Re: GEV on other worlds

Jerry wrote:

> I would think that increasing the atmospheric
> pressure would proportionately increase the ground pressure generated
by
> the GEV, so that if (for example) the vehicle is on a world with 1/2
> Earth's gravity and 1/2 the Earth's atmospheric pressure, these two
> factors effectively cancel each other out and the vechile operates
more or
> less the same as on Earth.

No, that's not true. Atmospheric pressure on Earth, at sea level, is
roughly 14.7 pounds per square inch. A square metre of surface has a
column of air on it that weighs about 10 metric tonnes. This does not
mean, though, that something that's a square metre in size weighs 10
tonnes! The reason has to do with buoyancy.

There is slightly less pressure at the top of your hovercraft than there
is at the bottom of your hovercraft, even on the Earth. The air below
the hovercraft is pushing up while the air above the hovercraft is
pushing down. The column of air is slightly heavier at the bottom than
it is further up, even if you're only talking a difference of a few
feet. (The air is also pushing in at the sides of your hovercraft, but
this cancels out.) The air at the top of your hovercraft weighs less
than the air at the bottom of your hovercraft by a tiny little bit. The
net effect _almost_ cancels each other out, but doesn't. Instead, there
is a net increase in _upward_ pressure. This effect is buoyancy. The
hovercraft weighs slightly _less_ on the Earth, due to the atmosphere,
than it would on a world with the same gravity that had a vacuum.

Another way of seeing this effect is to take a five pound weight and
attach it to a spring scale. See that it weighs 5 pounds. Now take that
same weight and submerge it in a swimming pool. You'll see that the 5
pound weight no longer weighs 5 pounds. (Its mass, however, has not
changed.)

(What you have to worry about with heavier atmospheres is the atmosphere
crushing the hovercraft. On Earth the hovercraft has about the same air
pressure inside as out, thus it doesn't get crushed. Our bodies have the
same weight inside as out, so we're not crushed by the atmosphere. If
you have a heavy atmosphere pushing down on the hovercraft, but the
internal pressure is the same as Earth, it could crush the hovercraft if
it's heavy enough.)

In fact, you'll find that a heavier atmosphere will probably make it
_easier_ to lift your hovercraft. The hovercraft works by aiming a
propellor straight down. It pushes air downwards, exerting a force
against the surface of the ground. The ground pushes back, exerting an
equal and opposite force against the hovercraft. The more air you push
through per second, the greater the pressure, until the pressure is
enough to offset the weight of the hovercraft.

This is where the heavier atmosphere comes in. You're moving more gas
molecules with each rotation of the propellor blade in a heavier
atmosphere than you are in a thinner atmosphere. The propellor doesn't
have to move as fast in order to lift the GEV in a dense atmosphere
compared to thin atmosphere. Aircraft see the same thing, but they also
have to deal with the heavier atmosphere causing drag, which is less of
an issue with hovercraft.

So, no, the heavier atmosphere does _not_ offset the gravity effect. I'd
imagine that a GEV with vacuum sealed cabin would work better on Venus
(with less gravity but a much thicker atmosphere) than on the Earth
(though, of course, you add weight to the GEV in order to make it air
tight, and if the cabin of the GEV is ever punctured the crew is toast).

> Now this ignores things like air resistance and concerns re:
combustion 
> engines, but aside from that, do the effects of gravity and air
pressure 
> work linearly and inversely to each other?

Nope, as explained above.

> Does atmos. pressure have any effect on the engine / turbines?

It has an effect on the blades, as explained above.

It also has an effect on the engine, assuming an internal combustion
engine that requires oxygen. A heavier atmosphere (heavier with oxygen,
anyway) results in more oxygen molecules in the cylinder heads, which
makes the engine more efficient. It's the reason for nitrous boosts
(adds more oxygen molecules directly to the cylinder head) and
turbochargers (increases the pressure, and thus the number of oxygen
molecules in the cylinder head) in high performance cars. It's also why
people who drive from low lying areas to the Rockie Mountains find that
their cars don't run quite as well.

If the atmosphere is dense with something other than oxygen it depends
on what you are using for an engine and what gas it does or does not
need in order to operate.

> If we 
> consider a given volume of air being pushed through the system, does 
> atmos. pressure matter?  I.e. 2x the pressure means the turbines spin
1/2 
> as fast to push the same amount of air through, 1/2 the pressure means

> they spin 2x as fast, but in either case, the same volume is being
pushed 
> through so the fuel / energy requirements are the same in either case.
 
> Right?

Wrong. If you are spinning your propellor (turbine, whatever) twice as
fast, you're using up more fuel in the same period of time. Even in a
100% efficient system, it stands to reason that you'll use up more fuel
spinning a propellor 1000 revolutions per second than if you were
spinning it 500 revolutions per second.

A propellor and internal combustion system, though, is nowhere near 100%
efficient. Lower atmospheric pressure means that you'll gain something
in less drag on the propellors, and less drag on the machine when it
moves (if you're using the same propellor system to vector forward
thrust) but that only partially offsets the inefficiencies in the
engine, the propellor shaft, etc. So, spinning the propellor/turbine
blades/whatever twice as fast will use up more fuel than if it spins
half as fast.

Also, as mentioned above, in an internal combustion engine the lower
atmospheric pressure hits you in two ways: you have to spin the
propellor faster to push the same volume of air, but you also get less
oxygen into the cylinder heads, so you have a less efficient engine.
(Okay, you'll also have less backpressure to contend with, but -- as in
a turbocharger -- is probably offset by the greater amount of oxygen.)
All this combined, I'd imagine you'd end up using up more than twice as
much fuel in a thin atmosphere if you were having to spin the propellors
twice as fast as in a dense atmosphere. 

Assuming, of course, that it's an internal combustion engine using
oxygen found in the atmosphere to power it.

If you're vectoring thrust from the same engine to propel your craft
forward, you'll gain something in not having as much drag on the craft,
but that won't offset the engine inefficiencies, particularly if the
craft is already pretty streamlined.

Allan

--
Allan Goodall		   agoodall@att.net
http://www.hyperbear.com   agoodall@hyperbear.com