"Jim Davis" wrote in message
. 1.4...
johnhare wrote:

Too close to perfection. The ALS (Advanced Launch System)
program
that cost $1B and ended in 1989, determined that dropping the
chamber pressure from 3000 to 2250psi, and the horspower of
the turbo pumps from 100hp per pound to 37, would result in a
specific impulse drop of 460 seconds to 420.

This doesn't sound right. The RL10 gets better vacuum Isp than
that with far less pressure.

But the RL-10 rarely has to work at sea level so it can have the high
expansion ratios to give high vacuum Isp at low chamber pressures.
The RL-10s for the DC-X which did operate at sea level had severely
truncated nozzles which would have hurt vacuum Isp badly.

The 460 seconds clearly was a vacuum Isp he was refering to. If it had
been 360 vs 320 seconds, then it would have been sea level performance.
Starting at sea level, you do need the high pressures to get performance,
which is your point. Out of context, I'm not sure what performance hit
was being refered to if not just the vacuum Isp.

Jim Davis