Fusion Propulsion Question

--- victoriatangoman wrote:
> Been searching the web looking for an answer to whether a far-
> future D-H3 or H3-H3 fusion system could be used for an earth
> launch. Most references I've come across limit discussion to a space
> only vehicle.

Fusion would require a large scale to actually work so the ship would have to
much larger then a chemical rocket although it might be able to be done.

By H3 do you mean Tritium (Hydrogen 3) or Helium 3. If you meant Helium 3 you
should have stated it as He3.

> Specifically, I'm curious about the exhaust temperature of the
> plasma. Would it be in the vicinity of the chemical propellants or
> would it be so high as to melt the concrete, and nearby supporting
> structures, basically destroying the launch pad and nearby
> environment. Could the temperature be mitigated with the inclusion
> of hydrogen, helium, oxygen in the gas exhaust to give extra thrust
> and thus lower the temp. I know most of this is pure speculation at
> this point, but I can't even find any intelligent guesswork.

You could add other substances into the exhaust but you would lower efficency
quite a bit. That could easily cause it to be not that much better then
chemical or possibly even worse considering that it may require a very heavy
engine.

> If fusion is destined to be a space-only means of propulsion,
> because of destructive (non-radiation) effects on the immediate
> launch environment, then what propulsive advances are in store for
> earth launched vehicles. Ion has too low of thrust, nuclear fission
> will never fly (couldn't help myself). Other than rail lauched or
> skyhook/sub-orbital shuttle, what else other than trying to eek out
> more performace from chemical launch (metallic hydrogen/metallic
> oxygen?)

Ion won't be used to get away from earth. Fission could be although the
reasons it probably won't be are political and not scientific. In terms of
getting into orbit it looks like we're stuck with chemical.

> Now, taking the speculation to more technical questions. Let's
> presume for this question that in the far future, one of the US
> military's Black programs has managed to solve the H3-H3 fusion
> process and has the H3 fuel,(Isn't this fun?) and are testing an
> earth launch system, but don't wish to tip their hand. What would be
> the tell-tale signs that an intelligent observer would see to
> distinguish such a breakthrough. I'm assuming spectroscopy of the
> exhaust would show a strong helium presence. There wouldn't be any
> radioactivity. They may even disguise the vehicle to make it look
> like a chemical launch vehicle with superfluous tanks, etc. But we'd
> be seeing helium rather than hydrogen/oxygen in the exhaust gas. Is
> that the dead give away? Or the temperature of the exhaust? The
> structural modifications/design of the launch vehicle?

I don't think I've heard people say that H3-H3 would be anygood, only D-H3 and
He3-He3 which may be what you're refering to.

If they're using D-T then they will give off a lot of radiation (probably more
so then a fission based system), D-He3 would only give off 2% of the radiation
of D-T but would be harder to ignite although the lower radiation may offset
that. He3-He3 would be even harder to ignite but the almost aneutronic nature
of the reaction would mean that if you can ignite it, it would probably be
better then the rest. Of course even He3-He3 would still give off some
neutrons from side reactions coming from contaminantes.

Finding a large amount of He instead of H2O in the exhaust may make people
think it's actually using He* although the He in the exhaust from a fusion
rocket will actually be in the form of alpha particles so it might be possible
to figure out the difference. There would also be large amounts of protons in
the exhaust (if using D-He3 or He3-He3) which whilst they can be directed
magnetically to come out the back could give it away if the rocket points its
exhaust at a radiation detection instrument which is likely to notice it.