[Static Test Stand] Just Give It A Little Character

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A5tr0 An0n
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[Static Test Stand] Just Give It A Little Character

Postby A5tr0 An0n » Wed Apr 22, 2015 2:03 pm

Introduction:

In a attempt to get more optimized and efficient propellants, a test stand was inevitable. I have put this off for some time but on the way home from my last flight [url="http://www.rocketryforum.com/showthread.php?124218-Post-Flight-1-Weekend-and-65-000Ns"](NM trip)[/url] a new flight design was thought up of that would also require a new propellant. A new propellant would require characterization to accurately predict the new ballistics parameters. Things such as Kn, web thickness and other grain geometries, ISP*, C*, density, molecular weight, chamber temperature, etc. can be theoretically calculated. However at this point in time parameters like the burn rate coefficient and burn exponent can only be determined by measuring the pressure and thrust over time which can be done via static testing. Once the burn rate coefficient [a] and the burn rate exponent [n] are determined a motor can be virtually scaled up to any size and the ballistics parameters accurately estimated.

I started this journey with zero knowledge of solid rocket motor characterization, so it has been a learning adventure and a subject that I am still learning about and still feel I have much to learn. I will not be going into chemical constituents or any of the ballistics collected from testing in detail. This is merely a paper about the test stand design, construction, and data acquisition of pressure and motor thrust. Something which I felt there was not as much information out there as there could be. Hopefully this will help other amateurs get started in solid rocket motor characterization and progress everyone as a whole.



Design:

After a quick Google search for test stand ideas (which resulted in little results) a page from [url="http://aeroconsystems.com/cart/ts-pics"]Aerocon Systems[/url] popped up with a collection of various test stands. I was interested in utilizing a test stand of horizontal descent as opposed to the motor being mounted vertically. With vertical test stands you have to account for the mass pressing down on the load cell due to gravity. Although relatively easy to account for, I wanted to avoid this all together by mounting the motor horizontally. When mounting a motor horizontally you must allow the motor to move slightly forward to allow the thrust to be measured by the load cell. There were not many horizontal stand concepts that I liked but ended up really liking the Aerocon Systems stand that can be found at the top of the page that I aforementioned. One major concept I liked about this stand is it is modular; in the event of a motor anomaly (such as a over pressurization) only the components that are damage have to be replaced. I ended up scaling down this stand, used different mounting techniques, and made a few modifications (slides, structural, etc.).


SRM Characterization Test Stand Constituents:

  • Test stand - I stuck with using 1 5/8" X 1 5/8" strut channel mainly because this eliminated having to weld, allowing the stand to be modular. I ended up using just over 20 feet of strut channel.

  • Solid Rocket Motor/Case (SRM) - To characterize solid rocket propellant you need a SRM and case to act as the pressure vessel. I choose to go with a threaded 2 grain (each 3" Lg) 54mm test case, machined from AL 6061. I found this to be a great compromise size for characterization. The threaded closure coupled with the thick wall cases allow this case to contain high pressure and stand up to the back to back testing stresses. [url="http://www.tclogger.com/"]For more information on the case you can find it here[/url].

  • Solid Rocket Motor Mount - This is the component that mounts the SRM to the test stand. I used [url="http://www.mcmaster.com/#catalog/121/1578/=wusdxw"]vibration-damping strut-mount clamps[/url] for this purpose. Hopefully this will cut down on some noise in the data from any vibrations during operation.

  • Load Cell - A load cell is a transducer that is used to create an electrical signal whose magnitude is directly proportional to the force being measured i.e thrust created by the SRM. This allows you to collect the thrust over time curve. For my stand a [url="http://aeroconsystems.com/cart/load-cells/50-kg-load-cell/"]50Kg load cell[/url] is all that is needed.

  • Single Channel Amplifier - Most load cells are not amplified, at least the ones most amateurs can afford, so the channel amplifier does what it sounds like, it amplifies the millivolt output from the load cell. Instead of making my own and turning a short build time into a long one, I purchased the [url="http://www.mewpcb.com/lca1/lca1%20info%20sheet%20v5.pdf"]MEW LCA-1 channel amplifier[/url] found at [url="http://aeroconsystems.com/cart/electronics/single-channel-instrument-amplifier/"]Aerocon Systems[/url].

  • Load Cell Mount - The load cell deflects under load, albeit barely and therefore needs to be mounted onto the test stand. The mount will need to be able to handle the expected load and also needs to be as flat as possible. I used a [url="http://www.mcmaster.com/#8910k659/=wusfju"]3/8" thick general purpose low-carbon steel rectangular plate[/url]

  • Pressure Transducer - A pressure sensor usually acts as a transducer as it generates a signal as a function of the pressure imposed. In this case it measures the pressure inside the pressure vessel (motor case) and allows one to plot the pressure over time curve. I ended up going with a 1V ~ 5V output and 2,500psi (max) pressure transducer. [url="http://www.digikey.com/product-search/en?pv144=5&FV=ffec55f0%2Cfff4001e%2Cfff800b3%2Cfffc00df%2C11c032d&k=pressure+transducer&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25"]More information on the sensor can be found here[/url].

  • Data Acquisition Unit - Data acquisition (DAQ) is the process of measuring an electrical or physical phenomenon such as voltage, current, temperature, pressure, or sound with a computer. A DAQ system consists of sensors, DAQ measurement hardware, and a computer with programmable software. I chose to utilize the Dl-155 as the DAQ module which has 4 channels, 13 bit resolution, and can sample at 10kHz/sec. As for the software I am using the WinDAQ software (unlocked version) which allows me to collect up to 10kHz for a single channel on my Win 8.1 PC. Another cool aspect of this unlocked software is that it has a real time link to Microsoft Excel, meaning all the data will be sent live during acquisition to a spreadsheet making characterization a little easier. [url="http://www.dataq.com/products/di-155/bundle.html"]For more information click here[/url].

  • Various bolts, brackets, pipe fittings, etc. - The nice thing about strut channels is there a quite of bit of available bolts, nuts, brackets, tube mounts, etc. made especially for the channels. They are like Legos for adults. I used 1/2" spring nuts, 1/2" square nuts, 1/2" bolts, 90 degree angle brackets, 4 hole connecting plates, U-style brackets, 45 degree closed angle brackets, closure strips, 0.002" x 3/8" OD 316SS shims, strut covers, 1/4" square pipe plug, and 1/4" Female × Female × Male Tee.


SRM Test Stand Dimensions:

Main Structure Length: 914mm (36") Lg

Main Structure Width: 356mm (14" Lg)

Main Structure Height: 356mm (14" Lg)

Diagonal Support Length: 559mm (22") Lg

Load Cell Mount Plate: 51mm (2") Width x 305mm (12") Lg x 0.5mm (3/8") Thick

Load Cell Mount Beam: 356mm (14") Lg



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Conclusion:

All in all this test stand was easy to build and source parts for. I am happy with the outcome and excited to use it in the near future. I will be posting what construction photos I have up shortly, but keep in mind there are not much. It is just struts so get creative. I did deviate a little from the CAD but only in the sense of brackets. If you have any questions, corrections, comments or concerns then let me know.

A5tr0 An0n
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Re: [Static Test Stand] Just Give It A Little Character

Postby A5tr0 An0n » Wed Apr 22, 2015 2:07 pm

I know you guys down under don't have much use for this in the since of a research stand but you can use it for commercial, sugar, hybrid, etc... idk. Good knowledge nonetheless. More to come.

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Re: [Static Test Stand] Just Give It A Little Character

Postby A5tr0 An0n » Wed Apr 22, 2015 3:22 pm

Forgot to mention that the stand will be staked in the ground via 4 concrete rebars through the channel. I will either bend the top to act as a hook or just leave a foot above the ground. Not to terribly worried, the stand is heavy and the stakes should provide enough support for less than 100lbs of thrust.

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Re: [Static Test Stand] Just Give It A Little Character

Postby air.command » Wed Apr 22, 2015 8:41 pm

Very nice. I like the idea that if you have a failure you can easily replace the damaged parts.
How do the solid motor mounts attach the motor? From the links it looks like they are a fairly solid connection although dampened with rubber? Does this let the motor move sufficiently without much resistance? Is the temperature rating of the rubber dampeners within the expected temperature range they are likely to encounter being in contact with the motor case?
I would have thought you would want to record any vibrations of the motor, as that may further characterize the burn. You should be able to filter the data later in software if you want a cleaner thrust curve.

Looking forward to the construction photos.

- George
Crop Circles: ... just a bunch of guys looking for their rockets ....

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Re: [Static Test Stand] Just Give It A Little Character

Postby A5tr0 An0n » Thu Apr 23, 2015 2:16 am

air.command wrote:Very nice. I like the idea that if you have a failure you can easily replace the damaged parts.
How do the solid motor mounts attach the motor? From the links it looks like they are a fairly solid connection although dampened with rubber? Does this let the motor move sufficiently without much resistance? Is the temperature rating of the rubber dampeners within the expected temperature range they are likely to encounter being in contact with the motor case?
I would have thought you would want to record any vibrations of the motor, as that may further characterize the burn. You should be able to filter the data later in software if you want a cleaner thrust curve.

Looking forward to the construction photos.

- George



Thank you very much and all good questions. So the outside of the motor case mount is 316 stainless steel and at 3.8mm (0.15") thick is plenty strong for the 54mm motors it will see. Now the inside is a thermoplastic material with a max service temperature of 135 degrees C (275 degrees F). The outside of the case will heat up but I am not worried about it, we use many materials for motor mount tubes in the actual rockets with lower service temperatures. I should of been more clear with the vibrations comment, I was meaning that I hope it will cancel out some of the noise (vibrations) of the stand itself.

Below you can see the motor case mount itself and how it functions with the strut channels. Depending on how torqued down the bolt is (on the top) the motor will either not move or move a lot and everywhere in between. As of now I have the two motor beams moving in parallel to allow the motor to press against the load cell for data acquisition. If that proves to ever bind or have issues then I will lock the two beams down and allow the motor case to travel just slightly within the mounts themselves.


Image
The bolt up top allows for easy torquing of the motor case in respect to the mount. This allows the user to specify the degree of axial movement.

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Here you can see the motor case mounts inside the channel. The mounts stay inside the channel and the bolt is loosened to remove the motor case from the mounts.

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Re: [Static Test Stand] Just Give It A Little Character

Postby CATO » Thu Apr 23, 2015 3:15 pm

More info on unistrut can be found here http://www.unistrut.com.au
"In thrust we trust"

AMRS 21 L3
TRA 07459 L3

Impulse:
2018: 14,767 Ns (44% N)
Ns 17: 5,973; 16: 34,558; 15: 35,955; 14: 6,016; 13: 10,208
PB - Gorilla N2717WC, H: 10,260', S: 1.14M

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Re: [Static Test Stand] Just Give It A Little Character

Postby martymonsta » Thu Apr 23, 2015 10:37 pm

CATO wrote:More info on unistrut can be found here http://www.unistrut.com.au


This unistrut stuff look like a good quick way to build.

On the design aspect of the static test stand, I'm a little concerned about the gussets being under tension and only secured by one bolt at each end and that only being a friction hold. While steel is stronger in tension then in compression but I would be dubious about welding that frame let alone bolting it together. If you have a 2000Ns 54mm motor that the equivalent of 204kg (450lb) secured by the friction of 2x 1/2" bolts. Sure it might hold up fine, but it might fail.
Many years ago the great British explorer George Mallory, who was to die on Mount Everest, was asked why did he want to climb it. He said, "Because it is there."
Well, space is there, and we're going to climb it

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Re: [Static Test Stand] Just Give It A Little Character

Postby A5tr0 An0n » Fri Apr 24, 2015 12:34 am

martymonsta wrote:
CATO wrote:More info on unistrut can be found here http://www.unistrut.com.au


This unistrut stuff look like a good quick way to build.

On the design aspect of the static test stand, I'm a little concerned about the gussets being under tension and only secured by one bolt at each end and that only being a friction hold. While steel is stronger in tension then in compression but I would be dubious about welding that frame let alone bolting it together. If you have a 2000Ns 54mm motor that the equivalent of 204kg (450lb) secured by the friction of 2x 1/2" bolts. Sure it might hold up fine, but it might fail.



It is a very quick way to build and rather fun. Their are actually 4 bolts per side in the actual build. I just didn't go back and edit the file to reflect that. The original stand design has been designed for up to 98mm motor, mine is for a 54mm 2 grain motor, albeit I originally built it for a 2550Ns case.

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Re: [Static Test Stand] Just Give It A Little Character

Postby A5tr0 An0n » Fri Apr 24, 2015 9:47 am

SRM Test Stand Construction:

Here are various photographs of the stand throughout the build and completion. The stand came together relatively problem free so not much to report on that front. I can say I am definitely a big fan of using strut channels now. As far as the build went I basically cut all the pieces to specification, inserted the spring nuts into their new home and then bolted the components together. All in all I ended spending about 1000USD for everything.



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The base and vertical structure complete.


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The SRM beams and mounts complete and sitting on the stand for gauging.


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Checking the diagonal supports before installing. Used law of sins to find the angles I needed to cut. Note that the vertical beam (for the load cell mount) has been rotated in the right direction.


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The T is installed into the SRM, this presses against the load cell and allows the fitting of the pressure transducer. Note that I have since abandoned the SS line to eliminate any dead volume that could otherwise cause a delay in the pressure transducers ability to rapidly detect the pressure. The load cell resting on the floor is to check the distance between the T, itself and the main structure.


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Both diagonal support beams are cut proper and ready to be bolted up.


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Painting with high performance enamel. Safety yellow seemed appropriate.


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Finished. The test stand is all bolted together, the load cell mount and the load cell plate are all put together, and the SRM beams are connected and in place.


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Top view. Note the holes in the channel cover; these are for the concrete rebar to stake it into the ground. There are for holes, two in the rear and two in the front.


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Probably not a good view to have when in action.


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A good shot of the major components, fore assembly connections, and front rebar holes up close.


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From left to right: 1) SS T fitting that is threaded into the front of the SRM for pressure measuring also focus the thrust to a smaller area on the load cell. 2) Load cell mounted to the load cell plate. The load cell uses two M6 bolts to mount to the plate. The load cell uses two SS 0.002" shims to allow the SRM to slightly deflect the cell and thus measure the deflection and the result of this will be given to me in LBS (thrust/time). 3) The load cell mounting plate is smooth and flat made from carbon steel plate measuring 2" x 12" and 3/8" thick. The plate itself is bolted to the vertical beam via two 3/8" thick SS bolts.

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Re: [Static Test Stand] Just Give It A Little Character

Postby rocket_troy » Fri Apr 24, 2015 10:25 am

Nice job! I have to say, your engineering and methodology does appear very sound to me from the little bit I've read in this thread.

Cheers,

TP
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A5tr0 An0n
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Re: [Static Test Stand] Just Give It A Little Character

Postby A5tr0 An0n » Fri Apr 24, 2015 10:51 am

rocket_troy wrote:Nice job! I have to say, your engineering and methodology does appear very sound to me from the little bit I've read in this thread.

Cheers,

TP



Thank you very much Troy! I will post more updates soon once I have the time to organize everything slightly.


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