I was thinking about extra-terrestrial settings / environments for DS2
and SG2 games this afternoon, and one thing I'm wondering about is GEV
vehicles and how they might operate or what changes might be made to them to
allow them to operate.
First I'm just considering the effects of gravity and atmospheric
pressure. Increasing/decreasing gravity changes the weight of the
vehicle
proportionately; increasing/decreasing the atmospheric pressure is what
I'm less certain about. 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.
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?
Does atmos. pressure have any effect on the engine / turbines? 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? Of course taken to *extremes* (near vaccuum, dense as water) I can
easily see this breaking down, but otherwise...?
Cheers,
> Jerry wrote:
> I would think that increasing the atmospheric
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
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
> they spin 2x as fast, but in either case, the same volume is being
> 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.
Hi Allan,
Thanks for the detailed input!
> agoodall@att.net wrote:
> No, that's not true. Atmospheric pressure on Earth, at sea level, is
Ah, yes, I agree; I was sloppy in my terminology: I meant the pressure
generated by the GEV cushion fans depends on the atmospheric pressure, not
that the vehicle itself was exerting greater pressure on the ground.
My bad.
> This is where the heavier atmosphere comes in. You're moving more gas
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? I.e.
energy is expended to move air -- in both cases it's the same amount of
air, so by conservation of energy arguments you don't get any advantage either
way. Or should that more appropriately be conservation of momentum...?
> So, no, the heavier atmosphere does _not_ offset the gravity effect.
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.
Or does it?;)
> Wrong. If you are spinning your propellor (turbine, whatever) twice
See above...
Thanks again.
Cheers,
> Jerry wrote:
> Right. The same volume rate can be achieved with fewer revolutions in
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
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!
> So, I suppose the easiest way to handle it is to assume that a GEV
The easiest way to handle it is to use tracks or wheels.;)
Or they just all run off a quantum nucleonic power plants that last for
months...
http://www.newscientist.com/news/news.jsp?id=ns99993406
--Binhan
> -----Original Message-----
> agoodall@att.net wrote:
> The amount of energy imparted by a propellor (or fan blade) on a
I don't want to drag the technical discussion out too much, since it
doesn't appear to be generating much interest :( -- but I did a little
scribbling and came up with the following:
I'm just considering a "standard vehicle" which can operate in different
settings characterized by their atmospheric pressure and gravitational
acceleration. I'm concerned only with lift and not propulsion, and assume the
engine will work under any conditions. The only variables are thus
gravitational acceleration, atmospheric pressure, and fan rotation speed.
Pressure under the skirt is a result of the lift fan moving air, and
there are two components to this -- how rapidly air mass is moved (mass
per second) and and how fast those particles are moving. I.e. we're concerned
with momentum here.
mass moved = m velocity of air molecules = v fan angular velocity = w
atmospheric density = n pressure under the skirt = p
dm/dt ~ w*n
v ~ w
and
p ~ dm/dt * v ~ w^2 * n
Since the pressure has to balance the weight of the vehicle we have
p = constant * g
or
w^2 * n / g = constant
From this, the atmospheric density and gravitational acceleration work
linearly in opposite directions. Doubling density and doubling grav. accel.
leads to no net change in operation of the GEV.
If we keep g constant but alter n, then we have to change fan speed to
compensate, proportional to the square root of 1/n. E.g. n drops to 25%
then we need to boost w to sqrt(1/0.25) = 2 times faster. Note however
that this means that the power required to run the fan, which is proportional
to w^2, goes up by 4. Which is to say the power requirement to run the fan
varies linearly with the inverse of the air density (g = constant).
Of course propulsion won't depend on g, so the power requirements to actually
move will vary according to the previous paragraph. And then
there's things like turning radius / rudders etc.
Anyway I started thinking about this in the context of Dirtside 2 scenarios in
the GURPS Transhuman Space setting, particularly on Mars (which has g = 0.38
Earth's, and in the setting has an atmospheric
pressure 45% of Earths after some 50+ years of terraforming) or even on
Titan.
Cheers,