[AR] Re: Portland State Aerospace Society
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Mon, 25 Apr 2016 16:36:12 -0400 (EDT)
On Wed, 20 Apr 2016, Kyle Meeuwsen wrote:
The Ethanol tank is currently aluminum.... Last year's senior design team
acquired it and the thought was that the Ethanol will not be present in the
tanks long enough to cause issues.
On a rocket, perhaps so. On a test stand, bad assumption. If nothing
else, troubleshooting problems on the test stand will be a lot less
stressful if people aren't anxiously watching the clock and worrying about
propellant corrosion eating away at tank safety margins.
(That said, a lot of rockets have used aluminum tanks for ethanol, without
difficulties that I know of.)
...I am not sure on the material for the LOX - I will look it up when I
get home. I believe the tank was previously used for oxygen in
industry.
Careful here, suitability for GOX and suitability for LOX are different
things, notably because of the cryo embrittlement issue with mild steel.
Lastly, how bad is it if the team wants to mount the engine to the stand
such that the exhaust outlet is parallel to the ground (mounted
horizontally). I have concerns about fuel pooling following testing.
Eliminating the possibility of liquid pooling in the chamber is a good
thing, all else being equal. Unfortunately, all else is rarely equal.
In particular, unless the engine is very small, downward firing is going
to require an exhaust deflector, and durable deflectors are surprisingly
difficult to make. Vertical test setups in general are bigger and harder
to build.
As I mentioned in previous mail, especially for a test stand, completely
eliminating problems is better than mitigating them. But it usually does
have a cost. You have to decide where to draw the line.
XCOR's experience certainly suggests that careful attention to reliable
ignition and purging can deal with this issue well enough. (Mind you,
their engines are mostly meant for horizontal firing in operational
service, so they needed to solve this one thoroughly anyway.)
Henry
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