STRUCTURAL ANALYSIS

Vent & Port Sizing

Altimeter static ports for accurate readings and pressure relief vents to prevent premature separation.

Vent Types

Type	Purpose	Typical Size
Altimeter ports	Pressure sampling for altitude	3-6mm diameter
Pressure relief	Prevent over-pressure	1.5-3mm diameter
Ejection bay vents	Post-deployment decay	3-6mm diameter

Ports must allow pressure equalization fast enough to avoid altitude measurement errors. The Stroick formula (widely used):

Volume < 100 in³

d = \frac{V}{400}

Volume ≥ 100 in³

d = 2\sqrt{\frac{V}{6397.71}}

d

Port diameter (inches)

V

Bay volume (cubic inches)

Minimum Ports

Evenly spaced

Recommended

Best gust immunity

Port Placement

Locate on cylindrical section parallel to airflow. Avoid proximity to nose cone shoulder, fins, and rail buttons.

Without pressure relief, altitude changes create internal over-pressure:

Premature Separation

A 4" rocket at 3000m can experience 254 N (57 lbf) force on the nose cone from pressure differential — easily overcoming friction fit!

For pressure relief (friction-fit nose):

Relief Vent Area

A_{vent} \approx 0.5-1.0\% \times A_{cross}

SOURCES

Static Port Holes from Nescience to Science — Stroick, G.
The foundational paper on altimeter port sizing methodology
Sizing static ports for altimeter sampling — Cambridge University Spaceflight
First-principles approach to port sizing with time constant analysis

Rocket Design (Tripoli Minnesota) — Stroick, G. (2010)
Presentation covering vent sizing and other design topics
Altimeter Port Sizing — VernK
Alternative sizing methodology with worked examples
How Big Does An Altimeter Vent Hole Need to Be? — Apogee Components
Peak of Flight #543 - Practical vent sizing guidance
Pressure Relief Holes — Apogee Components
Peak of Flight #68 - Preventing premature separation
Orifice plate
Theory behind orifice flow calculations
Choked flow
Understanding flow limitations through small openings
Calculation of Flow through Nozzles and Orifices — Neutrium
Detailed orifice flow equations

Static Port Hole Calculator — Stroick, G.
Excel spreadsheet implementing the Stroick method
Altimeter Vent Port Sizing Calculator — Rocketry Calculator
Online tool implementing common vent sizing methods
Vent holes calculator — Model Rockets UK
Alternative online vent calculator

Put this methodology into practice with the interactive calculator.