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Old October 27th 17, 11:17 AM posted to sci.space.policy
Jeff Findley[_6_]
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Default Were liquid boosters on Shuttle ever realistic?

In article ,
says...

Jeff Findley wrote:

In article ,
says...
Actually they haven't. Superchilled RP1 drives up costs in order to
improve performance. Going for reusability drives up manufacturing
costs and design costs because you have to make things that can be
used dozens of times without attention (so you need better materials
and tighter designs without pushing for extra performance, figure out
some way to avoid 'coking' on an RP1/LOX engine, etc). It's the
normal engineering evolution of launch vehicles that was stalled while
most payloads were government.


As far as engines go, this isn't really true. All liquid fueled engines
are designed to be fired multiple times, at least on the test stand.
That's how they're qualified. Henry Spencer used to say (paraphrasing
here), there is absolutely nothing fundamental about a liquid fueled
rocket engine that makes it expendable.


Well, actually it is. You go into engine design knowing how many
'refires' the engine needs to stand. If it needs to stand 3 with some
safety margin, the robustness required is a lot less than if it needs
to stand up to 36 of them with the same safety margin. That is going
to drive up manufacturing costs (which you hopefully get back through
the savings by reusing hardware).


A more reusable engine also helps to make qualification testing cheaper.
An engine that can be fired multiple times without tear downs is far
cheaper to certify than one designed to be "expendable" and therefore
needs to be torn down more frequently because the margins are thinner.

Making rocket engines single use is a false economy. Merlin seems to
bear this out considering how many times it's been test fired
(individually and on Falcons). The more test firings you can do, the
better confidence you have in the engine. As far as I know, no Merlin
1D has failed in flight, despite the crazy high number flown (10 on each
Falcon 9 flight). On top of that every one of those first stage engines
is test fired not only in Texas, but on the pad as well.

Falcon 9 has had a couple of quite spectacular failures, but neither was
due to a Merlin.


Same goes for tanks and plumbing. They pretty much have to be
"certified" for a certain number of fill and drain cycles even if
they're "expendable" in order to account for tanking tests, aborts, and
etc.


And they have to be "certified" for a magnitude (or multiple
magnitudes in the case of something like BFR) more fill and drain
cycles if they're supposed to be reusable.


Agreed. But is it really that much more expensive to certify, say, 100
drain and fills for a "reusable" versus, say, 10 for an "expendable"?
This is especially true if you consider you can certify it for 10, start
flying the vehicle, then continue the (ground) certification process
while you're flying the first generation (test) hardware. Aircraft
programs are sometimes run like this. You don't have to certify
everything for max number of cycles before you start flying (and making
money).

At any rate, my point is that liquid fueled stages are always reusable,
in principle. SpaceX has shown that with a small delta in cost and
effort, entire first stages can be recovered intact (without being
dunked in salt water).

Not trying to reuse liquid fueled first stages in this day and age seems
daft.

Solids on the other hand are single use, excepting for complete tear
down to component parts accompanied by extreme cleaning.


Solids are expensive for what they bring to the table when compared to
liquids, even if you fully expend all hardware. I really don't
understand ULA's love affair with them.


Also agreed.

Jeff
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