Matt's Page

My science project: Blast Off on a table Top!

Here is a summary of my science project from last year. I made a rocket engine test stand out of an old erector set motor and a metal quart paint can. I tested both Estes blackpowder motors and Vashon "coldpower" freon-powered motors. The paint can rotated during the test, with a rotation rate of about 4 seconds (on the lower rpm erector motor setting). This was just right speed to get a trace of the thrust profile of the engine.

Figure 1. The rocket engine test stand with a Vashon V-1 engine ready to test.

I tried three different propellants shown in the picture below. They were (as depicted from left to right in the photo): dichlorodiflouromethane (the old R-12 refrigerant), tetraflouroethane (the newer R-134a automotive refrigerant) and diflouroethane (sold as airbrush propellant).

Figure 2. The three propellants and their molecular structure.

Vashon and Estes used R-12 refrigerant for propellant and they called it RP-100. It has a molecular weight of 121. The molecular structure of the gas molecules are shown under the propellant cans. The atoms are coded: black=carbon, white= hydrogen, red=flourine, and green=chlorine. The R-134a propellant is ozone safe, although it is a greenhouse gas. Its molecular weight is 102. The airbrush propellant has fewer flourine atoms and therefore has the smallest molecular weight of 66. We actually did most of the coldpower tests on the kitchen table!

The rocket engine is tied to a plastic block that slides on a metal rod. The plastic block pushes against a spring to determine the engine thrust. A pen on the block is held against the drum with a streched rubber band to get a nice trace of the movement of the block. We found out that one must have some sort of resistance (i.e. damping) between the block and the rod or oscillation of the block and spring assembly will occur. (Click here to see the inital unstable results) To add resistance we put a nylon screw through the block and against the rod. The screw could then be adjusted to get just the right resistance.

Figure 3. Shot of the back of the test stand showing the resistance adjustment screw and the pen tension mechanism. Note the compression spring in front of the rocket block.

After we got things working fairly well we took data using the three different propellants.

Figure 4. Testing of the Vashon V-1 engine.

Here are the thrust profiles from the graph paper that was taped to the drum during the tests. Time is in the horizontal direction, and engine force or thrust is in the vertical direction:

R-12

R-134a

Air Brush

The variation in the thrust of the liquid V-1 tests is probably due to sloshing of the propellant in the engine tank. One could check this hypothesis by trying a vertical test. (We didn't have time to do this.) There was also a problem with the pen returning to the original baseline after the test. In particular, the air brush test did not return to the original baseline (eventhough the engine was pushed snug against the spring before the test). This may have been caused by non-uniform resistance of the screw on the rod.

The total impulse (Newtons*seconds) of the engine is given by the area under the test curve. The thrust areas for the three liquid propellants were: R-12 Area=590, R-134a Area=530, Airbrush Area=450. Since Airbursh propellant is almost half the weight, I think it should provide superior performance in the rocket than the other 2 propellants.

We also did tests (outside!) with Estes B and C class motors.

Figure 5. Testing of the Estes C6-5 engine. Note the trace being produced on the drum.

The results of the C6-5 test is given below.

This curve looks exactly like the thrust curves Estes publishes in its catalog! With the Estes motor, the pen would go below the baseline, probably because the block was not attached to the spring, and the fast shut-off of the Estes motor caused the spring tension to release quickly causing overshoot.

After measuring the spring constant of the spring using a ruler and a small weighing scale, we calculated the thrust of the engine by counting the graph squares under the thrust curve and using the following equations:

Convert graph measurements to time and force:

In the time direction, 1 inch equals 0.3 seconds.
In the force direction, 1.6 pounds compresses the spring 1 inch

1 inch compression = (1.6 pounds) * (0.45 kg/pound) *(9.8 m/s*s) = 7.15 Newtons

Impulse(N*s) = Area*(0.3 sec)*(7.15 Newtons)/ (400 squares per square inch of graph)
Impulse(N*s) = Area*0.0054

Area = 1829; Impulse = 9.9 Newton*seconds, which agrees with the definition of a C motor being between 5 - 10 Newton*sec!

The average compression for the C motor was 17/20 of an inch which gives an average thrust of (17/20 inches)*(7.15 Newtons/inch) = 6 Newtons.
This is where the 6 in C6 comes from!