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Old October 25th 17, 12:26 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Default Were liquid boosters on Shuttle ever realistic?

Jeff Findley wrote:

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.


Is it? When I look at cost numbers they're both around $13k/kg of
payload. ULA plays games with the cost of Atlas V by giving
'incremental cost' of an additional launch while charging all the pad
support, etc, to USAF contract.


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.


Actually, they aren't built by the same company (one is Boeing and one
is LockMart, although both are sold through the joint venture ULA) and
they want to get rid of Atlas V, too. In fact, Atlas V is the one
they really want to get rid of because it uses Russian RD-180 engines.


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.


But practically no one in the methane/LOX game is using common engines
among all their stages. ULA will be using BE-4 and SRBs on the first
stage and LH2/LOX on the second stage. New Glenn uses BE-4 on the
first two stages and LH2/LOX on the third stage (when used). There
are two versions of the BE-4, one sea level optimized and one vacuum
optimized. The same is true of the Raptor engine SpaceX will use and
once again SpaceX is the only one using the same engine everywhere.
SpaceX pays the price for not having a high energy LH2/LOX upper stage
by refueling on orbit.


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.


In the old days you almost had to go for performance because the
payloads and dry weight of the vehicles were always right at the edge.
Even SpaceX has done some of that, going with superchilled RP1 to
improve performance. I don't think Henry invented that phrase, as it
was pretty commonly used by a lot of people back in the day, but I
can't prove he didn't, either.


--
"Insisting on perfect safety is for people who don't have the balls to
live in the real world."
-- Mary Shafer, NASA Dryden