Gentlefolk,
At the AF rocket lab at Edwards AFB, almost 20 years ago, we were building a
rhenium tube heat exchanger for a solar thermal rocket, expecting to get 9 to
10 km/s exhaust velocity (=isp) with hydrogen. If memory serves, results
approximating that were achieved in testing a few years ago. Also, we were
looking at seeded plasma direct absorption schemes that would go
significantly higher; approaching gas core nuclear performance. Dr. William
Larson was doing that work, mainly aimed at laser-thermal propulsion schemes.
Anyway, one doesn't want to go much higher than 10-15 km/s for the Earth
to Orbit mission. While the optimum isp point varies with the choice of
parameters and technology, generally, what happens is that engine mass per
unit power tends to be constant, while the power needed for a given thrust
goes up as the square of isp. Eventually engine mass gains wipe out isp
related fuel & tank mass savings and for a given lift-off weight, the payload
mass curve drops off as isp increases beyond some maximum value.
--Best, Gerald
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