[AR] Re: Reaction Engines' Sabre Rocket Engine Demo Core Passes
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sun, 17 Mar 2019 02:10:00 -0400 (EDT)
On Sat, 16 Mar 2019, Rand Simberg wrote:
And as with any SSTO, Skylon would suck for off-nominal (higher
inclination/altitude) missions.
I think that's exaggerated. There's a real issue, but...
Yes, most any launcher is going to have to take a hit on TIM (total
injected mass) if a higher delta-V is called for, and because the other
components of TIM don't scale with payload, or don't scale much with
payload, the payload gets to take almost all that hit. E.g., if only 1/3
of the TIM is payload, then a 10% loss in TIM means something close to a
30% loss in payload.
This applies to TSTOs, expendables, etc. too. For example, an expendable
nuclear final stage will have a bad case of this, since it's got a huge
insulated LH2 tank, a very heavy engine, and complicated subsystems. But
whether that means it sucks for off-nominal missions depends on how well
it does in other ways. A LOX/LH2 upper stage has a more moderate case of
the same problems. The expendable final stage that suffers least from it
is probably a very aggressively-designed spin-stabilized solid, or maybe a
(non-LH2) biprop stage with balloon tanks and a very-low-Pc engine, but
those are nevertheless not everybody's choices, and for good reasons.
Reusables (SSTO or not) inherently suffer because they need to haul enough
extra dry mass along to bring the orbiter back down again intact.
However, comparing apples to apples here reduces the penalty. The biggest
advantage of a real reusable -- one whose life is long enough that you
don't hesitate to insert an extra flight or two when it looks useful -- is
testability: test-fly it before it carries a customer payload, test-fly
it after a major overhaul, do a systematic development program that flies
things like abort cases, etc. That is, treat it like a high-performance
aircraft instead of an artillery rocket, in the interests of achieving at
least jet-fighter-class reliability. (Airliner-class reliability is
farther down the road...) If you're going to compare the result to an
expendable, you should require that the expendable *also* achieve that
sort of reliability... somehow. That's going to mean, at the very least,
several engines and bigger design margins and more redundancy (and a long
costly test program), to achieve some degree of robustness and fault
tolerance. Figure that in, and surprise surprise, the expendable's
payload-fraction advantage isn't going to be nearly so big any more.
Similarly, the real point of an SSTO is simplicity and ops advantages.
Before saying that it sucks compared to a TSTO, the TSTO ought to meet the
same requirements. That won't be entirely easy. Note also that building
your rocket to break in half, reliably and repeatably, will itself incur
dry-mass penalties. (E.g., a structural joint that you can break on
command tends to involve concentrating loads, which adds structural mass,
as do non-destructive separation mechanisms.) The SSTO's penalty isn't
nearly as large as an oversimplified analysis would suggest. So, does it
suck for such missions? Depends on details; answer's not automatically
"yes".
In particular, it probably doesn't suck if it scores enough better on
other attributes that you'd rather fly it twice, once with the payload and
once with an electric-propulsion tug (or fuel for a reusable tug that's
already up there) to take the payload to its final destination, than fly a
more complicated system once. I could see that happening, although not
next week.
"Payload uber alles" is another relic of the artillery-rocket mentality.
If the goal is to do things right for a change, continuing to develop
systems that sacrifice everything else to squeeze in a bit more payload is
the wrong way to go about it. If the payload for a case you care about is
too small, then make the vehicle bigger, fly it more than once per
payload, or build and exploit support systems rather than assuming that
every launch must be a trek into virgin wilderness.
Henry
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