Vent holes...but not for the payload bay

[highpk]

I have read a considerable amount on the drilling of vent holes for use on altimeter-based deployments, and as a general practice to keep pressure changes and drag-related pressure buildup from popping a payload section before anticipated.

However, has anyone drilled holes aft of the forward motor centering ring? Specifically, in the dead space found between the forward and aft centering rings that is sealed off (presumably) by epoxy fillets. This space is sealed and is therefore susceptible and prone to pressure changes like all other spaces. Is it not as much of a worry given the materials and construction techniques? Has anyone seen this section of an airframe burst due to pressure changes (and not motor failure)?

 

[teepot]

I haven't seen a BT fail like that. Maybe one of the high altitude guys can answer your question.

 

[dhbarr]

I don't think we generally fly much of anything weaker than a balloon. If it can withstand the launch & recovery, it should be alright.

 

[SolarYellow]

It's on my checklist to make sure all those cavities aren't sealed. Can have a hole in a motor mount bulkhead. Could put holes in the MMT itself.

Best practice is to not expose the altimeter chamber to base drag through any leak path that could overwhelm the pressure ports (not much can be done about leakage past the motor in most cases), as that can affect pressure and data collection on the way up. Shouldn't affect deployment.

 

[Spacedog49Krell]

Unless you are reaching extreme altitudes, the air expansion between the centering rings will only make the body tube stiffer. If you are achieving altitudes >12,000 ft I would make a pathway to pressure equalize these volumes. Looking at my motor mount centering rings, I unintentionally incorporated equalization pathways.

 

[Steve Shannon]

There’s nothing wrong with venting the area, but I’ve never seen a forward centering ring blow out, even on high altitude flights. Many have a hole in them for a u-bolt or eye bolt to connect the shock cord anyway, so they’re not always sealed.

 

[Spacedog49Krell]

After thinking about this question overnight, I remembered Darcy's Law. This law is used to determine the 3 P's; porosity, permeability, and permeance of materials to fluids. Using numbers I found on the Internet for paper, the aerodynamic (Max Q) pressure on a paper rocket will exceed the Young's Modulus of a paper tube before the internal trapped pressure differential exceeds the hoop strength of the same thickness paper body tube. The permeability of paper to fluids makes it a very poor pressure vessel material.

 

[bschultz32]

I sheepishly admit that I've never thought of this potential problem!
Look at the photo that accompanies this Forum entry. That's me holding the booster portion of my LOC/Precision Mega-Magg, on the Bonneville Salt Flats. The annular spaces between the motor mount system and the body tube are filled with PML expanding foam. The foam displaced any air that would otherwise be trapped in the spaces. No air trapped - no problem! Plus, the result of this construction technique is a REALLY strong structure.

 

[86mustang408w]

I have read a considerable amount on the drilling of vent holes for use on altimeter-based deployments, and as a general practice to keep pressure changes and drag-related pressure buildup from popping a payload section before anticipated.

However, has anyone drilled holes aft of the forward motor centering ring? Specifically, in the dead space found between the forward and aft centering rings that is sealed off (presumably) by epoxy fillets. This space is sealed and is therefore susceptible and prone to pressure changes like all other spaces. Is it not as much of a worry given the materials and construction techniques? Has anyone seen this section of an airframe burst due to pressure changes (and not motor failure)?

Its a good question. The main purpose is a vent hole above the centering ring is so the nosecone doesn't pop off. A hole below that wouldn't make a difference but might not hurt to have one for extremely high altitudes. I've never seen or heard of that space expanding or it causing damage. I usually have two vent holes, one below the nosecone and one by the lug. Funny thing I still have a hole open on one rocket that I used to inject glue into.

 

[Spacedog49Krell]

Here are some calculated numbers of the expansion of a thin wall BT-60 paper tube with altitude. The expansion numbers for paper with thick walls, fiberglass, and other composite materials were negligible. These expansion numbers are for a perfectly sealed volume with no permeation losses. I'm still searching for the hydraulic permeability functions of paper for the Darcy's Law equation to determine the loss of pressure during the flight from permeation.

Altitude ft	Delta PSI	Diameter Expansion of BT-60	BT-60 Diameter
0​	0​	0​	1.63​
1,900​	1​	0.01​	1.64​
4,000​	2​	0.02​	1.65​
6,200​	3​	0.03​	1.66​
8,500​	4​	0.04​	1.67​
11,000​	5​	0.05​	1.68​