Re: Fusion + Antimatter Rocket Engines?

On Feb 21, 10:30 am, DarkHorse <Archaeus...@xxxxxxxxx> wrote:
Awesome, this is the kind of info I was after. I'd figured that beyond
a point it would all become "magic" but I was trying to keep to decent
numbers where possible.

Going by Nyrath's page, antimatter beam-core would max at an exhaust
velocity of 10^7 m/s and a thrust of 10^6 N. Inertial confinement
fusion (highest value on the list for fusion) is the opposite, 10^7 N
thrust and 10^6 exhaust velocity. I realize also that these are quasi-
fictional values, and frankly I hadn't considered the 'fudge factor'
that a mature amat-technology would allow. If it's just an issue of
"turning up the volume" to get that kind of thrust, then awesome. As
long as I can plausibly (not to be mistaken for realistically) get a
35 ton missile up to 300Gs for a decent burn or get a 40 ton
spacecraft to Saturn and back, then I'm OK.

I should have been clearer.
Energy is only the first part of a rocket; it must be converted to
momentum in reaction mass (light provides so little reaction per watt
as to make it almost worthless). So, while a gram of antimatter (and
a gram of matter) provides 42 megatonnes of power (1.798*10^14
joules), the energy will need to be captured and used to make thrust.
Capturing energy can be as simplistic as shooting a little bit of anti-
hydrogen into a vat of hydrogen and letting the hydrogen absorb the
energy, which will heat it up, and then exhausting it through a rocket
nozzle (inefficient) or having it explode inside a rocket-bell shaped
pusher plate (mildly efficient, except the energy is probably light)
or capturing it in a power plant and using the energy to power an ion
engine (which might be as much as 50% efficient with current
technology).

Now, a fusion reaction - say, hydrogen - provides only 6.15*10^11
joules per gram, almost three orders of magnitude less. This energy
is mostly in speed of the helium which is the final result. This
makes it slightly easier to use in a rocket-style system (although,
honestly, enough mass to make a useful rocket will quench the fusion
reaction in any reactor we could design today) - and slightly more
useful in a power plant.

In any case, if you want to have missiles with this sort of power,
you'll have to postulate a couple of technologies:
- a way to easily store antimatter (current method for storing
micrograms is in syncrotrons)
- a way to easily capture energy _or_ channel it - both these power
sources have a lot of energy released as light, which needs to be
turned into momentum in reaction mess
- a way to compress free ions without allowing them to lose energy for
fusion (current fusion designs lose energy so fast to the walls of the
reactor that they have not reached commercial break-even).
.

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