Improved Isp Rocketry II

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Mike Miller - 12 Dec 2003 14:52 GMT

Two questions this time:

1) Is it possible to get kerosene-LOX or propane-LOX to 395Isp? I've
seen numbers around 360 on www.astronautix.com .

2) How effectively do large, altitude compensating nozzles
(particularly aerospikes like on Bono's Rombus) maintain specific
impulse between sea level launches and orbit? For example, would it be
reasonable to maintain a LOX-propane specific impulse of 350 between
the ground and orbit?

Mike Miller, Materials Engineer

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Earl Colby Pottinger - 13 Dec 2003 01:27 GMT

cray74@hotmail.com (Mike Miller) :

> Two questions this time:
>
[quoted text clipped - 8 lines]
>
> Mike Miller, Materials Engineer

What happened to AlH3?

Earl Colby Pottinger

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Henry Spencer - 13 Dec 2003 21:29 GMT

>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.

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Mike Miller - 15 Dec 2003 12:44 GMT

> >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

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Space Forum / Space Flight / December 2003

Improved Isp Rocketry II