Richard Nakka's Experimental Rocketry Site

Richard Nakka's Experimental Rocketry Web Site

STS-5000 Static Test Stand
for Rocket Motors

Introduction

This web page presents details on the construction of the STS-5000 Static Test Stand. This is an apparatus that I have designed, built, and recently used for static test firing of my Kappa-DX (K-class) solid rocket motor (SRM). This test stand was designed to be versatile and capable of handling motors that produce up to 5000 Newtons (1100 lbs.) of thrust, such that sizes up to "M" class motors may be tested (or larger, depending on the maximum thrust produced). To verify the structural design, the completed test stand was hydraulically proof loaded to 105% of the design capacity (5300 N.) . The test stand with the Kappa-DX motor installed is shown in Figure 1.

This test stand was designed to be relatively simple to construct, using EMT (Electrical Metallic Tubing) as the main structural components. The "bolt together" design allows for simple construction, quick disassembly for transport and storage, as well as easy replacement of parts that get damaged when the inevitable CATO occurs.

For portability and simplicity, this test stand holds the motor in a vertical position, firing upward, such that the thrust force is reacted by the ground upon which the stand sits. This eliminates the complexities associated with horizontal firing test stands, such as the large mass (or other means) required for anchoring. Besides, there are no significant advantages to horizontal testing of motors. The weight of the motor (which rests on the load cell) can simply be subtracted from the measured thrust. In fact, the largest SRM ever built, the 260 inch diameter AJ-260X (SL-2) Space Booster was fired vertically in this manner.

The test stand featured on this page is equipped with a hydraulic load cell for motor thrust measurement, although any strain-gage load cell may be used instead. Also incorporated into the test stand is a provision for motor chamber pressure measurement. Both load cell pressure and chamber pressure are measured by analog pressure (bourdon) gauges mounted on the test stand. Recording of gauge data is done with a videocamera.

Figure 1-- STS-5000 Static Test Stand with Kappa-DX rocket motor, illustrating the main components

Concept

The basic structural form of the test stand is that of a tripod. A significant advantage to having three support legs is that the stand is self-leveling on any surface, with equal load distribution to each leg. The rocket motor is mounted vertically, nozzle upward, such that the thrust force exerts against the thrust plate. This plate sits atop the load cell, which bears against the Base Plate. The Base Plate is supported by a triangular arrangement of three Base Plate Supports which are attached at the ends to the three vertical Thrust Struts. These struts transmit the thrust load to the three Cross Beams. The tension load in the struts is beamed out to the three support Legs. The resulting compression load in the legs is then reacted at the ground surface. Wooden pads are placed under the foot of each leg to distribute the load in bearing.

The three Leg Braces prevent outward collapse of the legs. TheTension Rods serve to stabilize the structure against any loading normal to the longitudinal axis of the motor.

Construction Method

EMT is used to make the basic structural components. To make the struts, the tubing is cut to length by use of a standard tube cutter, as shown is Figure 2a. A vise is then used to form the lug ends by simply squeezing the tubing end until flat, as illustrated in Figure 2b. A 1/4" hole is then drilled through the lug. The lug ends of the Leg Braces, Cross Beams, and Base Plate Supports are then bent at an angle, as shown in Figure 4. The lug corners may be clipped to eliminate the potential hazard of a sharp corner. Figure 3 illustrates the struts and Table 1 provides the strut dimensions.

The three tension rods are made from 1/8" diameter steel rod, with a "hook" formed at one end, and the other end threaded to accept a #6-32 nut, as shown in Figure 5. The hooked end is installed into a blind hole in the Leg, and the threaded fitted through holes drilled through the Base Plate Supports. The nut is tightened such that the rod is under moderate tension.

The components are fastened together with 1/4" bolts as specified in Table 2. Machine screws (#10) are used as the nonstructural fasteners for mounting the rocket motor, Base Plate and load cell attachment brackets. Hardwood "mushroom plugs", drilled part way through to accept the #10 motor mount fasteners, act as motor guides. Two nuts, one on each side of the Thrust Strut, are adjusted such that these guides touch lightly against the motor. This allows for the slight downward motion of the motor while firing. This arrangement is illustrated in Figure 6.

Figure 2a and 2b--Cutting the EMT to length, then squeezing the ends by use of a vise. Holes are then drilled in the lugs for the attachment bolts.

Figure 3-- Struts (note that the Thrust Strut has end lugs that are at right angles to one another.

Figure 4-- Lug end detail

Table 1-- Tube cut and hole-to-hole dimensions

Figure 5-- Tension Rod

Table 2--Structural fasteners

Figure 6-- Detail of motor mounting

The Base Plate is made from a piece of 1/4" (6mm) steel or aluminum alloy plate. A triangular piece 4.75 in. (120mm) along each side is first cut out, then the corners cut off as shown in Figure 7. A hole is then drilled and countersunk for a #10 flat head mounting screw.

Figure 7-- Detail of Base Plate

The Thrust Plate is also made from 1/4" steel or aluminum alloy plate, with the size dependant upon the motor diameter. The plate sides should be at least equal to the diameter of the motor. For the Kappa-DX motor, the Thrust Plate has side dimensions of 2.75"(70mm). Note that a slot may need to be cut in the Thrust Plate to allow clearance for the pressure tap fitting.

Construction details

Detail A: Base plate and load cell mounting structure.
Detail B: Base plate and load cell mounting structure.
Detail C: Load cell mounting structure.
Detail D: Base plate securing bracket (view looking upward).
Detail E: Leg-to-brace joint.
Detail F: Cross beam connections.
Detail G: Tension stabilizer rods.
Detail H: Thrust plate with motor installed.

Instrumentation

The instrumentation that was used for measuring the motor performance consisted of:

Hydraulic load cell for thrust force measurement, which was interfaced to an analogue (bourdon) pressure gauge (the hydraulic load cell converts force to pressure). The gauge used was a 4" diameter 0-1000 psi unit. The load cell had a piston diameter of 0.75 inch (19.1mm), providing a hydraulic pressure/thrust force ratio of 2.264. The load cell is shown in Figure 8.
An "oil buffer system" allowed the motor chamber pressure to be measured. A 4"diameter 0-2000 psi pressure gauge was interfaced to the chamber via a piping system, filled with oil (SAE 30) to prevent damage to the gauge by the hot combustion gases. This system is illustrated in Figure 9. A pressure tap was installed in the motor bulkhead. An inlet shield covered the entrance, to prevent molten combustion products from entering and "freezing" within the piping. After mounting the motor in the test stand, the buffer system is filled with oil to a level that allows for nearly the entire piping system to be oil-filled.

Both gauges were mounted on a panel attached to the braces of the thrust stand. A videocamera, located at a distance of 10 feet (3m.) from the stand, recorded the readings during the firing. A frame-by-frame playback of the recording (transferred to VHS tape) allowed the pressure data from the two gauges to be extracted. The frame rate was 1/60 second.

In order to shield the videocamera, it was placed behind a 1/8" (3mm) plexiglas pane mounted in a wooden frame. This protects the camera, not only in the event of a CATO, but importantly, prevents distortion of the recording due to shock waves emanating from the supersonic flow exiting the nozzle during firing.

In order to improve the quality of the recorded image of the gauges, a "sun visor" was set up over the gauges. The purpose was twofold: to prevent glare off the glass face of the gauges, and to diffuse ambient light onto the gauges for better illumination. The visor consisted of a sheet of white corrugated cardboard stapled to the top of the panel at one edge, with the opposite edge supported by two poles.

An example of test data obtained from an actual motor firing is provided in Figure 10. In this example, pressure readings from the hydraulic load cell gauge have been transformed to thrust force.