Pressure relief hole

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Pyropetepete

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Not sure if I need to do this or not so best to ask. It's for my dual deploy build, so main and drouge sections. She's a 54mm bird

Airframe sections are made up from 2 of these....

4'' dia
20'' long

These are joined by the AV bay which is an 8'' long coupler, so 4'' is in each airframe, this making a total of 16''.

The Nose cone shoulder is 2.5'' for the main and the drouge will also have a coupler for the join to the booster which will also be 4''.

Main Airframe = 13.5''
Drouge Airframe = 12''

Hope this all makes some sense

Do I need to drill any pressure relief hole's? If so what size and where about along the length

Thanks
 
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As the rocket climbs, the outside air pressure diminishes. However, the air that's trapped inside the airframe stays at the same pressure. The pressure differential causes the outside air to get closer and closer to a vacuum, so the inside air responds by trying to fill the vacuum. Thus, at high altitudes, the air inside the airframe will push out against the nose cone or coupler, and try to force it out of the rocket.

The result of not having a pressure relief hole at high altitudes is that the nose cone or coupler may separate from the airframe. This means you get a chute deployment prematurely during acceleration, which is a bad thing.

Shear pins are the alternative to using a pressure relief hole, although you can use both methods in tandem.

If you use friction fit and a pressure relief hole, make sure that the sections of the rocket can support the weight of the rocket. Pick it up by the nose cone and shake it. If it stays in, your pressure relief hole should prevent separation. If it comes out, add some tape and try again.

Pressure relief holes should be a few inches below the upper component. Make sure they will not be in a location where they might easily be blocked by the parachute. As the air pressure tries to push out the upper component, you want that pressure to go out of the hole instead. So if it is too far south, it might not do much good.
 
Thanks for explain all that. Before I make any mistake and drill a hold in something I shouldn't. Where should I drill exactly

 
I'm confused. So where is best too locate the hole


I have mine at 1 inch below the NC shoulder and 1 inch below the AV bay. 1/8 inch is all I have ever used. You can place the hole anywhere you want as long as your "laundry" isn't blocking it. Makes sense though to keep it close to the top of your tubes as on the way up your "laundry" is going to be compressed down.
 
I have mine at 1 inch below the NC shoulder and 1 inch below the AV bay. 1/8 inch is all I have ever used. You can place the hole anywhere you want as long as your "laundry" isn't blocking it. Makes sense though to keep it close to the top of your tubes as on the way up your "laundry" is going to be compressed down.

1/8" (3m) it is then thanks for that.

Below the nose sounds good. Will have a look latter on mine but 1" below also sounds good.

Thinking to do the 2nd one for the drouge section 1" below the coupler for the AV bay.
 
I'm confused. So where is best too locate the hole
From a pressure relief perspective, it doesn't matter, but locating it at the top of the bay helps assure the hole doesn't get blocked by part of the recovery covering it during packing. This is why people are saying 1" from the top; that's an area unlikely to be densely packed.
 
From a pressure relief perspective, it doesn't matter, but locating it at the top of the bay helps assure the hole doesn't get blocked by part of the recovery covering it during packing. This is why people are saying 1" from the top; that's an area unlikely to be densely packed.

Thanks John. You guys are a massive help
 
The OP is asking about airframe pressure relief holes, not altimeter sampling holes.
I realize that. There's no harm in making them larger to result in zero differential pressure. Better "too safe" than having a drag separation caused by differential pressure on the base of the nose cone aiding recovery system forces pushing off the nosecone at burnout or the advent of significant negative Gs due to the motor's thrust curve. Why guess at what size to use? Use one that is known to be adequate for an altimeter bay unless minimizing drag is a must, which it usually isn't unless you're going after a record.
 
I always go by this excellent analysis of what is needed:

https://www.vernk.com/AltimeterPortSizing.htm

What I never understood with these charts, is why the maximum compartment length in inches gets shorter as the body diameter gets bigger????
For example: a 7.5" ebay only has a maximum length of 6.77inches? Or do the numbers not just go straight across?

Adrian

Adrian
 
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