Static Test Data

[Ray]

I static tested 4 motors Saturday evening. Results were mixed.

I had trouble calibrating my load cell, I think I have a wiring error. Next test will have better data.

First motor, a 3 grain 38mm formula called everclear, performed as I expected, but at lower than predicted values.

Second motor, a 75mm 2 grain aluminum formula, performed at much higher pressure than I expected and had a strange thrust curve that I don't fully understand. Actually with the pressure achieved, I am surprised that I didn't CATO the motor.

Third motor, a 3 grain 38mm formula called everclear at higher pressures, performed much as I expected, but at lower than predicted values.

Fourth motor, a 5 grain 75mm aluminum formula, performed better than expected, but at higher pressures than predicted. It also spit all five casting liners during the burn resulting in large pressure spikes.

All in all a good night, but I realize that I have a lot to learn about predicting the performance of the motors I run. The 2 grain 75mm really has me baffled. The software that I use to predict the motor burns tends to be optimistic by about 10-15%, so the two burns of the everclear formula are very much what I expected.

Our group is working to get good solid burns at around 1000 psi. We are creeping up on it slowly, learning a lot along the way.

Anyway, take a look at the data in the attached PDF file. If you have any feedback on the two grain 75mm thrust curve, I would be very interested in hearing it.

[Greg O'Bryant]

Ray;
First your experiment is out of my league. lf I were to try to make heads or tails out of the 2 grain aluminum engine. I would say perhaps the grain fragmented, but not completely so that the surface area was the highest at the start of the burn and decreased with time just my guess?

[Ray]

I was thinking that I may have experienced erosive burning...but the core to nozzle throat was within what I considered "safe". I'll get it figured out, just have to ask some of the more experienced folk in the hobby.

[Ray]

They are interesting results. What is the configuration of the grains (how are they burning)?

Can you give some information on your data acquisition hardware and software?

The grains are BATES style. 5" long in the 75mm land (core sizes listed in the graphs), and 1.9" long in the 38mm land.

Test stand is a horizontal affair with 2x2 steel tubing, a welded mounting plate for a home made load cell. Motor is mounted on a section of 1515 8020 brand rail that in turn is mounted on linear bearings made out of UHMW plastic.

A custom designed and built INA125 based instrumentation amplifier is used to amplify the signal from the strain gauges, fed into a DataQ DI-194RS A/D converter. I use a custom piece of software that I wrote in LabView to do the data aquisition, as a back up to my flawed software, I use WinDaq lite from DataQ to capture data too.

Data Reduction is tedious and involved, I do it manually in Excel.

The data in the 38mm is not smooth due to an error in wiring in the load cell that wasn't identified until after the test. There are about 4 counts worth of noise from the A/D converter. Voltage at maximum thrust was just over 1/2 a volt...instead of the approximately 3 volt value I was expecting at that thrust level.

If you have more questions, I can try to answer them.