Thrust and Impulse variance in commercial APCP motors

boatgeek

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I have heard of top TARC teams buying large numbers of motors, and then weighing them to try to weed out any that might have a tad bit more or less propellant.
The TARC team I mentor opens several motors (CTI, 24mm 2G), weighs grains, and then mixes and matches to get consistent propellant mass. That seems to work well. The max-to-min variation is on the order of 1-2% of total grain mass, including ignition pellet and casting tubes.
 

jqavins

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In a funny sequence of events, I did some googling to find just this sort of information (though I was looking for thrust variation, not impulse) and stumbled onto a NASA page about something else, the reason for the typical shape of BP engines' thrust curves. I found the answer I was looking for at ThrustCurve.org, then I came here to the propulsion forum to post a link to the NASA site, and here's this thread about motor performance variation. Look for X, stumble on Y, find X, share Y, stumble on X again!
Peak thrust and burn time will usually vary much more than total impulse though.
According to John, the limit for thrust variation is 5%. Actual typical variation I don't know.
 

sriegel

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This is one of the reasons full K and larger motors must be used with electronic ejection systems. We learned in the early days that pyro delays in large motors burned much longer in high altitude flights than when tested closer to sea level.
I had a J motor delay run a full 7 seconds longer than drilled in Alamosa, CO, last year. They're up around 7K MSL.
 

Grog6

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I just reread this thread, and I noticed something that caught my attention: apcp motors burn longer at elevated altitudes.
I have to wonder WTF? as the burning environment for the propellant is pretty isolated from atmosphere; it's at hundreds of pounds of pressure, separated from the rest of the world by a nozzle.
Any Ideas?
 

AlexBruccoleri

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I just reread this thread, and I noticed something that caught my attention: apcp motors burn longer at elevated altitudes.
I have to wonder WTF? as the burning environment for the propellant is pretty isolated from atmosphere; it's at hundreds of pounds of pressure, separated from the rest of the world by a nozzle.
Any Ideas?
After burnout the chamber pressure is quite low. Not sure how the delay grain/residual propellant pressurize the chamber though.
 

Johnly

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If the gas velocity through the nozzle throat is supersonic, the outside atmospheric pressure has no effect of the propellant combustion process.
The term often used for this condition is "a choked nozzle". Once the velocity though the nozzle throat goes subsonic it becomes "unchoked" and the atmospheric effects on the burn rate become apparent. as evidenced by the change in the burn rate of the tracking/delay grain at high altitudes.
 
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