On Dec 18, 12:03 am, Fred J. McCall wrote:
Len wrote:

:
:Another factor was the SSME, which used a lot of
:material rejected for the RL10 because of hydrogen
:embrittlement--not to mention a spindly shaft going
:through several stages of instability getting up to speed,
lus pressure gradients running from hot to cold, rather
:than vice versa. This was not a good starting design
:for a reusable engine.
:
:I remember being chastised by some NASA folks
:for calling the RL10 reusable, based upon hard test
:data. No. The RL10 by definition was expendable;
:the SSME by definition was reusable.
:
:Just invoking the word reusability does not ensure
:low-costs.
:

Exactly. Running hardware at 100% of design (or beyond) as routine
operation almost guarantees that a reusable vehicle will be quite
expensive (because of teardown, overhaul, and inspection costs).

We plan to derate the first-stage engines
to 80 percent for an order-of-magnitude
improvement in engine maintenance costs.
Even with de-rating, we can still fly on one
of the two rocket engines at critical takeoff
speed. The two DK-30 landing engines in
the carrier are sized for landing; thrust from
this source is incidental at takeoff.

At the moment, I am assuming about
90 percent derating for the orbiter LOx/kero
engine. One version of the orbiter has
additional RL10A-4 engines to allow limiting
acceleration to 3 gs and to pick up signifcantly
more payload; these look quite
reusable at 100 percent.

Len

--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw