AV bay vent holes (not a size question)

[Banzai88]

I’ve been looking over everyone’s rockets and rocket pictures, and I’ve noticed something:

Some folks put their altimeter vent holes in line with the fins; some put the avionics bay rivets/screws in line with the fins.

I get putting the vent holes 60 degrees out (for a 3 fin rocket) or 90 degrees out (for a 4 fin rocket) from the disturbed air of the fasteners to get them into clear air, but no one has ever explained to me why I want either my fasteners or my holes in line with the fins.

What do you do and why?

 

[markkoelsch]

Fins should not matter as they are below the holes. I try to keep the holes out of alignment with close by screws and such.

 

[Onebadhawk]

Tom,
I start a build by bringing the fin slots up the booster tube with a piece of angle iron...
After I'm done with the booster and I put the payload section on I bring the marks up to the nose cone,,
and onto the nose cone...
At the payload section -- av bay ( coupler ) joint,,
and at the nosecone shoulder to the nosecone joint there are steel screws threaded into steel "T" nuts ( "T" nuts epoxied in place )...
At the booster -- av bay ( coupler ) joint,,
and at the nosecone's shoulder -- payload section joint there are drilled and tapped ( 2/56" ) shear pin holes....
3 sets all the way up on a 3 fin rocket and 4 on a 4 fin rocket...

When this is done you have ample room to drill vent holes into the av bay with complete confidence that you'll get clean airflow over your vents...

Before I went to wireless switches I also did this so that 2 vent holes were over the sled mounted mechanical switches ....

Teddy

 

[cerving]

That's a really nice way to do it, Teddy. Another suggestion that I would have is to use an odd number of holes (3 is usually best) to reduce crossflow inside the AV bay, and don't line the holes up directly with the baro sensor on your altimeter. That helps minimize any turbulence-induced effects inside the AV bay.

 

[rharshberger]

Tom,
I start a build by bringing the fin slots up the booster tube with a piece of angle iron...
After I'm done with the booster and I put the payload section on I bring the marks up to the nose cone,,
and onto the nose cone...
At the payload section -- av bay ( coupler ) joint,,
and at the nosecone shoulder to the nosecone joint there are steel screws threaded into steel "T" nuts ( "T" nuts epoxied in place )...
At the booster -- av bay ( coupler ) joint,,
and at the nosecone's shoulder -- payload section joint there are drilled and tapped ( 2/56" ) shear pin holes....
3 sets all the way up on a 3 fin rocket and 4 on a 4 fin rocket...

When this is done you have ample room to drill vent holes into the av bay with complete confidence that you'll get clean airflow over your vents...

Before I went to wireless switches I also did this so that 2 vent holes were over the sled mounted mechanical switches ....

Teddy

+1 Teddy, I use the same method for most of my builds, and I only ever use 3 vents to prevent the unlikely cross flow Cris was referring too.

 

[mpitfield]

Well in my case it's "not rocket science", it's esthetics...three fins - three holes, four fins - four holes. I use the same technique as Teddy, use angle iron to mark everything out based on the fin location as a reference, then start to build, cut, drill, etc.

The only time I am concerned with the location of the sampling holes and breaking the laminar airflow or other disturbances that may have an effect on my altimeter baro, is when drilling nosecone sampling holes or holes below, camera shrouds. In that case I use a 2" below rule, which seems to work fine, and with a camera shroud I will also take the diameter into consideration.

The idea being if there is a disturbance then it only effects one hole, vs. two holes if it is 60° off-set. In my case I have only mounted a camera on one 3" rocket and based on the diameter I concluded that the camera shroud may effect two holes if off-set 60° between the holes, but that was eye-balling it on a 3" airframe. If it was a 4" airframe then a 60° off-set may of looked fine. With other appendages like canards so far the sampling holes have been forward so it has not been a concern...yet.

 

[dixontj93060]

I often use the holes to double for access to switches, so they kinda go anywhere. They are small and I usually have a paint scheme or decal set that hides any imbalance.

 

[Onebadhawk]

Ya know guys,, with turbulence inside the av bay in mind,,
I opt for more smaller holes as opposed to just one or 2 large ones..
I also put some holes high on the av bay and some low...
I still haven't mounted a camera yet,, so without that I feel I have
6 possible locations for a vent hole on a 3 fin rocket. .
3 high on the bay and 3 low..
I don't know if it sounds plausible but in my mind you're spreading
the turbulence around more evenly within the av bay...
A no turbulence hot spots kind of thing,, lol ???

Teddy

 

[SpaceManMat]

Ya know guys,, with turbulence inside the av bay in mind,,
I opt for more smaller holes as opposed to just one or 2 large ones..
I also put some holes high on the av bay and some low...
I still haven't mounted a camera yet,, so without that I feel I have
6 possible locations for a vent hole on a 3 fin rocket. .
3 high on the bay and 3 low..
I don't know if it sounds plausible but in my mind you're spreading
the turbulence around more evenly within the av bay...
A no turbulence hot spots kind of thing,, lol ???

Teddy

i think your not supposed to put holes a different heights. If there is a pressure difference between the 2 locations then you can end up with air streaming through your av bay.

 

[John Beans]

i think your not supposed to put holes a different heights. If there is a pressure difference between the 2 locations then you can end up with air streaming through your av bay.

But on the other hand you don't want pressure differentials. Avoiding flow by eliminating holes promotes a pressure differential, and multiple holes evens the pressure out, even if that's through flow.
Fair to ask: "If the pressures are different, isn't one of them more indicative of altitude than the other? Don't I just want to use the more indicative pressure?"
Yes, if you know which one that is.

 

[Onebadhawk]

I agree John,
I think the answer to so much is experience...
There certainly does come a point that something is being over thought..
This seems to happen quite often..

Teddy

 

[SpaceManMat]

But on the other hand you don't want pressure differentials. Avoiding flow by eliminating holes promotes a pressure differential, and multiple holes evens the pressure out, even if that's through flow.
Fair to ask: "If the pressures are different, isn't one of them more indicative of altitude than the other? Don't I just want to use the more indicative pressure?"
Yes, if you know which one that is.

I think you'll end up with a more average pressure, provided there isn't any strange aero effects. You'll certainly end up with more drag. I'd be a lot more worried about going supersonic due to shock waves. Also you're going to need to be sure that stream of air isn't running over your pressure sensor cause that could really mess it up.

 

[Dave A]

1) I have been using Perfectflites for 11 years and now use 1 Stratologger and 1 Strato CF in each of my 2 AV bays
2) Since that time, I have used on OOP Giant Leap Slimline AV bay.
It has a 3/4" hole near on end. It is modular so that it screws into EVERY rocket in my fleet from 3" up to 9".
Each rocket has the same corresponding hole in it. I do chamfer the outside edge of the hole to smooth the airflow, maybe.

I don't go close to Mach 1 but I assume because of the size of the hole the pressure is stable.
I never experience turbulence or poor readings.
It always works.

 

[rharshberger]

1) I have been using Perfectflites for 11 years and now use 1 Stratologger and 1 Strato CF in each of my 2 AV bays
2) Since that time, I have used on OOP Giant Leap Slimline AV bay.
It has a 3/4" hole near on end. It is modular so that it screws into EVERY rocket in my fleet from 3" up to 9".
Each rocket has the same corresponding hole in it. I do chamfer the outside edge of the hole to smooth the airflow, maybe.

I don't go close to Mach 1 but I assume because of the size of the hole the pressure is stable.
I never experience turbulence or poor readings.
It always works.
View attachment 301968

I like it!

 

[Onebadhawk]

Heck yeah,,
That's really slick Dave...
Nice and clean and neat...

Teddy

 

[OverTheTop]

I just position them for aesthetics. 3 fins = 3 holes, 4 fins = 4 holes. In line with the fins, just because it looks tidy that way.

My preferred method for switching avionics is RBF pins, so, as others have said, the holes also double up for those before launch.

I wouldn't put them at different heights as that encourages flow. Trying to guess what the pressure would be doing as the rocket changed orientation (different AoA) and spinning would be difficult. Remember the faster air is moving the lower the pressure, so air moving through a compartment is at a lower pressure than when static. Not sure how much or what the significance of it would be in our avionics.

 

[Dave A]

1) I have been using Perfectflites for 11 years and now use 1 Stratologger and 1 Strato CF in each of my 2 AV bays
2) Since that time, I have used on OOP Giant Leap Slimline AV bay.
It has a 3/4" hole near on end. It is modular so that it screws into EVERY rocket in my fleet from 3" up to 9".
Each rocket has the same corresponding hole in it. I do chamfer the outside edge of the hole to smooth the airflow, maybe.
View attachment 301968

I almost gave up on this system because the plates on the ends were difficult to keep centered while screwing on the end caps. "light comes on" Get some Wildman AV lids and cut them down with a step to keep them centered on the ends. That is why I threw away the 12" units because it was the reason they leaked. Wish I had kept them.
Each unit has 2 alts and 2 duracell quantum 9 volts. It can work all day and never have to open them. All you have to do is just fill the charge cups. I flew it at Red Glare 2015, never opened it and flew it again on a M-1780 at RG 2016. (it does it's own voltage check)
Next on tap: I have mini USB jacks I want to mount on the ends to change the program in the Stratologgers.