>I was wondering if the exhaust gas was heated in some type of a 'venturi
>augmentor' using propane as a fuel. Could such an addition increase the Isp
>to a level where the device would become 'interesting'?

Rocket specific impulse is proportional to the square root of absolute chamber temperature divided by the molecular weight of the working gas (then modified by the nozzle expansion based on pressure ratios), so yes, heating the air would increase the specific impulse.

However, heated air still doesn't look good compared to peroxide. 90% peroxide gives a 1400 F chamber temperature, and an exhaust composed of H2O and O2. Air is N2 and O2, so even if it was heated to the same temperature as peroxide, it would give lower performance for a given mass of propellant, due to the lower average molecular weight.

Compressed helieum or hydrogen actually has a damned impressive Isp as a cold gas, but it also has such a low molecular mass that you would get far less total thrust out of a given tank of compressed gas compared to air.

In any case, if you are going to burn something, you might as well make a proper bipropellant rocket engine, which can be roughly twice as efficient as a monopropellant peroxide engine. Propane / compressed air would work fine, but it would give a chamber temperature over 3000 F, so you would need to cool it some way. At that point, you are all the way to "real rocket science", a long way from just a compressed air rocket.

It is worth noting that nuclear-thermal engines are basically what you propose, just with a more efficient energy source -- they flow liquid hydrogen (low molecular weight) through a nuclear heater to get it up to a few thousand degrees, and let that run through a nozzle without any combustion taking place. The key is that the nuclear heater can run for a long long time without burning up any fuel, unlike a propane heater.

Even the very best rocket engines can't give the buck-rogers performance desired from a rocket pack. You could accelerate to speeds far faster than you would care to, but you couldn't hover for even two minutes with propellant you could carry on your back. Rockets are for accelerating, not cruising.

Advanced jet turbines have the potential for 10x the hover performance that rockets have, and solo-trek style vehicles could go even longer on the same amount of fuel. Rockets move small amounts of exhaust very very fast. Jets move a moderate amount of exhaust moderately fast, and get their oxidizer from the atmosphere. Rotors use atmospheric combustion like jets, but they move a large amount of atmosphere fairly slowly, which improves efficiency at the speeds they operate at.

John Carmack
www.armadilloaerospace.com