On Thu, Aug 17, 2017 at 09:31:38PM -0400, Jonathan Clark wrote:
> 1) Consider a helicopter hovering above a fixed point 1000 feet
> above the ground.  This is not in motion relative to the ground
> underneath it, nor to the local centre of gravity (considered as a
> point source, and the rotation is not, I think, relevant), and yet
> it is experiencing a force due to gravity. If it were to turn its
> engine off, it would swiftly start converting some of its potential
> energy into kinetic, with sub-optimal results (at least for the
> crew).

The power a helicopter must expend to stay at a constant height is not
due to some fundamental requirement inherent to keeping objects at a
constant height.  It is a side-effect of the specific means by which
it maintains that height: it continuously accelerates a stream of air
from near rest to rather high speeds.  Accelerating the air requires
power.

Likewise for rockets and jets (which also accelerate streams of air or
other gases).

Does Traveller contragrav employ a means that requires constant power
input?  We don't know.  All we know is that the vehicle design
sequence requires that contragrav lift units on vehicles be designed
to consume power.  This would be perfectly compatible with a device
that does not consume power for a stationary hover, but does consume
significant amounts of power when the vehicle is rising or
accelerating.  Vehicles are, after all, designed to move.  Moving
upward in gravity, or increasing speed does inherently require power,
while maintaining height does not inherently require power.

> All this is getting at the question of "if a helicopter needs to
> exert a downwards force in order to counteract gravity, then
> wouldn't a hypothetical spaceship using a CG drive need to exert a
> similar force?"

Force, yes.  Power, no.  The basic equation is

  power = force dot velocity.

If the velocity is zero (a hovering object), then the necessary power
required is zero.  If the velocity is perpendicular to the force (e.g.
an object in steady horizontal motion or circular orbit), then the
necessary power required is zero.  If the velocity is the the same
direction as the force (e.g. a vehicle rising, or accelerating) then
the power required is just the product of the two.

Other factors about the situation may cause power to be required, as
for a helicopter continuously accelerating a lot of air, or a lossy
magnetic levitation system that dissipates power in non-
superconducting wires.  We do not know if there are any such factors
in Traveller contragrav.

TL;DR -- There is no fundamental physical reason to consume power
while hovering.  There may be engineering reasons.  GM's call.

- Tim