Casting Rocket Engines 1

[indy1687]

Hi,
I am thinking about building a liquid rocket engine as a project. I have come up with a plan to cast the thrust chamber out of aluminium in the "full-mould" process. For those of you who do not no the process, its basical; you make the mould out of expanded polystyrene, cover the mould with sand in a moulding box and pour your molten metal on to the runner connected to the mould and the polystyrene should turn into CO2 and water vapour by the heat.

So my qustion is, is this a risky plan?, not in terms of casting but of the final thrust chamber, would it be safe to use?.

Finaly would I be better off to turn a thrust chamber on a lathe?.

Thank you for your time.

 

[IRstuff]

It's safe to use if you're planning on using the thrust chamber as part of the fuel. Or did you forget that aluminum, not only has a low melting point, but also has a tendency to burn?

Unless, of course, you're anticipating that the reaction temp will be less than 660ºC?

TTFN

 

[btrueblood]

Even if it doesn't burn, the strength of aluminum at relatively not-so-hot temperatures is pretty poor.

And, yes, a casting is inherently a less reliable pressure vessel than a similar structure made from wrought material. See the ASME Boiler & Pressure Cessel code, section VII.

 

[indy1687]

Thanks for the input, but how about using copper instead of aluminium

 

[IRstuff]

How about specifying what the maximum temperature is?

TTFN

 

[btrueblood]

In a general sense, a sand-cast pure copper (or any other metal) thrust chamber or other pressure vessel is still less reliable (more prone to inclusions, porosity and other stress risers) than one formed/fabricated from sheet or machined from wrought billet or drawn tube.

That said, I have helped design & test some rocket engines (well, okay, thrusters really) with investment-cast nozzles in Inconel alloys. These performed acceptably, and the casting process helped to lower the cost of the thrusters. But, an investment casting is a much cleaner, more controlled process (generally) than lost-foam and other sand casting processes. Also, we were willing to sacrifice some degree of material properties (yield strength primarily) for reduced manufacturing costs for the particular designs.

There are/have been quite a few rocket chambers made from copper (and various alloys of same), generally because of its high thermal conductivity and resulting improvement in regenerative cooling of the thrust chamber and nozzle throat. There have been quite a few "metal-augmented combustion" events using copper also, some of which I've participated in the post-mortem of.

As IRstuff says, if you want some good advice regarding a material selection, and/or design of rocket engine thrust chambers, come back with more information regarding your propellant choices, what kind of cooling scheme you are considering, what the mission for the engine might be (duration or total impulse delivered, thrust output, number of starts/restarts). The more info. you provide, the better your feedback on these forums will be.

 

[indy1687]

The fuel i would like to use is Aviation gasoline with hydrogen peroxide. I plan to use regenerative cooling by building a cooling jacket around the chamber and pumping the fuel through it. I was thinking of film cooling but it seams abit more complex. the mission plan is simple, its basical a one start lunch straigh up and a paracute ride back down, the duration i would hope for is around 100 seconds,the thrust output would be somewhere in the region of 5K to 10K newtons, i hope to use turbo pumps and a bonded platelet construction injector. i would like to use jet vanes as a means of control for the rocket. the max temperature for the chamber would be under 1000, around 800-900.

If i was to use wrough copper pipe for the thrust chamber, what thickness of copper would i need?.

Thanks for the advice irstuff and btrueblood

 

[btrueblood]

Why avgas? It will boil sooner, and provide not much better Isp than kerosene. Why peroxide?

What thickness? Thick enough on the outside to hold the fire on the inside, thin enough on the inside so that the heat can soak into the fuel without (most of) the fuel blowing out through the wall. Start researching. There are a lot of regen combustor technology reports available from NASA.

Your first course of business is to build a very thick concrete and/or sandbag bunker wall to stand behind during combustion chamber testing.

 

[IRstuff]

Copper supposedly softens at around 200ºC:

http://clickit.go2net.com/search?po...re&rawto=http://www.ansoniacb.com/c18150.htm,

so you might want to think about the cooling some more.

TTFN

 

[wolfkeeper]

Rocket engines can definitely be constructed with aluminum. There are some big advantages from doing so:

a) aluminum has great thermal conductivity (not *quite* as good as copper, but way up there)

b) aluminum is very light, 2.7 g/cm^3

c) aluminum is easy to work

The downsides are the low melting point and a tendency to combust in oxygen rich atmospheres. It also starts to lose strength very rapidly from about 200C; so keeping it cool is absolutely essential.

The Agena rocket engine had significant chamber and nozzle components made from aluminum- it works fine provided you cool it adequately.

 

[btrueblood]

The performance available from the Agena engine was pretty poor compared to modern day engines. Both for cooling purposes, as well as for stability reasons, the engine ran in a very fuel-rich mode, which limited its peak specific impulse and usefulness. It was also limited to one burn (but then, so are a lot of engines made of other alloys).

Interestingly, engine mass has a very small impact on launch vehicle performance. When optimizing a launch system, impulse (how much thrust*time you can get from each unit mas of fuel) and fuel density (how massive and drag prone is the tank/containment vessel gonna be) are found to be the big drivers. You also get different numbers if your goal is mass delivered to orbit or if it is dollars expended per launch.

 

[gwolf]

Try this guy's web site, his motors are smaller than the one which you plan but there is a good section on cooling and material properties. This site is one of the best that I have seen with lots of links too.

http://www.nakka-rocketry.net/

gwolf

 

[WKTaylor]

Indy1687

Metal castings are inherently hard to control for quality. I suspect You would have a hard time attaining a rigorous quality standard for repeatable structural performance. Also casting surface finishes [roughness] will be a major challenge.... relatively poor unless you have high quality dies made.

I suspect that without exhaustive testing and modeling, an aluminum chamber/nozzele would tend to erode disintegrate fast... even if machined from high quality stock and with cooling air fins.

Perhaps, though, ceramic coatings on the flame-side surfaces of a machined chamber/nozzel... and cryonic cooling on the bare-metal exterior... would work. For this arrangement, aluminum alloys MAY work... however I would tend to put my money on wrought Inconel or Monel alloys for heat and strength at temperature. Possibly, this composite chamber-nozzel system would be robust enough for Your purposes... Yet be relatively cheap(er) to build.

Hmmmm... perhaps chambers/nozzels could be cast in step-wise or symetrical sections... from a high strength casting alloy (then NDI'ed, "HIPed and heat treated)... then joined by friction stir welding.... and then be coated with high-temp ceramic by dipping or spraying [several cheap coating materials come-to-mind, such as SermeTel].

Regards, Wil Taylor