Fusion drives, fuel consumption, etc.

A thread of 2 messages, 28 January 1990 to 30 January 1990, in the order they were sent.

#864 Fusion drives, fuel consumption, etc.

Mark Gellis · 28 January 1990

For those making the ultimate mistake of playing around with a
ship design system (all right, all right, I'm glad I did because I
ended up with one I like, but it took a LONG time), and wondering
what kind of Isp, etc. you get from fusion drives, my method may be
helpful.

I cheated.

According to ON CIVILIZED STARS, and a few other sources, a Daedelus-type
fusion pulse starship can reach between 12% and 15% c as a terminal
velocity.  I understand the Isp for this vessel was about 1,000,000.  I
have been told by people who know physics a lot better than I do that
the D-He3 pulse system the Daedelus people talk about is inefficient,
that fusion drives using a containment system to raise the temperature
of the plasma to enormous levels and then direct the superheated plasma
as high energy reaction mass could give much higher Isps.

Suffice that, all things being said and done, I assume a standard fusion
drive and 30% of your ship's mass dedicated to fuel will give you a peak
velocity close to 2% c.  If you were loony enough to build a multi-stage
vessel with the Daedelus' 50-to-1 fuel-payload ratio, you could get a
terminal velocity (no way down) of about 30%-35% c.  Of course, if you
want to build a stl starship, the real choice is matter-antimatter, which
I have arbitrarily placed as being 10 times better than fusion--a single
stage vessel with 30% fuel can go to 20% c and decelerate down again,
multi-stage vessels can reach true relativistic speeds.

In other words, I assume that fusion drives with several thousand years
of technological improvements, scientific breakthroughs, and engineering
refinements behind them can do 2 to 3 times better than the early
prototypes being designed by people who were still (at the time) 50
years away from producing controlled fusion power.  Or am I being
too optimistic?

   Mark

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#882 Re: Fusion drives, fuel consumption, etc.

Leonard Erickson · 30 January 1990 · reply to #864

Slight problem. To a first approximation, Isp=Ve/g
(that is, the specific impulse equals the exhuast velocity divided by
the acceleration due to gravity at the Earth's surface)

This means that the *maximum* Isp is a bit over 3e7 seconds. And it
would be achieved by having an exhaust velocity of C!

Also your final velocity is determined by your exhaust velocity and
mass-ratio. I think it is Vf= ln(Mr)*Ve.

Ok, with an Isp of 1e6, Ve = 1e7. so for 2% of c as a final velocity
you need a mass-ratio of 1.8....

With Isp=6e5, you need Mr=2.7. not too shabby...

All opinions and material above is the responsibility of the originator.
Submissions: [email removed], or [email removed]
Administrator: [email removed] (James Perkins)
The TML is made possible by facilities provided by Tektronix, Inc.