How to size static vent holes for a rocket altimeter bay

Model rocket altimeters measure altitude from atmospheric pressure, which means the inside of the altimeter bay has to sense the outside air as accurately and quickly as possible. Too-small vent holes cause lag, the bay pressure lags behind the outside, so the altimeter reports apogee late, deploys the recovery charge late, and misses peak altitude readings. Too-large vent holes let pressure pulses and airflow-induced noise reach the sensor, which can cause false apogee detections and premature ejection. The formula used here (0.004396 × tube diameter × √(length/number)) is the long-established guideline developed for commercial altimeters and used by Perfectflite, Missileworks, AltimeterThree, Eggtimer and most other manufacturers as a starting point.

How many holes, and where?

Three or four holes spaced evenly around the tube circumference is the accepted practice. Multiple holes average out local pressure variations across the airframe, which reduces sensitivity to angle of attack, crosswind gusts and airflow disturbances from fins or rail buttons upstream. Holes should be placed at least three tube diameters away from any nose cone shoulder, transition or other change in cross-section, because pressure locally dips or spikes at those transitions. Centre them in the straight tube section that actually houses the altimeter.

Thick-walled or composite tubes

For tubes with walls of about 3mm or thicker (fibreglass, carbon, thick phenolic, printed couplers) the flow channel through the vent is long relative to the hole diameter, which reduces effective flow rate. Either drill slightly larger than the calculated value, or deburr and slightly round the inner edge of the hole to smooth the air path.

Supersonic and transonic flights

The simple vent formula assumes subsonic steady flow. Above Mach 0.8 shock waves and local flow separation create pressure readings that are significantly displaced from true static pressure. For supersonic rockets use flush-mounted static ports instead of drilled holes where possible, place ports well away from fins, rail buttons and other protrusions that cause local shock interactions, and use a larger internal bay volume with smaller ports to create a pneumatic low-pass filter that dampens transient pressure spikes. Always follow your altimeter manufacturer's supersonic-specific guidance.

Dual-deployment and backup altimeters

If you're running two altimeters in the same bay they share the same vent holes, so size based on the bay length and not per altimeter. Both units will see the same vented pressure; if anything, a larger bay with good ventilation actually helps the backup altimeter agree with the primary, reducing disagreement at apogee.

Frequently asked questions

What formula does the calculator use? Hole diameter (mm) = 0.004396 × tube diameter (mm) × √(tube length (mm) / number of holes).

How many vent holes are best? Three or four, equally spaced around the tube. Multiple small holes average out airflow noise better than a single larger hole.

Why 3× tube diameter from transitions? Pressure disturbances at shoulder transitions and nose cones decay with distance. Three diameters is the accepted minimum separation for clean static readings.

Do I need different holes for subsonic vs supersonic? Yes. The standard formula is only valid for subsonic flight. Supersonic rockets need flush static ports and manufacturer-specific sizing.