Analyzing motor performance

[Crimson0087]

So any tips on how to "estimate" thrust of my motor? I placed it face down on a kitchen scale and it pushed 45lbs but only briefly? I know that 45lbs would equal 200 Newton's but obviously the motor was not producing 200 Newton's for a full second. If I want to estimate how high my rocket might go on this motor how might I do this. Do I use average thrust, peak thrust, ect?

[stix]

This online calculator is reasonable:

http://www.unm.edu/~tbeach/flashstuff/RocketAltitudeFixedSize.html

 

Of course it's only going to be as good as the info put in.

 

An important input there is Total Impulse (in Newton Seconds), which is the average thrust (newtons) x burn duration.

 

[Reason for Editing]

I missed a couple of words out.

Edited October 20, 2018 by stix

[Baldor]

You can use a poor man test stand. Record a video of the test in the scale, then play the record frame by frame, noting time and thrust in an spreadsheet.

 

You can also make a proper test stand. A load cell and an openscale board works, Stix and I have been discusing this configuration, and Stix has ironed some kirks from the openscale board.

[stix]

@ Crimson

 

There comes a point in time when testing your own rocket motors, that you realise that in order to work out if your fuel is performing well, and to predict altitude, you need a proper test stand.

 

There are various ways to accomplish this (home-built, or purchased), regardless of which way you go, if your are serious, then it's an absolute must. Once you can record what you are testing, then you can advance more quickly. Otherwise it's just a guess.

 

As pointed out by Baldor, a SparkFun OpenScale board is very economical for the AD converter, then mounting the load cell (the physical hardware) connected to the electronics isn't that hard either. As far as software goes to interpret the results, I've been working on a software solution which Baldor has had some great input being a beta tester.

 

Unfortunately at this point, the software only shows Total Impulse and Specific Impulse and does not predict altitude (that's coming along next).

 

However, there are others on this forum, Peret and Dagabu (and others) that have alternate solutions.

 

I also have to say that if you are reading off a set of standard electronic scales, then there is NO WAY that they will be responsive enough to give accurate results. You'd be better off with spring scales and a camera as already suggested - even a spring on a stick would work better.

[Arthur]

Probably for sub second times a full electronic load cell and data capture system is appropriate. Record at 0.01 second intervals to get a big enough data set to be viable.

 

For long burn times, a video camera pointed at the kitchen scales is probably enough. You can single frame playback and get readings for every 25th or 30th or 60th of a second according to frame rate.

[stix]

Nup!!.

 

Arthur, electronic kitchen scales are not responsive enough. Or are you saying old school spring scales?... then I agree and apoligise for the assumption.

[Baldor]

I found 80SPS (Openscale max sample rate) is enough for getting a good feel of a subsecond burn time rocket performance (About 0.5s in some of the tests I posted). At least, it is enough to show you if you are improving your performance.

 

Imputing the data acquired in http://www.unm.edu/~tbeach/flashstuff/RocketAltitudeFixedSize.html , and eyeballing the rockets, seems you can be confident enough in your results to not cause a serious accident.

 

Some time in the future I want to make a 1000SPS or more test stand, using the ADC of an ARM uC. But is just because it can be made, not because it is needed.

[Baldor]

 

 

For long burn times, a video camera pointed at the kitchen scales is probably enough. You can single frame playback and get readings for every 25th or 30th or 60th of a second according to frame rate.

Calculate the time you will spend analizing a single test. Compare it to the cost of an openscale, a load cell, and some items from the hardware store. Add the time you will spend building the test stand. You have a full analysis with height estimation for various payloads before you finished counting frames of your first test.

[OldMarine]

Someone needs to build an Android compatible test stand with the data recording and analysis capabilities of Pete Hand's ACME test stand. We will beat a path to your door!

[shockie]

https://create.arduino.cc/projecthub/nmrsthrust/model-rocket-motor-test-stand-8cd2e7 just use a higher load cell.. 80sps

[SeaMonkey]

Arduino Rocket Motor Thrust Test Stand - Elemental Maker

 

 

Elemental Maker Channel

Edited December 1, 2020 by SeaMonkey