Sunday, September 30, 2018

What I learned from this year's Geezer TARC...

I spent a lot of time with my designs and simulations this year. Here's what I took away from yesterday's launch:

Reliant's flight profile from the PNUT altimeter (Click to enlarge).
1) As expected, the simulated altitudes are overly optimistic, by tens of feet in my case. Reliant was predicted to go 874 feet; it actually flew to 801 feet, a difference of about 9%. In contrast, Artemis was simmed to 880 feet and flew to 871, a difference of only 9 feet. I'm thinking the latter is due to a set of fortunate circumstances, as the 9% is more in line with my expectations of a 10% difference in altitude. So if I was simming a TARC rocket, I'd select configurations that give altitudes about 70-100 feet above the goal (926 - 956 feet).

2) The online descent rate calculations did an excellent job predicting the descent rates - the choice of two 12" octagonal parachutes was perfect for egg capsules weighing close to 300 grams. I got measured rates of 22 and 23 feet per second, which are right on the money.

3) The BAMA Recovery Systems TARC parachutes took the stress, and they are octagonal, which increases the parachute drag. Plus they are lighter and cheaper than comparable Top Flight parachutes.

4) TARC teams using F32's in BT-70 rockets may be adding a fair amount of ballast this year to drop altitudes. Do lots of sims and choose your motors carefully based on the numbers. Watch the delay times - 6 seconds is close, 4 is way too short. Ejection should occur just past apogee, not before (as in the Reliant flight profile depicted above).

5) You can do this year's challenge on a cluster of 2 Estes E12-6's - Artemis' flight proved this. The question remains as to how consistent black powder clusters are from flight to flight. If you have to cluster, 2 motors are the easiest (naturally).

6) If you use the Perfectflite Firefly (cheaper, smaller, and lighter than the PNUT), get the field display unit. The flashing LED is tough to read on bright sunlit days.The Firefly is not a logging altimeter, but is kinda like the Jolly Logic Altimeter 2. Here's the data from Artemis' flight:
  • Apogee: 871 feet
  • Speed: 428 ft/sec
  • Time to Apogee: 7.4 sec
  • Flight Time: 44.6 sec
  • Descent rate: 23.4 ft/sec
  • FDD Battery: 3.06 V
  • FF Battery: 2.63 V
7) An "egg pusher" (BT-70 coupler bulkhead screwed to end of 18 inch 1/2" diameter wooden dowel) or something similar is a must if you are to get the eggs in/out without breaking them. 3 eggs are a pain to pack.

8) The Apogee egg cushions work well and are not that heavy. I flew the 2 egg cushion and a 1 egg cushion in each payload section.

9) Pack your parachutes very carefully - I used Estes wadding to keep the chutes separated in the body tube. You do not want the egg capsule parachutes tangling up - that would be bad.

That's what immediately comes to mind - hopefully some TARC teams find something useful in this post.

No comments:

Post a Comment