Pneumatic Jet-Pack Calculations

[bwheeler96]

I would think someone would have thought about this already, I'm not taking physics so forgive my assumptions about water flow, thrust etc. but, If I were to say... take a 12" I.D. 24" section of PVC Pipe filled about 85% of the way, 6^2*3.14*24/231*.85 = so about 10 gallons, 80 pounds of water for our purposes. The remainder being air pressurized to about 200psi, the water then going through an opening of about 3in^2, for every square inch i should get 200 pounds of thrust to start with, so 600 total right off the bat, and then dropping off rapidly thereafter? Any corrections/comments, would this work?

 

[JaredJames]

Forget jetpack, are we talking about a bottle rocket?

 

[russ_watters]

If you have not yet watched the mythbusters episode on this, please do.

 

[bwheeler96]

If you have not yet watched the mythbusters episode on this, please do.

Will do, I understand the jetpack is a "better in theory" idea i just thought that with pneumatics, and i little innovativeness the weight of a jet could be eliminated and a substantial amount of thrust could still be achieved

 

[JaredJames]

Will do, I understand the jetpack is a "better in theory" idea i just thought that with pneumatics, and i little innovativeness the weight of a jet could be eliminated and a substantial amount of thrust could still be achieved

You mean "jet" as in airliner?

A substantial amount of thrust would provide a lovely boost, but unless you follow that with an amount of thrust required for flight it means nothing. If you don't you're just a glorified bullet.

 

[bwheeler96]

Amount of thrust required for flight must be > body weight, correct? And from there the more thrust you have, the faster you will accelerate/your top speed will increase? also if 150 p.s.i. of pressure is pushing water through a 3 square inch opening that mean 450 lbs of thrust yes?

Note: I understand that generally we should use kg for this type of thing but I am still getting used to using kg so please work with me here

 

[JaredJames]

Amount of thrust required for flight must be > body weight, correct?

Assuming we're talking vertical take-off (helicopter style) then yes. For hover, thrust = weight.

And from there the more thrust you have, the faster you will accelerate

Yes

your top speed will increase?

Not necessarily.

also if 150 p.s.i. of pressure is pushing water through a 3 square inch opening that mean 450 lbs of thrust yes?

Pressure = Force / Area

So Force = Pressure x Area = 150 x 3 = 450lbf. Sounds good to me.

 

[bwheeler96]

Thanks a ton, I really appreciate all the help. Now how would i go about calculating the rate of water usage? I can't find much on this and I saw a video with too much pressure and not enough water, the guy had plenty of thrust but this did not end well. Any formula for this?