Space Forum / Shuttle / September 2004

Take a look at real engineering analysis rather than a simple comparison
in a table - that sea-level ISP is hugely important given the mass you
need to lift off the ground *at sea level*.

With regard to the use of the F1 and its derivatives versus SRBs, well,
that's another whole kettle of fish.  Remember, the idea was that the
SRBs would be reusable - NASA (and the bidding contractor teams) had no
real grasp of the complexities of booster recovery and refurbishment. As
these became apparent, the image of reusability became more important to
NASA administrators than the reality.

The important engine parameter when designing a launch vehicle is
trajectory-averaged specific impulse (TASI). For the present shuttle with
solid rocket boosters and LOX/LH2 sustaining propulsion, the TASI has to be
in the 390-405 second range. The SRBs have pretty low specific impulse (Isp)
ranging from about 242 seconds at sea level to about 269 seconds at burnout
altitude (~ 26 n.mi reached 120 seconds after launch when the shuttle speed
is about 3,000 mph). SRBs make lotsa thrust, but are pretty inefficient in
terms of Isp.

To satisfy the TASI requirement, the shuttle's three SSMEs have to be
extremely efficient to offset the relatively inefficient SRBs. It turns out
that the Isp for the SSME has to be about 365 seconds at sea level and about
453 seconds at high altitude (vacuum conditions).

The kicker here is that the three SSMEs have to be reusable and have to fit
in the limited space available in the aft fuselage section of the orbiter.
There are two ways to raise specific impulse for a given propellant:
increase the chamber pressure and/or increase the expansion ratio of the
nozzle. Engines like the RL-10 and the J-2S that operate at relatively low
chamber pressure (500-1200 psia) can reach 450 sec Isp in vacuum by using
nozzle expansion ratios above 100:1. However, because of the design of the
orbiter, the SSME nozzle expansion ratio is limited to about 77:1, since the
nozzles have to have room to swivel in order to guide the orbiter on its
flight trajectory. The SSME nozzles also have to be sufficiently small to
"hide" behind the orbiter fuselage and the bottom flap so reentry heating
doesn't melt the nozzle material, which is uncooled during reentry.

So back in 1971 when the SSME development started, NASA and Rocketdyne had
to bite the bullet and design the SSME to operate at very high chamber
pressure (~3,000 psia). This meant that the discharge pressure of the high
pressure turbopumps would be in the 6,000-7,500 psia range. For the Saturn V
the F-1 engine operated at 1,100 psia chamber pressure while the J-2
operated at 763 psia.  It's a tribute to Rocketdyne that the SSME eventually
was flight qualified (in 1970 about 9 years after development began). But
these ultra-high pressures have taken a toll on engine parts, especially the
turbopumps, and have been a signficant factor in the high shuttle operating
cost. NASA spends about $200M per year (today's bucks) to keep the SSME
inventory ready for flight, but on average only makes about 6 flights per
year involving 18 SSMEs. So the annual operating cost is about $10M per SSME
actually flown.

Once the orbiter configuration/dimensions had been established at the end of
Phase B in Dec 1971, there was no way that the J-2 or J-2S engines could
satisfy the design requirements. Those engines had too low thrust, too low
Isp and too high nozzle expansion ratio. Besides, NASA realized during the
shuttle Phase A and B work (1969-71) that to retain the expertise at
Rocketdyne, which during the Apollo years had become NASA's preferred engine
contractor, a new, advanced engine like the SSME had to be developed in
order to provide the necessary challenge and incentive to keep it's core of
expertise intact. In fact, Pratt & Whitney competed for the SSME and
bitterly contested the award of the SSME production contract to Rocketdyne
in July 1971. However, P&W's formal protest was disallowed by both NASA and
the Government Accounting Office (GAO, now known as the "Government
Accountability Office"). The rest, as they say, is history.

That's enough for now.
Later
Ray Schmitt