Parachute Size

[Gregboy]

Can you please tell me if the size of a parachute depends on the height of the fall. For example could a parachute be much smaller than normal if the fall was below 1500feet. Does it also depend on the weight of the person using the parachute.

Many thanks and I look forward to hearing from you.

Gregboy.

[deleted-215416]

Hello and welcome to Science Buddies.

Yes, depending on the height of the fall, you could use a smaller parachute. The purpose of a parachute is to slow down a person or object's falling speed. Speed increases with how long an object is falling (velocity = gravity x time), so because a shorter fall would be completed in a shorter time, the speed would be less than a longer fall.

For parachutes designed for human use, the desired speed is around 4 meters per second (gravity is 9.8 meters per second).

This document goes into more detail and also gives you a formula to find the optimal area for a parachute - http://www.apogeerockets.com/downloads/ ... ter149.pdf. (Equation is on page 2).

Also, the weight of the person also has an affect on the size of the parachute needed.

Here is another website that also explains the concepts. http://www.nakka-rocketry.net/paracon.html

Best wishes!

[bradleyshanrock-solberg]

There are a couple of other concepts to consider - air is thinner at really high altitudes which can affect the formulas, for example, but the most important concept is "Terminal Velocity". This doesn't apply so much when using a model rocket, as in the links above, but is very important for humans falling out of airplanes.

If you drop a feather, it falls gently to the ground and will be undamaged by the fall. The "Terminal Velocity" for a feather is small.
If you drop a marble, it falls quickly to the ground and might bounce, sink into the ground or even break. The "Terminal Velocity" for a marble is large.

Parachutes try to change a falling human (or bleep like an aid package of food) into bleep that behaves more like a feather than a marble, so the person is not injured when they hit the ground.

Normally with a human, the fall can be controlled somewhat by manipulating the parachute, so human parachutes are usually designed around the worst case situation - a big heavy person carrying a lot of equipment, who is falling for a while before they deploy the chute. If a lighter person is using it, they can make it "smaller" by working with the parachute cords to spill some air out, and get to the desired speed. A person can also compensate to some extent against a cross wind, to land more or less where they want to, depending on their skill and the design of the parachute.

With a model rocket, you don't have that kind of control. Also model rockets are light and quite durable for their weight, a lot more like a feather to begin with than a human is. The risk is with too big a parachute, it may get carried off by the wind, so you want the smallest chute that's still effective at preventing damage to the rocket. The optimal area for a parachute in the above links balances those risks (you can lose your rocket to damage from the fall, or it being carried off by the wind. Either is bad).

Parachutes for humans are normally only used for quite large heights, largely because humans are like marbles...they fall quite a ways while a parachute is deploying. If you want to jump off of a tree or a building, you'll hit the ground before it changes you to be like a feather. Humans have other tools for those situations - bleep like a hang-glider works better on a cliff, and stunt-people use giant air-mattresses for jumping off of buildings...it's easier to pad the fall than to turn yourself into a feather at those distances. If you have ever jumped off a high-dive, that's the same idea. You sink into the water and it absorbs the energy of your fall slowly, instead of inflicting it all at once, as if you jumped onto concrete.