piclist 2001\01\19\010739a >
Thread: Rocket Altimeter Project
www.piclist.com/techref/index.htm?key=rocket+altimeter
face picon face BY : Bill Westfield email (remove spam text)



   From how I understand those motors are constructed I do believe age
   was the factor. The bottom of the engine is an epoxy based nozzle.

No, it's a pressed clay nozzle.


   If that nozzle cracks it will fly apart when the engine ignites. Since
   the whole contraption now has the whole diameter of the engine to
   flare out it burns much more violently.

No, when the effective nozzle size increases, the burn rate of (any)
propellant DECREASES (with some propellants, enough for the flame to go
out.)  OTOH, you have flames instead of a high-pressure stream of gas,
so it may LOOK more violent.

Pressed black powder motors such as the estes c6-5 are known to be
sensitive to temperature cycling.  The paper casing and the black powder
slug have slight different thermal expansion ratios, and repeated or
extreme temperature changes can cause the propellant to separate enough
from the casing to allow the flame front to burn between them.  When this
happens, the slug ignites all over, overpressurizes the casing, and
(usually) flies out the front of the motor/rocket like a roman candle star.
There's a rule of thumb that goes something like "don't launch at a motor
temperature more than 30F lower than the max temperature the motor has been
stored at."  (Didn't the original message come from someone up at Cornell,
where I imagine it's very cold at the moment?)

As for 30% lower than expected thrust...  I don't think you can accurately
measure thrust with an altimeter - a 30% error in predicted altitude is
likely to be mostly due to errors in calculating the rocket's drag, and/or
weight, and/or air density/etc.  On the other hand, the burn rate of black
powder varies rather dramatically with temperature, so a cold motor MIGHT
actually burn rather slower (and longer) than the same motor launched in
the heat of summer...

BillW

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In reply to: Your message of Thu, 18 Jan 2001 21:09:31 -0500
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