[AR] Re: fatigue life (was Re: Re: SpaceX F9 Launch/Update...)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Thu, 31 Dec 2015 15:37:23 -0500 (EST)
On Thu, 31 Dec 2015, Richard Garcia wrote:
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.
I prefer to observe a distinction of terminology: the usual (although not
quite universal) practice of adjusting the injector design to put a
fuel-rich layer near the wall is "curtain cooling", while "film cooling"
is reserved for concepts that put a low-velocity *liquid* layer on the
wall, typically by angled orifices in the wall itself (e.g., the V-2
engine did this).
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.
It has problems, both passage clogging and flow instabilities, but people
have made it work. If memory serves, P&W had used it extensively in an
experimental high-pressure engine, and thought it entirely feasible for
the SSME. Reportedly, MSFC dismissed it as unworkable without paying any
attention to the details of the P&W experience.
Beware of assuming that the design space has been thoroughly explored --
often it hasn't! There is a particular problem with people trying
something once, having trouble, and concluding that the whole idea is
infeasible.
Along similar lines, quite by chance today I ran across NASA CR-185257,
aka AIAA 90-2116, Quentmeyer, "Rocket Combustion Chamber Life-Enhancing
Design Concepts", which discusses half a dozen different concepts for
making channel-wall chambers more durable. Some seem like long shots,
some look workable.
(Sorry, don't have a URL for that because I ran across it in my own files,
but it ought to be findable...)
...the concept was ultimately abandoned. (of course with the exception
of the rigimesh injector face on the RL-10, and maybe some similar
engines)
All the operational US LOX/LH2 engines use Rigimesh injector faces, I
believe. (Rocketdyne was initially dead set against it on the J-2, but
kept having injector damage, and eventually grudgingly copied the idea
from the RL10.)
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.
Haven't thought about the details, but most likely you'd have to do a slow
start, ramping up pressures in a controlled way rather than trying to slam
everything wide open as most rocket engines do. It's unorthodox nowadays,
but it's been done.
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?
Don't have details on that one, I'm afraid. (For those who don't
recognize the number, that's the LOX/LH2 third-stage engine of the early
Arianes. The Europeans and the Japanese have both used dump cooling for
LOX/LH2 exhaust nozzles; hot GH2 exhausted through a simple nozzle has a
high enough Isp, even without combustion, that the performance penalty of
not burning the coolant is less than you might think.)
Henry
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