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Bye, bye, bye..
Use of parachute assumes that our target of investigation has an atmosphere and a gravity field like our own planet.   How do we know to assume that?   We're not really supposed to know that.   In any case, we do know.   And so we'll use a parachute to take advantage of the atmosphere and gravity to control the CanSat's descent rate.   The descent rate is important because it's directly related to the how many data samples the CanSat can take 'til touchdown.  

The Parachute
The parachute is made of nylon.   It's got a surface area of 4.5 feet sq.   We did not put a circular slit at the top of the parachute, though that may stabilize the can's swirling during its descent.   Actually, our payload doesn't care if the can is swirling or doing a hoola hoop.   (Go to the sensor page to find out what the payload is)  We want to maximize flight time, so no slit.

Resource Site on Parachutes / Skydiving

Atmospheric Descent with a Round Chute
Round parachutes descend at a constant rate.   The mathematical formula for computing the descent rate of objects with a round parachute is as follows:  

V = SQRT(2*W/[rho*Coeff*Sw])

where
W is the load,
rho is the air density
Coeff is the parachute's drag coefficient ~0.8 for a chute without holes or slits cut in the fabric

Sw is the parachute's surface area when measured on a flat surface.

With this formula, we can not only predict the descent rate, but we can compute descent time and the release point so that our probe will hit the desired ground aimpoint.   We set up a spreadsheet to do just that.  

To learn more about parachute computations, go here.


The Shell
Behold the name of the probe..."CanSat".   That should be a dead give away...  The shell is a standard can.   A beer can, if you prefer.   The design constraint specifies that it must be a 12ouncer.   The professor says no cheating... none of those "4ozs more free" cans (you know, them larger beer cans like the one on the left).

No structural reinforcements were added.   The can descends gingerly with the help of the parachute.   Once it gets close to the ground, the can's descent rate should slow down due to the ground effect.   The can should therefore be able to withstand the impact force at touchdown (longitudinal force).

Beer Can Collectors of America