# Designing a compressed air vehicle

1. Mar 13, 2015

### aerograce

I am doing a school project, building a compressed air powered vehicle to reach desired destination.
What I am thinking is:
I will use a paintball compressed air tank.
Four nozzles.
And an controller board to control valve opening.
Valve will only open for certain time like (50ms). After each round of opening, controller board will determine its location and compare it with destination. This will give feedback on which valves to be opened in the next round. But well, all those are just some background information.

My question is, for example, my compressed air is charged till 30 bar. And the air will eject to the ambient.
Based on bernoulli equation, I get this for exhaust air velocity from the nozzle:

Pg + 1/2 p Vg^2 = Pa
Pg=30bar
Pa=1bar
and p = air density = 1.225 kg/m^3
So what I get for exhaust velocity is around 3km/s.
OMG is this true?!?!

Schematic diagram is attached as follow.

And if it is not true, how do I get the exhaust velocity of the air? I need them to compute the final thrust of the air thruster.

#### Attached Files:

• ###### schematic.JPG
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2. Mar 13, 2015

### Staff: Mentor

Have you considered getting the thrust more directly, by directly measuring it? Fix your thruster nozzle to a scale, and see what the scale reads?

3. Mar 13, 2015

### aerograce

I'm doing initial design. I need some theoretical validation of my model:)

4. Mar 13, 2015

### cjl

Your equation is not valid for compressible flow, and going from 30 bar to 1 bar is definitely in the realm of compressibility. Look up "choked flow" for a more realistic set of equations.