Venting

[Normzilla]

Hey all,

I was wondering what the consensus is for what a good size would be of the vent hole on a rocket.

Thanks
Nathan

 

[blackbrandt]

Hello and welcome to TRF!

What is this vent hole for? Is it for an altimeter bay, or is it for pressure venting?

Also, what size rocket are you building?

 

[kevinkal]

Vent holes sizing is dependent on a number of factors. Volume to be vented is the major factor.
If you're simply venting a compartment without barometric sensors, then the vent hole would need to be sized with consideration of how much force (due to trapped pressure) the rocket's joints can handle. Here's a good reference for venting altimeter bays: https://www.vernk.com/AltimeterPortSizing.htm

 

[Normzilla]

Pressure.

Doing a play on the MadCow Tomach in 3".

Thanks

 

[tim cubbedge]

Madcow recommends 1/8” for most of their kits. I use same on just about everything I have ever built.


Sent from my iPhone using Rocketry Forum

 

[markkoelsch]

It depends to a certain extent on what avionics you are using.

That said, just off the top of my head I would go three 1/8" holes.


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[mccordmw]

I did a bit of research on this for my L3.

The rule of thumb was 0.25" static ports for every 100 cc of payload bay. That had no scientific foundation. It was just a general guideline based on some empirical observations. Those also didn't scale well with larger bays and gave too large of a recommendation.


The old equations also did not take into account the speed of the rocket. If I strapped my rocket to a balloon and let it drift slowly upward, I can practically get by with no holes to let the pressure normalize since it's not a hermetically sealed compartment. However, if I blast upward at high mach with a VMax motor, I'm going to need larger holes to let the rapidly escaping gas keep up with the atmospheric changes. Velocity of the rocket must be accounted for.


Also, lower altitudes have higher density air. The fluid dynamics of higher density gases would mean they escape more slowly through a static port. So starting altitudes closer to sea level will need slightly larger ports. Temperature also affects this.


So I used the more advanced calculator provided by OffWeGoRocketry. Here's their link.


https://www.offwegorocketry.com/userfiles/file/Calculators/Static Port Holes.xls

 

[Normzilla]

Thanks all!

I apologize for not using the search before I opened my trap....

I really appreciate the help.

Nathan

I did a bit of research on this for my L3.

The rule of thumb was 0.25" static ports for every 100 cc of payload bay. That had no scientific foundation. It was just a general guideline based on some empirical observations. Those also didn't scale well with larger bays and gave too large of a recommendation.


The old equations also did not take into account the speed of the rocket. If I strapped my rocket to a balloon and let it drift slowly upward, I can practically get by with no holes to let the pressure normalize since it's not a hermetically sealed compartment. However, if I blast upward at high mach with a VMax motor, I'm going to need larger holes to let the rapidly escaping gas keep up with the atmospheric changes. Velocity of the rocket must be accounted for.


Also, lower altitudes have higher density air. The fluid dynamics of higher density gases would mean they escape more slowly through a static port. So starting altitudes closer to sea level will need slightly larger ports. Temperature also affects this.


So I used the more advanced calculator provided by OffWeGoRocketry. Here's their link.


https://www.offwegorocketry.com/userfiles/file/Calculators/Static Port Holes.xls

 

[blackbrandt]

A little tip, when you search TRF, use google instead of the search feature. Use

site:rocketryforum.com search term here

Hope this helps!

 

[kevinkal]

...

So I used the more advanced calculator provided by OffWeGoRocketry. Here's their link.


https://www.offwegorocketry.com/userfiles/file/Calculators/Static Port Holes.xls

I opened this spreadsheet in Google Sheets, but it seems to be missing name definitions for the equations in cells B-35-38, 41, 41 and 44. Does it work for anyone?

 

[mccordmw]

I opened this spreadsheet in Google Sheets, but it seems to be missing name definitions for the equations in cells B-35-38, 41, 41 and 44. Does it work for anyone?

It's a Google Sheets incompatibility with the excel spreadsheet tool. I just opened it with Open Office's version of Excel, and it works.

You can try dropping in these values.

Atmospheric Pressure at Launch Altitude	29.0174182546	inHg
Air Density (Dry Air)	0.0756223786	lbm/ft3
Atmospheric Pressure Change	0.01216%
Pressure Equalization Rate	0.0082020997	s
Choked Threshold	1.8935326534
Altimeter Bay Volume	306.727197164	in3
Air Mass	0.0134232872	lbm
Mass Flow Rate	0.0001989955	lbm/s

 

[kevinkal]

Thanks.. I'll give it a shot.

 

[Zeus-cat]

....

 

[muddymooose]

I agree with a couple of the previous posts that when in doubt, go 1/8". The last few 2"-4" diameter rockets I've built have each had two 1/8" holes on opposite fore and aft sides of the main airframe and two 1/8" holes on opposite fore and aft sides of the payload compartment for the altimeter. I've heard 3 or 4 holes is better than 2, but these have worked so far up to about 3000 feet. These have all been SD...I would probably be more particular about sizing and spacing etc. if it was DD and the life of the rocket was literally riding on proper venting.

 

[muddymooose]

Vent holes sizing is dependent on a number of factors. Volume to be vented is the major factor.
If you're simply venting a compartment without barometric sensors, then the vent hole would need to be sized with consideration of how much force (due to trapped pressure) the rocket's joints can handle. Here's a good reference for venting altimeter bays: https://www.vernk.com/AltimeterPortSizing.htm

Good stuff, I definitely need to pay more attention to that in the future.

 

[OverTheTop]

I have used single 1/8" holes for venting each compartment in a 54mm Tomach. Configuration was two shear-pins on the NC, running single ended dual deploy. Cracked Mach 1.8 on the way to 21.5k', so rapid environmental pressure change. Deployment nominal.

I personally would use a single 5/32" port per compartment, based on my experience. Or maybe two 1/8".

 

[Tim51]

Thanks to the OP and all for an informative discussion. A supplementary question, if I may: usually when I load the burrito I see the blanket blocking the vent hole(s). This hasn't been a problem with low 'n' slow flights, but is it a factor to consider with trans/mach+ flights, and if so, how is it remedied?

 

[kevinkal]

Thanks to the OP and all for an informative discussion. A supplementary question, if I may: usually when I load the burrito I see the blanket blocking the vent hole(s). This hasn't been a problem with low 'n' slow flights, but is it a factor to consider with trans/mach+ flights, and if so, how is it remedied?

This issue gets my attention too. It seems to me that this issue is too often overlooked. What I have done, is to drill my vent holes where I anticipate no obstruction, or at a location that will be unlikely to be blocked by the chute protector or parachute. For example, I like to drill the vent hole in my payload sections just below the nosecone bulkhead. This assumes that launch acceleration will keep the recovery gear in the lower section of the tube, leaving a clear volume near the top of the section below the nosecone where the vent hole(s) is(are) drilled. I also have to assume that deceleration forces after burnout will not shift the recovery gear enough forward to block the vent hole(s).
Another thing to do is to size the shear pin holes such that even if the vent is blocked, the captive pressure cannot cause separation. (For the payload/main parachute section, also consider adding some shear pin strength margin for potential shock forces when the cord snaps tight at apogee deployment.)

I have seen a nicely done spreadsheet that covered venting and shear pin sizing. It considered trapped pressure due to unvented volume, and also deployment shock forces etc. I havne't seen it for a while and don't seem to have a link for it. Perhaps someone could post link to it.

 

[Normzilla]

Wow, a ton of great info and discussion.

Thanks for that!

Nathan

 

[woferry]

A supplementary question, if I may: usually when I load the burrito I see the blanket blocking the vent hole(s). This hasn't been a problem with low 'n' slow flights, but is it a factor to consider with trans/mach+ flights, and if so, how is it remedied?

I'm fairly positive I got bit by this once. The first flight of my Go Devil 38 (H148R), where even the drogue is a pretty tight fit in the airframe, had a separation during ascent, fortunately no damage, didn't make any changes to the rocket and flew it again on the same motor successfully, only difference the second flight was I ensured when I packed the chutes that the shroud lines lined up with the vent holes (one in each chute section) rather than the chute burrito. If I happen to see a line directly over the hole I have a small hex wrench I can put through the hole to push things around. Have had 8/8 successful flights on that rocket since. So that's become my procedure for this rocket, and I'd repeat it on any others where things pack tightly (though as noted above, putting the hole close to a bulkhead would decrease the chances of a hole getting blocked).

 

[mccordmw]

I've never had an issue with blankets or chutes blocking the holes. However, I tend to put the pressure vent holes higher up in each tube that needs them. That way, the laundry in the tube is usually pushed down below the hole.

 

[Tim51]

@kevincal, @woferry @mcordmw - thanks very much for sharing your thoughts/ tips/suggestions on this problem. An extra vent hole up near the NC shoulder BP and poking a little space between the blanket and the vent hole with an allen key will now be on my prep checklist!