Are barometric altimeters suitable for rocketry?

[Chad Brown]

This might be a dumb question, but its one I've had for a while. I've seen a lot of people use barometric altimeters to measure the altitude of model rockets. As I understand, the relative velocity of a gas and its pressure are inversely related so wouldn't the rockets velocity and the changing in its velocity through out the flight make altitude predictions based on the pressure readings inaccurate?

 

[cerving]

That's why there are multiple vent holes in the AV bay... so the pressure equalizes between the outside and inside of the AV bay. You also want to locate the vents away from high/low pressure areas of the rocket if possible... although head-end-deploy AV bays just behind the nose cone don't seem to be a problem. About 99% of the altimeters in use rely on baro pressure measurements for altitude determination and deployments, so they seem to work just fine.

 

[Buckeye]

The principle is based on static pressure ports. The static ports are generally placed on the straight airframe, perpendicular to the rocket motion/airflow, and not behind any protuberances. The airflow gradients here are negligible. Same concept as a pitot-static tube. One hole can work, but 3 or 4 are better to even things out.

https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/pitot-static-tube-speedometer/

 

[fyrwrxz]

This might be a dumb question, but its one I've had for a while. I've seen a lot of people use barometric altimeters to measure the altitude of model rockets. As I understand, the relative velocity of a gas and its pressure are inversely related so wouldn't the rockets velocity and the changing in its velocity through out the flight make altitude predictions based on the pressure readings inaccurate?

Think of this. What is the barometric pressure at your location? Think of sea level. Air is 14.7 psi give or take for weather systems. Now, look at a barograph, that's a recording barometer. I have one, but not at home, can't get a pic. Now, air pressure varies according to changes in the density of the air, "fronts" if you will. These can be precisely charted by millibar changes, used on weather prediction. Ignoring aerodynamics, boundary layers and shock waves, the rocket speed has no calculation in the atmospheric pressure as sensed by the Altimeter. That, along with some tricky software, decodes the altitude change from the base input to the sensor on board. Deployment is based on negative changes (or positive, depends how you think) indicating a thickening atmosphere. By using simple time/speed/distance permutations coded in a chip, you can get a flight profile. Accelerometers as basically just that, useful in conjunction with other data. It's a hoot to find out how hard your motor hit for how long, and that is partly why we have thrust curves published. When your go chip pulls up to the stop sign at 18k, he opens the door via a circuit signal and kicks everybody out of the bus. FWIW. Straight smoke and good chutes.

 

[Titan II]

Yes

 

[cls]

Read the instructions for your baro altimeter. Run the calculation for the sample port size. Stick to that.

My experience is, very oversized large holes make a resonance in the ebay, like a flute or recorder, which will trigger the altimeter before apogee.

Way undersized or blocked holes will keep a positive pressure in the ebay, for some time after apogee..
Chute comes out late.

 

[John Beans]

Google "static pressure port" to learn more about how one or more holes on the side of a rocket or aircraft are used to measure the ambient pressure accurately even when the rocket or aircraft is moving rapidly. In use for many years and quite reliable.