This concerns a feasibility study for a small helium-filled R/C blimp.
It will have to navigate through holes of 60cm diameter and in a
relatively constant magnetic field of 1-2 Teslas. Because of induced
voltages and eddy current, an alternative to DC motors could be
propulsion by gas. Furthermore, the use of magnetic materials is
stricly prohibited (no iron, nickel, cobalt, ferrites etc.) However to
keep the blimp dimensions reasonable, the max payload is in the order
of 400 grams.
An existing 180g blimp is propelled by DC motors and hand-made
propellers delivering 5 to 6 grams of thrust, so I would need a thrust
system able to deliver 50-100 milliNewtons.

Knowing the general thrust equation (F = me*Ve + (pe-p0)*Ae), how
should I dimension the system to ensure required autonomy (to navigate
at max 1 m/s during 1-2 hours)? Of course it will vary on the number
of impulses given, but I would like to know the formulas to estimate
that too.
- What gas: CO2, N2, Helium? Can we talk about Isp (specific impulse)
for such cold gas?
- How to calculate the initial tank conditions (pressure, size),
knowing that after each impulse the pressure will have dropped, and
the temperature too?. I guess a differential equation must come.
- What material should be used for the tank, to make it the lightest?
(Regarding tank wall thickness formulas, it's OK)
- What about nozzles, can they work at initial tank pressure (around
70-100 bars I guess), or do they need a lowered pressure?

My guess is the nozzels will produce more ISP per gram of gas at 1000
bar than at lower pressure and 1000 bar may be practical if you can
tolerate slight risk of explosion. Concider hydrogen if a slight fire
hazard is exceptable. My guess is helium is next best. CNT = carbon
nano tubes may be available now and provide an outstanding energy to
weight ratio. How many seconds of 50 milliNewtons do you expect to
need for a 2 hour flight? Is that equal to 5 grams of thust?   Neil