One of our experiments this year on STR-03 is the fluid flow velocity $latex V$ measurement with a Pitot-Tube. The principle of this device is the pressure difference between the ambient pressure (static pressure $latex p_s$) and the dynamic pressure $latex p_t$ on the tip.

We measure this gap with an differential pressure sensor (ASDXRRX0) which is connected via $latex \mathrm{I^2C}$ to the flight measurement system (FMS).

The formula to get the velocity is:

$latex V = \sqrt{\frac{2 (p_t – p_s)}{\rho}}$

The missing term – the density $latex \rho$ – can be calculated with $latex \rho =\frac{p_s}{R_s T}$. $latex R_s=287.058~\frac{J}{kg K}$ is the specific gas constant of air and $latex T$ is the absolute temperature.

There are two assumptions with this method: the air is incompressible ( good assumption up to $latex 800km/h$) and the air is dry (otherwise the density would also depend on air humidity).

To safe our pitot tube from damage there is a linear servo actuator inside the top section of STR-03 which withdraw the tube in a safe position at descend.