Re: GEV on other worlds

Jerry wrote:

> Right.  The same volume rate can be achieved with fewer revolutions in
a 
> thicker atmosphere.  But doesn't the engine have to work just as hard 
> because it is after all pushing the same amount of atmosphere?

The amount of energy imparted by a propellor (or fan blade) on a column
of air is equal to 1/2mv**2. In order to move the same mass of air in a
less dense atmosphere in the same amount of time, you have to move the
air twice as fast. However, that means that the energy needed to move
the mass goes up due to the formula squaring the velocity. So, in order
to move the same mass of air in the same amount of time, you have to
move it more quickly. This means that you end up spending more energy to
move the same amount of less dense air.

One way to move a greater mass of air without increasing speed is to
increase the size of the fan blades. Bigger blades move more air, thus
you can decrease your speed. However, greater surface area means greater
drag, which decreases the fan blade's efficiency. I don't know enough
about fluid mechanics to know the formulas, but I'd expect that for peak
efficiency you'd really want different fan blades for different
atmospheres. Assuming the same fan blade size, you will use up more
energy moving air in a thin atmosphere than in a dense atmosphere.

I also haven't talked about the drag on the internal components of the
fans. The fans would be mounted in some form of a cylinder, with the air
drawn in one end and shot out the other. Less dense air would result in
less drag on the these parts, but I suspect that the drag issue is less
of a factor than the velocity issue, with respect to a lift.

If your engine is an air-breathing internal combustion engine, you will
get _way_ less power out of it in a less dense atmosphere for each drop
of fuel used. So not only are you using more energy to move the same
mass of fuel, you're getting far less efficiency out of the fuel you're
using. (This won't be an issue with an electric engine or a fusion
engine. An electric engine would have problems if the temperature went
down, and all other things considered, temperature drops as pressure/air
density drops.)

Another thing to take into consideration is the inefficiencies in the
moving parts within the engine, but that's getting pretty specific for
something that's hypothetical.

> Again, though, isn't this sort of like running in a swimming pool? 
The 
> GEV fans have to push against very thick atmosphere (requiring more 
> energy), but they don't have to turn as fast to push the same volume 
> rate (requiring less energy) -- so it comes out even in the end.

See above. In a dense atmosphere you can get away with thinner fan
blades, thus lowering your drag on the propellors and moving the same
mass of air.

All of this is thinking in terms of lift. Your comment reminded me that
a GEV that hovers but doesn't move isn't much use. Once you start moving
the craft, that's a whole new ball of wax, and you're getting into even
hairier aerodynamic issues. In that case it would take less energy to
move the GEV, but more energy to lift it. Do they cancel out? We don't
have enough information to tell that.

So, I suppose the easiest way to handle it is to assume that a GEV uses
the same amount of fuel regardless of atmosphere. I _suspect_ that isn't
the case, that a GEV would use less fuel on Earth than on Venus and less
fuel on Venus than on Mars, but that's just a guess. 
 
> Thanks again.

No problem!

Allan

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