In a previous Instructable, I shared my experience of building a small hybrid engine. While that hybrid seemed to work effectively on a static sawhorse, my haphazard tests had no way of measuring its thrust accurately. While math can be used to estimate the thrust, I'm not sure I trust my back-of-the-envelope calculations. So before I embark on any other dangerous rocket projects, I need a tool to measure engine thrust.
That's exactly what we're building.
This rig is something I designed to be strong, compact, and versatile. It's strong so that it can withstand the force of whatever engine I attach to it. It is compact so that I can store it easily or transport it should I need to test an engine in a different location. Lastly, it's versatile enough to handle all sorts of different engines: from small Estes models and sugar rockets to hybrids, as well as supporting different spring scales to accommodate each engine.
Let's get building!
Step 1: Materials and Tools
- 3 long pieces of angle aluminum (18 in. each)
- steel plate (I used a nail plate)
- stainless steel hinges (the stronger the better)
- steel angle brackets
- heavy duty sliding drawer-pull hardware (anything big and metal that slides back and forth will work well)
- (2) small hose clamps
- (20) 1/4 in. hex bolts (roughly 1 in. long)
- (20+) 1/4 in. hex nuts (obviously)
- (20+) 1/4 in. lock washers
- a set of spring scales
- drill press (with a 1/4 in. bit)
- flat-head screwdriver
- JB Weld
- a weekend, or at least a few hours of work and some time for the glue to set (another few hours)
- I spent about an hour a day every day for a week making this (that includes lots of mistakes and a few minor design changes)
Step 2: Build the Legs
I guess we should start by building the bottom of this thing.
The legs are pretty simple, each one is just a piece of angle aluminum with an angle bracket at one end and a hinge at the other. So long as all the holes are drilled in the right places the legs will work just fine.
So... it's just a measure-twice cut-once sort of thing. It's worth taking the extra time to measure twice, as fixing mistakes often consumes more time than doing it right the first time (I have learned from experience).
The tripod design has three legs (obviously). Each leg will support a different side of the stand, so I angled the hinges accordingly. The hinge attached to the leg supporting the front is straight, while the hinges attached to the left and right rear legs are angled to the right and left, respectively. You can see this by looking carefully at a picture of the entire rig, such as the one in the previous step.
Step 3: Make the Central Plate
Again, this isn't too difficult so long as everything is in the right place. I would recommend putting each end of the hinges up against the bottom of the nail plate and tracing each one. Make sure to mark which side of the plate is which. In my case, the sides were not symmetrical! This led to some confusion and some holes drilled in the wrong place. Tracing out each hinge is worth the time as it will save you from the headache of retracing steps and fixing mistakes.
When determining the position of each hinge, make sure to leave plenty of room to mount the sliding rail. For maximum stability, mount the legs around the exterior of the plate and the sliding rail in the middle. In some places, it may be necessary to share a connection and use one bolt to secure the leg hinge and the sliding rail hardware with the steel plate sandwiched between them.
When drilling holes, ensure that you remember which side of the plate is which via the method of your choice. I put a small piece of tape on the top side to prevent the confusion of drilling holes in the wrong spot. I would have designed everything with a bilateral symmetry, except that the hinges were not symmetrical. Hence, I had to keep track of which side was which.
Step 4: Attach the Engine
There was very little to no room for bolts to go through the top of the sliding rail, so I used some JB Weld to secure two bolts and some extra angle aluminum onto the top of the sliding rail. From here, I bolted an additional piece of aluminum on so that there was space to attach a set of hose clamps. The smaller hose clamps allow Estes solid fuel engines to be secured to the sliding rail - though make sure everything is very tight before testing the engine.
The attachment I devised could be scaled up to accommodate larger engines, such as the hybrid I made in a previous instructable. Though I would not rely on hose clamps for anything that could produce more than 20 Newtons of thrust.
We also have to attach the spring scales. I used JB Weld to secure a black angle bracket to the rear sliding portion of the sliding rail attachment. Near the center of the steel plate on the side of the sliding rail, I placed a bolt with an additional nut to hold the opposite end of the spring scale. Nearly any of the classroom-style spring scales can be placed between those two points.
Step 5: Fire!
But first, a note on safety:
Rockets are super dangerous. Though this rig is very study and safe if used correctly, static testing of engines is not covered by NAR safety codes. https://www.nar.org/safety-information/model-rocket-safety-code/
As such, it may be worth consulting local regulations before testing an engine. A list of those regulations and the US States they apply to can be found here: https://www.nar.org/find-a-local-club/section-guid...
Some common-sense guidelines:
- Keep a fire extinguisher nearby. Though model engines are designed not to catch their surroundings on fire, the risk is still present. Hoses are also a good safety measure but they are not a replacement for a fire extinguisher
- Safety goggles might be a good idea
- Not standing within 15 or so feet of this thing might also be a good idea
- not a toy, don't let children play with it, etc.
- you are responsible for your own actions. Don't do anything stupid.
Now that we have gotten that over with, let's begin testing!
I started with an Estes B6-4 engine and consulted some rocketry literature. A B6-4 engine outputs an average thrust of 6 newtons for a total of 0.8 seconds, meaning the impulse of the engine is 4.8 Newton-seconds, placing it well within the B impulse class. According to this model engine chart, the B engine has the potential maximum thrust of 12 Newtons, more than double its average. Though this may only be true for a tiny fraction of a second, before the engine averages out to 6 N.
Before testing the engine, it is necessary to select an appropriate spring scale. Since there was a possibility that the engine would output 12 Newtons, I selected the 20 Newton spring scale. This proved unnecessary, and I would recommend selecting a spring scale closer to the range of the engine's average thrust (I should have chosen the 10 Newton spring scale).
When the rig has been set up and the engine securely attached, be sure to use the appropriate electric ignition system. Make sure that you have taken the delay and parachute ejection charge into account, as the engine may spew flaming material out of the wrong end after the fuel is consumed. Make sure there is nothing flammable anywhere near this thing.
To record the results, set up a camera a few feet away and make sure it is focused/zoomed in to read the scale.
Make sure the camera is recording, issue the countdown, and pound the launch button.
Play back the video and watch the spring scale move as it the engine begins to recoil.
Step 6: Going Further
So this rig can easily test pre-made model engines. But that isn't the whole reason I built it. Some of you may be familiar with my hybrid engine project. If so, than you will probably notice that I had no clear data on the engine's performance. I had no way of directly measuring the engine's thrust, so I had to make a rough estimate based on some other rough estimates. Now that I have a way of actually measuring the thrust, I can see just how wrong my initial estimates were.
NASA has some interesting articles on calculating other aspects of rocket propulsion, which I would highly recommend reading if you are interested in that sort of thing:
If you enjoyed this instructable, consider following me here and subscribing to my YouTube channel, as I have some more rocketry videos planned. If you have any questions, please leave them in the comments below and I will do my best to give an accurate answer.