### Re: Altitude Records Online

Posted:

**Thu Apr 26, 2012 7:42 pm**makeit wrote:If you are getting close to Mach 1 then it will go higher if you add more weight.

hey PK, if you have the time and you are not joking could you please help me understand that?

If you take a mass up in the air then it gains energy. To understand this, first place a brick on you head. Then place the same brick on your head by having someone drop it from the second floor, the brick had more energy when it was on the second floor....

So a rocket is really a simple machine for turning the potential energy of the chemicals in the motor, into the potential energy of the rocket at altitude.

The more efficient we can make that energy conversion, the higher our rocket will go.

Drag is a force that comes from energy that is transferred from the rocket motor, via the airframe and into the air. Once imparted, the air is warmer, or moving faster, and that energy is no longer available for gaining altitude. Drag is a function of the speed of the rocket squared.

So a rocket doing 200Km/h is experiencing 4 times the drag of a rocket doing 100Km/h. So more speed=WAY more drag; ergo speed is bad right? Well almost...

There's one more thing, and it's the tricky bit. "As soon as a rocket leaves the ground, it is in freefall and has to generate more thrust than its weight (plus its drag) if it wants to Accelerate.

Imagine a 10Kg rocket on the pad with a D12 in it... What's going to happen when you push the button?

5..4..3..2..1..Zip, Nada, Nothin'.

You need 100N of thrust just to stop that rocket falling backwards..... So.. The slower a rocket flies, the longer it has to pay the price of gravity... If that 10Kg rocket got to 1000 feet in 1second, then it would have wasted 100NS of impulse(energy) on overcoming gravity. If it took 10 seconds, then it would have wasted 1000NS (a J class motor). Our 10Kg rocket with a D12 wasted 20NS and got nowhere!

So more speed = less gravity penalty; ergo speed is good right? Well almost...

From the above you can see that there is an optimum velocity to travel at to thrust to a maximum altitude.

BUT! You should consider that the vast majority of a rockets flight is taken up by the coast phase. And here comes the clincher:

During Coast...

The drag issue still applies: More speed is bad.

But the more mass you have at burnout, the further you go.

This is why bullets aren't made from polystyrene. For the same diameter and speed, a lead bullet has heaps more mass than a polystyrene bullet. It will go further and do more damage when it gets there.

This works out quite well for us. We can slow down our rocket by adding mass until drag comes down to an acceptable level. Then, at burnout, that mass helps our rocket coast further..

It takes a bit of simulating, but try a few different weights for your rocket an see if it makes a difference...

PK