(Henry Spencer) wrote in message ...
In article ,
Mike Miller wrote:
1) Is it possible to get kerosene-LOX or propane-LOX to 395Isp?

I have my doubts, but I wouldn't call it absolutely impossible without
doing some numbers. It would certainly be difficult.

Hmm. And 514-520 Isp is not likely for hydrogen-oxygen rockets. GURPS
is usually very good about researching the basic numbers it cites for
vehicles and near-future systems. Maybe the writer(s) just got ahold
of a basic rocketry book and found a list of "absolutely ideal Isps by
fuel."

Getting that sort of Isp at high altitude is not a big problem, as witness
some of the Russian LOX/kerosene upper-stage engines. 350s at sea level,
however, would be quite a challenge for any LOX/hydrocarbon mix. I think
you'd need a very high chamber pressure indeed to get that high an
expansion ratio without overexpansion and flow separation.

And using specific impulses averaged over altitude or flight time is
less than accurate. Is there a relatively easy means of estimating
engine performance for SSTOs, like "treat the engine as being at sea
level up to 20 miles, then in vacuum for the rest of the flight", or
should I just dig up some back pressure/Isp calculations?

Mike Miller, Materials Eningeer