On 24 Feb 2018 at 13:16, Jeff Zeitlin wrote:

> >Given fusion reactors (hot fusion, not cold fusion) you can create
> >quite a rocket. If "raw" fusion rocket is a bit much for you (ie the
> >exhaust is basicly plasma from the reactor run thru an MHD "nozzle")
> >you can just use the reactor to heat liquid hydrogen to very high
> >temperatures and run it through a more normal nozzle.
>
> >The higher the temp of the exhaust, the better the Isp. That is, the
> >higher the exhaust velocity the less fuel you need for the same
> >acceleration.
>
> So, Drake more-or-less got it right with his plasma thrusters (using
> any room-temperature liquid as 'working fluid') for getting through
> the atmosphere, and powering the ship through a 'fusion bottle'.

Well, hydrogen is best because you want the lowest possible molecular
weight in the exhaust (basically, the lighter the particles the
higher the exhaust velocity for a given energy input, and thus the
higher the thrust)

Water is ok, but given the molecular weight is 18 vs 2, you lose
efficiency.

> (That, of course, leads to the question of whether his "high drive",
> pretty much the same thing but done with a matter/antimatter reaction,
> is plausible for an exoatmospheric reaction drive. And ignores the
> question of where the antimatter comes from.)

As Dr. Robert Forward pointed out in several places, an antimatter
rocket has an "interesting" property. Depending on the mix of
matter/antimatter you get different exhaust velocities. Which means
that a mass ratio of 4 works for the whole range of thrusts!

> (Your other ideas, while interesting, are for different settings
> entirely - but thanks for the ideas; good ones are hard to find...)

As I've said before, any fools that think a planet with space travel
is a pushover because they don't have CG and M-drives can often be in
for a *rude* shock.

Especially if they use laser launch systems. Those things can pretty
much interdict anything in line of sight out to geosynch orbit. So
given enough launch sites well distributed around the planet, you are
in for a world of hurt if you don't play nice with the locals.

And, of course, the aliens in Footfall learned a nasty lesson as
well.

Picture some planet that used Orion drives to put up stations and
launch colonies, but used something more normal for regular shipping.

Now picture them being, or having been recently, balkanized. Along
those Orion drive space battleships sitting around.

Launch a few of those and any would-be space vikings are going to
need a change of underwear.

Other scary as hell ideas are the 12 km long asteroid that has a
fusion reactor taking up much of the inside (at that size you can
actually get away with "simple" magnetic mirror confinement) Fire
that up as a rocket and watch people worry.

Sort of like Troy in Ringo's Troy Rising series. Have fun trying to
take out something that has a km or so of nickel iron as a hull.

ps. I forget to include various mass driver launchers in my list,
btw.

--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com