Motor Performance Measurements

True or False – a rocket motor’s performance is quantifiable and there can be no disagreement as to what that performance is. This must be true – you can put a motor on a load cell, burn it and gather performance data. What is there to disagree about? Well, let’s take a closer look.

NFPA 1125 sets the guidelines that motor testing organizations use for the key measurements of burn time, total thrust and average thrust. This and the other NFPA documents that seem to rule rocketry are a mystery to many given the cost of the documents and their triennial updates. NFPA 1125 says that burn time is measured from the time that the thrust initially exceeds 5% of the maximum thrust, until such time as it drops below the same threshold. Both tails of the thrust curve are cutoff, so to speak. Total impulse uses the same starting point, but continues to the point of last measurable thrust prior to ejection. So burn time and total impulse are measured over two different time periods. Average thrust as labeled must be within 20% of the average thrust calculated during the burn time using total impulse measured over the burn time.

There are no doubt some good reasons behind the NFPA definitions. And in fact, the results from different measurement periods are immaterial in many respects. Total impulse rarely changes if measured over the full burn. The NFPA defined burn time is shorter, but from a practical perspective, close enough. However, if the numerator (total impulse) doesn’t change, but the denominator (burn time) changes, the result – average thrust, does change. And not insignificantly. Take the case of an 80 N-S motor with a total burn time of 1.36 seconds (rounded to 1.4) but an NFPA defined burn time of 1.24 seconds (rounded to 1.2). Is the average thrust 57 N, 59 N, 65 N or 67 N? And now, that 20% range doesn’t seem so large.

There are other reasons performance can be measured differently. Load cell performance can vary (based on load cell capacity, etc). Temperature and altitude impact performance, and while attempts are made to control for such, some error is introduced. A larger sample of motors burned by a manufacturer may also give slightly different results than the smaller sample burned for certification testing. And then there is rounding!

Total impulse and maximum impulse are not open to much debate. Burn time and average thrust, though, are whatever you want them to be (within limits). Roadrunner measures performance over the entire period of measurable thrust with some minor rounding. And it is printed right on the motor label where it is easy to find.