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> 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
