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.

2) How effectively do large, altitude compensating nozzles
(particularly aerospikes like on Bono's Rombus) maintain specific
impulse between sea level launches and orbit?

Careful here. Altitude compensation, in principle, maintains optimal
expansion regardless of outside pressure. That doesn't mean retaining the
same Isp! In general, the higher the expansion ratio the higher the Isp,
and if chamber pressure stays constant, an altitude-compensating nozzle
has to back off on the expansion ratio when outside pressure is high.

For example, would it be
reasonable to maintain a LOX-propane specific impulse of 350 between
the ground and orbit?

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.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |