Most rocketeers that use altimeters are aware of the need to use vents of the correct size. Fewer are aware that in some cases vents are required for other parts of the rocket.

As an example of the problem consider a 6" diameter rocket that uses 3 2.56 nylon screws as shear pins and flies from sea level to 10,000'. The pressure difference between ground and apogee is about 4.6 psi. If the higher pressure air inside the rocket were not able to leak out, the force on the nose cone would be 130 pounds. Since the shear pins fail at an average of 25 pounds, this exceeds the 75 pounds required to shear three of them and the nose would come off.

So how much vent do you need? Excellent question.

First we need to know something about what effect the vent has. Bob Krech has posted a formula for computing the time constant for an altimeter bay vent. This was posted on Rocketry Online but because of a major meltdown, it is lost.

L is the compartment length, D is the compartment diameter, d is the vent diameter, and T is the time constant in seconds. (All dimensions in inches.)

Since I studied electrical engineering I tend to think of systems in terms of their electrical analogs and this system is no different. The vent is like a resistor, the compartment like a capacitor, and the pressure difference is like a voltage. This forms a simple system that behaves like a RC filter.

The equation describing a simple RC circuit is:

The value "RC" is the time constant. This is analogous to the time constant in the first equation. Of course this equation doesn't help us too much as we are not starting from a simple constant pressure but have a time varying pressure. I could go back to the basic differential equation:

But that isn't really needed. Instead, I will create an equivalent digital low pass filter to pass pressure data through. That pressure data will come from Rocksim.

Wiki has a handy page discussing this if you want more detail but the filter turns out to be an old favorite of mine: recursive. The basic filter equation is:

Where alpha is:

The first step is to export your simulation data from Rocksim. Select only "Pressure" in the simulation export menu. The output will be in a text file in comma separated format (.csv). This will be the input to the filter program. (The filter program is provided as C source code. You will have to compile this yourself unless this Linux version works for you. The filter is pretty simple so you could do this with a spreadsheet.)

Here is an example using a simulation of my level 3 certification rocket flying on a M1850. It is six inches in diameter with a 1/8" vent hole. The main parachute compartment is in the nose so I just guessed at a length of 24".

The time constant in this case was 13.8 seconds. I have plotted the atmospheric pressure and force and you can see that the force peaks well before apogee. I used 2 4-40 nylon screws as shear pins and a good thing as 2-56 would not have worked. Note that the pressure does not decrease very much by apogee. This pressure will add to the drogue deployment stresses on the shear pins and should be reduced.

Switching to a 1/4" hole reduces the time constant to 3.5 seconds, cuts the peak force in half, and greatly reduces the residual force at apogee. I don't like the idea of a 1/4" hole so I will use four 1/8" holes instead.