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Old October 25th 17, 11:31 AM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Were liquid boosters on Shuttle ever realistic?

In article ,
says...

Yes, but the difference isn't as preposterous as your case makes it
sound. LH2/LOX is going to be around 50% more expensive for engines
for 'similar' performance. The ability to stage higher and faster
because of Isp differences makes up for some of that (because you're
either carrying a smaller mass of fuel or because you're carrying the
same mass of fuel and it burns longer). You then have to adjust that
for tank weight and aerodynamic drag from the need for bigger volume
tanks. There are 'sweet spots' in there where LH2/LOX is competitive
and there are hypothetical cases that make LH2/LOX look even worse
than one would expect.

GENERALLY, a higher density exhaust (from a denser fuel) makes sense
for a first stage, but that's not always the case or Delta IV wouldn't
look like it does.



While you're making valid points, do note that Delta IV is the (far)
more expensive launch vehicle compared to Atlas V. LOX/LH2 may not cost
much more in the engine department, but the costs start mounting when
you consider everything else which has to go into the (larger, better
insulated) first stage and all of its associated plumbing. That
includes all of the headaches involved with buying, storing, and loading
large amounts of LH2 into the first stage at the pad (the upper stage is
tiny by comparison).

That's why ULA wants to drop Delta IV as quickly as it can. It's too
expensive and simply can't compete, even with Atlas V, made by the same
company. LOX/LH2 makes very little sense as a first stage propellant
combination due to the many disadvantages that quickly drive up costs
compared to pretty much any other (sane) liquid fuel.

LOX/methane is the fuel of choice for new development today because it
offers a good compromise on density and performance. Plus in its liquid
form it can share a common bulkhead with LOX without a lot of (any?)
insulation on the bulkhead. And yes, having common engines and common
fuel/oxidizer on both the lower stage and upper stage simplifies
manufacturing and operations, lowering costs. So that's a win for
LOX/methane as well.

In a world where SpaceX has set the bar very low on cost, everyone is
scrambling to optimize on cost rather than performance. Henry Spencer
used to call optimizing for performance the "performance uber alles"
mindset which came along with former German engineers who were used to
designing missiles rather than launch vehicles. That still holds today.

Jeff
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