Sunday, July 13, 2014

The old guys take on TARC!

Work travel has been kicking my rear lately - Finland, Houston, and soon Washington. It has definitely put a damper on rocketry activities, but August should see a return to a more normal routine. I certainly hope so, because I have been challenged.

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).
The Mark 2 is a more conventional (i.e., non cluster) design. It is a minimum diameter (1.8") bird powered by a single 24 mm composite motor, like an Aerotech E15 or E30. Open Rocket shows that I have over 200 feet of altitude margin, so I will probably have to add a little weight to this puppy if I choose to fly it. Yep, you heard right - I intend on building both of these rockets and will select the model that will fly against the others on the day we launch. Never hurts to have choices.

Open Rocket simulation of the Mark 2 (click to enlarge).
Open Rocket depiction of the Mark 2 in flight (click to enlarge).
The next few weekends are going to see a lot of rocket building.

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