A Homemade Rocket Motor Test stand

The electronic equipment is accurate and easy to use. but it’s expensive. And though prices may come down in the future, at the time of this writing, to buy everything new would cost S500 to SI.000. Thanks to the physics of rocket motor operation, a motor’s performance parameters are mathematically interrelated. Therefore, if you’re testing a motor who’s design and propellant formula are the same as one of the motors in this topic, you can adjust its performance to the approximate specs, quoted in this topic by simply insuring that it generates the same maximum thrust. When testing motors up through F. you can measure the maximum thrust with the device shown in Figure 18-3.
It consists of a long wooden arm that hinges on a plywood platform, mounted on a flat plywood base. The free end of the arm pulls down on a spring supported by an arrangement of pipe fittings. On top of the arm. a few inches away from the spring, is a rocket motor holder made from a plastic water pipe cap. By switching pipe cap sizes, you can accommodate various motor diameters, and by switching springs, you can test motors with various thrusts.
To one side of the arm is a vertical piece of sheet metal that holds a piece of note paper. An ultra-fine point marking pen with water-based ink is mounted on the underside of the arm with a rubber band, so that is scribes a vertical line on the paper when the arm moves up and down. At the time of this writing, a Pilot (brand name) “Razor Point liquid ink marker” with black ink works well. A sheet metal disk moving up and down in a 3 lb. soft margarine tub filled with heavyweight gear oil acts as an oscillation damper. Note that in the photo above. I’ve left-out the margarine tub to give you a clear view of the damper’s metal disk. For a photo of the stand with the tub in place, see Figure 18-24 on page 483.
You can buy the materials at a hardware store. Important note. Pick a hinge that is loose and exhibits some side-to-side wobble. The one I bought is a Stanley cabinet hinge, but any brand will work, as long as it swings loosely. The springs in this example are made by the Gardner Spring Co. of Tulsa, Oklahoma. They come in a variety of sizes, but you don’t have to use these exact ones. The Gardner springs are 6″ long (including the end loops), but you can use longer or shorter springs if you adjust the height of the other parts accordingly. I bought the gear oil at an auto parts store.
When setting up the stand, try to pick springs that stretch no more than 2 to 2-1/2 inches under a full-thrust load. This will keep the maximum travel of the pen within the limits required by the chart recorder described in the pages ahead. The following instructions explain how to make the stand shown in the photos. Figure 18-4 on the page 471 is an exploded view of the stand’s components, and the capital letters in the materials list refer to the letters in Figure 18-4.
An exploded view of the test stand's components. The capital letters in this drawing refer to the capital letters in the materials list on page 472.
Figure 18-4. An exploded view of the test stand’s components. The capital letters in this drawing refer to the capital letters in the materials list on page 472.
 A rubber band is stretched between a wood screw on top of the hinge platform and another screw
Figure 18-5. A rubber band is stretched between a wood screw on top of the hinge platform and another screw (not visible in this photo) on the underside of the thrust arm. The rubber band pulls the arm slightly to the side, and holds the pen against the paper.
MATERIALS LIST
A. Test stand base: One piece 3/4″ CDX or better grade plywood, 12″ wide x 36″ long.
B. Hinge platform: Two pieces 3/4″ CDX or better grade plywood, 4″ wide x 6-1/2″ long.
B. Hinge platform: One piece 3/4″ CDX or better grade plywood 4″ wide x 6″ long.
C. Thrust arm: One piece clear pine or hardwood “one-by-two”, 3/4″ thick x 1-1/2″ wide x 24″ long.
D. Thrust arm hinge: One 1-1/2″ wide Stanley cabinet hinge (or similar) with mounting screws.
E. Spring support assembly: Two 3/4″ galvanized floor flanges.
F. Spring support assembly: One 3/4″ x 18″ galvanized pipe nipple, threaded on both ends.
G. A selection of 6″ extension springs in various widths & wire sizes.
H. Motor holders: One each of 1/2″, 3/4″, 1″, and 1-1/2″ PVC plastic, “slip fit” type pipe caps. J. Spring support assembly: Two 1/4″ x 3″ eyebolts with 2 nuts & 2 washers for each bolt. K. Motor holder assembly: One 1/4″ x 2″ eye bolt with 2 nuts & one washer.
L. Support rod for oscilfation damper: One 1/4″ x 6″ length of threaded rod, or a same-size carriage
bolt threaded its entire length, with 3 nuts and 3 washers. M. Oscillation damper: One stiff, sheet metal disk, 4-1/2″ dia., with a 1/4″ dia. hole drilled in its center. N. One ultra-fine point marking pen with water-based ink. P. Four #12 x 1″ F.H. wood screws.
Q. Sheet metal paper holder: One piece of stiff aluminum or galvanized sheet metal, 4″ X12″. Four #8 x 3/4″ R.H. wood screws. Assorted nails and woodworking glue.
Oil container for oscillation damper: One plastic 3 lb. soft margarine tub with lid. 1-1/2 quarts of SAE 140-weight gear oil.
1. Cut the 12″ x 36″ plywood base (Figure 18-4-A) to size. Mark the positions of the hinge platform and the pipe fitting spring support, and cut a 2″ dia. hole directly below the future location of the PVC motor holder. Cut the plywood pieces for the hinge platform (Figure 18-4-B). Assemble them as shown, and mount them at the indicated location on the plywood base. Use small nails and glue to insure a rigid, sturdy, and permanent structure.
2. With a pipe wrench and a vise, screw the 3/4″ floor flanges (Figure 18-4-E) onto the ends of the 3/4″ x 18″ pipe nipple (Figure 18-4-F). and make sure that they are tight. It is important that the spring support stand vertical, so when buying the floor flanges, examine them carefully, and try to avoid flanges that are warped. If necessary.
tmp8B-11_thumb[1]
you can true up the face of the base flange on a metal lathe. To do so. screw it firmly onto one end of the pipe nipple. Mount the nipple, with the flange attached, in a metal lathe, and clean up the bottom of the flange with a face cut. Then mount the finished pipe assembly at the indicated location on the base with the four #12 x 1″ F.H. wood screws (Figure 18-4-P).
3. Cut the “one-by-two” pine (or hardwood) thrust arm to length (Figure 18-4-C ). and predrill the mounting holes for the spring support’s eyebolt (1/4″). the motor holder’s eyebolt 111 A”). and the oscillation damper’s carriage bolt (1/4″). Locate and mount the two #8 x 3/4″ R.H. wood screws that hold the pen’s rubber band, and the two #8 x 3/4″ R.H. wood screws for the rubber band that keeps tension on the thrust arm (Figure 18-5 ). Then connect the opposite end of the arm to the platform with the hinge (Figure 18-4-D).
4. Drill a 1/4″ hole through the center of each plastic pipe cap (Figure 18-4-H ). and mount the selected pipe cap on the thrust arm (Figure 18-4-C) with the 1 /4″ x 2″ eye bolt (Figure 18-4-K ). Mount the 1 /4″ x 6″ carriage bolt {Figure 18-4-L) in the indicated position. Cut a 1-1/2″ dia. hole in the center of the margarine tub’s lid. and slip the lid over the bolt (Figure 18-6 ). Then mount the 4-1/2″ dia. sheet metal disk (Figure 18-4-M ) on the bolt’s lower end with two nuts and two washers (Figure 18-7 i.
5. Fill the margarine tub with approx. 1-1/2 quarts of 140 weight gear oil. Place it on the base of the test stand. Lower the disk into the oil. and adjust the position of the tub. so that the disk moves freely up and down without touching the sides of the tub. Snap on the lid. and firmly affix the tub & lid to the test stand’s base with two half-wide strips of duct tape.
6. Important note. Extension springs are manufactured with their coils prestressed. so they remain tightly closed when at rest. It takes a significant amount of force just to begin to pull an extension spring open, and testing a motor with a spring in this condition will generate false data. You must, therefore, pre-stretch each spring before you can use it.
To relieve the stress in a small spring, slip one of the end loops over a punch or a screwdriver clamped tightly in a vise. Insert the shank of another screwdriver through the opposite loop, and pull gently (Figure 18-8). Allow the spring to relax. Then look closely at the coils. If the coils are closed, perform the stretch again, pulling harder, and pulling the coils farther apart. Then repeat the stretch with increasing force until all the coils remain slightly open when the spring is at rest (Figure 18-9). When all the coils remain open, the stress has been relieved, and the spring is ready to use.
Important safety note. With larger springs, use a lever to perform the actual stretch, and be careful to keep your body, your arms, and your hands well out-of-the-way of the spring to prevent injury in the event that the spring breaks loose from the vise, and flies back in your direction.
Springs used for motor testing must be pre-stretched to remove the stresses created during the manufacturing process.
Figure 18-8. Springs used for motor testing must be pre-stretched to remove the stresses created during the manufacturing process.
The spring in this photo has been stretched to a point where its coils are slightly open, and is now suitable for testing rocket motors
Figure 18-9. The spring in this photo has been stretched to a point where its coils are slightly open, and is now suitable for testing rocket motors
The spring is connected to the thrust arm and the spring support with two eye bolts.
Figure 18-10. The spring is connected to the thrust arm and the spring support with two eye bolts.
tmp8B-15_thumb[1]
6. Select the appropriate spring, and connect it to the thrust arm and the spring support with the two eye bolts (Figure 18-10). Use the nuts and the washers on the eye bolts to adjust the position of the thrust arm until the arm is level. Eye bolts are manufactured with their end loops tightly closed. Before you can use them, you’ll have to clamp them in a vise, and pry them open just a bit.
8. With a vise as a helper, bend the sheet metal paper holder to shape (Figure 18-4-0). Set it on the test stand base, and hold it in place with a spring clamp or a heavy weight. It should not be mounted permanently. Mount the paper in place with two strips of masking tape. Strap the marking pen tightly in position with a rubber band, and back near the thrust arm hinge, mount the rubber band that keeps tension on the arm. and holds the pen against the paper (Figure 18-5).
Important note. This equipment is suitable for testing motors up through F. but the “one by two” thrust arm (Figure 18-4-C). and the Stanley cabinet hinge (Figure 18-4-D), are not strong enough to safely handle anything larger. Before you test a G or an H motor, replace the “one by two” arm with an arm made from a “one by four” (a piece of wood 3/4″ thick x 3-1/2″ wide), and replace the Stanley cabinet hinge with a larger hinge accordingly.


The Test for Maximum Thrust

Before you can use the stand, you have to calibrate it. That is. you have to find out exactly how far down the pen moves when subjected to the force of a known weight. The weight should be an even number close to the maximum expected thrust of the motor-to-be-tested. In the following demonstration I’ll be testing an experimental NV6-powered motor with a 3/4″ i.d. casing, a 1/4″ dia. nozzle throat, and a propellant mass (m ) of 61.0 grams. I’m adjusting the propellant’s baking soda content to achieve a maximum thrust of 10 lbs., so I’ll use a 1(5 lb. calibration weight. To calibrate the stand, and perform the test, proceed as follows.
1. Place the stand firmly on a pair of saw horses or sturdy chairs.
2. Wrap the front of the motor with masking tape until it fits snugly into the motor holder (Figure 18-11).
3. Tape a piece of paper to the sheet metal paper holder. Slide the paper up against pen’s tip. and mark the point where it touches the paper with a short, horizontal line (Figure 18-12). I call this the zero line, and it represents a thrust of 0 lbs.
tmp8B-16_thumb[1]As the rocket motor fires, it pushes down on the arm. and the pen makes a vertical streak on the paper.
Figure 18-15. As the rocket motor fires, it pushes down on the arm. and the pen makes a vertical streak on the paper.
 An engineering scale, graduated in tenths of an inch, measures the length of the pen streak, and the distance between the zero line and the calibration line.
Figure 18-16. An engineering scale, graduated in tenths of an inch, measures the length of the pen streak, and the distance between the zero line and the calibration line.
4. Buy a small bucket (with a handle), a pair of S-hooks. and a short length of lightweight chain. Place the bucket on an accurate scale, toss in the S-hooks and the chain, and fill the bucket with sand until the total weight of the bucket plus the sand plus the S-hooks plus the chain equals 10.0 lbs. (Figure 18-13).
5. With the hooks and the chain, hang the sand-filled bucket from the eyebolt directly under the motor holder, so that the chain passes through the hole in the test stand’s base. Once again, slide the paper up against the point of the pen. and draw a second line where it touches the paper {Figure 18-14). I call this line the calibration line, and for this test, it represents a thrust of 10.0 lbs.
6. Slide the paper away from the pen. and remove the bucket, the hooks, and the chain. Slide the paper against the pen one more time. Then retire to a safe distance, and fire the rocket motor with an electric igniter. As the motor fires, it will push down on the thrust arm. and cause the pen to make a vertical streak on the paper (Figure 18-15). When the test is finished, remove the paper from the sheet metal holder, and take it to your desk for analysis.


Interpreting the Results

With a ruler or an engineering scale divided in tenths of an inch, measure the length of the pen streak, and the distance between the zero line and the calibration line (Figure 18-16 ).
To calculate the maximum thrust (F „ ). divide the leneth of the pen streak (L ) by the distance between the zero
v max’ *~ r v pen ‘ J
line and the calibration line (Lca|). and multiply the result by the calibration weight (Fcal). EXAMPLE 18-1:
In this case the pen streak is 1.65″ long. The distance between the zero line and the calibration line is 2.18″, and the calibration weight is 10.0 lbs. The motor’s maximum thrust (Fmax) is calculated as follows:
tmp8B-19_thumb[1]
The motor’s maximum thrust is therefore 7.57 lbs. The ideal thrust for this motor is 10 lbs., so this motor is underpowered. In this case, I’M keep building motors, testing them, and decreasing the propellant’s baking soda content in 1% increments until I achieve a maximum thrust of approx. 10 lbs.
When mounted on the test stand in Figure 18-3. this homemade chart recorder will record a rocket motor's thrust-time curve. It is amazingly accurate, and it provides very useful data.
Figure 18-17. When mounted on the test stand in Figure 18-3. this homemade chart recorder will record a rocket motor’s thrust-time curve. It is amazingly accurate, and it provides very useful data.

Next post:

Previous post: