piclist 2001\01\19\013708a >
Thread: Rocket Altimeter Project
www.piclist.com/techref/index.htm?key=rocket+altimeter
face picon face BY : Sean H. Breheny email (remove spam text)



Hi Bill,

At 09:52 PM 1/18/01 -0800, William Chops Westfield wrote:
>     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.

That's what it looks like to me, a clay material.




>     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.

Makes sense, but in my case, it WAS violent, it didn't just look it.



{Quote hidden}

Wow, I think you hit the nail on the head. Those motors had been through
many cycles of temp change, in my basement (where my workshop is and I
frequently heat it up to 75 F and then let it drop down to 40 F or so when
finished working.) Your argument makes sense, AND the symptoms are exactly
what I saw (a piece of something, which was burning brightly, flew up in a
large arc when the engine exploded. Also, when slowed down, the video shows
a large jet of flame coming out the top of the rocket). Also, on the day I
launched, it was about 25 F outside, and I had just brought the engines
from my house which was at about 65 F.

BTW, yes, I'm now at Cornell and it is cold. However, I conducted the tests
in Moscow, PA (near Scranton), where it is also cold :-)


>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...

Well, my altimeter doesn't just read peak altitude. In fact, it is actually
an acceleration datalogger which I used to obtain altitude. I got the
measured thrust from the acceleration data directly. Actually, the measured
drag was within 1.6% of the predicted value! Cold could have played a
factor here, too, though, because the burn time was 1.83 seconds, instead
of the normal value of 1.62, and the delay period was 5.83 seconds instead
of 5. So, overall, the entire operation of the motor was slowed down.

Thanks for the explanation!

Sean


>BillW
>
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<4.3.2.7.2.20010119011944.00d2ae50@postoffice2.mail.cornell.edu>

In reply to: <CMM.0.90.4.979883577.billw@flipper>
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