Physics Behind Why Rockets Fly

Physics Behind Why Rockets Fly

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In order for any rocket to fly, it must obey some basic rules of physics. No rocket can escape the cardinal rule that the center of gravity must be in front of the center of pressure.

Center of Gravity

The Center of Gravity or Cg is the point that behaves as if all the mass was concentrated in that one spot. Simply put it is the point at which you could balance the rocket on your finger. The Cg is also the point around which the rocket rotates. This becomes very important for the rocket's stability as you will soon see.

Center of Pressure

The Center of Pressure or Cp is similar to the Cg. The Cp is the point at which the forces from the air pushing on the vechicle seem to be focused.
What is a Cg or a Cp?
Basic Rocket

Why Does This Matter ?

When a rocket is in flight, the force of the air pushing on it keeps the Cp behind the Cg. This is important because the rocket is pushed by the force of the exhaust coming out the back. If the Cp was in front of the Cg, the rocket would want to spin around (so that its back was pointing forwards).. However, because the exhaust is always pushing the rocket forward, it spins around again and again... and you get the picture from there.


So now that you know all about Cgs and Cps, it should make perfect sense what fins are for.

Still a little confused? The fins are extra surface area near the back of the rocket. This brings the Cp further back on the rocket without significantly moving the Cg..

Calculating Cp

Calculating a Cp is rather involved. The easiest way to do it is by using Barrowman's Equations, although these are still fairly complicated. I was going to explain them, but I found a site that does a better job than I could ever do.

The UAF Student Rocket Project builds and flies sounding rockets with help from Wollops Flight Facilty.

Our latest mission, SRP4, is a student designed and built payload. It will be carrying a science instrument designed by students and professors at Toyama Prefectual University in Japan. Our goal is to measure the D-region of the ionosphere.

SRP4 will fly atop an Orion sounding rocket motor that will be supplied by NASA's Wollops Flight Facility.

We expect to launch from Poker Flat Research Range in March of 2002.

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We calculated the Cp of the motor and payload to be located 166" from the nose tip (or NEP).

To have the stability we desire, we need to have the Cp 1.3 calibers (or 1.3 times the diameter of the payload) behind the Cg. This comes out to about 19". Which means that the Cg must be no farther back than 147" NEP. We had to move some of the internal electonics around a bit to accomodate these requirements, but we were able to achieve our necessary Cg without adding weight to the front of the rocket.

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