by Bill Cooke

NAR and the AIAA have just released the objectives for the 2013 TARC season. In brief, they are

1. Altitude mark of 750 feet
2. Flight Duration of 48-50 seconds
3. 15″ parachute must be used to recover the egg and the altimeter
4. Egg must be placed horizontally (“lying on its back”)
5. Section of rocket containing the egg must be at least 60 mm (2.362″) in diameter
6. Gross liftoff weight of under 650 grams
7. Motor impulse must be 80 newton-seconds or less

At first glance. this seems simple; after all, each of these has been used in past years. No surprises, so pretty easy challenge for the TARC teams, right?

Wrong…

Consider the fact that most of the local teams do not have much difficulty hitting near the altitude mark; rather, it is the duration goal that presents the most problems, usually with the rocket not staying aloft long enough. This was true for last year’s challenge, with targets of 800 feet altitude and 43-47 seconds duration. How much more so will it be this year, with the target altitude 50 feet lower and the duration goal lengthened to a minimum of 48 seconds? Oh yeah, don’t forget that you have to achieve that 48 second flight time with a 15″ parachute – last year had no such limitation on chute size.

So how difficult will it be? To find out, let’s consider some weights for the payload section components:

• Egg = 2.1 ounces
• Altimeter = 0.3 ounces (PNUT) or 0.6 ounces (APRA)
• 8″ BT-80 body tube = 0.7 ounces (3″ for altimeter, 3″ for egg, 2″ for nose cone/coupler shoulders)
• 4″ Balsa nose cone = 1.3 ounces
• 2″ Balsa coupler/bulkhead = 0.2 ounces
• 3″ BT-20 and 2 fiber centering rings for altimeter bay =  0.1 ounces

Total: 4.7 ounces

The above includes no additional weight from paint or the packing material for the egg. We can now go online and use one of the many parachute descent rate calculators to work out how long it will take to descend from 750 feet under a 15″ parachute:

• http://www.onlinetesting.net/cgi-bin/descent3.3.cgi gives a rate of 17.55 feet per second or 42 seconds from apogee to ground.
• http://www.rocketreviews.com/descent-rate-calculator.html gives similar results

It will take about 6 seconds to reach apogee, so this gives a total flight time of 48 seconds, barely within the duration window. A payload section heavier than this will fall faster, and the team will incur the hefty 4x multiplier on the time difference – Not good. I expect that a lot of time will be spent working out how to get the egg capsule to stay up there on a 15″ parachute.

NAR was pretty smart this year. In mixing goals from past years, they came up with a challenge that seems deja vu, but is in actuality going to be fairly tough to meet. I am looking forward to see how the teams do; there will undoubtably be some clever solutions.