# Could one use Air Pressure as a form of stealth Propulsion in Space?

This is my first question here so I would like to apologize if this is sloppy or in the wrong place or the like. I have searched and found nothing that answers the exact question I think, so thought to ask. I also apologize if the tag is wrong as well.

Let's just hand-wave the heat issue of stealth in space for the sake of this question. The nation this question pertains to uses Nanocarbon tubes to turn the waste heat into what amounts to the background radiation of space or the energy used by the opponent's sensors.

Anyways, now that that's out of the way, here's what I'm looking for and the like:

Could one propel a ship using air pressure or through similar means without having to power up or the like to do so?

If yes, I would like an explanation, same for if not.

EDIT: Yes in space, sorry for not clarifying.

• sorry to clarify, this is for space travel, or travel inside an atmosphere? Aug 14 '19 at 22:16
• In outer space? Yes. Fill an airlock with air and then open the door and vent it to space. For every action there is an equal reaction so the ship will move. If you have a thruster port you can push out air or anything else - but make sure it is super cooled or it will be visible in the infrared. I think that is how ion propulsion works: nasa.gov/centers/glenn/about/fs21grc.html Aug 14 '19 at 22:17
• @JerryJeremiah No, that's not how ion propulsion works. An ion thruster ionizes the gas (hence the name) and then uses an electric field to accelerate it to a velocity far out of the reach of simple pressure. Aug 15 '19 at 14:13

Technically, yes. Newton's Third Law is not suspended for compressed air. And indeed, this is pretty much exactly what a cold gas thruster does--it's just a nozzle attached to a tank of pressurized gas.

But practically speaking? Absolutely not. The energy densities are just way too low. Sure, you can use a compressed air thruster for fine adjustments to attitude and docking alignment, but you won't be managing any significant orbital transfers with such a device.

• Another issue is that it might not be "stealth" if the opponent's sensors were finely tuned enough, as the air would give off thermal radiation just like any other form of propellant the ship spews out, though because of the lower energy densities and temperature the radiation would be much weaker. Aug 15 '19 at 14:14

As others have pointed out, you can move yourself in space using compressed air. However, it is unacceptably inefficient. You wouldn't actually be able to appreciably accomplish any goals.

One of the most important measures of a fuel is Specific Impulse. Specific Impulse linearly related to exhaust velocity, and it will be easiest to talk in terms of exhaust velocity because its tangible. The space shuttle SRBs have an exhaust velocity around 2500m/s. LOX/LOH engines can get up to 4400m/s. Ion thrusters, known for their efficiency, get to 30,000m/s or faster.

Compressed air exiting at the speed of sound clocks in on the order of 300m/s. So it's 1/10th as efficient as the engines we use today. The limit for Nitrogen gas is around 730m/s, so you should expect your thrusters to produce on those ranges.

Of course, this gets worse, thanks to the tyrany of the rocket equation. Another key feature of space travel is the mass fraction -- this is how much of your mass is your payload, versus how much has to be fuel. The equation is $$\Delta V = v_e\ln\frac{m_0}{m_f}$$, where Ve is the exhaust velocity. A mission that requires some quantity of delta_V to accomplish requires a mass fraction to match. If you have 1/10th the effective velocity, you have to have a smaller mass fraction on the order of $$e^{10} =22026$$. If you've got engines closer to the theoretical maximum, you're going to be on the order of $$e^4=42.598...$$ or $$e^5=148.413...$$ worse mass fractions.

This means that to move any given payload via air pressure rather than chemical rockets, you need to have a rocket that's literally hundreds or thousands of times larger. That's why we don't see them used for large maneuvers.

It would be stealthier because it wouldn't show up in visible light, as much, compared to external combustion. But you could still see it in infrared. Barring extreme metamaterials that just straight-up make you magically invisible, there's no stealth in space.

Gases do cool on expansion, so stealth is possible. But you can't conceal the movement itself. I believe automated watch for all detritus nearby will be standard on spaceships, using radar lidar etc. Any unexpected movement will cause an alert, regardless of whether there is thermal signature or not. So, the answer is, no.