You do not operate the actual metal above it's melting point. If you
did or even got close, you would quickly get some engine-rich
combustion (aka a tribrid). For the record you don't really care about
the melting point directly when you are designing an engine. All
materials have a strength vs. temperature curve, where most materials
slowly grow weaker as you increase the temp. 6061 Aluminum for example
has only about 10% of its room temperature strength at 400-450 degrees
F. So when you design your cooling solution you make sure that
whatever you are doing keeps the chamber wall temp below the
temperature where your material strength is no longer strong enough
for the hoop stress, or the stress at the throat.
What you are basically suggesting is that you should properly design
your engine so you don't melt it.
A properly designed regen cooled liquid engine (or pretty much
anything other than ablative) will only be life limited by corrosion
you get from your combustion products or propellants, metal fatigue
and creep from the stress and thermal cycling, and erosion (if you
have it) of your throat/nozzle. Use the right materials, and cooling
design and there is no reason you couldn't run a liquid for as long as
any other heat engine we make. Doing so typically dosn't make too much
sense at the moment as even a few hours of run-time is a bit excessive
for any of the launch systems we currently make. So trading the
thousands of hours of run-time for some extra performance or reduced
cost makes sense.
What you are actually decreasing when you make your chamber bigger
isn't so much the temperature. Even if you go from 1000 psi chamber to
300 psi chamber, the combustion gas temperature is still way too
gorram hot. You are trying to reduce the heat flux, which is a
function of temperature and pressure.
I don't know if you want to put nitrogen into an engine if you can
help it. At high enough temperatures (like inside an engine) it very
much does participate in combustion reactions. Most of the materials
formed are endothermic, and some don't play nice with the stuff you
make your engine out of. There have been engines made that had a
non-interacting third component as an energy carrier. Getting an ISP
boost by using a light molecule, and having an energetic reaction that
produced heavy combustion products to produce the heat. The Li-LF2-H2
engine comes to mind.
Running rich on your O:F ratio is standard practice, as the most
efficient ratio at a system level often isn't operating at the ideal
O:F anyway. You are limited to how rich you can go, depending on the
fuel combo. As some will start sooting things badly, promote some type
of combustion instability, or have some other form of adverse reaction
if you go rich enough.
On Tue, Mar 13, 2018 at 12:08 AM, Robert Clark <rgregoryclark@xxxxxxxxx> wrote:
I was referring to Henry’s statement that jet engines operating at
temperatures above the metals melting points, which therefore use
regenerative cooling, still get long lifetimes. My argument was that the
extent by which the rocket engine temperatures are above the melting points
shortens rocket engine lifetimes. I suggested instead use lower combustion
temperatures to extend rocket engine lifetime.
By the way, I didn’t discuss how to get the reduced combustion chamber
temperatures. Three ways come to mind. First, you could use a larger
combustion chamber than usual to reduce pressure and therefore also
temperature. This though would increase engine weight which is what we need
to avoid.
A second way would be to use actual liquid air, including the nitrogen, for
the oxidizer. The nitrogen doesn’t take part in the combustion, so would
cool the reaction. The ISP would be low though in this case. From memory
it’s in the range of 253 s vacuum ISP for kerosene fueled. Perhaps some
other inert component would allow a better ISP.
My preferred method though would be to use a lower mixture ratio for
kerosene/LOX propellant. This is running the engine fuel rich and is known
to cool the reaction. That necessarily means you get a reduced ISP, but the
idea is to get a very high mass ratio, i.e., very low structural fraction,
instead.
Bob Clark
On Sunday, March 11, 2018, rebel without a job
<rebelwithoutajob@xxxxxxxxxxx> wrote:
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>
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> 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
