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.
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
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.