Creative Method of Measuring Thrust?

[CoalesceRockets]

So I'm not sure if I can afford or put together a proper thrust measurement rig and had a interesting thought...

 

Could you estimate thrust of a static engine burn using one or more decibel meters at a fixed distance?

 

For example, I place a $20 handheld meter half a meter away from the engine and record the setup. Then playback footage and note decibels with respect to time (s).

 

I don't think the relationship is perfect but maybe there are in fact conversions out there for vibrational energy to thrust? Otherwise the number could still be used as a relative benchmark over multiple engines. I think I would trust this more than whatever gross spring contraption I could build. Obviously, you would want to minimize erratic background noise as well.

 

What are your thoughts? Has this been done?

Edited November 13, 2017 by CoalesceRockets

[AustralianPyromaniac]

I have never heard of such a contraption and highly doubt it would work. Simply, too many variables come into play relating the dB to thurst to get any kind of reasonable relationship.

 

Although I have never done it myself this would be my idea for a thrust meter on the cheep.

 

Get a mechanical kitchen scale, one of the old-timey ones from before I was born and attach the rocket to it facing up; you could use many things for this, maybe a buck of sand or some kind of clamp. Then, zero the scale so it reads, well, 0kg/ 0lb. From there you just have to light the rocket. It will push down on the scale and it will read some weight. There, just divide the answer by 9.81 if the scale reads in kg and 4.45 if the scale reads in lb which will give you the force the rocket applies in Nutons. Film all this of course and then just play back the film, pick an interval, say 0.2 seconds and calculate the readings at these points. Then graph this in Excel and there you go, you should have a Thrust (N) vs. Time (sec) graph which is decently accurate.

 

I think this would work, not entirely sure but I don't see why not.

 

Regards, AP

Edited November 13, 2017 by AustralianPyromaniac

[stix]

Yep, agree with AP on this. I can't see how using a decibel meter would work well, if at all - or be consistent over many different motors.

 

What AP has suggested using the old mechanical scales would yield more accurate results. That method is exactly what I did when I first started out. Recorded on Video then when through every frame (@25fps) and plotted to graph paper. Then worked out the total impulse... using a calculator... this was before personal computers where mainstream... Gulp!!! Wow Gee I'm getting Old

 

Those sort of scales are probably getting harder to find. So check out opportunity shops, garage sales, flee markets - or ask the little old lady over the road if she still needs hers - but best not to say what it's for.

 

Alternatively, It shouldn't cost too much to build a "proper" thrust meter:

• Load Cell: $10
• Load Cell Amplifier: $20
• Arduino Development Board $20

 

Then get some help putting it all together - there are people on this forum who would be able to give advice.

 

Cheers.

Edited November 13, 2017 by stix

[CoalesceRockets]

Do you mind posting purchase links for those three components (at ~those prices)? What is needed/used on the software end? Power supply? I have a soldering iron, wires, and Matlab...

 

Thanks,

CR

[stix]

Load Cell:
https://www.ebay.com/itm/40kg-Electronic-Scale-Parallel-Beam-Weighing-Load-Cell-Sensor-/171341900967?

Load Cell Amplifier:
https://www.ebay.com/itm/4-20MA-Load-Cell-sensor-Amplifier-Transmitter-strain-gauge-transducer-0-5V-0-10V/262675931462?hash=item3d28b4a146:g:Ec4AAOSwh2xYAjaW

Arduino Uno R3:
https://www.ebay.com/itm/ATmega328P-CH340G-UNO-R3-Board-USB-Cable-Compatible-with-Arduino-LW/172350624428?epid=1184299414&hash=item2820e5d2ac:g:EBcAAOSw8gVX44KH

The above are just examples.

The Arduino can be picked up from most hobby electronics stores.

 

You could also check this out:
https://www.sparkfun.com/products/13879

 

btw. You won't need Matlab. Just some simple soldering is required. The hardest part will be mounting the load cell securely and making the mount for the motor. All doable but needs a bit of thought.

 

[EDIT] Sorry, I didn't see the other bit.

 

This is how mine is setup:

• I have on an old cheap laptop running WIN7
• The Arduino is loaded with some software I wrote - I'll give it to you
• The Arduino is tethered to, and powered from the Laptop
• The Load Cell Amp needs more voltage, so I have it running off a 12VDC power supply
• The laptop is running some software that records, creates a graph then displays the performance result

Couldn't help but post the current look of the software - it's almost done

 

Cheers.

Edited November 13, 2017 by stix

[NeighborJ]

Stix, can your system be set up to record and graph more than one input? Like maybe a pressure transducer for case pressure and the loadcell of coarse? This looks like a worthwhile project.

[CoalesceRockets]

@Stix

I purchased two of Arduino R3 boards, two of the load cells, a 12V DC power supply, and one of the red SparkFun load cell amplifiers. Hopefully, everything arrives in time so that I can build a testing rig next week and be ready for December tests (winter break). I will probably PM you once it comes time to put everything together.

Thank you.

[stix]

No probs CR.

 

I'm not that familiar with the setup for the sparkfun amp, but there will be similarities to my setup so I'll do my best to help out. Meanwhile you could check out these details:

https://learn.sparkfun.com/tutorials/load-cell-amplifier-hx711-breakout-hookup-guide

 

Stix, can your system be set up to record and graph more than one input? Like maybe a pressure transducer for case pressure and the loadcell of coarse? This looks like a worthwhile project.

 

That capability will be introduced in ver.10.10.9... as in no

 

Yes, it "could" be done, but considering that it's taken me over 18months (approx. 10hrs per week) to get to this stage, then I can't see myself having the will to complicate things more than they already are.

 

After reading your post, I did toy with the idea that you could run two instances of the application setting the serial port different on each, but then that would cause issues the writing of files, preferences etc. The only other way I can think of would be to have two seperate laptops running the software, recording at the same time... complicated.

 

Regardless, my software only allows one graph to be shown at the one time, so you wouldn't be able to overlay both graphs which is what I expect you would want to be able to do. I suppose you could export both files and import into Excel but that sounds like a long winded process.

 

There are data-loggers and software out there that allows you access multiple ports, but the software is probably more generic, whereas with mine, it's pretty much a simple matter of an initial port setup, mount the motor, tare the device, press record, light the fuse, stop record. The performance & results are instantly shown.

 

Cheers.

[NeighborJ]

Thanks for the reply Stix. Most of my rockets these days are for fireworks and would be a challenge to create a tap for pressure anyway. Just figured I'd ask. This is still a very useful tool and is certainly on my to do list.

[pyropro]

so much scientific thrust measuring and what not.If your making rockets bp particularly press or ram three rockets of same size.Simply place dummy hearders weighing same as your shell you have just finished ontop of your motor fire it see what it does. One pound rockets have parimeters so make appropiate weight shells.If said shell is to heavy for a one pounder after its been fired go up to a next pound size motor attach fire you should know soon which weight shell for which motor.Its call trail and error.I never wasted a good shell on a motor it couldnt lift to appropiate height.Shells with fast burnig charcoal stars no inserts need not be as high as say report shells or serpent shells.give the later more room to perform with safety.

[stix]

Cheers pp.

 

I love your comment - I've always wondered and have been surprised that this simple remedy hasn't been brought up before. You are correct, in that just do a few simple practical tests. But some people like to delve further into the "why" in order to improve something.

 

I should point out that quite a few threads under this "Rocketry" category are started by people who are into "sugar motors" and/or model rocketry and therefore may not be of a purely pyrotechnical nature. Maybe there should be two rocketry categories: Rocketry Pyrotechnic and Rocketry Other? Dunno.

 

Some people like to explore and experiment - it can be very useful.

 

Some examples: making your own charcoal, nozzle mix, convolute tubes, glues, shell pasting machine, ball mill, visco machine... the list goes on. Sometimes it's the journey that drives us, not just the final result.

 

This is my version of what you could/should have said:

What a bunch of crap! I've never read such over-indulgent rubbish in all my life!! All these scientific mumbo jumbo measuring machines - absolute wankery!!

 

Simply put the bloody motor on the end of a stick with a mock weight and do a few tests. You'll soon know where you're at.

 

Also, I hope you guys can put a bit of money together and compensate me for all the beer I've just wasted spurting it over my screen in disgust when I was forced read this sh*t.

 

Cheers

Edited November 15, 2017 by stix

[pyropro]

thanks and yes I was kind but on here I tend to stirr the hornets nest,lol! I use many tried and true but I more so used my own common sense approach to things.The expert,run from.

[pyropro]

Case in point any batch of bp I made for motors was always tested. I see one of many a master made many rockets up, put up shells ontop of these untested motors and found the motors were not able to perform. Many hours of precise shell making were waisted just because a step was missed.Test your motors if ye dont want to be grossly dissapointed as he was.

[stix]

We all have our methods.

[stix]

Case in point any batch of bp I made for motors was always tested. I see one of many a master made many rockets up, put up shells ontop of these untested motors and found the motors were not able to perform. Many hours of precise shell making were waisted just because a step was missed.Test your motors if ye dont want to be grossly dissapointed as he was.

 

Yep. And your case in point is what testing is about. Some people may seem to take "testing" to extremes, but then it's really a matter of a point of view.

 

Here's an analogy that's more on the pyro side of things:

 

You launch your 4" shell on top of your proven rocket motor, the shell breaks at the time you would like, but you don't like the burn of the stars.

 

You could make an adjustment to your composition and/or prime and build another 4" shell to test again. Lots of time, lots of money spent. Somehow there is a nagging thing in the back of your mind that says "there must be an easier way". Yes there is, it's called a Star Gun.

 

So, the point here is that there is a good use for testing and creating test equipment, it allows us to make a reasonable judgement before investing further.

 

Cheers.

 

[EDIT]

 

And as far as anyone knows, perhaps the images I've posted on this thread are purely concocted. Maybe there is no such software at all. I just happen to be good at using Adobe Illustrator and PhotoShop.

 

Anyway I've got to go. My internet is getting slow - it's probably due to me downloading the latest version of PhotoShop.

Edited November 15, 2017 by stix