Re: GEV Physics and GEVs for engineers

Thomas Barclay wrote:

> Odd question for the thinkers:
> Assume we have one AFV of mass X.
> On wheels, it probably has 8 wheels to
> distribute its weight between thus giving a
> certain ground pressure. On tracks, it has even
> more area to divide its pressure across, giving
> a lower ground pressure (assuming drive
> systems are roughly comparable, thus allowing
> mass X to stay about the same). Now, is this
> taken to an even greater extreme with GEV?

Pretty much.  The air cushion does not need that much pressure to lift a
very heavy object if the area is great.  A 200 ton craft with a
footprint of 200 square metres would need an aircushion with a guage
pressure of about 10 kilopascals (1.4psig).  This hypothetical craft
should float, as the air cushion need only form a depression in the
water with an average depth of one metre (why I love metric).  Hammer's
Slammers style hovertanks have a problem (with floating) that, even if
they could generate the requisite aircushion, their center of bouyancy
and center of mass are too far apart for stability (like standing on
someone's shoulders, as they stand in a canoe).

>
>
> I'm a little unaware of the physics of a plenum
> chamber. Is pressure concentrated around the
> skirts, where air is contained? Is it greatest
> under the fans? Is it equally distributed?

As the air goes from the fan to the edges of the skirt, the laws of
fluid mechanincs dictate that the air pressure is greatest in the
middle, and lowest at the edges.

>
>
> My question was this:
>
> Given that a GEV is higher off the ground than a
> conventional tank, given that a GEV may have
> significantly lower ground pressure, is it
> possible that GEVs will have better luck
> penetrating minefields?

Like everything else in the world, it depends.	If I do not operate any
aircushioned vehicles, but know that you use them to the exclusion of
all other vehicles, my mines will be triggered barometrically.	AC
vehicles will not get far, but my wheeled, tracked, walking units are in
the clear (assumption: weather on the particular planet is not
extreme).  A Hammer's Slammers hovertank is an unspecified size, but
known mass.  If they are three by six metres, and 180 tonnes, they apply
a whopping 1atm of pressure.  Although this is slightly less than half
of the pressure exerted by a car against the road, or only three times
the ground pressure of a typical infantryman, there are still
circumstances when these hovercraft will bog (Therefore, HS hovertanks
have a larger footprint than eighteen square metres).

>
>
> I assume with mine pressure detectors that you
> set AT mines for a certain minimum pressure,
> which probably makes them harder to sweep
> and less likely to go off on a jeep or false and
> go off on their own. If you crank this down, so
> you can target a GEV, then don't the mines
> become easier to sweep?

Current landmines are set for a given weight.  Serious overpressures can
take the place of a vehicle (why explosives can clear mines), but pretty
much if a sufficiently heavy object is placed on a mine, it detonates.
To bury mines, some account must be made for the weight being spread
out, but that is need to know information that I do not care to find
out.  Barometric mines are harder to sweep, if they can differentiate a
shockwave from a passing hovercraft.  Gravimetric mines are the hardest
to sweep, as they are set to go off when a sufficiently dense and
massive object comes near.

    Hovercraft are particularly vulnerable FAE mines.  On detecting a
passing hovercraft, they emit a blast of butane and ignite it after the
lifting fans have mixed in oxygen.  Smaller hovercraft are sent sailing
through the air, and heavier ones are flipped over.