On Tue, Oct 07, 2014 at 11:28:24AM -0700, Craig Berry wrote:
> Yes, gravitics change a lot. But you still need streamlining to
> operate in an atmosphere -- both per the rules, and per reasonable
> extrapolation. A streamlined shape will move through the air more
> easily, with less turbulence.

That matters very much less at high Mach numbers than at low,
surprisingly.  The main reason why our supersonic jets are highly
streamlined is that our engines are very weak and extremely
fuel-hungry by Traveller standards, so every little bit of drag
reduction helps.

Consider a 10-dton fighter with 6 gee drives, a mass of 150 tonnes,
and shaped essentially like a brick.  It doesn't need aerodynamic lift
or control surfaces, since it has its grav and thrust plates.

At Earth's sea level, it has enough thrust force to get beyond mach 2.
At 10 km altitude, it can approach mach 5, and at 40 km it can exceed
orbital speeds.  The peak temperatures depend upon Mach number and are
essentially the same for any shape.  It's also likely to be
surprisingly stable, since the aerodynamic center for that shape turns
out to be very close to its center of mass when the long axis is not
too far from the airflow direction.

The main problem would be the ear-buggering shockwave it makes as it
rams the air in front of it to much higher density -- but that's going
to be true for any craft of comparable size and speed.

I'm not saying that a brick is the optimal shape for an atmospheric
fighter, but I am saying that it matters a great deal less than our
pathetic technology permits.  I suspect that retaining a relatively
compact shape would be of great benefit, both in reducing the
cross-section that can be hit from arbitrary angles for given volume,
and in permitting thicker armour while hardly affecting performance.

Proecting surface components from high- or low-temperature airflow is
an issue that vacuum-only fighters don't have to address, but they
would probably want to armour them against point-defense lasers,
sandcasters, and occasional plasma and microfragments in the space
battle environment anyway.

So with only a few very small modifications, a vacuum fighter would
easily be able to conduct multi-Mach operations in atmosphere.  A
fighter of the specifications above could be launched from low orbit
and reach any point on the surface of an Earthlike world within twenty
minutes.  It could patrol a smaller region from the low stratosphere
at mach 15, reaching anywhere within 300 km on two minutes notice.

Making it heavily streamlined won't help either of those numbers by
more than a few percent, and will hurt other operational specs by a
lot more.

- Tim