Of Load Cells and Fourier

I ran across a post on LLC competitor SpeedUp's website about vibrations in their test stand. The post reminded me of some similar tests I did to measure vibrations in thrust measurements.

As you might be able to tell from the picture in this post , I'm using a beam load cell to measure thrust. It has the advantages of being relatively easy to mount and requiring only one load cell. It has the disadvantage of turning the engine into a vibrating, cantilevered beam supported by the load cell. Fortunately, my data sample rate was 10kHz, so I was able to spot the low frequency ringing by comparing the thrust and chamber pressure data.











In order to determine which frequencies were caused by the test stand and which were "real", I did a hammer test. My test stand is mounted on a steel I-beam, so I turned on the DAQ system and hit the beam with a hammer. The next two charts show the raw thrust data and a FFT of thrust data. The FFT shows vibrations at 31, 33, and 85 Hz, so I can use a digital filter to remove that frequency content from my test data. I use and recommend Sigview to do plots, FFTs, and filtering.

Regression Rate

When I started this project, I chose nitrous oxide and HTPB for propellants. However, at the time there was no published data on regression rates for that propellant combination. So I started out with a small lab motor that is shown in the previous posts. It was a simple design, similar to motors used in high power rocketry with the exception that the case was 4130 steel instead of aluminum.

That motor turned out to be a good way to do many more things than simply collect regression data. Finding vendors, setting up data acquisition, and learning plumbing best practices are all non-trivial tasks. In keeping with the test a little, build a little philosophy, I find it much easier to incrementally add features as you go. Not worrying about all the features a motor (or any system for that matter) needs to have will often generate more progress than a "single step to solution". Or as my brother says, sometimes you need to just go build something even if you're not sure what it is.

In the intermediate time between when I started and now, a group at Stanford has started publishing data for nitrous oxide hybrid motors. You can read the first page of their papers here and here. Fortunately, their data and mine compares reasonably well as you can see in the chart.


Once I had reasonable regression data and a working test stand it was time to start building a flight worthy motor. However, before you design a motor you have to choose a vehicle architecture. A VTVL vehicle has a much different motor than a sounding rocket. I'll talk a little about motor applications in a future post.

About this blog

First, "Moonlighting" is a bit of a misnomer.

moon·light unˌlaɪt/ Pronunciation Key - Show Spelled Pronunciation[moon-lahyt] noun, adjective, verb, -light·ed, -light·ing.

5. to work at an additional job after one's regular, full-time employment, as at night.

You have to get paid for it to be moonlighting. I'm not, so it's not moonlighting.

I've been working on building hybrid rocket motors in my garage for about two years. Having made some small measure of progress and lacking any eminent prospects for starting a rocket business, I thought I would share some of my lessons with the community. I hope that what I can share may be of some small help to others.

To start, here is a picture of my first motor.

Stay tuned.