TARCing through my hat

I’m building the Bad Cholesterol for our club’s May contest — D dual egg loft duration. But it seems there’s another dual egg loft contest later in the season; September’s contest, the schedule says, is “TARC rules”. I’m assuming that means we go by the rules (possibly a bit more relaxed) of the 2014 Team America Rocketry Challenge; the rules are summarized as follows:

  • Launch two eggs and recover them, undamaged
  • Get as close as possible to 49 seconds total duration
  • Get as close as possible to 825 feet (251 meters) maximum altitude

You get a 4 point penalty for every second under 48 or over 50, and 1 point for every foot above or below 850.

So I’m thinking, how reasonable would it be to use the Bad Cholesterol, suitably modified, for the September contest?

Right away there’s an unnerving problem. Let’s suppose you hit the duration and altitude targets precisely; let’s further assume your rocket takes 7 seconds to get to apogee. Then you have 42 seconds for recovery from 251 meters. That means an average speed of 6.0 m/s. That’s fast! Hitting the ground at 6.0 m/s is about equivalent to a free fall drop from 1.8 meters… head height. I try to design for 3.5 to 4.5 m/s normally, and that’s without eggs aboard: about equivalent to a free fall drop from 60 cm to 1 m. At 6 m/s, never mind the eggs, would the fins survive?

Of course 6.0 m/s is the average descent speed. Not that I know much of anything about dual deployment, but the whole point of DD is to give you a slow descent at the end of recovery but a faster average over the whole drop. But I don’t think the TARC rules are compatible with DD.

And you could in principle get away with a slower speed if you had a longer time, which you could buy with a faster ascent. But, face it, you won’t shave more than a couple seconds (if that) off the ascent time — and you’d be subjecting the rocket and payload to higher g forces to do it — and stretching your descent time from 42 seconds to 44 doesn’t do you a lot of good on the impact speed.

Another approach is to take the not unreasonable attitude that penalty points are bad but disqualification is worse, and you may be better off coming in slower and eating penalty points. If so, should you shoot for too low an altitude or too long a recovery? Suppose you wanted to come in at 4.0 m/s; then if you hit the altitude target of 251 m it would take you 63 seconds, plus 7 for the ascent makes 70 seconds. 20 seconds over 50, for an 80 point penalty. On the other hand, at 4.0 m/s, if you hit the duration target with a 42 second drop you’d need a peak altitude of of 168 m or 551 feet, a 274 point penalty. Clearly the game to play in this approach is to aim for the target altitude and let the duration suffer!

If we were launching from hard packed dirt I’d probably do that. Fortunately we have a fairly forgiving grassy landing surface, mostly, at our field. I’m inclined to think I’ll aim for the 6 m/s recovery, in which case, want to build those fins strong! And I’ll probably want more padding than the Mark I egg capsule is going to provide, so I’ll think about a larger Mark II. The altimeter will have to be accommodated too, of course.

 

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