Just for the record, IMTU there are actually three separate grav/thrust systems on a ship:

1. CG, which "shields" or "cancels" some fraction between 0 and 1 of the gravitational attraction felt by the ship. A ship with its CG at 100% is effectively weightless, and might e.g. drift upward off the ground if a strong wind pushed it that way. But you can't control ship movement with CG alone, other than modulating your acceleration straight downward.

2. Thrusters, which push the ship around. These are reactionless, operating on pure handwavium magic. You push a lever, and the ship experiences a force of x Newtons in a particular direction.

3. Acceleration compensation/artificial gravity. Related to thrusters, but used inside the ship to cancel out net accelerations being experienced by the ship as a whole, and add back in an x G component perpendicular to the decks. Again, pure handwavium on how this works.

So e.g. to hover in a dirtside docking bay, you'd crank the CG to 1.0, and set the thrusters to cancel out all ship accelerations relative to the local ground (you do want to follow that patch of ground as it rotates around the axis of the planet. :) ). You'd turn off AC/AG if the local gravity is tolerable, to avoid disorientation and edge effects when entering or leaving the ship. Or you'd leave it on if local g is too high or low for comfort. (Futurama had a hilarious portrayal of what it's like walking from ship to planet g in the "local g too high" scenario.)

I found that this setup provided enough detail to keep players happy, matches up with Traveller canon pretty well, and didn't lead to weird unintended consequences too often. :)

On Fri, Aug 18, 2017 at 9:04 AM, C. Berry <xxxxxx@gmail.com> wrote:

Yep. But again, all of that could just be fuzzing together power requirements for all CG operations -- rising quickly, descending, hovering, and depending on how you handwave thruster/cg coupling, perhaps moving sideways as well.

The key point to understand from this whole conversation is that remaining motionless in a potential field doe not require energy input. Moving to a different potential requires or dissipates energy equal to the potential difference.

It may well be the case that power input is required to hover, just to keep the heterodyne graviton resonance field properly collimated in the Frobman interocitor assembly. :)
On Fri, Aug 18, 2017 at 8:26 AM, Jerry Barrington <xxxxxx@gmail.com> wrote:
Example of hovering without using power:
https://www.youtube.com/watch?v=Ws6AAhTw7RA

On Thu, Aug 17, 2017 at 11:00 PM, Tim <xxxxxx@little-possums.net> wrote:
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

-----
The Traveller Mailing List
Archives at http://archives.simplelists.com/tml
Report problems to xxxxxx@simplelists.com
To unsubscribe from this list please go to

http://www.simplelists.com/confirm.php?u=z4ykj54zpoNxz3pUaE773cJHeATwsgSu
----- The Traveller Mailing List Archives at http://archives.simplelists.com/tml Report problems to xxxxxx@simplelists.com To unsubscribe from this list please go to http://www.simplelists.com/confirm.php?u=PltOdItWBSgOP4y0Q6abkGbDI1eus0lz
--
"Eternity is in love with the productions of time." - William Blake

"Eternity is in love with the productions of time." - William Blake