|Open Rocket visualization of the Reliant - still trying to decide decor (Click to enlarge).|
So what am I looking for in my Geezer TARC entrees?
1) Simple and relatively easy to build - hard to cut straight with my eye condition, so I need fins with simple geometries, like swept trapezoids or deltas. No elliptical shapes, even though those are less draggy.
2) BT-70 based - BT-80 tubes have too much area, hence too much drag (remember we are trying to get to 856 feet this year, 56 feet higher than last year's mark). Centuri ST-18 tubes are "minimum diameter"; a regulation size egg will barely slip into them, but there is no room to pad the egg sides. OK if you land nose down, but bad if you hit on the side of the capsule, especially with 3 eggs inside. A BT-70 is a good compromise between the two. Another plus is that Apogee makes some relatively lightweight egg protection for this tube diameter.
3) A weight around 520-525 grams fully loaded without motor - The fact that this year's rockets have to be painted means added weight. Throw in the additional egg and higher altitude and you may find it extremely hard to make altitude with a BT-80 based bird. Too much drag, too much weight.
4) Assuming the weight range above and a typical rocket drag coefficient of about 0.75, a F motor should get me to about 850 feet. Open Rocket has the model going about 150 feet higher, but it always over estimates altitude.
|1st-order altitude prediction for the Reliant (Click to enlarge).|
|Open Rocket design of the Reliant (Click to enlarge).|
|Reliant ready for primer (Click to enlarge).|