Compressed Air Rockets – The Physics Behind the Flight

3 10 2008

I’ve been thinking about how the rockets work and which forces are involved in making the fly.  After some help from Will, Matt, Ben, and Blake, I was finally able to come to the equation that governs the flight of the paper rockets.

h = (P.rocket – P.atm)*A*x/(m*g)    eq. 1

h = height of the rocket

P.rocket = Pressure of the rocket inside or the pressure of the rocket launcher compressed air tank

P.atm = Pressure of the atmosphere

A = Cross-section of the rocket body (not the fins)

x = the length of the rocket body that is in contact with the launch tube

m = mass of the rocket

g = gravity

  • Note 1 – m*g = Weight,  So for those working in English units you just way your rocket and subsitute it for the m*g term.
  • Note 2 – make sure that your units are consistant.  NASA has made catastrophic errors and lost millions of dollars due to conversion errors don’t let this happen to you!  =)

Using the equation

In order to keep things simple I added in a conversion factor so that the answer will be in feet after you plug in the values. The equation then becomes

h = (P.rocket – 14.7)*A*x/(12*W)    eq.2

where

h =(ft) height of the rocket

P.rocket = (Psi) Pressure of the rocket inside or the pressure of the rocket launcher compressed air tank

14.7 psi = (Psi) Pressure of the atmosphere (@sea level)

A = (in^2) Cross-section of the rocket body (not the fins) = (π*d^2)/4

x = (in) the length of the rocket body that is in contact with the launch tube

W = (lbs) weight of the rocket

Examples

I’d run through some calculations but I need a digital scale that is small enough to measure the rocket.  I’d suggest buying one that has the capability to measure in grams because you will be a better resolution.  Then you can convert it over to pounds.

More to come!

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Compressed Air Rockets (Part 3.5)

2 10 2008

I made a rocket last night in about 30 mins even while documenting it.  They are really simple to make.

Materials List:

Heavy duty Paper 2-3 peices

Tape

Pen

Scissors

Large Circular object  (i.e. Pot Lid, coffee can lid, etc.)

PVC Launch tube