It is one thing to develop a rocket motor that produces useful thrust, but it is another thing to know just how much thrust is generated. What is the shape of the thrust-time curve? Is the thrust constant throughout the burn, or does it vary greatly? What is the total impulse produced, and how does the actual specific impulse compare to the theoretical (a measure of design efficiency)? In my attempts to answer these questions regarding the rocket motors that I developed, a number of devices were devised. In this web page, I am presenting an overview of these.
One of the first successful rocket motors that I developed was the one that I used for my first amateur rocket flight. It was relatively small, about 1.8 cm diameter by about 22 cm length. It was based on a similar, but shorter motor, that appeared to develop useful thrust, judging by the short burn time (about a second) and by the frightfully loud sound (at least compared to the model rocket motors that I had been familiar with!). But now that the motor seemed to be powerful, how could I find out just how powerful -- what thrust did it actually develop? Since thrust is a force, as weight is, why not use a scale? So my first static test stand (not counting the stands that I had used earlier simply to restrain the motor) consisted of a 25 lb (11 kg.) capacity scale against which the motor would push. I dubbed this contraption a "thrustometer" (Figure 1). I had simply secured a piece of pencil lead to the dial, which would etch against a sheet of paper that was taped to the face of the scale. As the dial would rotate under loading, it would etch an arc. In this way, the maximum thrust was recorded. This "thrustometer" worked surprisingly well, and was used for well over a dozen static tests. It finally met it's fate one day when a test motor built up excessive pressure, severed the safety pin, and blew out the end plug. The resulting "thrust" was well beyond the scale's capacity, and broke the dial off!
In a sense, the demise of the thrustometer was timely. Recording the maximum thrust was a big step in the right direction, and the burn time could be estimated from a tape recording of the firing. However, there was no way of telling what the shape of the time-thrust curve (thrust function) was. This was just as important to know, since the total impulse of the motor is basically the area under the time-thrust curve. Just how high a rocket will fly depends on the total impulse, and even more so, on the particular shape of the time-thrust curve rather than on the maximum thrust. As well, it is necessary to know the total impulse produced in order to determine the actual specific impulse of the propellant.
The type of instrument normally used to record a time varying function (such as that produced by a rocket motor) is a chart recorder. I decided to build my own version of such a device, which would measure the thrust directly, and for the full duration of the burn. I coined this apparatus that I built a "thrustograph", which is shown in Figure 2.
The rocket motor was mounted horizontally on a sled which was allowed to move forward a short distance (under motor thrust) along a set of rails. The sled was restrained by a pair of powerful extension springs. Attached to the sled was an arm, which was attached at the other end to a pen holder. The pen was held such that the tip was in contact with a sheet of recording paper atop the thrustograph table. This recording paper (which was stored in rolled form) advanced along the thrustograph table by means of a feed mechanism, which consisted of a pair of steel rollers fitted with rubber rings, between which the paper was fed. These rollers were driven by a 120V electric (furnace) motor turning at 1720 RPM. The rotational speed of the rollers was reduced, however, through a series of belts and pulleys such that the paper feed rate was 4.50 inches per second (11.4 cm/s.).
Just prior to firing the motor, the paper feed was activated. As the motor fired, the thrust would overcome the spring force and move forward. The actual distance the sled would move was determined by the spring constant (stiffness). Typically, the springs were selected such that the maximum movement was no greater than about 6 inches (15 cm). The paper width was 8.5 inches (21.6 cm). The movement of the sled combined with the feeding of the paper over the table caused the pen to trace a curve which corresponded to the thrust-time curve of the motor. An example of such a plot is shown in Figure 3.
Calibration of the springs was done by either of two methods. A pair of angler's scales of 50 lb. capacity (each) was used to extend the thrustograph springs by a certain distance (d) and by observing the total force (F) required, the spring constant (k) was determined (k=F/d). The second method was simply to remove the springs from the thrustograph and hang weights from the end, and measuring the displacement to obtain the spring constant. These methods were fine for motors of, say, less than 200 lbs (900 N.) thrust, but were impractical to do with springs of greater stiffness.
Overall, the thrustograph worked well, and was used for many static tests. It's main limitation, however, was that it was pretty much limited to motors of relatively low thrust ( < 200 lbs.). Fitting the device with stronger springs that would be necessary for motors of several hundred pound thrust seemed to be beyond the practical limits of the thrustograph. Clearly, it appeared necessary to devise a more universal static testing rig that would be suitable for motors of just about any size and power.
Construction of the Static Test Rig (as I came to refer to it ) was begun in early 1982. This device was built as a replacement for the thrustograph, and, of robust construction, was intended as a tool that would have the capability of handling rocket motors of much greater thrust than its predecessor could ever handle. The principle of how this device worked was significantly different -- rather than directly plotting the thrust-time curve, as the thrustograph did, this device would convert thrust to an electronic signal that would be collected and processed by computer . At the heart of the system was a force transducer, salvaged from a digital bathroom scale. This was essentially a small cantilever beam fitted with four strain gauges. As the beam was deflected, the strain gauges would undergo a change in electrical resistance. A conditioning circuit would convert this to a change in voltage, an Analogue-Digital (A/D) converter circuit would convert this to a binary signal, which was read by computer and stored on digital tape, for postprocessing. The signal sampling rate was a generous 580 samples/sec.
|The rocket motor was mounted vertically, with the nozzle facing upward, in a tubular holder. The bottom of the holder sat on a deflection bar which acted as a beam supported at both ends, with the load (motor thrust) acting downward at the middle of the beam (detail). The force transducer was mounted such that it's end was in contact with the deflection bar near the middle. As the motor would fire, the thrust would force the deflection bar to deflect downward, and in doing so, also deflect the beam of the force transducer.|
During trial testing of the rig with simulated loads applied, it was found that oscillation was a significant problem. Instead of the smooth curve that was expected, the curve was full of high frequency oscillations, as a result of being highly underdamped, much like a spring that would oscillate up and down if a heavy weight was suspended from it. After much consideration (including the possibility of eliminating the oscillation through software means) it was decided to design and build a hydraulic damping device which would be attached to the deflection bar. The damper was made such that the amount of damping could be easily adjusted, as required.
The deflection bar chosen on the basis of deflection at it's midspan, due to the motor's maximum thrust. The deflection is given by:
Calibration of the static test rig was performed with the aid of a calibration arm with various weights hung at its end. The arm effectively amplifies the force due to the weights, with this force applied at the deflection bar midspan (where the motor thrust acts), as shown in Figure 6.
Example of Thrust-time curve from data obtained using the Static Test Rig.
The initial test firing of any newly designed motor was conducted in a large gravel pit in order to eliminate a fire hazard in case of a blowout. If the test turned out to be successful, it would certainly be useful to know what the maximum thrust achieved would be. This would allow for selecting springs of suitable stiffness for the thrustograph, or in the case of the Static Test Rig, this would allow for selecting a suitable deflection bar. To obtain an estimate of the maximum thrust, a simple, highly portable device was constructed, similar in operation to the thrustometer. It consisted of a wooden plank supported at both ends, acting as a "simply supported" beam (Figure 7). The motor would be held (vertically, nozzle upward) such that the thrust force would push downward on the the middle of the plank, causing it to deflect. A marking pen attached to the plank would etch a line on a paper target taped to a spike driven into the ground. In this way, maximum displacement would be recorded. It was then possible to determine what the corresponding force was to cause the deflection. By calibrating the device (by applying weights to it and plotting the corresponding displacement), the maximum thrust could be estimated with sufficient accuracy.
A new static test rig which I have deemed the STS-5000 Static Test Stand has been designed, built and used for my latest series of static tests for the Kappa-DX (K-class) solid rocket motor. This test stand was designed for testing motors with a maximum thrust of up to 5000 Newtons (1100 lbs.).
Although static testing is the conventional method to determine a rocket motor's performance, it is possible to estimate the performance of a motor through flight testing. This method has the advantage of not requiring specialized equipment that is needed for static testing. Strictly speaking, the only equipment that is required is an accurate altitude measuring device, which is simple to make, and a video camera.
In the absence of atmospheric drag, the equation of motion that governs the trajectory of a rocket is relatively simple. From the equation of motion, it is possible to estimate the total (and specific) impulse of the motor, knowing the altitude achieved, burn time, ascent and descent time, etc. But how can drag be neglected and still arrive at a reasonably accurate estimate of the performance? For a rocket with purely vertical flight, the equation of motion may be written as follows, where a=acceleration of the rocket at any instant:
This method of estimating a rocket motor performance will be covered in greater detail in a future web page.