Hi All,
My name is Anthony Ransford and I am working in an undergraduate research program a JPL in Pasadena. Another student and I have been tasked to build a capsule that will fall from a balloon at around 35000 meters. Our drift window goal is 500m^2 and it became quickly apparent that the vehicle would have to be guided. Currently our preliminary design is a nose down facing capsule approximately 1 meter long and 15 cm in diameter with a mass of 1kg. The goal would be for the vehicle to use flight surfaces to maintain the GPS position from where it was dropped (ardupilot mega). As there are many challenges and plenty of opputunitys to go wrong I was wondering if i could get some experienced knowledge on what size surfaces we may need, what torque servo motors, and In general any helpful informations thanks!
Replies
Not being experienced in dropping 1 kg capsules from 35 km altitude I'll start with some modeler's common sense...
Given your dimensions and reasoning that your speed range would be around 200 to 300 km/h, I would consider
1. attaching three or four small and low aspect ratio - say 10 x 10 cm ballpark "wings" pretty much to the COG of the capsule. Cropped delta would be my first guess planform.
2. four stabilizers at the tail with small - about 10 - 15% control surfaces on each. Cropped Delta there as well.
Pay attention in very stiff constructs. You do not want to get the wings in flutter. The same applies (even more) to the control surfaces. Build the leading edge of the control surfaces heavy and the trailing edge light. You may even want to have counter balancing weight to these surfaces to ensure they remain stable. The further aft from the hinge line is the COG of the control surface the more risk you are running. Forward of the hinge line is stable against flutter but does require counter balancing. Remove all the the slop from the hinges and linkages. Use ball link ends.
You do not need any extreme deflections and no super fast control surface movements either. max deflections around 10° should be plenty.
All this would allow you to use rather small (15g - 20g) servos.
Given that you are not building a plane per se and probably not extremely tight on the weight budget I would not save at the servo weight but go for standard size servos (about 40g). Bigger is stronger than small and has less risk of failing mechanically under a possible surprise. Buy good quality servos and favor torque over speed. You do not need top of the line in terms of centering precision or servo resolution.
My 2 Euro Cents...
-mikko