Atmosphere
The atmosphere is the gaseous envelope surrounding a planetary body. For aerospace engineering, Earth’s atmosphere is the operating medium for aircraft and the environment rockets must transit during ascent and reentry. Its properties — pressure, density, temperature, composition — vary with altitude in ways that directly affect vehicle design.
Standard atmosphere (ISA)
The International Standard Atmosphere defines reference conditions:
| Altitude (km) | Temperature (°C) | Pressure (kPa) | Density (kg/m³) | Notes |
|---|---|---|---|---|
| 0 (sea level) | 15.0 | 101.3 | 1.225 | Reference conditions |
| 5 | -17.5 | 54.0 | 0.736 | General aviation cruise |
| 10 | -49.9 | 26.5 | 0.414 | Commercial jet cruise |
| 15 | -56.5 | 12.1 | 0.195 | HALE UAV altitude |
| 20 | -56.5 | 5.5 | 0.089 | Stratosphere (constant T) |
| 30 | -46.5 | 1.2 | 0.018 | Balloon ceiling |
| 50 | -2.5 | 0.080 | 0.001 | Mesosphere begins |
| 80 | -86.3 | 0.001 | ~10⁻⁵ | Edge of space (mesopause) |
| 100 | -73 | ~0.00003 | ~5×10⁻⁷ | Kármán line (conventional space boundary) |
Key design implications:
- Density halves every ~5.5 km. A UAV designed for sea level produces half the lift at 5.5 km unless it flies faster or has more wing area.
- Temperature drops to -56.5°C by 11 km then stays constant through the tropopause. Materials and batteries must function at these temperatures.
- At 20 km, density is 7% of sea level. HALE platforms need enormous wings (wing loading below 5 kg/m²) or very high speed.
- At 100 km, the atmosphere is effectively vacuum for aerodynamic purposes but still produces enough drag to deorbit satellites in months to years.
The Kármán line
The Kármán line (100 km altitude) is the conventional boundary of space. It is defined as the altitude where an aircraft would need to fly faster than orbital velocity to generate enough aerodynamic lift to support itself — above this altitude, orbital mechanics governs flight rather than aerodynamics.
Related terms
- Dynamic Pressure — the aerodynamic pressure that depends on atmospheric density and velocity
- Mach Number — the speed of sound varies with atmospheric temperature, changing the Mach number at a given velocity
- Reynolds Number — decreases with altitude as density and viscosity change