> On Fri, 27 Nov 1998, Sean Bayan Schoonmaker wrote:
> executive summary: fusion fuel is not an issue. reaction mass is
[snip]
> the question is, how are you going to turn this energy into thrust? if
Right. So if you're using a 'reactionless' drive, you need very little fuel
with either fission or fusion reactors to power the drive.
> if you are going to use a reaction drive (ie a
ummm, from what I've read, a fusion rocket produces thrust in a two part
process: first the frozen fuel (H or an He-Deuterium mix depending on
the type of reaction you're using) pellet drops into a magnetic confinement
chamber. Second, your fusion reactor powers up a laser or particle beam which
zaps the fuel pellet and produces a fusion explosion that the confinement
chamber channels out the back end of the ship as thrust. This kind of fusion
pulse drive was used in Pournelle and
Niven's _Footfall_ by the aliens. Again, depending on the reaction you
use, you can achieve ISPs from 5 to 40 times more efficient than the best
theoretical designs for ion drives. Fuel use is moderate, and
gives credence to the gas giant/ice moon/comet refueling stations. If
you want to use ion drives, all your refueling stations have to be on planets
or asteroids (or maybe out of the deep atmosphere of a gas
giant) where you can pick up the heavier ionizates- Argon or whatever.
Sources: Advanced Propulsion Study, Robert Forward (incidentally inventor of
the Forward Mass Detector), Hughes Research Labs 1987.
The Future of Flight, L. Myrabo and D. Ing, 1985.
and probably some others.
> Probably the easiest way to create reaction "mass" is to use a
What the heck is a 'photon' drive? How does this produce thrust?
Curious
> At 02:55 PM 11/27/98 -0500, you wrote:
[snip]
> What the heck is a 'photon' drive? How does this produce thrust?
A photon drive basically uses a directed beam of light (or invisible
electromagnetic radiation). The momentum of the photons (h*f/c where
h=planck's constant, f=frequency of the light and c=speed of light) is
transferred to the light source. Normally, the light is emitted in all
directions, producing a net result of zero, hence the need for reflectors to
direct the beam. Another example of a space drive that uses the same principle
is the solar sail, but that relies on the solar wind instead of a light source
integrated into the spaceship.
This limits its application to non-military uses where arbitrary
maneouvering is the exception rather than the rule.
An interesting consequence of this principle is that you can calculate
(although not measure) the amount that your car slows down when you turn on
your headlights:)
> Curious
Hopefully this makes sense!
> On Fri, 27 Nov 1998, Sid Jones wrote:
if
> >you have grav drives or photon drives, then it is complicated but
if
> >you are using a super-high-velocity ion drive, then your reaction
oooh, *that* kind of fusion drive. right. there are two types of fusion drive,
which for want of a better term i will call internal combustion and external
combustion.
the type you describe is external combustion: like a chemical rocket, the
products of the heating process are expelled to generate thrust. there are two
subtypes of this: the Orion type, where nuclear bombs are exploded
behind a big shock-absorbing plate (as used by the humans in Footfall),
and the Daedalus type, where fuel pellets are compressed by lasers to start
fusion (inertiall confinement fusion, i think this is called). the internal
combustion drive generates power which is then used to run a motor; this is
somewhate like the nuclear fission rockets that have been proposed, as in the
NERVA study in the 60s. there are again two ways of doing this: you can run a
reaction fluid in contact with your reactor to superheat it, producing a jet,
or you can collect the energy as
electricity, and use that to power an ion drive or a thermal-expansion
rocket using an electric heater (no, not like in a kettle; a maser or
something.
> This kind of fusion pulse drive was used in Pournelle and
all i remeber is that they had a ramscoop at the start and then got rid of it.
i suppose they could have been collecting hydrogen with the scoop, making
pellets and feeding those into the drive.
> Sources: Advanced Propulsion Study, Robert Forward (incidentally
well, i have a PDF (sorry!) on gas-core nuclear rockets, which are
roughly
comparable to contact-heating internal-combustion fusion rockets, on me
website:
users.ox.ac.uk/~univ0938/gzg/rockets/gcnr.pdf
i'm not sure who it's by; i can't read pdf on this machine...
> What the heck is a 'photon' drive? How does this produce thrust?
elementary, my dear Schrodinger! photons have momentum, due to... er
...
quantum things. if you shoot photons in one direction, you get a force on your
photon emitter. at the power outputs in everyday life, the force is quite
small, but if you have a really, really big emitter, the momentum change of
all those photons shooting off in one direction imparts a momentum change to
the shooter. a cousin of the solar sail.
hope (a) this is right (b) this helps.
Tom
> Tony Christney wrote:
Photon drives represent a theoretical ultimate in rockets. After all, how can
you beat a rocket where the exhaust velocity is the speed of light (short of
some kind of tachyon exhaust).