Static port calculator

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bigone5500

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Uploaded excel spreadsheet. See below.
 

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  • Rocketry Resources.xlsx
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Try this (from the Eggtimer Users Guide, p. 35):

A quick formula for the diameter of the holes, assuming that you are using three is:
1) Length of Payload Bay in inches: __________________
2) Diameter of body tube in inches: __________________
3) Square of #2 __________________
4) Multiply #1 and #3 __________________
5) Divide by 1500 __________________
6) Take the square root: __________________
This is the diameter of the holes you need


Is there an online calculator anywhere?
 
Related to Holes for Altimeter Bays:

See this link: https://www.vernk.com/AltimeterPortSizing.htm

Related to Holes for Airframe Venting:

Thinking through the problem. The purpose of these ports is to relieve the internal pressure of the airframe so that the airframe does not break apart before the scheduled events.

The hole sizing is determined by:


  • the amount of internal airframe volume to be relieved
  • the amount of force needed to break apart the component (e.g., push the nose cone off of the airframe)
  • velocity (i.e., how fast the rocket is going determines how quickly the internal pressure builds up)

In other words, the hole needs to be sized to bleed enough air.

For example, all things being equal you would need a larger hole for a rocket going to 10,000 feet in 7 seconds, than you would for the same rocket going to 5,000 feet in 12 seconds.

Greg
 
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For example, all things being equal you would need a larger hole for a rocket going to 10,000 feet in 7 seconds, than you would for the same rocket going to 5,000 feet in 12 seconds.

Greg

This is true if you are wanting accurate climb rates or accent data. Not so much for detecting altitude attained.
Apogee V is really what matters if you are looking for an accurate Altitude obtained, or detecting apogee.
Aerodynamics are a real factor for higher velocity apogee. IE. is the port located where the speed of the rocket induces an area of high pressure, or an area of low pressure? this will alter the percieved altitude. For instance, If your nose cone is where your device is, is it better to port via tube to a place down on the airframe?

Verns page discusses 3 vs 4 ports, and essentialy tells you, the volume of the port, and the ability to normalize with the ambient atomosphere is key. the fluid dynamics of the port "size" and "flow rate"...So, a thick port, may need to be bigger in diameter than a port that is only 1/3rd the depth.

Its always neat to see data where low pressure is transient over my port locations. it's just cool.
 
Thanks for the clarification.

Vern's site has to do with holes related to altimeter vents, whereas my discussion was geared to pressure relief (to prevent the unwanted disconnection of airframe components due to over-pressurization).

Greg
 
ohh... my bad, i am off in lala land today! :)

I guess i didn't read to good. "static ports"... i have a weird shaped bay in the nose of a rocket, i guess, i should have closed my thinking box while reading this....
appologies for the tangent.
 
No, Clay, I wasn't clear. So I will make a note of correction.

Greg
 
Updated excel file...
 

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  • Rocketry Resources.xlsx
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bigone5500,

I reviewed your file and added a few things. Vern's formulas were not that clear (at least to me), so I added named cells so that the spreadsheet, so that the formula was a little clearer. I also added an OD field, in case one wanted to mark the outside of the compartment for hole orientation.

But, you had it all correct. Nice work.

Greg

View attachment Static Port Sizing Calculator Rev 1.xlsx
 
Thanks Greg! Nice work on the spreadsheet BTW. I see there are many things I need to learn about excel. I copied the sheet to my resources file and I added your name to the credits.
 

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  • Rocketry Resources.xlsx
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Combine that with a data sheet of BT sizes and you can get a nice user interface that fills in the ID and OD fields based on selecting the BT name from a drop down.

kj
 
All the above calculations provide 1 sq. inch of venting for 2000 cu. inch of internal volume. If you divide the volume of air trapped inside a rocket compartment by the volume flowrate of air through a hole and do the dimensional analysis, you will find the ratio is in units of seconds. Volume /( volume/second) = seconds.

The conductance air flow through a hole is ~10 liters per second per square centimeter, or ~4,000 cubic inches per second per square inch. Since the VernK formula uses 2,000 cubic in. per sq. inch. and the venting rate is ~4,000 cubic in. per sq. inch per second, the VernK time constant is 2,000/4,000 ~ 0.5 seconds. In 1 tiime constant, the pressure in the confined has reduced to 1/e = 0.367 = 36.7% of the actual outside pressure difference. For example, if the outside pressure difference is dropped -1.7 psi in 1 second (0-Mach 3 in 1 second), the internal pressure is ~0.624 psi higher than the outside. The time constant for the VernK is ~0.5 seconds, so the change per second is 2 time constants = (0.367)^2 = 0.135 = 13.5%. For the same example, the internal pressure would be only 0.23 psi.

Translated to a 4" diameter rocket, if the airframe is not vented, in 1 second the internal separation force would be 1.7 psi x 12 sq. in. = 20 pounds. Using the VernK formula the internal pressurization separation force would be 0.135 x 20 pounds = 2.7 pounds. If the rocket continued upward at the same velocity in 3 second, the internal pressurization load would be approximately 3 x 20 pounds = 60 pounds, about what it takes to break (3) 2-56 shear screws. If vented by the VernK formula, the load would still be only 2.7 pounds.

This should illustrate why the VernK calculation is so useful, and why you need to vent high acceleration, high flying rockety even when you use shear screws.

Bob
 
Thanks Bob. This was very educational to me.

All the above calculations provide 1 sq. inch of venting for 2000 cu. inch of internal volume. If you divide the volume of air trapped inside a rocket compartment by the volume flowrate of air through a hole and do the dimensional analysis, you will find the ratio is in units of seconds. Volume /( volume/second) = seconds.

The conductance air flow through a hole is ~10 liters per second per square centimeter, or ~4,000 cubic inches per second per square inch. Since the VernK formula uses 2,000 cubic in. per sq. inch. and the venting rate is ~4,000 cubic in. per sq. inch per second, the VernK time constant is 2,000/4,000 ~ 0.5 seconds. In 1 tiime constant, the pressure in the confined has reduced to 1/e = 0.367 = 36.7% of the actual outside pressure difference. For example, if the outside pressure difference is dropped -1.7 psi in 1 second (0-Mach 3 in 1 second), the internal pressure is ~0.624 psi higher than the outside. The time constant for the VernK is ~0.5 seconds, so the change per second is 2 time constants = (0.367)^2 = 0.135 = 13.5%. For the same example, the internal pressure would be only 0.23 psi.

Translated to a 4" diameter rocket, if the airframe is not vented, in 1 second the internal separation force would be 1.7 psi x 12 sq. in. = 20 pounds. Using the VernK formula the internal pressurization separation force would be 0.135 x 20 pounds = 2.7 pounds. If the rocket continued upward at the same velocity in 3 second, the internal pressurization load would be approximately 3 x 20 pounds = 60 pounds, about what it takes to break (3) 2-56 shear screws. If vented by the VernK formula, the load would still be only 2.7 pounds.

This should illustrate why the VernK calculation is so useful, and why you need to vent high acceleration, high flying rockety even when you use shear screws.

Bob
 
The closest thing I have to a .071 diameter drill bit is a 5/64". That small bit of extra circumference shouldn't hurt anything should it?

5/64" = 0.078125
 
I have updated the spreadsheet. Changes to it are consolidation of the scale calculator and scale chart sheets.

I am looking to add more functionality to the spreadsheet. If you have any suggestions please post them here.

View attachment 124862
calculator did not calculate hole or circumference sizes in inches
 
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Thread necromancy here.

I have a nose cone alt-bay with a volume of about 135 cubic inches.

Can anyone tell me how big/how many static pressure ports I need? I can't find a calculator that works on volume

I plan to put them equally spaced around the perimeter of the nose cone 1" above the break between nose cone and body tube.
 
Thread necromancy here.

I have a nose cone alt-bay with a volume of about 135 cubic inches.

Can anyone tell me how big/how many static pressure ports I need? I can't find a calculator that works on volume

I plan to put them equally spaced around the perimeter of the nose cone 1" above the break between nose cone and body tube.

All the calculators are based on volume.

https://www.vernk.com/AltimeterPortSizing.htm

"A good rule of thumb for the static pressure port is to use a 1/4" diameter hole for every 100 cubic inches in the altimeter bay compartment that is being vented."
 
If you are going to put the holes in the nose cone, I would also keep them 1/8" to 1/4" above the break, just so you get as much "vertical" surface above the holes as possible. That's based on my "logical" thinking, not and specific data.
 
I was going with 1" above the break to account for the swell of the shoulder. The nosecone is a laid-up fiberglass gelcoat one from Polecat Aerospace and I'm trying to get an unrestricted/clean flow into the nosecone.

By my calculations three 3/16" diameter holes should do the trick.

Now the trick will be to get holes lined up so that I can see the RRC2+ status LED flash.
 
I was going with 1" above the break to account for the swell of the shoulder. The nosecone is a laid-up fiberglass gelcoat one from Polecat Aerospace and I'm trying to get an unrestricted/clean flow into the nosecone.

By my calculations three 3/16" diameter holes should do the trick.

Now the trick will be to get holes lined up so that I can see the RRC2+ status LED flash.

My calcs:

Port.Calcs.135.ci.jpg

Greg
 
I'm looking at three holes at slightly less then 1/8" diameter should be about right for 135 Cubic inches of volume. My 4" dia. 9" long bay is only about 110 cubic inches and three 3/32" holes has worked fine.

I don't think being slightly too big is much of an issue. Being too small can be. Have you ever seen DD rockets that arched over the top and were heading down before the apogee charge went off? that is because the holes were too small and it took too long for the inner and outer pressures to equalize.
 
Does it matter how close the avionics/sled is positioned to the static ports? I am just wondering if the ports absolutely need to have total clearance from obstructions relative to their geometrical positions in the av bay?
 
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