What happens if a parachute is too small for a rocket?

Discussion on ground control/GSE and recovery equipment. This includes launch pads, triggers, chutes, streamers etc. Includes other items such as simulation and other computer software, etc.

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lucifer911
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What happens if a parachute is too small for a rocket?

Postby lucifer911 » Wed Sep 30, 2015 4:22 pm

I use the SCR parachute which can be modified to several different sizes.
I have lost my bigger parachute and was wondering what would happen if the parachute is too small?
would it descend faster? if so then wouldn't it be a better idea to use smaller parachutes without a hole in them?

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Happy Heyoka
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Re: What happens if a parachute is too small for a rocket?

Postby Happy Heyoka » Wed Sep 30, 2015 5:07 pm

lucifer911 wrote:I use the SCR parachute which can be modified to several different sizes.
I have lost my bigger parachute and was wondering what would happen if the parachute is too small?
would it descend faster? if so then wouldn't it be a better idea to use smaller parachutes without a hole in them?


The spill hole makes them more stable if you care about that... but it does also make it fall faster compared to the same diameter chute without slits or a spill hole.

Less surface area will make them fall faster... you probably do care about that unless you love repairing fins.

This looks like a pretty informative link:
http://www.pcprg.com/rounddes.htm

OpenRocket seems to simulate descent based on the Cd (co-efficient of drag) and diameter of the chute... leave the Cd at 0.75 and play with the diameter (eg: sim for the same motor, same weight of chute etc and watch the descent time)

I have far more technical stuff I could point you at (there's a whole world of stuff that gets delivered by parachute)

Examples of why you might care about descent stability are if you had a camera on board.
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Re: What happens if a parachute is too small for a rocket?

Postby SpaceManMat » Wed Sep 30, 2015 5:20 pm

Spill holes do allow more air through but make surprisingly little difference to the decent rate. As HH said have a play around with a simulator. Weight and chute size are the main considerations, but build strength and landing surface should also be considered.
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Re: What happens if a parachute is too small for a rocket?

Postby Lister » Wed Sep 30, 2015 8:06 pm

Spill hole only really makes the chute descend in a more controlled and stable way than no spill hole.. unless you make it too big then you have a streamer.. the spill hole lets air pass through the top of the chute instead of the air having to escape via one side whuch makes the rocket spin around to allow the chute to tilt and release trapped air.

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Re: What happens if a parachute is too small for a rocket?

Postby lucifer911 » Wed Sep 30, 2015 8:37 pm

so if a parachute has no spill hole it would likely sway from side to side in an uncontrollable fashion?
how big should a spill hole be for the most controllable descent?

In this video I made a square spill hole 10mmx10mm in the middle of the chute.
The bigger the spill hole the closer it will land to my launch pad assuming if I fly it perfectly vertical with no wind?


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Re: What happens if a parachute is too small for a rocket?

Postby Viking » Wed Sep 30, 2015 8:51 pm

From memory(dodgy) the spill hole is supposed to be 20% of the _inflated_ diameter. Which I think works out to only a 4% reduction in area, hence the negligible effect on descent rate.
So for example, a 450mm dia chute would be a 90mm spill hole.
Last edited by Viking on Wed Sep 30, 2015 8:59 pm, edited 1 time in total.
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Re: What happens if a parachute is too small for a rocket?

Postby Lister » Wed Sep 30, 2015 8:55 pm

Where it lands depends on a few bits other bits aswell as chute/streamer choice.. on the size rockets your using I dont think you will see much difference with spill or no spill.. but it will make them come down a bit straighter.. yes less side to side sway.. I like to keep the spill hole size within 10-15% of the diameter of the chute.

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Re: What happens if a parachute is too small for a rocket?

Postby vance2loud » Wed Sep 30, 2015 8:58 pm

20% of diameter is the rule of thumb that I've always worked to. For interest sake as well, an overly large hole actually ends up being a ring which has a very stable descent but requires considerably more material and cord than optimal for a parachute. Unless it tangles and then it's a streamer.
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Re: What happens if a parachute is too small for a rocket?

Postby Lister » Wed Sep 30, 2015 9:08 pm

Thats a kite isnt it?

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Re: What happens if a parachute is too small for a rocket?

Postby vance2loud » Wed Sep 30, 2015 9:12 pm

Yep but the same principles apply, I couldn't find the picture I had seen of a parachute designed like it.
For a basic starter on parachutes, https://www.fruitychutes.com/help_for_parachutes/how_to_make_a_parachute.htm
There is a lot more indepth articles out there and a few gore calculators but fruity chutes make well known chutes for our hobby so I figure their guide is a good starting point.
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Re: What happens if a parachute is too small for a rocket?

Postby Lister » Wed Sep 30, 2015 9:17 pm

Its a cool kite :) im about to make some chutes tonight.. gotta make something big for the athena 4 and something for our mate who started this thread (yeah its coming ryan :P)


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