[AR] engine life (was Re: Re: SpaceX F9 Launch/Update -- Live Link)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Tue, 29 Dec 2015 20:15:47 -0500 (EST)
On Sun, 27 Dec 2015, David Weinshenker wrote:
Addenda: Flight F-1s were fired an average of 12 times before flight.
At the time, folks thought that on a reusable vehicle, an operational
life of 20 flights ought to be straightforward...
What's the primary life limit on such engines - creep and distortion
due to repeated heat-cycling of the chamber, I suppose? That sounds
like something that could -probably- be addressed at the design level
if one were determined to do so.
Aside from things like pump bearings and valve seals, which (given good
design) you can just replace regularly, the usual issue is metal fatigue
in the inner walls of the cooling passages.
The outer wall of a passage is cold and doesn't expand much during
startup, especially since it's usually thick to carry pressure loads,
while the inner wall gets hot and wants to expand. The result is that the
inner wall is in compression due to its own thermal expansion, and it will
try to bulge inward into the chamber, or thicken, or both. And of course,
running hot reduces its yield strength. Moreover, there's quite a thermal
gradient across the wall, so its gas side is also seeing more expansion
than its liquid side. Then the engine turns off and cools down, and the
inner wall wants to go back to its previous shape. Keep repeating this,
and eventually the inner wall will crack in the middle.
Tube-bundle walls get some of this, especially in the fancier forms with
flattened passage cross-sections, where it's harder for the inner wall to
bulge elastically. Milled-channel walls are still more rigid and have it
bad, especially since one reason to go with milled channels is to use
weaker materials like copper. Thermal shock from fast starts and stops
can aggravate it.
And yes, you can definitely address this at design time. It's just an
extra complication on top of an already difficult design problem, so if
you want longer fatigue life, you'll pay a price somewhere.
Henry
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- » [AR] engine life (was Re: Re: SpaceX F9 Launch/Update -- Live Link) - Henry Spencer