Static Test Rocket Motor Analyzer

For my research motors I've finally gone and acquired the load cells for proper static test thrust measurement.

I put together a package and now I can wirelessly monitor my test fires, and immediately get motor specifics. It will tell me things like burn time, average thrust, peak thrust, total impulse, sampling specifics, and even a motor designation! The sample data is also conveniently able to be pasted right into Excel for nice graphs.

Next weekend I'll be doing my next series of static tests and look forward to utilizing this new hardware.

More information on the project page here: https://hackaday.io/project/7942-rocket-motor-analyzer

Have you considered adding a pressure sensor and take internal pressure readings during the burn?

I think that would be very useful. I watched one of our club members do a test burn on his stand and the pressure was close to 1200 psi. He didn't bother with the next burn with the next size smaller nozzle.

The point of the test stand is to test propellant, not motors. The members test stand uses a 2 grain 38mm snap ring motor for all tests. The forward closure has a SST tube attached with the pressure sensor. To test and characterize a propellant, he usually does three burns with three different nozzle sizes. With the thrust and pressure data and known motor and nozzle data, he can characterize the propellant quite well.

I have thought about doing a combustion analysis on the fuel, but I would probably go about doing strand burning to characterize the propellant. I have thought about adding pressure gauges, but thrust right now is what I'm interested in. I've always operated with quite a bit of safety margin in my designs to limit CATO possibilities.

I've been successful with the modeling of the fuel I'm utilizing and matching prediction to empirical data, but I'm going to see if I can squeeze a few more percent by playing with expansion and cone angles (and hence nozzle weight and efficiency)

IF you are trying to characterize something you need pressure. Pressure is the cause, thrust is the effect. Thrust can be off due to many factors and pressure is nearly always true.

Edward

While I do agree that pressure is probably the best information on internal workings of the motor, I'm trying to test different convergent and divergent angles and expansion ratios. This cannot be done with pressure alone.

But you would need pressure as a known variable. For instance, when you change the C/D angles, you will need to know what your pressure is to simply rule it out in case of an anomaly. The pressure in the motor will change when you change the C/D angles.

The pressure in the motor will change when you change the C/D angles.

Click to expand...

Why would that change the pressure? Pressure is defined by the propellant type and Kn. Thrust coefficient is defined by divergent cone angle and expansion ratio.

Jeff the convergent angle is not really important as long as the contour is smooth. A divergent angle of 15 degrees will provide a shock-free expansion of the exhaust gases. This engineering was figured years ago. I don't think you'll find any improvement with different C/D angles but I suspect you will probably try anyway.

Steve G.

I know that 12° divergent angles should only be up to a couple of percent better, but at 200lbs (my large motor design is in this class) thrust that equates out to almost 5lbs of thrust added, which easily compensates for the extra nozzle weight. But yeah... This is all part of the fun.

As a side note, ran my first small motor through the equipment:





You can see the erosive failure of the nozzle that didn't cure long enough. I was curious if it would fail, and was happy when it did to see the graph. This motor was not intended to fly or have a high initial thrust.

5 pounds @ 200 pounds is 2.5%, which is probably going to be lost in the noise of your setup unless you purchased NIST-traceable load cells. Again, I would add a pressure transducer. Why measure the effect when you can have the cause.

Edward

The nozzle width and burn area define KN, when the chamber pressure rises it modifies the amount of gas density that chokes when it reaches sonic(mach 1) velocities. Chamber pressure is altered by propellant variables such as burn rate, etc, Once the exhaust is choked at a specific pressure it's choked and there is no way for anything after the choke to change the pressure inside the chamber. It's going supersonic after the choke and begins expansion in the divergent region, depending on the expansion we get pressure differences, and depending on nozzle divergent angles we get off axial thrust of varying degrees.

I don't think I'm misunderstanding anything here... Divergent angles don't change chamber pressure unless I'm grossly misunderstanding the principles of choked flow.

And as I side note I did finally locate a viable 1000 psi pressure transducer for a reasonable price, it will arrive later this month from China and be integrated into this analyzer

sghioto said:

Why would that change the pressure? Pressure is defined by the propellant type and Kn. Thrust coefficient is defined by divergent cone angle and expansion ratio.

Click to expand...

I stand corrected.
I do have a question though; In answering, you did not address the effects of changing the convergent angle (Obviously weight is one effect, but that is outside of this question.), could you shed some light on this? (I hope I don't step on anyone's toes here.)

It had been shown by others that the convergent angle has very little impact vs other factors. The difference between flat (just a choke in a bulkhead with slight chamfering) and 45° is just a percent or so better in thrust. This is due to the nozzle choking (add it supposed to do). The convergent angle does play a role in erosion of the nozzle though, and if not smooth some heat and kinetic energy will be lost. My interest in convergent angle is primarily erosive in nature.

Cool. You learn something new everyday.
Btw, your projects are very interesting and are exactly the things I try to do on occasion. How well does that sparkfun loadcell amp work?

The loadcell amp isn't sparkfun specific, this one was from Amazon (where I obtained the load cells as well) for cheaper. Besides having to move a surface mount resistor to get a faster data poll and rewriting half the library it works great.

I've had some other amateur rocket enthusiasts express interest in obtaining this setup, and will probably be making a small run of PCBs.

Adding the electronics to the new stand... I like the wheel attachments

I we able to complete 4 static tests today at the MDRA launch, I was able to close the wireless link quite well and was able to obtain some great data: this motor is a 50% I motor, I486, with one second burn:

How are you graphing out the data after it has been stored in a spread-sheet? Could you post the links to the loadcells, loadcell amplifier and the pressure transducer?

Have you considered taking temperature data?

Over my wireless link I send all the telemetry, on the other end (this case a laptop) I log all the communication. I can then copy and paste into Excel for graphing. The analysis I can view on the field during testing without doing anything.

I will probably be making a GUI for this so it's all in real time, but not necessary for accurate measurement. Just ease of use.

If you follow the link in my signature to the analyzer you can see the components used. However, I'll probably be doing a limited run of PCBs and selling some kits because I've had a decent amount of interest in this from the amateur rocketry community.

I'll be adding chamber pressure monitoring here when my pressure transducer cones in later this month, and I have thought about case temperature, but finding fast enough response sensors is a little difficult.

Here's a 24mm-ish 5 grain motor test, about an F58 that I'm working on for MPR stuff:



I think there was probably a slow ignition of the grains, I might need to prime between them (sugar)