# Vent holes

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#### cobra1336

##### Well-Known Member
What size vent holes in the atlimeter bay ? Bay is 7.5" X 12". Can't seem to find a chart to follow.

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I found that but it doen't show a length of 12". Have built level 2 rockets and understand the three / four rule.

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Not really a chart, but this should be more accurate.

Volume (cubic inches) = Bay Radius (inches) x Bay Radius (inches) x Bay Length (inches) x 3.14

Single Vent Diameter = 2 x SQRT ( Volume / 6397.71 )

Single Vent Area = ( Single Vent Diameter / 2 ) x ( Single Vent Diameter / 2 ) x 3.14

Multi Vent Diameter = 2 x SQRT ( ( Single Vent Area / # of holes ) / 3.14 )

I then would look on a drill chart and find the closest one to the diameter calculated.
https://www.arizonasilhouette.com/images/Decimal_Conversion_Chart_web.jpg
https://microadvances.com/guide/DrillChart.gif

Does this help?

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I'm going with 4 1/4" holes. Sound good ?

"Do NOT use one..."

Why do you think one is NOT a good idea???

Justin

Simple. That one vent is going to control all the air flowing into your ebay and its pressure. If there is some type of turbulence around that hole or it is in some way blocked, you can have a problem because the pressure inside the ebay will actually be different than what the outside (altitude) pressure actually is. Your altimeter will, therefore, use anomalous sensing data with which to determine proper deployments. With multiple smaller holes (which together have the same hole area as the one large hole), if one is blocked or is disturbed by unusual turbulence around THAT hole, the other holes can compensate and the ebay will properly pressurize and eliminate the anomalous sensing problem.

I'm going with 4 1/4" holes. Sound good ?

Yes...it will fly beautifully! That's what I used on mine.

Simple. That one vent is going to control all the air flowing into your ebay and its pressure. If there is some type of turbulence around that hole or it is in some way blocked, you can have a problem because the pressure inside the ebay will actually be different than what the outside (altitude) pressure actually is. Your altimeter will, therefore, use anomalous sensing data with which to determine proper deployments. With multiple smaller holes (which together have the same hole area as the one large hole), if one is blocked or is disturbed by unusual turbulence around THAT hole, the other holes can compensate and the ebay will properly pressurize and eliminate the anomalous sensing problem.

Actually one is good.
Numerous fliers I know use one large hole that doubles as access point for screwdriver to turn on internal switches, besides being the vent.
Some high performance kits even come with 1 hole pre-drilled in the vent band. They all seem to fly just fine.

Gotta be careful 'bout 'dem blanket statements Mark. I found that out the hard way. There is always somebody, somewhere that will prove us wrong. LOL
After all it is rocket science, and bees aren't supposed to fly, but they do!

What altimeter are you flying? The instructions should tell you how to calculate the vent hole sizes and placements.

"Gotta be careful 'bout 'dem blanket statements Mark. "

Bimbo.

I have seen a few instance where too many (more than one) vent holes in a rocket causes high winds to "flow" through the bay and confuse barometric altimeters. These false positives go boom on the ground and make for a depressing and unintentional ground test.

I have flown upwards of mach 2 with a single vent hole with no issues at all. Provided your avionics can breathe, all is well.

Go with a single hole configuration. There is no need for more than one.

Justin

A single hole is more sensitive to the wind blowing across the rocket while sitting on the pad. The pressure coefficient on the surface of a cylinder varies from -3 to +1 around the cylinder. The +1 occurs on the side facing the wind and -3 occurs at the points 90 degrees away from the wind. With one hole, the sensed pressure will depend on the direction the hole faces. With either 3 or 4 holes, the average pressure coefficient is -1 no matter which way the wind blows. If the altimeter uses a rapid change in pressure to sense launch, a single hole will be more prone to false triggering, but it would have to be a really strong gust to do it.

A single hole is more sensitive to the wind blowing across the rocket while sitting on the pad. The pressure coefficient on the surface of a cylinder varies from -3 to +1 around the cylinder. The +1 occurs on the side facing the wind and -3 occurs at the points 90 degrees away from the wind. With one hole, the sensed pressure will depend on the direction the hole faces. With either 3 or 4 holes, the average pressure coefficient is -1 no matter which way the wind blows. If the altimeter uses a rapid change in pressure to sense launch, a single hole will be more prone to false triggering, but it would have to be a really strong gust to do it.

...... and if the wind is blowing that hard, you shouldn't be flying rockets anyway. I've flown countless rockets with one hole, never had a problem.

Well, I didn't want to come off sounding judgemental, but I agree. In case you haven't figured it out, I'm an engineer, an aerospace engineer to be precise, and anything worth doing is worth over-doing.

Hi Alt 45K

The manual is at https://www.perfectflite.com/Downloads/HA45Kman.pdf and page 12 claims that you should use H=D*L*.006 to calculate the size of a single vent hole. In your case, unless I botched the math, a single vent hole would be 0.54 inches. You should read that section as it has their take on the "the good of the one versus the good of the many" (sorry, couldn't resist that Spock quote. Was watching Trekkies with the cats while having lunch 8) argument and on the general placement of vent holes. Gook luck with the flight!

Sorry, I should have mentioned that if you use some of the other techniques in this thread you get essentially the same answer. That's probably a good thing in that it might be providing a reality check on your calculations.

Looks like I'm good. Only a week to finish before Red Glare VI. 3/4 of the way done.

Well, I didn't want to come off sounding judgemental, but I agree. In case you haven't figured it out, I'm an engineer, an aerospace engineer to be precise, and anything worth doing is worth over-doing.

Ahhhhh.... OK, that 'splains it.

I know some machinists that have some names for engineers that I can't put on a family forum. LOL!

In reality though, both sides have their merit, but I *have* seen rockets with three vent holes deploy on the pad on a breezy day......

...... and if the wind is blowing that hard, you shouldn't be flying rockets anyway. I've flown countless rockets with one hole, never had a problem.

I was observing at a HPR launch last year, and not 20 feet away from me, while I was standing in the next EZ-up tent, the next guy over's ejection charge went off while he was ground testing and setting up the altimeters... This was on a 4 inch plus model, and the guy I was facing in the tent, who was building a motor at the time, nearly took a foot and a half long heavy plastic Ogive nosecone to the head point first, with a big balled up chute pushing it along... luckily he was sitting about one foot forward of where the cone went; it flew behind his lawn chair and clattered to the pavement about ten feet further down beside him... He'd have taken it square to the temple, too, as I was standing directly in front of him across the worktable and saw the whole thing firsthand.

The guy programming his altimeters too the kickback to the gut. The upper section of the rocket kicked back like a 12 guage shotgun. The altimeter board was attached to two balsa or basswood 'struts' sticking out of the aft bulkhead, with the breadboard in between, edge on, and when it hit his gut it hit hard enough to snap the struts and breadboard off the bulkhead, and the tube itself slammed him the in the gut before coming to rest. THe guy collapsed in the hatchback of his Suburban/Yukon/Escalade whatever it was with the wind knocked out of him and barely conscious... I was with the rest of the guys coming to his aid, and lady with the first aid kit. When they pulled up his shirt, he had a perfect "Ghostbusters" (O with slash across it) symbol tattooed across his gut from the impact of the breadboard and tube. Knocked the wind out of him but good-- they took him to the RV and let him lay down awhile, and after about an hour he got his second wind back...

Reason for the accident?? Gusty winds were blowing, probably blew across the baro sensor holes and gave a false trigger signal, firing the deploy circuit. Can't remember offhand if it was multiple or single holes (but I'm ALMOST sure it was multiple) and the wind, while not very strong, was a bit turbulent and GUSTY that day.

Lesson learned-- use 'remove before flight' shunts to isolate BP ejection charges until the rocket is ready for liftoff...

Program altimeters and electronics BEFORE loading/hooking up ejection charges in the rocket...

Never stand "in-line" with live hooked up ejection charge compartments-- always stand to one side, and make sure the nosecone is pointed in a 'safe' direction and nobody is standing in front of it or walking across in front of it; Do not line up the altimeter bay/parachute section with the other tents so if you DO have a mishap, you don't take out half the people at the adjoining tents!

Perhaps cover altimeter ports with tape or something in gusty conditions until the rocket is set up and ready for liftoff; remove like shunts before liftoff...

Safety is the word! OL JR

Lesson learned-- use 'remove before flight' shunts to isolate BP ejection charges until the rocket is ready for liftoff...

I don't use shunts, but I NEVER power up my altimeters until the rocket is on the pad AND vertical! It's a bad idea to power up altimeters in the prep area, especially when the charges are already loaded.

Program altimeters and electronics BEFORE loading/hooking up ejection charges in the rocket...
Agreed!

Perhaps cover altimeter ports with tape or something in gusty conditions
Not necessary if you don't powr up the altimeter until it's on the pad.

Safety is the word! OL JR
Absolutely.

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