Tom’s Vashon Rocketry Site

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My Vargúlf (Werewolf) design is a convertible rocket that can be flown one either standard 18mm solid propellant model rocket motors, or on a Vashon V-2 cold propellant motor. The rocket has exactly the same external configuration in both cases. It was designed as a research vehicle to make comparison flights to measure the total impulse of the V-2 motor via flight tests as accurately as possible. The model carries an electronic altimeter in its payload section. By flying the model with B6-4 and C6-7 motors to accurately determine the drag coefficient of the design, I can then fly it on the V-2 motor to get the total impulse of the V-2 using R152a propellant.

The rocket is in three main sections. The bottom section consists of the main body tube and fins. The body tube is Centuri-sized ST-10 (available from Semroc). The fins are 1/32-inch waferglass.
     To fly the model on cold power, the V-2 motor slip fits into the main body tube. An adapter ring is taped to the rear of the V-2 motor, and small screws through the rear of the main body hold the assembly in place. The very top of the motor's recovery adapter sticks out the top of the main body, ready to grab the parachute compartment (the middle section of the rocket) when the motor is pressurized.
     To fly the model on solid propellant motors, an insert made of Centuri-sized ST-9 tube slip fits into the main body and is secured with the small screws near the base. The insert contains an 18mm engine mount and has a standard shock cord mounted in the front. A balsa bulkhead fits very tightly into the middle section above, and fairly loosely into the main body. The parachute is housed in the main body for solid propellant flights.

The middle section of the rocket is ST-9 inside of ST-10. It acts as the parachute compartment for cold power flights. The tube end is hardened with thin CA so that the motor's recovery adapter can grip it securely. The compartment incorporates a spring mechanism to force the parachute out.

The top section of the model is an ST-10 payload section with a balsa bulkhead at the bottom and a balsa nose at the top. It can accommodate a Perfectflite altimeter. There are three 1/16-inch vent holes into the payload section to allow the altimeter to function.

In Norse mythology, a vargúlf is a werewolf ...maybe. The Wikipedia page on Norse mythology lists "varulf" as a beast, and the link from that word goes to their page about werewolves. But my brother John thought "vargúlf" might be more correct. Either way, close enough for a rocket that can transform between two different propellants (and it doesn't even need a full moon to do so).


Below are all the parts of the Vargúlf model. At the top of the picture is the 1-mil Mylar parachute. Below that is the V-2 motor (note the adapter ring at its base attached with clear tape) and the parachute ejection spring mechanism. Next is the Vargúlf lower and upper body sections (the middle and top compartments come apart below the small vent holes in the payload section). The Perfectflite altimeter is seen below the payload section. At the bottom of the picture is the solid propellant power insert with its balsa bulkhead. (Click the image for a larger picture.)

Below is the Vargúlf ready to fly in solid propellant mode.

And below we see the Vargúlf transformed into cold propellant flight mode.

The Vargúlf was flown at the June and July 2007 launches of the Zia Spacemodelers rocket club. The June flight was made using R-152a airbrush propellant and the altimeter reported an altitude of 365 feet. Temperature was 87.8°F, the barometer read 24.44 inches of Hg, and wind speed was about 6 mph. The model initially tipped downwind when it came off the rod, but had weathercocked back toward vertical before parachute deployment, which occurred shortly before the rocket reached apogee of its trajectory. Deployment was somewhere between 10 and 20 degrees from straight overhead.

The July flights were to be on B6-4 and C6-7 solid propellant motors, but only one flight was made on a B6-4 because the model broke a fin upon landing. The altimeter reported 265 feet (temperature was 89.8°F, Barometer 24.63 in. of Hg, wind speed 3 mph at launch). The launch photos below are by Mark Hamilton and Tom Beach.

Flight Analysis: When I analyzed the B6-4 flight data to obtain the drag coefficient (Cd) for the model, I found that the Cd result was VERY sensitive to the total impulse value I chose to use for the B6-4, so I really couldn't get a good Cd value (and I really wanted more solid propellant flights to get a more accurate Cd value anyway).

But, the results of the B6-4 flight suggested that the total impulse value I could derive from the earlier cold propellant flight would be insensitive to the Cd value I used. This turned out to be the case. When I plugged a variety of Cd values into my OSX Altitude program (which is based on the Malewicki equations) I found that any Cd in the range of 0.75 to 0.50 resulted in a total impulse between 8.54 and 8.34 N-sec for the V-2 motor using R-152a propellant. That's a pretty narrow range, so I was happy.

I was concerned that there might be an error in the analysis due to the fact that ejection had happened before the model reached the apogee of the trajectory, so I also did a more complicated analysis using RockSim (allowing RockSim to calculate a variable Cd from the design as it normally does). I had RockSim calculate the altitude vs. time plot for the model using various total impulse values for the coldpower motor, and then I fit these plots to the altimeter data (for the times before parachute deployment). See the plot below. A total impulse of 8.1 N-sec is cleary too low, but the results for 8.3 and 8.5 N-sec seem to bracket the correct value; a total impulse of 8.4 N-sec looks good, which agrees nicely with the Malewicki-based calculations. (The reason the Malewicki-based calculations agree so well with RockSim even though deployment was short of the trajectory apogee is probably because the altimeter's reported altitude of 365 feet was due to the small peak in the data that occurred at deployment.) Because the flight was not perfectly vertical, the actual total impulse of the V-2 motor should be somewhat higher than 8.4 N-sec, which agrees nicely with the 9.0 N-sec total impulse I got from thrust stand tests.

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Page last updated July 20, 2007