Static Vent Port Sizing

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jjwb22101

Flying on a student budget
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Hey all, I was wondering if there was a good way to estimate the size needed for a vent port on high-performance rockets - the ones that are put in the parachute bays to prevent separation due to differential pressure on high altitude flights, not the ones that are used for pressure sampling for barometric altimeters in avionics bays (I've already got tons of info on those from various altimeter manufacturers' manuals and other published resources). Thanks!
 
sort of depends.
how high
how fast
diameter and length of compartments

I'm just finishing up a 98mm min dia. I put an 1/8" hole in each compartment

Tony
 
sort of depends.
how high
how fast
diameter and length of compartments

I'm just finishing up a 98mm min dia. I put an 1/8" hole in each compartment

Tony

Looking to be about 13,100' at about 1,400 ft/sec (4" rocket on an M1780).


Thanks! I'll take a look through the spreadsheet and try to back out what it's doing - looks like it's basically an iterative method, so it won't be easy to turn it into a formula for calculating the necessary vent hole size, but at the very least it should tell me what I need to know, even if it doesn't give an elegant way to find it (I'm a physicist, I like elegant solutions haha)
 
Just a gross look at the question: if a hole is big enough to accurately measure the pressure outside for altitude, then the pressure inside is close to outside, and it must be big enough to prevent separation. So the same formulae/rules should do just fine.

Shear pins (and coupler friction) mean you can surely get away with smaller holes for static venting. But the altimeter holes are pretty small, so why overcomplicate things?

Am I being terribly naive here?
 
Last edited:
That's a good question. I would think some items to consider are:
1. how big the holes will be for the much larger volumes that the booster and payload tube volumes typically have. Altimeter bays typically have much less volume that the parachute compartments, but not always.
2. how will the larger holes impact the deployment charge's ability to pressurize the volume for deployment separation?
 
That's a good question. I would think some items to consider are:
1. how big the holes will be for the much larger volumes that the booster and payload tube volumes typically have. Altimeter bays typically have much less volume that the parachute compartments, but not always.
2. how will the larger holes impact the deployment charge's ability to pressurize the volume for deployment separation?
Honestly, I'm significantly more concerned about 1 than I am about 2. The other major difference between this and sizing a hole for an altimeter bay is that it doesn't have to match the outside pressure perfectly, just enough to prevent the rocket from separating (IE, it's fine if there's a couple PSI difference between inside and outside. As long as it doesn't shear my pins, I'm fine). I'm not super concerned about the effects on ejection charges for two reasons. The first is that an ejection charge changes the internal pressure to a much greater extent (I typically aim for ~15 PSI, which is almost 3x the pressure difference I'm expecting in flight), much more quickly (fractions of a second vs just over 30 seconds), allowing a lot less time for these vent holes to do their thing (and meaning they'd have to vent a lot more pressure). The second reason is that any sort of issue with the deployment should be pretty easy to square away in ground testing. If it's enough of an issue, I can just add a bit more BP until it works.
 
This is a good topic. If someone has a decent formula, I could make an online calculator.
 
This is a good topic. If someone has a decent formula, I could make an online calculator.
Unfortunately, I think, given the fact that the separation force is so heavily dependent on the exact position of the vehicle as a function of time (if I'm doing this right, I think the term that you end up needing to maximize is the external pressure times the velocity), that it will be hard to make a simple online calculator. It might be something that could be incorporated into an openrocket extension though!
 
Unfortunately, I think, given the fact that the separation force is so heavily dependent on the exact position of the vehicle as a function of time (if I'm doing this right, I think the term that you end up needing to maximize is the external pressure times the velocity), that it will be hard to make a simple online calculator. It might be something that could be incorporated into an openrocket extension though!

I am close to a guide calculation. I am thinking about giving a range.
 
I am close to a guide calculation. I am thinking about giving a range.
That could work. Are you trying to just use expected apogee and volume of the compartment, or are you also using the max velocity as an input?
 
I started with a estimate found in the Adept manual. Will adjust front here.
 
But what's the harm in oversizing the holes? The holes for altimeters are not really big, and they'll be safe for anti-deployment.

Physics is about understanding the underlying theory and getting the answer just right. Engineering is about getting it to work, work well, and meet the spec. Sometimes that starts with the physics, and sometimes it starts with a known, acceptable solution. (All too often, engineering managers and program managers only want it to work and meet the spec; In this case, an altimeter sized hole really won't keep the rocket from working well.)
 
I always oversize and also have a hole to turn on the altimeter, works for me in over 300 flights
 
I always oversize and also have a hole to turn on the altimeter, works for me in over 300 flights
The OP is referring to the vent holes used to prevent overpressurizing the parachute/payload bays, not the altimeter static ports. I generally use a 1/8" hole for payload bays.
 
The OP was asking for information on how to size static vent holes to equalize air pressure during ascent, to prevent separation, not about sizing ports for altimeters.
 
Yeah, airframe vents and flight computer sample ports are two different things, but can be calculated the same way. Static vent ports calculators are designed to give a vent port dimension that keeps a tube atmosphere equalized between the inside and outside environment in a manner that minimizes sensor lag. This is similar to what we want to do to prevent tube pressure pushing out a fuselage or nose cone shoulder prior to a commanded event due to built up air pressure.

I've always solved this question of what size airframe vents to use like this: take your favorite static vent port calculator, online or otherwise, and solve the equation for the same way you would for static vent holes for your flight computers, but solve the equation for ONE (1) hole. Just be sure to use the dimensions of the tube between the applicable bulkheads. Drill holes in line with the rail buttons on the back side of the rocket for aesthetics, and pack to make sure that the laundry doesn't plug the hole.
 
That's oversizing them, and I approve. It doesn't take the strength of the shear pins into account; the shear pins mean the rocket can tolerate some pressure differential, and can theeefore use a smaller hole.

That's the physics. The engineering says "Who cares? A hole sized for an altimeter isn't so big. So, it gives you more design margin, where's the bad?"
 
What's the estimate? (I don't have an Adept altimeter and they've gone out of business)

Jasper,

Sorry for the delay . . . Adept info !

https://web.archive.org/web/20070805004922fw_/https://www.adeptrocketry.com/staticports.htm

QUOTE :

Adept Rocketry - Information on Static Ports
Copyright © 1999, 2005, All Rights Reserved

An altimeter must be installed in a “sealed” chamber with a vent or vents to the outside. A sealed bulkhead below the altimeter chamber is necessary to avoid the vacuum caused by the aft end of a rocket during flight. A sealed bulkhead above the altimeter chamber is necessary to avoid any pressure fluctuations that may be created at the nose end of the rocket. The vent (also known as a static port) to the outside of the rocket must be in an area where there are no obstacles above it that can cause turbulent air flow over the vent hole. Do not allow screws, ornamental objects, or anything that protrudes out from the rocket body to be directly in line with and forward of a vent hole. The vent must be neat and burr free and on an outside surface that is smooth and vertical where airflow is smooth without turbulence or obstruction.

Some rocketeers use multiple static ports (vent holes) instead of just one. Very strong wind blowing directly on a single static port could affect the altimeter. Multiple ports evenly spaced around the rocket tube may help cancel the effects of strong wind, the pressure effects of a non-stable (wiggly) liftoff, or the pressure effects that occur due to flipping and spinning after deployment. If you wish to use multiple ports, then use three or four. Never use two. Ports must be the same size and evenly spaced in line around the tube.

The general guideline for choosing port size is to use one 1/4 inch diameter vent hole (or equivalent area, if multiple holes are used) per 100 cubic inches of volume in the altimeter chamber. For instance, an eight-inch long four-inch diameter tube has a volume of about 100 cubic inches. Use one 1/4 inch port, or three or four 1/8 inch ports evenly spaced around the tube. Area wise, one 1/4 inch hole is equivalent to four 1/8 inch holes. An altimeter chamber two inches in diameter and eight inches long (25 cubic inches) needs one 1/8 inch vent hole or three or four 1/16 inch vent holes. For a chamber 1.5 inches (38 mm) ID by 4 inches long (7 cu. in.) use one 1/16 inch hole or three or four 1/32 inch holes. Keep hole sizes within -50% or +100% of the general guideline. Do not make the holes too small, and especially do not make them too large. Obviously, a vent or vents in a BT-20 (18 mm) body tube will be quite small.

Vent holes should be a minimum of four body diameters below the junction of the nosecone with the rocket body. This is necessary with high performance (high speed) rockets. The tremendous pressure on the nosecone leeches down the rocket body as much as four diameters before it dissipates.

www.adeptrocketry.com, 11-18-05

END QUOTE :

Dave F.
 
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