International CANSAT Competition
As part of an independent team of mechanical engineering students from the University of Hawaii, I co-led the designing and building of a CanSat (can-sized satellite) for the international Cansat Competition held in Texas. The CanSat competition is essentially an egg-drop competition with a handful of extra engineering-requirements tagged on. As part of the competition requirements, the team was also required to present a preliminary design review (PDR), critical design review (CDR), and post flight review (PFR) to NASA engineers. I led the fabrication and integration of parachutes and electronics into the acrylic body of the CanSat.
I learned that there are actually many different types of parachutes that function in slightly different ways. Flat disc parachutes (also called parasheets) are made from a single piece of fabric cut into a circle. Shroud lines attached to the edges of the parasheet pull the edge down and air forces the center upward into a hemispherical shape. This type of parachute is the easiest to construct but comes with several disadvantages: Since parasheets are flat, they tend to act more like a wing and often result in more drift than desired. Also, when the flat sheet is distorted by the tension of the shroud lines, folds can manifest in the material. Due to the unpredictable nature of parasheets I decided that they would be inadequate for the CanSat.
In contrast to the parasheet, a perfectly hemispherical parachute has improved drag properties, however, there is a tradeoff between parachute weight and hemispherical perfection. This depends on how many gores are used; the less gores one uses, the more distorted the hemisphere will be. However, the more gores you use, the more sewing has to be done, resulting in more time, mass, and opportunity for error.
After testing several types of parachutes, I decided to move forward with a cupped-parabola parachute. This design had several advantages that made it more desirable over the hemispherical parachute: The cupped parabolic shape was simpler to fabricate than the other designs; It also provided a more stable descent with minimal drift and a more predictable trajectory. The geometry allowed it to be folded easily into the small space that the parachute occupied within the CanSat.
For the purposes of the competition, I assumed a constant air density of 1.22 kilograms per cubic meter. Since the CanSat mass, descent rates, and air density were fixed, we were able to get specific surface area requirements and sizing for each parachute. Although we met all payload requirements for the competition, the satellite failed to deploy on launch day – despite the free fall, however, the egg still remained intact upon retrieval!
One can't come to Texas without heading to a shooting range to save damsels from zombies. : )