Fusion rockets...

In Archive-Message-Number: 4516, Steve Higginbotham writes:

>A million ton battleship (moderately light by MT standards) pulling
>3g's requires 5000Kg/s of fuel, which produces an energy of 9E16
>joules.  At a range of 2000-odd Km, that appears about as bright as
>the Sun viewed from Earth-orbit.  At 1AU, it is about 16 times as
>bright as AlphaCent viewed from Earth-orbit.  At combat ranges
>(500,000Km or less), it is over 1000,000 times as bright as AlphaCent
>as viewed from Earth (that's brighter than the moon!).  This will NOT
>fade into the background!

Now *subtract* the portion of that that is kinetic energy of the ship
and of the exhaust. Only the remaining energy will be available for
radiating.

>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)

10Kg/s at Isp 600,000 produces energy equivalent to a 50 KT bomb every
second.   This will NOT produce "*small* holes".  Unless you define
*small* as comparable to Hiroshima or Nagasaki.  Keep in mind that it
takes you 6g rocket 6 seconds to reach 1Km+ altitude.  So you'll dump
the equivalent of a 300 KT bomb on the launch site.  This will not be
dealt with be "blast pits".

Once again, you are assuming that the energy is "just there". Look upon
it as a plamsa arc torch with a 10 Kg/s flow rate. It'll vaporize the
hell out of anything that gets in the way, but it is *directed* energy
(at least at first).

Compare this with a Saturn 5. The first stage of the Saturn V was good
for quite a few kilotons if it blew. But in a normal launch, the the
exhaust damage wasn't that hard to contain. A fusion rocket with the
same thrust (6 million pounds) would use 10 *pounds* of fuel per
second. The energy release is of the same order of magnituide., it's
just more concentrated. (yes, I picked that thousand ton ship in the
original message *specifcly* because it was "equivalent" to a Saturn 5!)

Stop and think, a 10 kg/sec jet is going to be fairly small. *Easily*
less that 10 meters. More like *one* meter. Especially considering the
exhaust velocity (roughly 2% off lightspeed!). (anybody know enough
plasma physics to rough out an answer? If not, I'll just treat it as
hydrogen gas, and work it out that way...)

It may bore a hole 100 meters deep in the rock of a class X starport,
but the hole will be fairly narrow.