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.