Spacecraft propulsion using a reusable propellant

[Iggyz]

The compressed air thruster is probably a silly idea so I am offering my apologies upfront should feel I wasted your time.

How the compressed air thruster might work:

The propulsion module (please see attached picture) contains a compressor (A), pressure vat (B), electrically controlled high speed valve (C), cylinder (D) and high speed vacuum pumps (F).

Vacuum pumps evacuate cylinder (D). This air is compressed immediately by the compressor (A) to, for example, 100 bar and pumped to the pressure vessel (B).

Part of the stored air is injected into the evacuated cylinder as an air burst (E) by the electrically controlled high valve (C).

The vacuum pumps immediately evacuate the injected and expanding air before it reaches the rear wall of the cylinder and applies pressure. This might create some pressure in / near the vacuum pumps but it will probably negate itself since the vacuum pumps are fitted opposite of each other side wall.

The compressor will simultaneously, at the same speed at which the vacuum pumps extract the air, compress the same amount of air and send it to the pressure vat.

For this to work the vacuum pumps and compressor will have to operate simultaneously and started just before the air injection starts.

[edzieba]

No net thrust.

Imagine the behaviour of a single molecule of gas. It will enter the pressure tank through the pressurisation pump, fly quickly out the nozzle into the expansion chamber, be recovered by the vacuum pumps, and returned to the pressurisation pump. It has travelled in one loop. There has been no net movement of mass, so no net thrust.

[Crispy]

Conservation of momentum must be obeyed. All you are doing is moving gas around in a circle. The same mass of gas goes forwards as goes backwards, therefore there is no change in momentum.

This is the same as sitting on a wheeled chair and shaking a weight back and forth.

[Iggyz]

Agree the same mass of gas goes forwards as backwards but it was fired as a burst (it is not a continuous stream) like a canon ball, and the gas traveled some distance before it is moved backwards again. Also the gas probably will move backwards at a lower speed than the speed it had the moment it left the valve. I figured all this would be enough to create a change in momentum. Looks like I was wrong.