On Thu, 6 May 2004, Bill Clawson wrote:
> One question I have is how to figure out how much
> weight gets added with a pressure fed system, for the
> added strengthening of the tank and for the added
> pressurant -- helium or nitrogen at 600 PSI should be
> denser than the same gas at 40 PSI (or what have you).
To a first approximation, N times the pressure requires N times the tank
wall mass and N times the gas mass.
Multiplying the wall mass by N will be a little bit conservative, because
a low-pressure tank may be sized by ground handling loads, pre-flight wind
loads, or flight hydrostatic loads to some extent.
Multiplying the gas mass by N is technically right only for an ideal gas,
but it's often a reasonable approximation. The one place where it can hit
trouble is for high pressures and low temperatures, e.g. pressurizing a
pressure-fed tank full of LOX. If you need precise numbers, particularly
for hydrogen and helium -- the "quantum gases", whose behavior deviates
significantly from the behavior of most other gases -- it's desirable to
have a gas-properties chart for that particular gas, to get the true
density for a given set of conditions.
The fun part for pressurizing cryogenic fluids is guessing the average
temperature of the pressurant. (This can come up for non-cryogens too,
if you heat the gas to reduce its density.) It's somewhere between the
temperature the gas enters the tank at, and the temperature of the liquid.
For small vehicles, which have a lot of liquid surface per unit volume of
gas, a safe guess is that it'll be pretty close to the liquid temperature.
Henry Spencer
[EMAIL PROTECTED]
_______________________________________________
ERPS-list mailing list
[EMAIL PROTECTED]
http://lists.erps.org/mailman/listinfo/erps-list
