Ns variation in CTI motors?

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Well-Known Member
Oct 18, 2016
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Hey everybody-

I know there's variation allowed in certified rocket motors, but how much is typical? Suppose I buy 3 different I224 P29 motors...how close will they be?

If bought from same batch/time of manufacture, virtually identical.

From different times/batch... can vary a bit. I don't know percentages.
Dealers usually rotate stock, so there may be various batches in stock at same time.
Just ask when you buy, for motors all from same batch.

Batch codes are stamped on every motor reload tube. Competitions such as TARC teams, those with experience anyhow....buy 10-25 motors from same lot to ensure testing and final performance yield same results.

What are you trying to accomplish?

If for clusters, they will be fine from any batch. If entering contest, then get same.
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Per NFPA 1125 up to 6.7% variation in total impulse is allowed, but the certifying organization only has to test 2 motors for an HPR motor like this one, so it's hard to argue they are getting meaningful statistics. Anecdotally composite motors have pretty tight tolerances on total impulse, probably better than the requirement in most cases.
You can check the data sheets at NAR and CAR. Some contain ave, stdev, maybe even the raw data samples. However, the amount of info given for each motor varies, and as noted above, may not be statistically significant.
Good idea. I just looked at CAR for the I224 and I243, but didn't see any variation data. Their motor testing doc says 3 samples of new motors are required for testing. However, their thrust curve just shows one line. Unfortunately, the certification letters don't spell anything out for those motors either.

mikec, yeah...sample sizes of 2-3 aren't statistically relevant. It seems like manufacturing processes these days are good enough that the motors would be really close though.

Another noob motor related question...where do I look up the core design for specific motors? People here make reference to some motors having offset cores, etc, but without having a motor in hand I haven't found a way to find that info.

Another noob motor related question...where do I look up the core design for specific motors?
Usually not documented -- why would it be? Nearly all motors are BATES; long-burns are either offset core or C-slot. There are a very small number of finocyls. And a few end-burners.

Aerotech drawings sometimes have this; assembly instructions for some motors have cutaways.
I'm working with a college team that is developing a solid motor. They have done 6 static tests, each a 3 grain 38mm motor. Range in the mass of all six motors was 0.3 grams. Range in Ns impulse between six motors was 4 Ns. Six still isn't a large sample, but with good processing and keeping the mass of propellant consistent they have had good results.

Running 2 or 3 tests is really only good for making sure the one test wasn't an outlier. 10 gives a reasonable uncertainty estimate. 30 tests would be doing it "right," but who has money for that testing?
This might be why delays are no longer available for large motors... Didn't Aerotech used to offer motor ejection in 98mm?

They were unreliable at high altitudes and nfpa required another means of deployment once electronics became used. Plus costs are better if you only burn 2 big motors instead of 6.