# Help with compressed air thrust

## Main Question or Discussion Point

I'm working on a kinda-sorta confidential device. I would like to know how much psi is required to lift approx. 400 pounds 120m (about 393ft and 8 13/32 inches) into the air. The thing I'm working on is still in the designing stage, so don't worry about the specifics of individual thrusters and stuff since I'll work it out.

P.S.
This is just something I hope to complete as something to add to my college résumé.

## Answers and Replies

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Danger
Gold Member
Welcome. The pressure required depends upon the cross-sectional area of the piston (therefore the diameter). Psi x area = force. How high it can go is solely determined by how long the cylinder for the piston is and how large an air supply you have.

(One caveat: this is on paper. In reality air is compressible, so you might not get exactly the calculated performance from your constructed machine. More load resistance in the form of weight leads to more compression of the air in the cylinder. That can also lead to thermal issues.)

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Welcome. The pressure required depends upon the cross-sectional area of the piston (therefore the diameter). Psi x area = force. How high it can go is solely determined by how long the cylinder for the piston is and how large an air supply you have.

(One caveat: this is on paper. In reality air is compressible, so you might not get exactly the calculated performance from your constructed machine. More load resistance in the form of weight leads to more compression of the air in the cylinder. That can also lead to thermal issues.)
For the cylinder I'm thinking of a foot and a half long and a diameter of 6 inches. Air supply is something I'm trying to work out as well.

etudiant
Gold Member
The job does not seem doable to me with the size cylinder you have in mind.
For one, you want to push about 400 lbs upward against gravity, so you need to have enough thrust to accelerate the 400 lbs against a downward gravitational acceleration of 32 ft/sec**2. So if your nozzle(s) allowed 4 pounds of air to escape per second, it would need to flow at 100x32 ft/sec to balance the weight, more to move it upward. Volume of a 1.5 ft long cylinder 6" in diameter is about 0.3 cubic feet, which holds about 0.025 pounds of air. So for 4 pounds of air in that volume, you need it to be at 160 atmospheres pressure. Squirting that out gets you momentary hover, much more is needed to rise to 400 feet. As 'danger' has pointed out, compress air a lot and it gets very hot, similarly let it expand a lot and it gets very cold.
Even if you could compress enough air into that tank to theoretically lift your weight, it would cool to liquid as it came from your nozzle.
There are possible fixes, hydrogen for example warms as it expands, but that has its own issues. See:

The job does not seem doable to me with the size cylinder you have in mind.
For one, you want to push about 400 lbs upward against gravity, so you need to have enough thrust to accelerate the 400 lbs against a downward gravitational acceleration of 32 ft/sec**2. So if your nozzle(s) allowed 4 pounds of air to escape per second, it would need to flow at 100x32 ft/sec to balance the weight, more to move it upward. Volume of a 1.5 ft long cylinder 6" in diameter is about 0.3 cubic feet, which holds about 0.025 pounds of air. So for 4 pounds of air in that volume, you need it to be at 160 atmospheres pressure. Squirting that out gets you momentary hover, much more is needed to rise to 400 feet. As 'danger' has pointed out, compress air a lot and it gets very hot, similarly let it expand a lot and it gets very cold.
Even if you could compress enough air into that tank to theoretically lift your weight, it would cool to liquid as it came from your nozzle.
There are possible fixes, hydrogen for example warms as it expands, but that has its own issues. See:
I only just realized that the weight that I had given was incorrect, as I will be using a motor for another function, the true weight is approx. 200 pounds.

Danger
Gold Member
I only just realized that the weight that I had given was incorrect, as I will be using a motor for another function, the true weight is approx. 200 pounds.
Hang on a sec... something needs clarification. I just re-read your original post and noticed that you used the term "thrusters". I couldn't understand the complexity of etudient's response until now because I had based mine upon the supposition that you were asking about a pneumatic ram rather than rockets. (And yes, I know that 120 metres of lift would make for one hell of a long cylinder, or one with a lot of gearing, but I thought that maybe this was for a novel or short story which would allow some leeway as opposed to a working project. Sorry about that.) Most of the stuff that I mentioned is still somewhat applicable, but not nearly to the extent that I figured.

etudiant
Gold Member
Thanks for the clarification that the weight is only 200 pounds. It helps some.
Liquid air is about as dense as you can compress and it is almost 90% the density of water.
So the tank will hold about 20 pounds of liquid air. Assuming you eject 4 pounds/sec at 3200ft/sec you would accelerate you 200 pound weight upward at about 1 g, 32 ft/sec**2. The time t needed to rise 400 ft at 1 g is the solution to 400 = 16* t**2, so t**2 is 25, so t = 5 seconds. As you have 20 pounds of liquid air, your setup has just enough propellant to get there.
That indicates your gizmo can work theoretically, except that the liquid air won't leave the nozzle at 3200 ft/sec by itself. But the idea is the same in a rocket, the liquid air is burned with some fuel to liberate the needed exhaust energy

I also forgot to mention that it will not be a continous flow of air, rather 10-20 second bursts to provide the proper thrust required over a certain distance to get to a certain height, as the it will be following a triangular vector system.

etudiant
Gold Member
Do note that your tank holds only enough for 5 seconds of the needed thrust even if filled with liquid air.
For a triangular course which includes lifting a 200 pound weight to a 400 foot height gain, you have nowhere near enough propellant.
The requirements cannot begin to be met with the setup you are describing, at least imho.

Then
Do note that your tank holds only enough for 5 seconds of the needed thrust even if filled with liquid air.
For a triangular course which includes lifting a 200 pound weight to a 400 foot height gain, you have nowhere near enough propellant.
The requirements cannot begin to be met with the setup you are describing, at least imho.
Then what size tank would you suggest? After all, I only gave 1ft and 6in. as a baseline. And I think that I'll lower the height to about 300ft. Depends on my budget later, I'll let you know.

Danger
Gold Member
Is there a particular reason that you can't use electric propellors or even something like JetCat engines? I can't help thinking that either one or both might be simpler, more reliable, and maybe even cheaper than what you're trying to do.

etudiant
Gold Member
The basic problem is that compressed air does not have enough energy to do the work you want.
For one, compressed air is a terrible source of energy. It cools as it expands, so it does not give much push.
It is fun to see a balloon jetting around the room when you let go the nozzle, but for lifting a few hundred pounds a couple of hundred feet you need ridiculously more power.
Look up Isp, specific impulse, to get an idea of how much propulsion energy you can get from various sources. Compressed air is worse. My guess is that your model cannot work, because the air does not have enough oomph to lift its own tank, much less any added weights. You need a different propulsion system.

Is there a particular reason that you can't use electric propellors or even something like JetCat engines? I can't help thinking that either one or both might be simpler, more reliable, and maybe even cheaper than what you're trying to do.
I don't think I can attach propellers or an engine to my hips.

Danger
Gold Member
I don't think I can attach propellers or an engine to my hips.
oo)
You're looking at a whole different set of problems there, in addition to the one that we're already discussing.

It's going to be tough to accomplish this using compressed air. Are you able to change your fuel? (I don't think we can technically call it "fuel"...owell). If so then it might be worth looking into other ideas currently in use. Compressed Hydrogen Peroxide is being used in current jetpack designs and works fairly well for the amount of "fuel" you would be restricted to.

http://www.gofastsports.com/GoFast_JetPack/equip.html [Broken]

It is also safe, seeing as you intend to strap this thing onto yourself (though I might suggest you pick the least favorite group member to do the testing)

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Danger
Gold Member
The best jet packs have a maximum flight duration of 30 seconds. Also, the peroxide isn't compressed; solely for the sake of storage density it is "pumped" by compressed nitrogen. It doesn't become a propellent until it's catalyzed by a platinum (or similar) screen. The peroxide is the fuel; steam is the reaction mass. The only thing that comes close to what he wants (but so far at much lower altitude) is the water jet pack used for recreation on beaches and at some theme parks. In that case, the water is provided through a tether hose from a honkin' powerful jet-ski.

The thing I'm building isn't really a jet pack, it's more of a zip line attached to a harness with a forward thrust

Danger
Gold Member
Oh, that's way different then. You don't need lift; that's provided by the harness/line combo. Propulsion should be easy., and I assume that you're already using the lowest-friction wheels and axles on your attachment point to keep the resistance down. There's a guy that I just saw on Daily Planet a few days ago (but it was recorded, so I don't know when it aired) who zips around on skis using just a couple of ducted electric fans (such as used in model jet aeroplanes) attached to a bar that he holds. He gets up to something like 30 or more kph. I see no reason why you couldn't incorporate something like that into a belt, or even better the harness. Go to the Daily Planet website (discovery.ca/dailyplanet) and snoop around to find the clip. It might help you a bit.

The thing I'm building isn't really a jet pack, it's more of a zip line attached to a harness with a forward thrust
Perhaps we need some more clarification, do you intend to use the thrust to lift the object vertically to your specified height? This makes it sound like you will use it for horizontal thrust.

As for the H2O2 jetpack, I stand corrected. I even wrote a little paper on those back in college, shame on me for actually forgetting how the damn things work (don't tell my employer I forget things easily).

The emission nozzle I built is tilted at a -6 degree angle to help provide a somewhat diagonal thrust, the main direction of it being horizontal, so I guess it could be classified as horizontal. The height I provided was actually supposed to be the distance of the diagonal vector in the system, but I've been using my iPad a lot lately since my PC is out of commission, so I blame autocorrect.

Danger
Gold Member
Jeez... pick something and stick with it, already! You're making me dizzy. :p
Yeah, I hate autocorrect. I even started a thread about it in the Feedback section. As I found out there, there should be a way to turn it off but still leave the spell-check working.
Anywho... a totally different approach just came to mind, which probably won't appeal to you but must be let out so it doesn't hurt my brain. It seems a lot simpler to me if you were to just build an electric traction motor into the zipline carriage. Worth investigating?

*sigh* :s

Back to the drawing board...

:(

Danger
Gold Member
Now, now... don't cry. It was just a suggestion...

Five years and \$400 down the drain!

Woe is me.

Anyway, changing the topic of thruster to something else I'd like to ask, how much psi for the same size canister I mentioned is needed to propel a grappling hook at a 60 degree angle with a mass of 0.094 kg with an initial launch velocity of 45.2 m/s (v final is 0) and max height of 104 m. Normally I'd solve these things out myself, but lately I haven't been feeling so hot, headaches and constant sleepiness. Ugh, I hate relying on others when it comes to physics, but I'm in too much pain to deal with the math.