Vapor as rocket propellant and coolant

Iain McClatchie wrote:

A lightweight engine designed for gas-phase combustion
will explode if you fill it with LOX and liquid methane
and ignite it. So at startup you'd have to release small
amounts of gaseous oxygen and methane into the combustion
chamber to get it going. If you want to avoid propellant
valves that can throttle the engine (which sound hard to
get right), you'll want to just bang open the propellant
valves.

The easiest way to heat up the propellants
to the boiling point temperature is to make
a small rocket engine and vent its exhaust
into the propellant tanks.

"Andrew Nowicki" wrote in message
...
Iain McClatchie wrote:

A lightweight engine designed for gas-phase combustion
will explode if you fill it with LOX and liquid methane
and ignite it. So at startup you'd have to release small
amounts of gaseous oxygen and methane into the combustion
chamber to get it going. If you want to avoid propellant
valves that can throttle the engine (which sound hard to
get right), you'll want to just bang open the propellant
valves.

The easiest way to heat up the propellants
to the boiling point temperature is to make
a small rocket engine and vent its exhaust
into the propellant tanks.

..but then you wouldn't have a rocket - you have a bomb!

Heat + oxidiser (traces from your rocket exhaust) + fuel (in the tanks) =
bang, with the right mix.

AIUI, one of the reasons in favour of *liquid* propellants is that they are
harder to ignite (further away from their ignition temperature) than their
respective vapours, and hence safer and easier to handle - not being
vaporised until the last millisecond.

Cameron:-)

Mike Miller wrote:

Alloy segregation, grain size, heat affected zones, fusion, lack of
fusion, slag inclusion, the effects of multiple weld passes, the
problems with work piece orientation...welding is COMPLICATED,
especially when dealing with high strength alloys fit for large
pressure-fed rockets. It's so complicated that it's not yet possible
to automate the welding of large submarine hulls, which use more
forgiving alloys than the alloys considered for pressure-fed big dumb
boosters.

Yes. Some alloys are easier to weld than others.
What do you think about weldability of high-strength
aluminum-lithium and titanium alloys, especially
Weldalite 049-T8, 2195 alloy (external tank of the Space
Shuttle is made of this aluminum-lithium alloy),
and russian aluminum-lithium 1460 alloy?

Some cylindrical tanks have very few welds. The Atlas
rocket launchers had so called balloon tanks which had
extremely simple design and very low weight. The balloon
tanks did not have any stiffeners; they relied on internal
pressure to prevent buckling and to make the tanks rigid.
They were stabilized for ground handling by maintaining an
internal pressure of three to five psi. During flight, all
liquid propellant launch vehicles depended on internal
pressure in the tanks - typically 20 to 50 psi - to support
thrust and launch loads, as well as to feed the propellants
to the engines.

Mike Miller wrote:

You're probably using an overly strict definition of
"flimsy." Fuel tanks for pump-fed rockets do survive
re-entry without a heat shield (usually pancaking into
some frightened farmer's field).

Are you talking about the first stage, the second stage,
or the third stage? The external tanks of the Space
Shuttle do not survive the reentry, and even the orbiters
do not always survive the reentry. On the other hand the
Shuttle's solid rocket boosters do survive reentry and
splashdown.

A reusable first stage is easy to make, and yet only
Russians seriously think about it. A reusable last stage
seems impossible and unnecessary because it will be
eventually replaced with a rotating space tether or another
orbital device. The best design of the reusable first stage
is determined by politics rather than technology. If NASA
and their subcontractors determine the design, it will be
another boondoggle -- the Space Shuttle Main Engine has
70,000 parts! The simpler the design, the better.
KISS = keep it simple stupid

PS. I do not believe that rocket entrepreneurs can
compete with the big government contractors. If I had
a rocket launcher and offered it for free, there would
be no takers.

Mike Miller wrote:
Welding is not rocket science.

Well, if you're using high-strength alloys with little tolerance
toward welding, it can be as complicated as rocket science.

Alloy segregation, grain size, heat affected zones, fusion, lack of
fusion, slag inclusion, the effects of multiple weld passes, the
problems with work piece orientation...welding is COMPLICATED,
especially when dealing with high strength alloys fit for large
pressure-fed rockets. It's so complicated that it's not yet possible
to automate the welding of large submarine hulls, which use more
forgiving alloys than the alloys considered for pressure-fed big dumb
boosters.

As a slight rebuttal to this, though...

It is possible to braze a lot of the Al alloys with much
less complications. See "Alumiweld".

It's becoming possible to friction-stir weld a lot of things,
definitely including essentially all Aluminum alloys, and at
least some of the high strength steel ones, with a lot less
complications as well. Delta-IV common booster core main tank
wall segments are FSW'ed; it was initially used as an emergency
recovery test when manual welding wrecked one tankset, and it
worked so well that they abandoned the manual welding.

FSW has been done on equivalently high strength steels as
the HY-series used for sub hulls, but I don't know of any
serious program to try FSW on subs themselves. Among other
problems, the sub hull plates are a lot thicker than what
I have seen successfully reported as being FSWed.

A propellant tank, however, would be somewhat thinner.

Thicknesses up to 75mm of Al have been reported as being
successfully FSWed right now.


-george william herbert

Andrew Nowicki wrote in message ...

Yes. Some alloys are easier to weld than others.
What do you think about weldability of high-strength
aluminum-lithium and titanium alloys, especially
Weldalite 049-T8, 2195 alloy (external tank of the Space
Shuttle is made of this aluminum-lithium alloy),
and russian aluminum-lithium 1460 alloy?

I don't know offhand, and I'm away from my usual metallurgical
resources. George Herbert has brought up some plausible sounding
alternatives to welding.

Some cylindrical tanks have very few welds. The Atlas
rocket launchers had so called balloon tanks which had
extremely simple design and very low weight. The balloon
tanks did not have any stiffeners; they relied on internal
pressure to prevent buckling and to make the tanks rigid.
They were stabilized for ground handling by maintaining an
internal pressure of three to five psi. During flight, all
liquid propellant launch vehicles depended on internal
pressure in the tanks - typically 20 to 50 psi - to support
thrust and launch loads, as well as to feed the propellants
to the engines.

However, the Atlas was not a pressure-fed rocket. It consistently used
pump-fed engines. Is 20 to 50psi adequate for your needs?

Mike Miller, Materials Engineer

(George William Herbert) wrote in news:ccdh0m$m90$1
@gw.retro.com:


Thicknesses up to 75mm of Al have been reported as being
successfully FSWed right now.

Is there any pre-heating or does the tool friction do
it all?

--Damon

Andrew Nowicki wrote in message ...
the Space Shuttle Main Engine has
70,000 parts! The simpler the design, the better.
KISS = keep it simple stupid

There's an advantage to KISS, sometimes, but then again...

How many parts does a typical car engine have?
How many parts does a Boeing 777's GE90-115B engine have?

And how many of the SSME's 70,000 parts are fasteners?

Mike Miller, Materials Engineer

Iain A lightweight engine designed for gas-phase combustion
Iain will explode if you fill it with LOX and liquid methane
Iain and ignite it. So at startup you'd have to release small
Iain amounts of gaseous oxygen and methane into the combustion
Iain chamber to get it going.

Andrew The easiest way to heat up the propellants
Andrew to the boiling point temperature is to make
Andrew a small rocket engine and vent its exhaust
Andrew into the propellant tanks.

Cameron .but then you wouldn't have a rocket - you have a bomb!

It's been done. I remember reading about this in "Modern Engineering
of Liquid Propellant Rocket Engines".

I don't agree with Andrew's idea of boiling all of the propellant
in the tanks. [Not that my opinion really matters all that much,
of course. ] I don't see how you regulate the pressure in the
tanks with that scheme, short of regulating the heat flow into
the tanks. Someone please feel free to educate me about cheap,
lightweight ways to regulate multi-megawatt-level heat flows.

I do like the idea of boiling some of the propellant in the tank
in order to pressurize the tank. If most of the propellant mass
exits the tank as a liquid, the tank pressure can be regulated
with a gas-phase pressure regulator with reasonably small flow
rates, which can then be reasonably small. Variations in heat
flux cause variations in this small mass flow of propellant into
the engine, which should cause small variations in the heat
output of the engine and so any positive gain in the feedback
loop should be very small.

Earlier I suggested that self-pressurizing oxygen and methane
would be heavy. Now I'm not so sure. You might be able to get
interesting Isp (maybe 80 seconds or so) by venting the tanks
through the engine. 80 seconds sucks compared to burning the
stuff, but the comparison here is not to a high pressure liquid
fueled rocket, but to *no thrust at all*.

You might arrange things so that by the time the engine shuts
down it's main burn, the small amount of fluid in the tank is
reasonably warm, maybe 100 K or so. As you vents the tanks
(sequentially, lets say, oxygen first, then methane), the
pressure drops and this remaining fluid boils.

A more aggressive design might combust some of the oxygen and
methane vapors. It might not be worth the trouble, since the
liquid methane cooling system is basically gone. But maybe
you can get oxygen-methane to burn rich and cool enough to
not kill the engine.

My overall point is that self-pressurization leads to the
interesting possibility of extremely small burnout propellant
loads, which might actually save considerable weight over a
traditional pressurization system. It might be simpler too.

And finally, since the last few seconds of thrust are very
low acceleration, you have the potential for higher precision
cutoff, and maybe even precision maneuvering once in orbit
using cold gas thrusters operating off the left-over fully
vaporized fuel.

What is the advantage of putting the engine in the tank?

If the engine is cooled by the (initally liquid) methane flow,
that flow is pressurized and has to be contained. There are
three surfaces under consideration he the interior of the
engine, the exterior of the engine, and the tank. The idea
would be to use the exterior of the engine as the interior of
the tank.

If we want the propellant valves to be before the flow cools
the engine (so the valves can be small), the coolant passages
have to be empty before the engine starts. So the exterior
of the engine is a pressure bearing surface.

Once the engine is firing, the coolant channels are at near-
tank pressure, and the interior of the engine is a pressure
bearing surface. Less so in the combustion chamber, more so
in the nozzle.

The savings from having two pressure surfaces instead of
three is not very compelling when the cost is the loss of
volume of the tank, and added complexity.

To boil fluid from the tanks, you might tap off some of the
heated fuel flow and run it through pipes through the
propellant tanks, and then dump it in either the nozzle or
(if you can get a tap point with both hot fuel and a
significant subsequent pressure drop to the combustion
chamber) the combustion chamber. Dumping into the nozzle
sounds simpler, but once again I just threw away some Isp.

Alternatively, you might tap both fuel and oxygen lines after
they get down to the engine but before the regulating
orifices. Small flows could go to heat exchangers, boiling
the fluid which is then led back to the tank. The trouble
here is getting positive flow, and not just dumping the
tank pressurization gas into the engine intake by reversing
the intended flow. Any positive flow will be a consequence
of a pressure drop caused by the difference in weight of
the column of fluid (in the feed line) and the column of
gas (in the gas line going back to the tank). This sounds
hokey to me... I see one-way valves and problems.

Iain Some sort of tank-pressure loop will need to supply
Iain heat to the oxygen tank to make it self- pressurizing.

Andrew I like the idea of physical contact between the oxygen
Andrew tank and the methane tank. Fans inside both tanks churn
Andrew the propellants.

Think about your propellant valves. How big are they? And
how do you control pressure? I think you'll find you want
liquid propellant going through the main valves.

And that leads you to boiling just a little of the propellant,
and leaving the rest as a liquid (cold).

I keep coming back to the problem of getting a heat path from
the engine to a small portion of the tank. Maybe you have
two oxygen and two CH4 tanks, all long cylinders, and you
nestle the engine between them. Maybe you have physical
contact between the engine and the tanks. I keep thinking the
vibration is going to beat the tanks into premature failure,
and ruin the thermal contact as well. There must be some
way around this problem. The LH2 in the Shuttle's tank
apparently heats quite a bit from recirculating engine exhaust,
and they have insulation all over the thing. Of course, its
also a lot colder...

Andrew How did you get the 28.2 kg/m^3? Do you have the gas
Andrew tables for oxygen and methane?

http://www.airliquide.com/en/busines...x.asp?GasID=41

Iain You have only crude control over the heat input to the
Iain tank...

Andrew Fans inside both tanks churn the propellants and thus
Andrew increase the heat flux between the engine and the
Andrew propellants.

This doesn't address my point. I claim you aren't going to
be able to predict the heat flux into the tanks to within 20%.
The flux may vary during flight by 2x. How are you going to
maintain pressure control?

Andrew Do you think that cryogenic valves would be too
Andrew expensive?

Think about different kinds of valves. It's easy (and so
cheap and light) to just open or close. Actually, at low
temperatures even that is hard, but move along. It's much
harder to build a valve to pass only a certain volume flow
at a particular pressure. It's way more difficult to build
a valve which passes a particular mass flow when the
pressure, temperature, and viscosity are all varying.

Iain Across a 30 cm orifice that's 46 m/s. At this
Iain velocity there will be large drag pressure losses.

Andrew You exaggerate. 46 m/s is close to average car
Andrew speed on a highway. 1.5 MPa equals 153 meters
Andrew of water head.

Do I? Your gas is at high pressure (and so density),
tearing around corners, travelling through long, tiny
little tubes in the cooling passages. I would guess the
SSME loses 5 MPa through it's coolant flow. A lower
pressure engine has less heat flow per unit area, so
the problem isn't quite so bad, but I suspect you may
lose 20-40% of your tank pressure through the coolant
system. That's still bad, because
(a) the gas flow gets larger, so everything must get
bigger, and
(b) your expansion ratio in the atmosphere just went
down a lot, reducing your Isp.