Vent holes?

[majordude]

I bought a minimum diameter 18mm rocket, the Alien Interceptor BT20 Minimum Diameter from Mach1.

https://www.mach1rocketry.com/product-page/alien-intercepter-bt20-minimum-diameter
In this video the builder adds vent holes.



Is a vent hole really necessary on a LPR-MPR rocket? (if so, what diameter, and should it go above or below the recovery device?)

 

[Rob Campbell]

I never used them on LPR/MPR Rockets and never had a problem.

 

[majordude]

I never used them on LPR/MPR Rockets and never had a problem.

Same. But I thought I was missing something!

 

[heada]

Its a form of insurance for rockets that can see fast changes in altitude and thus air pressure.

Unless you seal sections of the rocket so well that they can hold pressure, they're not absolutely required but adding a 1/32" hole as a vent won't hurt and helps equalize pressure.

 

[majordude]

Its a form of insurance for rockets that can see fast changes in altitude and thus air pressure.

Unless you seal sections of the rocket so well that they can hold pressure, they're not absolutely required but adding a 1/32" hole as a vent won't hurt and helps equalize pressure.

I really don't think I'll need it for a C or D but who knows until it goes wrong. Where does it go? Above or below the recovery device (or does it matter)?

 

[heada]

Exact placement doesn't matter as much as long as it is somewhere in the section of the rocket. I put as small a hole as I can just below where the shoulder of the nosecone would be. Using a thumb-tack to pop a small hole is enough.

Vent holes are an important part of an av-bay when using a barometric sensor. In that case the number, size and location are important and there are a few online calculators for that.

 

[Voyager1]

Exact placement doesn't matter as much as long as it is somewhere in the section of the rocket. I put as small a hole as I can just below where the shoulder of the nosecone would be. Using a thumb-tack to pop a small hole is enough.

Vent holes are an important part of an av-bay when using a barometric sensor. In that case the number, size and location are important and there are a few online calculators for that.

^^^ This is what I’d recommend, too. However, to be clear, the vent hole in the upper airframe to prevent internal pressure premature separation is generally only an issue when using a friction coupling, and is a separate issue to the barometric sensor vent(s) in an avbay. Probably a good idea to include them anyway, even if you’re employing shear pins.

 

[Brian Ditmer]

I bought a minimum diameter 18mm rocket, the Alien Interceptor BT20 Minimum Diameter from Mach1.

https://www.mach1rocketry.com/product-page/alien-intercepter-bt20-minimum-diameter
In this video the builder adds vent holes.



Is a vent hole really necessary on a LPR-MPR rocket? (if so, what diameter, and should it go above or below the recovery device?)

that's Ian, he likes to launch em as high as possible

 

[Handeman]

I think vent holes for LPR is another one of those "group think" things that becomes "you need to do this" because it gets propagated through the forums as helpful suggestions by people that want to post something and ends up becoming "the proper way to do things".

It is like swivels on parachutes. It started with people using snap swivels so they could use the snap part to quickly change out chutes on LPR rockets. It's grown into this, "you must have a swivel on your parachute" that is so popular and "common knowledge" now that even parachute vendors are saying you need them. I think it's because they sell more chutes with swivels than because the swivels are actually needed, but I can't blame them for that. I've never used a swivel on any L1 - L3 parachute and I've never had a parachute twist up, but you can go with swivels if you think that's best.

If you want to put vent holes in your LPR/MPR, go for it. You might want to do a little "rocket scientisting" and leave some without holes as a control group to see for yourself if the holes really make any difference.

 

[tsmith1315]

I think vent holes for LPR is another one of those "group think" things that becomes "you need to do this" because it gets propagated through the forums as

That’s a reasonable conclusion. I read about vent holes in Modern HPR, or something similar, way back when I first found there were motors bigger than D’s.

Since then, anything I expect to go higher than a couple thousand feet gets vent holes, it’s quick and easy. No thought involved as to whether or not they’re actually needed.

 

[NateB]

If you want to put vent holes in your LPR/MPR, go for it. You might want to do a little "rocket scientisting" and leave some without holes as a control group to see for yourself if the holes really make any difference.

The biggest difference I have noticed with vent holes on LPR and MPR is getting cleaner data from my altimeter.