On Wed, Aug 30, 2017 at 10:26:17AM -0400, Jeffrey Schwartz wrote:
> There was a comment earlier in the thread about "Hot Air Balloon" in
> reference to CG
> I think that one is the most on-track I've seen.

The premise of the thread was about whether power consumption is
mandated by the laws of physics to maintain a constant height.  The
answer is no.  Any at-rest power use is some form of inefficiency due
to particular details of some technology.

> What about approaching power usage this way:
[details given]

That's one approach, sure.  I like the idea of expending energy
up-front to build up a field.

> I think this removes the ability to build a flywheel perpetual
> motion machine

Not that it really matters since so much of the rest of Traveller
technology blatantly ignores conservation laws, so why should
contragrav obey them?  But anyway - not as stated, but some variations
on the idea will work.

In particular, the device must over the long run consume at least as
much power as the rate of gain of gravitational potential energy
(proportional to the local gravity, the speed of ascent, and the mass
of the object supported by the device).  Presumably, more powerful
contragrav units would be able to support greater ascent rates for the
same mass.

If the power required exceeds the specifications of the device,
presumably the field has to fail in some manner.  Someone has to
decide whether that would be as uneventful as the object gradually
regaining some of its weight, or as catastrophic as sudden field
collapse dumping all its energy into the grav plates and turning much
of the vehicle around them into flaming wreckage.

I tend to favour the former, as contragrav is portrayed in Traveller
as being very reliable.  It also fits in with the idea of stored field
energy being "drained" into gravitational potential energy as the
object rises, and having to be replenished if the object is to remain
fully shielded from gravity during the ascent.  It's stable in the
sense that failure to provide enough power is self-correcting: the
returning weight of the vehicle from a weakening field would generally
slow the ascent and reduce the power requirement.

One consideration not yet mentioned for a "field-like" device, where a
field counters the weight of objects inside it, is that it would
presumably also counter the weight of air inside the field but outside
the hull.  This would cause air around the vehicle to rise, much like
the updraft from a bonfire, with the speeds proportional to the height
of the field.  A large vehicle could cause quite a powerful updraft
zone around it.

The power for accelerating the air would have to come from somewhere,
though it might be small by comparison with most vehicle power
requirements.

- Tim