holes in rocket body tube - how they affect aerodynamics

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nils
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holes in rocket body tube - how they affect aerodynamics

Postby nils » Sat Oct 07, 2017 4:15 am

I would like to make 2 elliptical holes in my rocket body tube. One for GoPro Session camera and one for parachute side ejection. I have no idea how much these holes affect rocket aerodynamics and stability. Do I absolutely must make window for camera and door for parachute? Or can I just make holes?
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Rex Fynns
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Re: holes in rocket body tube - how they affect aerodynamics

Postby Rex Fynns » Wed Oct 11, 2017 12:03 am

G'day Nils,

In the absence of any other responses I'll weigh in with a thought or two, however I know very little about rockets but a bit about holes.

I suspect the true answer will only come after a test flight as there are so many variables and unknowns at play, having a Perspex window over the Go Pro hole is pretty easy to do and will remove the head scratching of will it or won't it work of an open hole.

The chute deployment hole might be a problem and whilst mostly it's an unknown a couple of things are known, one is drag will increase and it will be asymmetric drag being only on one side, the drag will be the result of turbulence from the opening and this turbulence will have some effect on the fins ability to do what fins are meant to do. All this leads to a possibility of yaw and an arcing flight profile, whether that pans out as significant in reality is an unknown, at least to me.

Other considerations are the turbulence created within the tube and the pressure differential, static air has a higher pressure than air with velocity, also as the rocket ascends the air pressure behind the chute/wading etc. if the chute/wading is reasonably sealed against the internal body could move your chute/wading up and toward the hole as it tries to equalise. At the hole you will have air flowing out near the front of the hole and air flowing in at the rear of the hole, consider an open car window as an example.

If the rocket does yaw or is subject to an initial crosswind component at launch and the hole is on the dead side or lee side then this will accentuate the low pressure over the hole.

What all this could lead to is your chute being sucked out at velocity.

The only positive to having holes in your rocket is it might whistle quite loudly, testimony to drag being apparent.

Personally I'd be avoiding holes and looking for an alternative simple solution, perhaps a jettison able hatch that is held in situ by internal lugs that are strong enough to withstand not being be sucked off in flight but weak enough that the deployment charge can still blow the hatch away. A bit of BP and some simple Estes fuses to run a few ground tests might be the key to it.

The other way might be to cut the rocket in half and glue a coupler to one side and a neat slide fit with the other side such that the deployment charge blows the rocket into two pieces (attached with joining line) and out comes the chute.

As I said I know little about rockets but perhaps there might be something in waffle that helps.

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Re: holes in rocket body tube - how they affect aerodynamics

Postby OverTheTop » Wed Oct 11, 2017 6:20 am

Rex has it pretty well covered.

Not many people do side hatches for parachute deployment, I think just because it is harder to make and there is some uncertainty with things like you ask. Give it a go :)

I think I have seen someone using a servo to release a hatch on a design somewhere.

For the camera I think as long as there is nothing protruding your results will be better. If the lens pokes out a bit you might want to consider adding a similar shape elsewhere to make it symmetric.
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Re: holes in rocket body tube - how they affect aerodynamics

Postby crom » Wed Oct 11, 2017 1:37 pm

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Re: holes in rocket body tube - how they affect aerodynamics

Postby Kryten » Wed Oct 11, 2017 2:46 pm

OverTheTop wrote:I think I have seen someone using a servo to release a hatch on a design somewhere.

You're probably thinking of a system used in water rockets.
Check out the AirCommand website for details of various deployment mechanisms
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Re: holes in rocket body tube - how they affect aerodynamics

Postby nils » Fri Oct 13, 2017 7:48 pm

Thanks for the answers!
I decided to make a door for parachute. Parachute rubber is bonded to the door and then to the inner side of the rocket tube so the rubber works as hinge. Door lock is a piece of tape which is strong enough for flight but is weaker than ejection charge.
This rocket is now ready. I will test it tomorrow.
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