Mike Borgelt
September 30th 03, 01:42 AM
The pressure difference across a thin vertical slice in a fluid is
dP = rho x g x dh where rho is density, g = acceleration due to
gravity, dh is the vertical thickness of the slice.
So by rearrangement we get
dP/dh = rho x g
dP/dh is the vertical pressure gradient(rate of change of pressure
with altitude)
So where the density is low the rate of change of pressure with
altitude is low - this in fact is what happens in the Earth's
atmosphere.
Also where g is low the same applies.
So on Mars where g is about 0.38 of Earth and the surface density is
low (about 1% of Earth) the vertical pressure gradient is low.
The density of a gas also depends on its absolute temperature but
vertical temperature gradients are limited to dry adiabatic or
thereabouts except for short periods(you get convection which then
ensures dry adiabatic by mixing - assuming no condensation).
This does mean that the density with altitude doesn't drop off as fast
as the pressure with altitude which I've just shown is much slower
than on Earth so the conclusion is that if your Mars airplane can fly
at all at the Martian surface it will probably have a useful operating
ceiling.
No astrophysics required, just freshman physics 101 and a little
meteorology.
Mike Borgelt (B.Sc Physics, post grad Diploma meteorology)
dP = rho x g x dh where rho is density, g = acceleration due to
gravity, dh is the vertical thickness of the slice.
So by rearrangement we get
dP/dh = rho x g
dP/dh is the vertical pressure gradient(rate of change of pressure
with altitude)
So where the density is low the rate of change of pressure with
altitude is low - this in fact is what happens in the Earth's
atmosphere.
Also where g is low the same applies.
So on Mars where g is about 0.38 of Earth and the surface density is
low (about 1% of Earth) the vertical pressure gradient is low.
The density of a gas also depends on its absolute temperature but
vertical temperature gradients are limited to dry adiabatic or
thereabouts except for short periods(you get convection which then
ensures dry adiabatic by mixing - assuming no condensation).
This does mean that the density with altitude doesn't drop off as fast
as the pressure with altitude which I've just shown is much slower
than on Earth so the conclusion is that if your Mars airplane can fly
at all at the Martian surface it will probably have a useful operating
ceiling.
No astrophysics required, just freshman physics 101 and a little
meteorology.
Mike Borgelt (B.Sc Physics, post grad Diploma meteorology)