# Static pressure

When estimating altitude with a barometric pressure sensor, it’s very important to choose carefully the position of the static hole as the air is perturbed by the nose cone of the rocket. These outside holes must be perpendicular to the direction of flow, so the sensor is pressurized by the local random component of the air velocity. The pressure in these holes is the static pressure ps discussed in Bernoulli’s equation,

$\displaystyle p_s + \frac{1}{2} \rho V^2 = p_t$

If we also were to measure the total pressure pt, the difference in total and static pressure would yield the dynamic pressure q which could be solved for velocity. It’s important to keep in mind that this equation holds for incompressible flow only. We don’t expect our rocket to reach very high subsonic speeds.

In conventional Pitot tube designs, denoting the diameter of the tube by d, a number of static pressure taps are arrayed radially around the circumference of the tube at a station that should be from 8d to 16d downstream of the nose. An amateur rocket shouldn’t be that much different, though our rocket will not be so large. Thus, some analysis should be undertaken as to place the sensor in its optimum position.

I’ll try to carry out some CFD research and publish in this blog my results, but this post is the most I got to achieve during the summer holidays.