A couple of weeks ago, Duane challenged me, Nate, and Woody to a rocket contest. We are to build rockets capable of meeting this year's TARC goals and fly them for the first time at a launch in mid August; no practicing allowed. The dude with the best score wins the bragging and swaggering rights. In case you don't know or remember, here's a quick summary of the 2015 TARC challenge:
- Fly 1 raw hen's egg (weight of 2.1 ounces) to 800 feet. Altitude is measured by a Perfectflite APRA or PNUT altimeter.
- The rocket must be at least 25.6 inches in length and weigh less than 650 grams (22.9 ounces) fully loaded.
- The payload section containing the egg and altimeter must separate from the sustainer. It must descend by parachute, whereas the sustainer can use any safe means of recovery.
- The payload section must land the egg safely (no cracks) 46-48 seconds after first motion on the launch pad.
- Rockets are restricted to less than 80 newton seconds total impulse (F class or less).
Not easy, but I think I can win this. As a matter of fact, I have already come up with 2 designs. Neither have names (yet) - right now, they are known as Mark 1 and Mark 2. The names will come as I build.
The Mark 1 is a BT-70 (2.17" diameter) based rocket powered by a cluster of 4 motors - 2 24 mm and 2 13 mm. Standing 30" tall, it features 3 smallish fins to minimize turning into the wind. Stability is good at just over 1 caliber when loaded with 2 D12's and 2 A10's. As you can see from the Open Rocket simulation below, I should be able to make the 800 foot target with just 2 D12-5 motors - IF the weight is as expected, the drag coefficient is right around 0.75, and the motor thrusts are nominal. Too many things need to be right, so that's why I have the 2 A10's. These little motors provide just enough thrust to buy me a margin of an extra hundred feet in altitude, which hopefully will compensate for higher drag and/or lower thrust.
|Open Rocket simulation of the Mark 1 (click to enlarge).|
|Open Rocket depiction of the Mark 1 in flight (click to enlarge).|
|Open Rocket simulation of the Mark 2 (click to enlarge).|
|Open Rocket depiction of the Mark 2 in flight (click to enlarge).|