In Archive-Message-Number: 4514,
[email removed] writes:
>First, if I were concerned about the passive sensors of potential
>hostiles, I would not be cruising around at 3G on fusion rockets.
>They must be very easy to detect, especially by passive IR.
Not necessarily. Fusion rockets don't use all that much fuel, so there's
not that much exhaust. And space is big. There's a lot of IR sources out
there too.
>Second, if I were a concerned world government, I might not allow
>fusion takeoffs from or landings on the world surface. Damage to the
>environment, population, etc.
Once again, you have to consider the *scale* involved. A fusion rocket
giving 6 gs of acceleration to a *thousand* ton (mass, not
displacement!) would be "burning" 10 *kilograms* of fuel per second.
(assuming the more reasonable Isp of 600,000 instead of the atrociously
higher one in the rules)(note that a higher Isp *reduces* the fuel used)
I think that even at fusion temperatures we can deal with that small an
exhaust. It may burnm *small* holes in the field, but more likely,
they'll have "blast pits" to handle that.
And since the exhaust is hydrogen & helium, it's not exactly a hazard.
It *will* create some nitrogen oxides as it reacts with an oxy-nitrogen
atmosphere.
The only real hazard is the brightness.
>So, what would be wrong with having two maneuver drive systems in your
>spacecraft? One fusion rocket drive for in-system performance, and
>one grav drive (not thrusters) for planetary use. OK, so this adds
>some extra hardware. But the grav drive just needs to be big enough
>to produce the highest G to get you off a world, no more than 2G. The
>two drives could both be used in-system, with a 50% reduction in
>efficiency in the grav unit beyond 10 diameters. And when you don't
>want your exhaust lighting up the passive sensors for half a parsec,
>cruise on the grav unit (at 50% efficiency).
Well, you won't light up the sensors for *that* far. And even if you
did, you'd have 1.6 *years* before they noticed.
Heck, at a "mere" million km away, you've got over 3 seconds lag *each
way*. At 1 AU (approx 150 million kn), it'd take 500 seconds for the
heat/light/etc from your drive to get to their sensors. And even longer
for any sort of response to get back...
"Well, 8 minutes ago, he was *there* and made a course
adjustment. Now he's running stealthed again. He could be
anywhere in this area, according to the tacitcal computer... "
(computer displays plot showing possible locations of ship
given position of burn and best estimates of original velocity
and the delta-V from the burn)
Game it out with the vector movement system or the Mayday rules. If you
can't be sure how fast he was moving, nor the magnitude *or direction*
of his velocity change, you can only maker an educated guess. And the
area keeps growing...
Oh yeah, at 1 g that thousand tonner can travel 1 AU in a bit over 48
hours. Doing so, it'll use 288+ tons of fuel. And be travelling at 1732
km/s (about .5% of light speed!)
Don't ask me how he's gonna slow down!