[AR] Re: fatigue life (was Re: Re: SpaceX F9 Launch/Update...)
- From: Richard Garcia <GalaxyNGC1672@xxxxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Thu, 31 Dec 2015 06:01:43 +0000
Of course, the ways to fend off hot gas, like film cooling, all come at a
price; it may be worth it.
Most LPREs have film cooling, and my previous comments where made assuming
there would be at least some film cooling involved.
P&W's SSME proposal used transpiration cooling
It's my understanding that transpiration cooling, while successful in a
laboratory setting, ultimately proved to difficult to build flight weight
versions that was reliable enough. All those tinny cooling passages turned out
to be too troublesome, and the concept was ultimately abandoned. (of course
with the exception of the rigimesh injector face on the RL-10, and maybe some
similar engines)
Put a final pump stage between the cooling jacket and the chamber, ...
Clever ideas, but.... then how do you deal with start up pressure? Fuel will be
running through the regin passages before combustion starts. So even If your
regin passages and thrust chamber are at the same pressure during steady state
you still have to deal with holding back all that pressure trying to implode
your chamber during startup and shutdown. This is typically the limiting case
for pressure induced stress on a thrust chamber. Also purge pressures would
need to be higher as well.
or do dump cooling with LH2
I could see starting the thrust chamber and cooling channels at the same time
with dump cooling. It would of course add complexity and sensitivity to the
ignition sequence. Any idea what the fatigue life or ignition sequence on the
HM7 is? (As a side note, my old boss kept referring to "a lot of film cooling"
as "dump cooling" that drove me nuts.)
Of course, there's always the option of backing off on performance. ... In
aviation, it's routine to back off on performance to improve operating life
and safety
I definitely agree. Especially when trying to build a low cost system. If you
try to build a Volkswagen bug that performs like a Ferrari, it will wind up
costing just as much. Also My comments about reusing milled channel thrust
chambers with a conventional fatigue life was meant to demonstrate that
fatigue, at least for thrust chambers, is not any kind of roadblock to reducing
launch costs via reusability.
-Richard
________________________________________
From: arocket-bounce@xxxxxxxxxxxxx <arocket-bounce@xxxxxxxxxxxxx> on behalf of
Henry Spencer <hspencer@xxxxxxxxxxxxx>
Sent: Wednesday, December 30, 2015 8:54 PM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: fatigue life (was Re: Re: SpaceX F9 Launch/Update...)
On Thu, 31 Dec 2015, Richard Garcia wrote:
Allowing the inner wall to expand may ameliorate the problem but does
not eliminate it...
That's why I said "much of the problem". :-) How much remains, due to
differential expansion in the wall itself, depends on details of materials
and design.
... What would really help is reducing the thermal gradient across the
wall. The best way to do that would be to make it out of something very
thermally conductive (i.e. copper) and to make it as thin as possible...
I would say the *best* way to do that would be to keep hot gas away from
the wall, reducing heat flow through it, since the gradient is more or
less thermal resistance (fixed by materials and dimensions) times heat
flux. Of course, the ways to fend off hot gas, like film cooling, all
come at a price; it may be worth it. For example, P&W's SSME proposal
used transpiration cooling to give its inner walls essentially unlimited
life -- the SSME spec called for 400 cycles and P&W felt that this was
impossible with just milled channels.
That thinness is where you start to run into trouble.
Making the thrust chamber walls thinner than some copper milled wall
chambers already are (less that 0.075") will limit what chamber pressure
you can run at, sacrificing performance.
One caveat: if I'm not mistaken, there's a hidden assumption here, that
there's a significant pressure difference (proportional to the chamber
pressure) across the inner wall. That's true of orthodox designs, but
it's not absolutely inevitable. Put a final pump stage between the
cooling jacket and the chamber, or use a separate fluid loop and a
propellant heat exchanger for cooling, or do dump cooling with LH2 -- just
exhaust it through an auxiliary nozzle rather than trying to burn it.
All of these concepts have their own problems, but they're not ridiculous
(dump cooling in particular has been flown), and all permit nearly zero
pressure difference across the wall (perhaps only at one particular point
along its length).
State of the art, high performance, milled channel , nickle backed
thrust chambers can usually get around 100 runs before cracks start
showing up from thermal cycling induced fatigue.
Of course, there's always the option of backing off on performance. As of
1990 -- don't have more recent data -- there were RL10s at P&W with over
200 cycles on them, still in good shape, with stainless-steel tube-bundle
walls. They thought then that qualification for 300 cycles of operational
use was feasible at the original 15klb thrust, although fatigue would
become an issue at higher thrusts (as in the RL10B) -- that engine was
somewhat overbuilt for 15klbf.
In aviation, it's routine to back off on performance to improve operating
life and safety. Nobody's delighted about doing it, but it's just
necessary, so nobody blinks at it. Rocketry has some way to go yet. :-)
Henry
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