On Oct./30/2018 at 06:47, Jeff Findley wrote :
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.

I would be surprised if the mixture ratio was a real problem here. If
you don't have the right ratio you might have less performance but that
isn't much of a problem. Not to say, that you should just ignore that,
before flying it you should know that the mixture ratio is not a
problem, not just think that it isn't a problem.

Without knowing if this really is a problem, I think one should check if
having in the mix some butane, with a melting point of -140C which is
quite higher than the boiling point of oxygen -183C, will cause some
problems. If you have a tonne of frozen butane in the tank that clogs
some pipe, you can have a bad day. I don't know if this is really a
problem, maybe solid butane is soluble in liquid methane. Maybe it would
make the mix only a little slushy and it would still flow through the
pipes without a problem. Also, this fuel mix might still be liquid but
become very viscous when passing in pipes near liquid oxygen. Maybe,
maybe, maybe... Before flying such a thing one should know what will
happen, not just rely on the fact that LNG is similar to liquid methane.
Maybe, it will fly, maybe not.


Alain Fournier