Simple Rocket Motor Test Stand?

The Rocketry Forum

Help Support The Rocketry Forum:

Joined
Apr 21, 2014
Messages
7
Reaction score
0
How might I go about testing the thrust of a relatively weak engine? It will put out a maximum of 45lbs of thrust. Keep in mind this will be a water rocket, so there are no hot or toxic elements involved. I would prefer to stay on a budget of under $50 if possible.
Google has left me with a few commercial products which are more expensive than I would like, and not much information on making my own test stand.
If you have any info on constructing a test stand, please link me to it.
 

daveyfire

Piled Higher and Deeper
TRF Lifetime Supporter
Joined
Jan 26, 2009
Messages
3,218
Reaction score
55
Location
thank u, next
When I was in 8th grade I built a test stand that consisted of a bathroom scale and a video camera. Run the motor, videotape the scale output, and trace the force frame-by-frame to reconstruct your thrust curve at 30 Hz. Of course, there's all sorts of issues with time constants, hysteresis, and the fact that it's kinda tough to read a bathroom scale on a video camera, but it worked well enough that I got performance numbers roughly in the area of what I expected.

You might be able to reverse engineer a modern digital scale-- take the strain gauges out and use them directly. Better yet, Aerocon has some cheep load cells (this 10 kg'er is $15: https://aeroconsystems.com/cart/load-cells/10-kg-load-cell/ and Bob is hilarious and amazing to deal with) that you could combine with a home-built INA125 amp (if you really need to stick to $50 cheap; does Burr-Brown still make that chip even?) or you can buy one for $30 from Phillips Instruments (https://picom2.com/) but then you still need some sort of A/D converter. A sound card is a cheap A/D converter but matching levels would be tough. A DATAQ DI-145 is $29 and would do just fine. That puts the total at something around $80 with batteries and stuff, plus whatever it costs to build the widget that holds your motor in place. More than a scale and the camera on your cell phone, but also way more accurate and repeatable.
 
Joined
Apr 21, 2014
Messages
7
Reaction score
0
Would the use of an Arduino make this system a bit less complicated? This is my preferred method as I have a good deal of experience with them.
And, I don't see why not, but I will ask anyway; can the motor can be pointed straight down on top of the load cell (with proper support of course)? Is it just as simple as that? This seems like a dumb question to me, but I want to make sure.
 

daveyfire

Piled Higher and Deeper
TRF Lifetime Supporter
Joined
Jan 26, 2009
Messages
3,218
Reaction score
55
Location
thank u, next
Would the use of an Arduino make this system a bit less complicated? This is my preferred method as I have a good deal of experience with them.
And, I don't see why not, but I will ask anyway; can the motor can be pointed straight down on top of the load cell (with proper support of course)? Is it just as simple as that? This seems like a dumb question to me, but I want to make sure.
Yes, absolutely, wow I can't believe I didn't think of that. (I spend my day bathed in NI and AD DAQ equipment, I think it's something subconscious.) All you need is something to do A/D and record the data. You can certainly fire the motor into the load cell but you're going to have to make an approximation for the propellant mass flowrate through the nozzle and the ensuing weight loss. If your propellant mass is small compared to the thrust, you can probably ignore it.
 
Joined
Apr 21, 2014
Messages
7
Reaction score
0
First, please correct me if I'm wrong, but I believe that the Arduino is capable of analog to digital with it's analog ports.
The propellant mass should be around 5kg, however if the motor under-performs, tests could be done with 20+kgs. How is testing performed without having to make an approximation for the weight loss over time? I'm imaging a system where the motor is on it's side, and one where the fuel is stored off of the stand.
 

daveyfire

Piled Higher and Deeper
TRF Lifetime Supporter
Joined
Jan 26, 2009
Messages
3,218
Reaction score
55
Location
thank u, next
Yes, exactly, an Arduino is definitely capable of doing A/D and is a great low-cost option for it. We are in agreement! Sorry for wording it poorly.

For more complex stands, the chamber is usually mounted to a load cell independent of the tankage and then coupled using flex lines to minimize the thrust transmission into the feed system. This, of course, leads to all sorts of interesting acoustic instabilities as the whole thing shakes around during testing.
 

Winston

Lorenzo von Matterhorn
Joined
Jan 31, 2009
Messages
9,363
Reaction score
1,498
How might I go about testing the thrust of a relatively weak engine? It will put out a maximum of 45lbs of thrust. Keep in mind this will be a water rocket, so there are no hot or toxic elements involved. I would prefer to stay on a budget of under $50 if possible.
Google has left me with a few commercial products which are more expensive than I would like, and not much information on making my own test stand.
If you have any info on constructing a test stand, please link me to it.
Sample time on a digital bathroom scale video recorded might be an issue. Would probably depend on the specific model. May not even be a factor. Fish scale recorded during burn might give you what you need. When I was a kid, I used a 60 RPM AC motor to rotate a drum made from a section of a large paper tube, a fish scale, a felt-tipped pen, and graph paper, initially calibrated by generating traces with different weights attached. When we transitioned to large Zn/S, KNO3/sugar, and GALCIT based motors, we just launched and measured altitudes with triangulation.
 

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,353
Reaction score
37
You can also use a vertical test stand and a horizontal scale.

From https://aeroconsystems.com/cart/ts-pics
courtesy of NASSA

We built a simple stand out of wood, using a small "baby" scale to register thrust. With the help of a video camera we where able to test fire the sample motors and record the readings of the scale. Back in the shop we viewed the videos and with stop motion, we timed the thrust registered on the baby scale with the time on the video. The test stand was quite easy to construct as can be seen in the photo. When the motor fired the upward thrust forced the opposite end down onto the scale and we got our readings. The stand is designed to reconfigure. Using a 25 lb. scale, we are able to double the reading by shifting the center post to give us a 2:1 reading. We have since gone to a 50 lb. scale and can now read 100 lbs. of thrust. 100 lbs. of thrust is NOT necessary for "test" motors!!

Bob
[h=1][HR][/HR][/h]
 

pyrobob

Well-Known Member
Joined
Dec 6, 2013
Messages
2,410
Reaction score
31
When I was in 8th grade I built a test stand that consisted of a bathroom scale and a video camera. Run the motor, videotape the scale output, and trace the force frame-by-frame to reconstruct your thrust curve at 30 Hz. Of course, there's all sorts of issues with time constants, hysteresis, and the fact that it's kinda tough to read a bathroom scale on a video camera, but it worked well enough that I got performance numbers roughly in the area of what I expected.
Worked for us, too. Although we opted for $5.99 Wal Mart fish scale. We had no budget and I was 14 or so, but it worked for Science Fair purposes....
 
Top