Oh my sweet summer child, what do you know of turbines.
If your melting point is 1200 C and your engine is operating at 1500 C, without
cooling your engine will fail. The surface will melt and it will neither have
the structural integrity nor aerodynamic shape necessary to do its job - direct
and expand hot gas in a useful fashion.
In a turbine or piston engine, you can get away with using more common
materials. There are plenty of VW Beetles on the road today and they are made
of alloys less exotic than my silverware. The M701 I mentioned earlier is a
single stage centrifugal flow jet engine, again made of dinnerware-grade metal.
The reason you can do so is that gas engines have the benefit of an environment
with a continuous flow of fresh cooling fluid (and working fluid, for that
matter) that allows you to dump excess heat into your surroundings quickly and
efficiently. Piston engines have cooling fins or radiators, turbines have air
and fuel cooled oil coolers. This means that you can control your temperature
by regulating your cooling apparatus.
In space, no one can hear you scream.
A consequence of that is that you don’t have a fluid to cool your heat
exchanger. The only cooling fluid and working mass you have is that which you
have brought with you. Now your only choice is which mass you want to cool
with, and how you wish to do so (ablatively, with the liner, or regeneratively
with your fuel)
Similar problems exist with the COPV (or perhaps composite overwrapped
combustion chamber) - any composite structure wrapped tight enough to provide
structural support is going to be wrapped tight enough to conduct heat, with
all the issues of heating composites and differential expansion that entails.
On Mar 11, 2018, at 12:22 AM, Robert Clark
<rgregoryclark@xxxxxxxxx<mailto:rgregoryclark@xxxxxxxxx>> wrote:
I suspect that is a matter of degree, literally. It’s one thing for the
melting point to be, say, 1,200 C and the jet engine to be operating at, say,
1,500 C, and quite another thing for the melting point to be 1,200 C and the
rocket engine to be operating at 3,000 C.
But let me ask the reverse question. Instead of getting exotic materials with
melting points above the engine operating temperature of 3,000 C, can we get
the engine to operate at lower temperatures say 1,000 C so more common metal
alloys can be used that are above the operating temperature? Then we can get
thousands of hours of use out of a rocket engine like for jet engines and for
automobile engines.
In this case the ISP would be significantly reduced, but you could still get a
orbital rocket by having very high mass ratio. The high temperature but
lightweight ceramics I mentioned might be able to get the T/W ratio for the
engine into the range of 300 to 1. But if you’re using these ceramics you might
as well just use their high temperature properties as well to retain the high
ISP.
But another method might be able to get even higher T/W for the engine when
the operating temperature is reduced to ca. 1,000 C. Use common metal alloys
for the engine but only for the surfaces exposed to the high temperature, then
use insulation around the engine, then finally use carbon fiber composites for
strength to contain the high pressure.
The carbon fiber doesn’t have very good temperature properties but the idea is
during a launch the engine might be operating less than 10 minutes. In this
case we’re also operating at a lower temperature, a third that of usual. Then
besides that we’ll have very effective insulation between the metal exposed to
the combustion temperatures and the carbon fiber. In this scenario since you
need only a thin layer of the metal inner surface, and the other materials for
the outer layers are lightweight, you might be able to get a markedly reduced
weight for the engine, perhaps an order of magnitude lower, bringing the T/W up
1,000 to 1(!)
We still need to get also the tanks reduced in weight multiple times. I’ll
discuss this in the next email.
Bob Clark
On Thursday, March 8, 2018, Henry Spencer
<hspencer@xxxxxxxxxxxxx<mailto:hspencer@xxxxxxxxxxxxx>> wrote:
Robert Clark wrote:
But suppose we had a ceramic that had a melting point even higher than
the combustion temperatures? Then regenerative cooling would not be
needed and then like jet engines, rocket engines could operate for
thousands of hours, giving rockets reusability comparable to jet aircraft.
Modern jet engines use regenerative cooling *extensively*; in particular, their
turbine inlet temperatures routinely exceed the melting point (never mind the
maximum service temperature) of the turbine-blade materials. So the idea that
avoiding regenerative cooling is the magic that will confer long operating life
seems questionable.
By the way, conservatively-built rocket engines like the RL10A and the XCOR
engines already have reusability comparable to many jet engines (allowing for
the fact that one mission is hours of run time for a jet and minutes for a
rocket). The short useful lives of most large rocket engines have more to do
with their design philosophy (performance uber alles!) than with anything
inherent in rockets; jet engines designed the same way -- e.g. for cruise
missiles -- similarly have short lives and poor reliability.
Henry
