johnhare wrote:

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.

The point is, John, that you are unfairly comparing an engine that is
designed almost exclusively for vacuum (the RL-10) to one that has to
operate at sea level also (the ALS/NLS engine). If an engine has to
operate at sea level, chamber pressures have to be high to avoid
gross overexpansion and flow separation with the expansion ratios
necessary to acheive high Isp in vacuum. If the engine is going to
operate only in vacuum overexpansion is not a concern so low chamber
pressures are not inconsistent with high Isp.

In other words, when the chamber pressure of the ALS/NLS engine was
lowered, the expansion ratio had to be lowered as well to avoid gross
overexpansion at sea level. The reduction in expansion ratio hurt
vacuum Isp.

Jim Davis