Altimiter Sampling Ports

[Theory]

I'm laying out the electronics bay for my Caliber ISP, and want to ensure that my sampling ports are not too large for an RRC3.

the bay will have an internal diameter of 2 5/8" and will be 7.5" long (a 3 inch LOC stiffy coupler). The ports will be located approximately 10.5" from the base of the nose cone. I am thinking of using two 11/64" ports located 108 degrees opposed to each other.

I am basing this on the guidance set forth by Missile works in their "quick start guide" where they state that a 3" X 9" bay would be served by 3 5/32" holes. as my bay is smaller, I cut down the are by a carefully thought out, yet still a touch arbitrary amount.

thoughts???

 

[DaveW6DPS]

Here is some pretty good info:
https://ahrocketry.com/AHRocketry/Alt_St_Port_1.html

I tend to round up in drill size to the next one in my drill index.

 

[Keisling]

You should be fine with the 3x 5/32" holes. If you follow the math that Missileworks includes in the RRC3 user manual it comes out to something like 0.151498615887918" per hole. That rounded to the nearest common drill bit is 5/32".

 

[Curtis Enlow]

Is it possible to have too large of a port?

I ask because it seems very few switch access options are airtight...?

Having said that, is there an easy way to permanently mount rotary switches as flush (and airtight, if necessary) as possible?

 

[mbeels]

Is it possible to have too large of a port?

I ask because it seems very few switch access options are airtight...?

Having said that, is there an easy way to permanently mount rotary switches as flush (and airtight, if necessary) as possible?

As far as I understand, the only downside to ports that are too large are more noisy readings. But I don't think that shows up much in the data, most logging altimeters have pretty good filtering. I end up with oversized sampling ports (for reasons you mention), and the logged altitude data still looks pretty good. It could perhaps be an issue if you're trying to detect a certain velocity. It shouldn't impact the altimeters ability to detect apogee.

 

[Bat-mite]

Better too large than too small. Too small will prevent accurate sampling and produce false readings. I always use a calculator, then round up. IOW, if it says three ports at .237 inches, I make 3 at .25.

 

[Theory]

this is all very good to know. not interested in the most accurate data, more just making sure that deployment occurs as required. if velocity or peak altitude readings are off, that not a big deal at all.

looks like it will go with three at 5/32, or even down to 9/64.

 

[DaveW6DPS]

Is it possible to have too large of a port?

I ask because it seems very few switch access options are airtight...?

Having said that, is there an easy way to permanently mount rotary switches as flush (and airtight, if necessary) as possible?

If you don't go really big, it isn't a problem. I use one of the static ports for screwdriver access to the power switch most of the time.

There are flush mount switches on the market. I wouldn't worry about them being air tight. Again, unless you are incredibly sloppy it shouldn't make much difference.

Incidentally, peak altitude accuracy should not be affected much by hole size, since the rocket speed should be very low. The same is true for main altitude on dual deployment.

Barometric speed readings are affected mostly by how straight your rocket flies, how well the edges of the holes are finished, and how consistent the hole size is. If you get much air flow through the e-bay your speed readings can be seriously effected.

Make your holes consistent in size and spacing, and make the outer edge of the hole as clean as you can. A 10 to 15 percent variance in hole diameter from the rules of thumb should not hurt anything.

The main thing is make sure your rocket flies straight. An angle of attack that is not straight into the relative wind can screw with the best altimeter installation.

 

[Handeman]

From the way I understand it, the calculators are determining the size based on how fast the air will flow out of the bay to equalize pressure as the rocket ascends. The internal pressure is always behind actual altitude pressure by some amount of time because of hole size. The smaller the holes, the faster the rocket, the more behind it is. The calculators are trying to determine a size for an acceptable delay.

Too small of holes and the air won't leave fast enough and you will be after apogee and coming back down before the pressure equalizes and a baro altimeter will detect apogee because the pressure stops dropping. If they are too large then the pressure drop will be quicker and apogee detect will be more accurate. I don't really know what the negatives are for too large of port holes. I had heard it was false launch detects because of wind across the ports, but I don't think that is really an issue with today's altimeters. At least I've never seen a rocket fire apogee deployment on the pad because of wind blowing.

Personally, I use holes about twice the size of the calculator recommendations and have never had an issue. Not with premature deployments or late apogee deployments. I have seen rockets that have arched over and looked to be heading down pretty fast when the apogee charge blew and I suspect it was because of too small of port holes.

 

[rharshberger]

From the way I understand it, the calculators are determining the size based on how fast the air will flow out of the bay to equalize pressure as the rocket ascends. The internal pressure is always behind actual altitude pressure by some amount of time because of hole size. The smaller the holes, the faster the rocket, the more behind it is. The calculators are trying to determine a size for an acceptable delay.

Too small of holes and the air won't leave fast enough and you will be after apogee and coming back down before the pressure equalizes and a baro altimeter will detect apogee because the pressure stops dropping. If they are too large then the pressure drop will be quicker and apogee detect will be more accurate. I don't really know what the negatives are for too large of port holes. I had heard it was false launch detects because of wind across the ports, but I don't think that is really an issue with today's altimeters. At least I've never seen a rocket fire apogee deployment on the pad because of wind blowing.

Personally, I use holes about twice the size of the calculator recommendations and have never had an issue. Not with premature deployments or late apogee deployments. I have seen rockets that have arched over and looked to be heading down pretty fast when the apogee charge blew and I suspect it was because of too small of port holes.

Twice the diameter or twice the area? Huge difference, twice the diamter is like 4.5-5x the recomended area whereas twice the area is slightly larger in diamter than the recommended size from the calculator? Think 1/8" recommend vs 1/4" ( twice diameter) ir 1/8" vs like 5/32" (example only, havent done the math for the area of 5/32 vs 1/8, and currently to tired [emoji3]).

 

[Handeman]

Twice the diameter or twice the area? Huge difference, twice the diamter is like 4.5-5x the recomended area whereas twice the area is slightly larger in diamter than the recommended size from the calculator? Think 1/8" recommend vs 1/4" ( twice diameter) ir 1/8" vs like 5/32" (example only, havent done the math for the area of 5/32 vs 1/8, and currently to tired [emoji3]).

Usually close to twice the diameter. I have mostly 3" & 4" bays so the recommended area works out to like 4 - 1/16" to 3/32" holes. Since I use pull pins, I usually have two 3/32" holes and two 3/16" holes for the 1/8" dia. pull pins. The two big holes are usually filed a little oblong because the alignment for the pull pins are never quite perfect and needs to be adjusted. Also, I have two 1/4" to 5/16" holes into the bay for access to screw switches. These are in the coupler part that fits inside the fincan tube. They are covered during ascent, but exposed during drogue decent. They never caused a problem either.

BTW, I've seen four 1/4" holes on a 4" x 9" bay. Obviously much bigger than needed, but it worked just fine too.

 

[Dave A]

FWIW, I have several common aluminum av bays that work in every rocket I have from 3" to 9"+.
There is one hole in the side of the bay that is 3/4". The bay is mounted about 1/2" from the side of the rocket. Therefore I reach through the 3/4" hole with an ink pen to access 2 switches for the altimeters.
So there is one 3/4" hole in every payload section I have.
For 12 years I never had nothing but smooth readings and deployments. I don't fly anything above 500mph, not sure if approaching mach would be any different. Bigger is better than smaller.

 

[UhClem]

The one time I tried to measure the effect of port size I got nothing.

I had one pressure sensor plumbed using some tubing and a manifold to three ~1/8" static ports as a reference. Another sensor was connected to a test volume with a single tiny port. The data showed almost no difference. Given the tiny port size I expected something a lot more obvious. I managed to find the RDAS data file but not any notes. A plot is attached.

 

[Banzai88]

I've seen 3 rockets pop apogee charge on the pad with light to variable winds, all modern current production flight computers. One thing in common with all 3 was that they all had 4 sample ports.

I've used 3 and 4 sample ports, and I really NEVER feel like they're large enough, but they all seem to work well enough. Recently I've changed over to using screw switches exclusively, and that necessitates a port large enough to fit the tool through, which is always larger than the 'calculator'....but they seem to yield cleaner data and apogee deployment just at the rocket lays over.

 

[mbeels]

Nice test and data (and I think I recognize gnuplot). I also have waaay oversized sampling ports for a 33mm AV bay with screw switches, and the data looks fine. The calculator gave extremely small hole sizes, and I needed to get a screw driver through for the two switches, plus a reset safety plug.

 

[mbeels]

The one time I tried to measure the effect of port size I got nothing.

(meant to quote this)