Static Port hole question

The Modern High-Power Rocketry 2 book tells me to use 3, even spaced, static port holes if I am going to use more than one. I have a situation where two static port holes work,.. and not three. Will there be a problem with pressure differences with my e-bay design using only two?? These two port holes will be located on sides exactly opposite one another.

Thanks for the help!

I think High power Rocketry 2 is a bit dated, especially where electronics are concerned. I would rely more on the instructions that came with the altimeter. Just my :2:

Walldiver7 said:

The Modern High-Power Rocketry 2 book tells me to use 3, even spaced, static port holes if I am going to use more than one. I have a situation where two static port holes work,.. and not three. Will there be a problem with pressure differences with my e-bay design using only two?? These two port holes will be located on sides exactly opposite one another.

Thanks for the help!

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Use this calculator

I don't like the Term, "Static Port Hole". It suggests that there are other Types of Port Holes that are not Static, and able to move.

Makes me want to start a Thread called "Ambulatory Port Hole Question".:surprised:

The general theory is that having two holes exactly opposite would affect the pressure in the av-bay when the wind blows over the holes and each hole would have an amplifying effect. The more holes, the more the effect of wind is negated. How much that theory had to do with early electronics and sensors is something I can't answer.

TopRamen said:

I don't like the Term, "Static Port Hole". It suggests that there are other Types of Port Holes that are not Static, and able to move.

Makes me want to start a Thread called "Ambulatory Port Hole Question".:surprised:

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You know,.... there are other types of port holes out there that move a lot. I've chased them for years!!!!!!!!

Handeman said:

The general theory is that having two holes exactly opposite would affect the pressure in the av-bay when the wind blows over the holes and each hole would have an amplifying effect. The more holes, the more the effect of wind is negated. How much that theory had to do with early electronics and sensors is something I can't answer.

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I think it's time for a low-power experiment on this. I was of the opinion that this information might be dated also.

I have an Adept Altim1, and it will read a false liftoff when the wind blows hard enough ~10 mph in a bay with 1 port.

My deployment altimeter bays (for an RRC3) are made with 3 or 4 holes, so that they may be aligned with the fins for purely aesthetic reasons. I also make them one size larger than the Vern Knowles table. I'd rather over do it than have a charge blow on the pad.

Cl(VII) said:

I have an Adept Altim1, and it will read a false liftoff when the wind blows hard enough ~10 mph in a bay with 1 port.

My deployment altimeter bays (for an RRC3) are made with 3 or 4 holes, so that they may be aligned with the fins for purely aesthetic reasons. I also make them one size larger than the Vern Knowles table. I'd rather over do it than have a charge blow on the pad.

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I've always, in the past, had three port holes in my av bays and have had no false/premature ignition of charges. This time, I will have to go to 4 holes, if two will cause problems... thus the reason for my inquiry.

TopRamen said:

I don't like the Term, "Static Port Hole". It suggests that there are other Types of Port Holes that are not Static, and able to move.

Makes me want to start a Thread called "Ambulatory Port Hole Question".:surprised:

Click to expand...

Static air pressure I like the humor!! How about dynamic port hole...

"Static port" is a carry over from the aviation industry where they are used for sensing static pressure and temerature. Dynamic ports are known as pitot tubes. They face the front of an air or water craft and are connected to sensors which measure the increase in pressure of the fluid created by motion of the vehicle. This is used to calculate speed.

Cl(VII) said:

I have an Adept Altim1, and it will read a false liftoff when the wind blows hard enough ~10 mph in a bay with 1 port.

My deployment altimeter bays (for an RRC3) are made with 3 or 4 holes, so that they may be aligned with the fins for purely aesthetic reasons. I also make them one size larger than the Vern Knowles table. I'd rather over do it than have a charge blow on the pad.

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Another reason for going slightly larger, is so the pressure equalizes quickly. If you've seen DD rockets that arc over and actually start back down before the apogee charge blows, it's because the port holes are too small and it takes until after apogee before the pressure drops enough to sense apogee.

How many static port holes can be as important as their location and what is in front of them; airflow wise. In subsonic aircraft barometric altimeters, we always try to maintain smooth well defined laminar airflow over the port. Hence no sharp edges sticking out into the airflow just ahead of the static port. Multiple port holes is one way to obtain a more locally averaged air pressure profile for the small region where the static port sampling is located.

Then there is the whole issue of calibrating the system which is needed to verify correct operation.

Just my 2 cents.

Thanks for the explanation of the use of the Term "Static" as it pertains to this Application.
It makes much more sense now.
Pitot Tubes would be like what's on the Nose of the TALOS Missile right?

If my limited research is correct, the nose of the TALOS missile had an array of 4 antenna for radar guidance.
Wikipedia has a pretty good explanation of pitot tubes here.. https://en.wikipedia.org/wiki/Pitot_tube

Here's a related question i need the hive mind to chime in on: How finicky are the static port hole sizes? I have a MD nose cone av bay with nose weight and a bulkhead on a extended shoulder in order to get an Adept ALTS2 in w/switch. I rotated the baro sensor end down to get it as far from the nose cone shoulder as possible. Should i subtract the volume taken up with the sled, switch and alt from the internal volume of the nose cone to come up with port size or does it really matter? My switch hole has to be included in my calcs. Everything on paper seems to be okay as far as no mach flight goes (depending on the load) , but I've never located my ports so high up. Looks to be about 56mm off the N/c tangency. I'm worried about shadowing and low pressure area approaching transistion. What say the gurus?

Getting nervous- time to drill holes-need an expert here!

I've always disregarded the volume taken up by av-bay internals. The static ports sizes are often rounded to the nearest available drill bit anyway...


Later!

--Coop

fyrwrxz said:

Getting nervous- time to drill holes-need an expert here!

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I'm no expert and I do believe it is altimeter dependent... But I can only say that I have never had a problem locating sampling holes in or near the nosecone. When in that location of the airframe though I take the calculated hole size and increase it by 50%. Why? I don't remember. I'm sure I had bulletproof logic when I started doing it a decade ago, but hey, it works for me.

Regarding overall volume I don't subtract sled, mounting hardware, etc.

Below is an example of my PML Bulldog, 6" airframe, altimeter in NC with tether release. Note the large size of the sampling hole in the orange band. Two of these, one on each side. In this case mongo volume as essentially it is the whole nosecone internal volume that is the av-bay.

Good to go, guys- thx! Flight report in two weeks.

T-Rex said:

If my limited research is correct, the nose of the TALOS missile had an array of 4 antenna for radar guidance.
Wikipedia has a pretty good explanation of pitot tubes here.. https://en.wikipedia.org/wiki/Pitot_tube

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Yup, you are right. I had this Picture in my Head, which shows the "Ram-Static Pressure Probe".