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Old October 30th 18, 11:47 AM posted to sci.space.policy
Jeff Findley[_6_]
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Default US Air Force explores space-based cargo operations, confirms talks with SpaceX

In article ,
says...

JF Mezei wrote on Sun, 28 Oct 2018
14:02:38 -0400:

On 2018-10-27 17:51, Alain Fournier wrote:

BFS will burn liquid methane. I'm not sure what would happen if you
replaced the methane with LNG. Possibly it would fly, possibly it would
go kaboum. Even if LNG is mostly liquid methane, there typically is near
10% of ethane, propane, butane and small amounts of other stuff. In some
cases the difference can have consequences.


Are rocket engines extremely sensitive to fuel density? or do they allow
for a range of density so that the turbines don't go kablooey if fed
with slightly different fuel?


The density difference between liquid methane and LNG is (at most)
around 10%. It's typically much less. Navy fighter planes use JP-8
ashore and JP-5 at sea. The two fuels vary in density by about 5% and
have different properties. Given that, I wouldn't expect a rocket,
which is a much simpler machine than a jet, to have problems over
minor differences in fuel density.


It's also worth noting that for this role, BFS would almost surely be
trying to return empty. So that mitigates the problem somewhat in terms
of the performance required. You could further mitigate that by
planning on hopping BFS a couple of times on the way "back to base"
which would give you more margin on the performance.

So the only thing you'd really need to do is to insure that the engines
are properly instrumented and have the proper controls in place in order
to handle these small variations in propellant density. In other words,
you've got to tweak the mixture ratio a bit in this case, which really
shouldn't be too hard to do.

Jeff
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