane to start burning with our
peroxide biprops, but kerosene always lit right up. It is possible
that propane may not be as easy to start as you hope, and if your
peroxide concentration isn't up to 90%, it will be even harder. I
think kerosene will auto-ignite at 85% decomposition temperatures, but
below that, you may need to look at ether-like additives to lower the
startup temperatures. You could always use some external ignition
source, but then you throw away much of the benefit of a peroxide
motor.

I would suggest experimenting with kerosene over LPG.

John Carmack
www.armadilloaerospace.com

(John Carmack) wrote in message
I STRONGLY suggest that you buy unstabilized semiconductor grade
peroxide for your 50% feedstock.

I live in New Zealand. There is no semiconductor grade peroxide here.
I would need to import it myself. And that is expensive as new zealand
is very isolated and shipping costs are high.


Note that a stainless engine isn't going to last very long when you
light up a hydrocarbon in it. For simplicity, you would be better off
with a big hunk of copper.

Indeed. I was going to try graphite. but I have a idea for a type of
film cooling instead. I hope to have some lab grade experiments done
by the end of 2004 and write a paper on the results.

We had a hard time getting industrial ethane to start burning with our
peroxide biprops, but kerosene always lit right up.
[snip]
I would suggest experimenting with kerosene over LPG.

Thanks for the tips. I'm still working on small stuff compared to your
work. I have a very crude (at this time) web site at
www.rocketIntel.com

Greg