Space Forum / Shuttle / December 2003

Fuel handling, for one.  There's a whole sequence of tradeoffs, and
I'll defer to those more knowledgeable on which is "better" for a
particular application, but one advantage of kerosene is that it's
a fairly dense noncorrosive liquid at standard temperature and
pressure.  The fact that it's liquid at STP means it doesn't need
pressurized or insulated tanks.  Its density means the tankage can
be smaller than the tankage required for a similar weight of
liquid hydrogen.  Since a most of a launch vehicle is
propellant/oxidizer tankage and propellant itself, it makes
sense to pay attention to the structural mass required in the
tanks.  For some applications, the easier ground handling of
kerosene versus LH2 is very important.

However, liquid hydrogen has a significantly higher specific
impulse, meaning less fuel mass is required.

See
http://users.commkey.net/Braeunig/space/propel.htm
http://www.hq.nasa.gov/office/pao/History/conghand/propelnt.htm

--Rich

I assume that you're referring to Rocketdyne's work on the RS-84
LOX/kerosene engine.  That engine uses staged combustion similar to the SSME
and to the Russian RD-180 which powers Lockheed Martin's Atlas V.  IMHO,
Rocketdyne is playing catchup with the Russians and is using the RS-84 to
become competitive in  staged combustion LOX/kerosene engine technology in
case that's the way the LV market swings.

Rocktdyne touts the RS-84 as a "reusable" engine. But with 2800 psi chamber
pressure (in the same ballpark with the SSME 3000 psi chamber pressure), I
don't know how reusable that engine will be. It took Rocketdyne nearly 14
years (1971-85) to qualify the SSME for 20 reuses. (The original SSME spec
called for 7.5 hours of operation, equivalent to about 55 shuttle flights,
before major maintenance). Of course, NASA wouldn't think of flying a SSME
20 times in succession without major maintenance at frequent intervals.
Prior to the Challenger disaster of Jan 1986, the SSME turbopumps were being
removed after each flight, disassembled, inspected and refurbished before
returning to the flight-ready inventory. From 1988-2000 NASA spent nearly
$2B (current dollars) developing the Pratt & Whitney turbopumps to replace
the original Rocketdyne pumps used on the SSME. I recall seeing a blurb
recently that now NASA removes the P&W turbopumps after 5 or 6 flights for
major maintence, repair and overhaul (MRO). But this is still a long way
from 55 reuses before major maintenance, which is how the SSME was
originally hyped when the shuttle program was being sold to Congress.

The big advantage of kerosene is that it's high density (similar to that of
LOX) makes it a lot easier to pump than low density liquid hydrogen (LH2).
The SSME LH2 high pressure hydrogen turbopump spins at nearly 30,000 rpm in
order to supply LH2 at the specified flow rate. By comparison, the LOX and
kerosene turbopumps of the F-1 engine that powered the first stage of von
Braun's Saturn V moon rocket spun at only 5500 rpm.

In fact, the F-1 engine was quite reusable, even though it's considered a
single-use expendable engine. On the test stand one of the qualification
engines was restarted 20 times and operated for a total of 2250 seconds
without major maintenance. The operating time was equivalent to 14 Saturn V
launches. This was accomplished in the mid-1960s. One of the reasons that
the F-1 had this level of reusability was it's conservative design. It
generated 1.5 million pounds of sealevel thrust with only 1100 psi chamber
pressure and used a simple once-through gas-generator cycle. A later upgrade
of the F-1, called the F-1A, generated 1.8 million pounds of sealevel thrust
in the late 1960s, which is still the world record for a single nozzle
liquid-fuel rocket engine.

Later
Ray Schmitt

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