Pressure relief vent hole

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Steven88

Well-Known Member
TRF Supporter
I’m wondering what size of pressure relief holes my 5.38” diameter Loc Big Nuke 3E should have in the booster section and main section? It will fly up around 10,000 feet. Both sections are also pinned with 2-56 nylon sheer pins. Is there a calculator or rule of the thumb for sizing and placement of these holes? I should obviously place them in a spot where they won’t be blocked off by recovery gear as well

Steve Shannon

Well-Known Member
TRF Supporter
All you should need is a small 1/8"-"3/16" hole just below the nose cone shoulder, or just below the bulkhead of the avionics bay. It allows the air to vent out instead of being trapped, thus pushing against the nose cone or other bulkhead…. You can easily search on the forum.... More info

That’s all I do. I use a 5/32” drill because it’s the first one I find.
Size isn’t critical unless you have a super large compartment you’re venting.

richP

Well-Known Member
Just keep each of these holes somewhat forward so that the laundry has less chance of plugging it. 1/8" should be fine.

JLebow

Well-Known Member
I just did these calculations for my L3 project packet, so it is still fresh in my mind. The conclusion is unless the rocket is sealed with o-rings, you are going very high, or very fast, the natural venting of the rocket is going to be adequate. That said, I don't like to depend on incidental design and chance, so I drill a hole with the smallest drill I have on hand - 1/16".

Here's the math.

This is atmospheric pressure vs altitude.

My launch site is at 4500 above sea level. My rocket will go 20,000 feet above ground level, so I look at the pressure difference between 24,500 and 4,500 feet. That is a pressure drop of 6.5 psi from pad to apogee.

If the rocket is sealed, on a 3" diameter nose cone bulkhead there will be pi*r^2*pressure = pi*(1.5 in)^2*6.5 psi = 50 lbs force pushing to separate the nose cone. Using the output file of my simulation for the flight, I can see over time how the pressure builds. The pressure at launch altitude gets sealed inside the rocket when the rocket is assembled and remains constant over the entire flight (gray trace), The atmospheric pressure drops as the rocket goes up (orange trace) and the shape of the curve depends on the velocity of the rocket. The pressure acting to separate the rocket is the pressure difference between inside the rocket and outside the rocket (blue trace).

Now if there is a hole in the airframe to equalize pressure, air will flow when there is a pressure difference between the rocket and atmosphere. It will flow faster with a bigger pressure difference, or a bigger hole. The rate of equalization is slower with a larger rocket volume. There is an equation that describes this flow, given these conditions - air mass flow through an orifice in a pipe. With a 1/16" hole, the pressure will reach a maximum 1 psi difference, during the maximum velocity of the rocket (during boost), with a separation force of 7 lbs. My shear pins can hold the force, along with help from aerodynamic pressure on the front of the cone traveling at Mach 1.8. By the time the rocket is well into coast, the pressure difference is negligible.

Well-Known Member
make sure whatever you decide balances with your altimeter documentation. The Stratologger documentation dedicates a whole page and table to vent holes.