Measuring the thrust of rocket motors...

I had some spare cash a few months back, which enabled me to fulfill a goal I have had since a kid - build an experimental setup to measure the thrust curve of rocket motors. I have wanted to do this since I saw articles on thrust measurements in rocketry magazines and catalogs (see this one from the 1966 Rocket Development Corporation catalog), and it was finally time to drain the bank account - er, I mean, take the plunge. So I did the usual googling and came up with a system consisting of parts from Vernier:

• Dual-range force sensor (measures up to 50 newtons, good for LPR and some mid power motors)
• LabQuest 2 interface/data collector
• Logger Pro 3 software

All-in-all about 600 bucks of stuff. It turns out I didn't really need the Logger 3 software, which would have saved a couple of hundred, but it sure is a nice to have. I placed the order and the gear arrived within a week; I have to commend Vernier for their excellent customer service, as they not only shipped quickly but also responded very fast to my inquiries about how to make a personal, non-education purchase of their products.

The next step was to incorporate the force sensor into some sort of test stand. I suck at building such things, so I turned to the able engineer Duane, who whipped me up a fairy nice contraption in a couple of days:

The motor test stand, engineered and built by Duane (Click to enlarge).

You will note that the force sensor has a toggle allowing you to switch sensitivity from 10 newtons full scale to 50 Newtons. Most LPR rocket motors have peak thrusts near or exceeding 10 newtons, so I anticipate operating with the 50 newton setting practically all the time. The motor tube is 24 mm inside diameter, and you can see a red plastic Estes 18/24 mm adaptor sticking out of the top. The motor is placed in the tube, and fires straight down towards the ground, thereby minimizing the risk of the stand moving around. Got to work on the motor retention at ejection though.

The test stand plugged into the LabQuest2 unit. This is all I need to measure the thrust curve of a rocket motor
 (Click to enlarge).

The force sensor connects straight into the LabQuest 2 unit, which does the data recording. It also does a heck of a lot else, such as serve as a wireless hub to stream the data out on a local network to computers and tablets (there are free apps for those), enabling a bunch of people to participate in the experiment. It can also analyze the data and send the results out by email or SD card. I have not even begun to unravel all the functionality, but it was easy enough to figure out how to zero the sensor, take data,  and send it out to my computer. I'm sure I will discover many new things as I gain experience with the unit, which appears to be much smarter than me.

Since I have a ton of Estes A8-3 motors, I figured Saturday's TARC practice launch would be a good time to make some thrust curve measurements. After I finished flying my rockets, I loaded an A8-3 (with igniter) into the motor tube, connected the launch controller and the LabQuest 2 unit, and placed the test stand on the ground. I set the LabQuest to record measurements for 10 seconds at 50 samples per second - I wanted to catch the ejection charge, to see when it occurred after motor burnout. As the count from 5 progressed, I started data recording on the LabQuest and saw the following plot appear as the motor fired:

LabQuest2 screen capture showing the thrust curve of an Estes A8-3 motor. The sharp spike near the end
is the motor ejection charge (Click to enlarge).

Success! I happily loaded a second A8-3 (from the same pack) and repeated the process; I would have made more measurements, but it was approaching 2 PM and my stomach was telling me I had missed lunch. However, upon reaching my apartment, I quickly transferred the data from the LabQuest2 into my mac, whereupon the fun with numbers commenced. I discovered that I could use the Logger 3 software to replay the measurement run:

and zoom in on the motor thrust curve, at which point I could tell the software to integrate the area under the curve to get the motor's impulse (2.12 newton-seconds):

The Logger 3 software can integrate the thrust curve to derive the motor impulse (Click to enlarge).

Once you have the impulse, the average thrust is simply the impulse divided by the burn time of the motor. After I had analyzed the data from both motor firings, I then created a plot comparing the measured thrust curves to the published NAR test results (from 1995). As expected, the measured curves are similar - after all they are from the same box, with the same date code (A011216). However, they differ a bit from the NAR average curve, leading me to wonder if the A8's have changed a bit over time. The difference is not much, but noticeable. You will also note that the A8 is no such thing - it has a tad over 3 newtons average thrust, not 8. It should be labeled A3.

Thrust curve comparison (Click to enlarge).

Finally, here are the results in tabular form. I'm looking forward to making more measurements!!!!!

A8-3 thrust test results (Click to enlarge).