Aircraft Oxygen System Sizing

Aircraft Oxygen System Sizing (vehicle-design/sizing/aircraft-oxygen-system-sizing)

Use when the task is sizing the supplemental oxygen system of a transport aircraft at the conceptual level: passenger continuous-flow chemical oxygen generators (one unit per passenger, TSO-C64 style 22 minute protection) against a crew diluter-demand gaseous oxygen system, the oxygen demand volume at standard conditions from the per-occupant flow rate and protection duration, the stored oxygen mass, and the crew high-pressure bottle volume from the ideal gas law. This leaf implements that sizing in pure Python, stdlib only. It pairs with vehicle-design/sizing/environmental-control-sizing, which produces the governing cabin pressure altitude that this leaf consumes as the demand driver (the schedule itself lives there; this leaf takes its altitude result as an input). The aircraft oxygen system sizing never covers surface thermal protection (ice-protection-sizing) or seat layout (fuselage-sizing).

Domain quick reference

• Passenger continuous-flow demand: V_pax = n_pax * q_pax * t_pax in standard litres (SL), with q_pax = 5.0 SLPM per passenger and t_pax = 22 min of protection per generator unit. The architecture supplies one chemical generator unit per passenger.
• Demand mass: m = V * 1e-3 * RHO_O2_STP with RHO_O2_STP = 1.429 kg/m3, the oxygen density at 0 C and 101.325 kPa (1.429 g per standard litre), so m_kg = V_SL * 1.429e-3 exactly.
• Crew diluter-demand: V_crew = n_crew * q_crew * t_crew with q_crew = 2.5 SLPM average per crew member and t_crew = 120 min (2 h crew protection); same volume-to-mass conversion.
• Crew storage: the gaseous oxygen bottle holds the crew demand mass at the service pressure. Ideal gas law V = m * R_O2 * T / p with R_O2 = 259.8 J/(kg K), T = 288.15 K (15 C storage) and p = p_psi * 6894.757 Pa. The bottle volume is inversely proportional to the service pressure at fixed mass and temperature.
• The design cabin pressure altitude sets the demand context: cabin partial pressure of oxygen at an 8000 ft cabin altitude is about 15.8 kPa versus 21.2 kPa at sea level, which is why the altitude drives the demand. The altitude itself comes from environmental-control-sizing.
• All demand volumes are standard litres at 0 C and 101.325 kPa; pressures enter as psi and are converted internally.

Workflow

1. Fix the occupant split: passenger count n_passengers and crew count n_crew (sources: fuselage-sizing for seats, the operator's crew complement) and the governing cabin pressure altitude from environmental-control-sizing.
2. Decide the delivery architecture: continuous-flow chemical generators for passengers, diluter-demand gaseous oxygen for the crew.
3. Compute the passenger demand with passenger_demand (volume_sl and mass_kg at the default 5.0 SLPM over 22 min) and the generator unit count with generator_units, one unit per passenger.
4. Compute the crew demand with crew_demand (default 2.5 SLPM over 120 min).
5. Size the crew storage with bottle_volume from the crew demand mass, the service pressure in psi and the storage temperature; it returns volume_m3 and volume_l (for layout, one bottle or two half-size bottles).
6. Roll everything up with oxygen_summary(n_passengers, n_crew, service_pressure_psi), which returns the passenger demand, the generator count, the crew demand, the crew bottle volume and the total stored mass.
7. Confirm the deterministic checks with the contract test scripts/test_aircraft_oxygen_system_sizing.py.

Worked example

Reference transport: 150 passengers, 6 crew, crew bottle service pressure 1800 psi. Real module outputs:

• passenger_demand(150): volume_sl = 16500.0 SL (150 * 5.0 * 22), mass_kg = 23.5785 kg (spec bound 23.58 within 1e-1).
• generator_units(150): 150 units, one per passenger.
• crew_demand(6): volume_sl = 1800.0 SL (6 * 2.5 * 120), mass_kg = 2.5722 kg (spec bound 2.57 within 1e-2).
• bottle_volume(2.5722, 1800): volume_m3 = 0.015516 m3, volume_l = 15.52 L within 1e-2 of the spec value (2.5722 * 259.8 * 288.15 / (1800 * 6894.757) = 0.015516 m3). For layout, one 15.5 L bottle or two 7.76 L bottles.
• oxygen_summary(150, 6, 1800): total_mass_kg = 26.1507 kg (23.5785 passenger + 2.5722 crew), bottle_volume_l = 15.516 L.

Pitfalls

• Using one architecture for both cabins: passengers get continuous-flow chemical generators (one unit per passenger, 5.0 SLPM over 22 min) while the crew runs on diluter-demand gaseous oxygen (2.5 SLPM over 120 min); swapping the flow rates or durations mis-sizes both.
• Forgetting the unit conversion in the demand mass: m_kg = V_SL * 1.429e-3 exactly (one standard litre of oxygen masses 1.429 g at 0 C and 101.325 kPa), so a standard-litre volume quoted directly as grams is off by 1000.
• Sizing the bottle at the wrong pressure convention: pressures enter in psi and are converted internally (1800 psi in the worked example); the ideal-gas volume is inversely proportional to the service pressure, so a psi-versus-Pa slip mis-sizes the bottle.
• Confusing standard litres with ambient litres: all demand volumes are at standard conditions (0 C, 101.325 kPa); an ambient-volume demand does not convert through the same density.
• Neglecting the cabin altitude driver: the design cabin pressure altitude sets the demand context and comes from environmental-control-sizing; this leaf consumes that altitude result as an input rather than re-deriving the schedule.
• Feeding non-positive occupants, flows, durations, mass, pressure or temperature: every one of these raises ValueError, including through the oxygen_summary convenience call.

Verification

• Confirm passenger_demand(150) returns 16500.0 SL and 23.5785 kg, and that doubling the passenger count doubles both the volume and the mass.
• Confirm generator_units(150) returns 150 and generator_units(1) returns 1.
• Confirm crew_demand(6) returns 1800.0 SL and 2.5722 kg.
• Confirm bottle_volume(2.5722, 1800) returns 0.015516 m3 and 15.516 L, that doubling the pressure halves the volume and doubling the mass doubles the volume.
• Confirm the mass round trip: one standard litre of oxygen masses exactly 1.429e-3 kg (mass_kg = volume_sl * 1.429e-3).
• Confirm identical inputs give identical outputs (deterministic).
• Confirm every non-positive n_passengers, n_crew, flow, duration, mass, pressure and temperature raises ValueError, including through oxygen_summary.
• Confirm the oxygen_summary keys match the documented contract exactly: passenger_demand_sl, passenger_mass_kg, generator_units, crew_demand_sl, crew_mass_kg, bottle_volume_m3, bottle_volume_l, total_mass_kg.
• Run the contract test offline: python3 scripts/test_aircraft_oxygen_system_sizing.py (34 tests, deterministic).

Related leaves

• vehicle-design/sizing/environmental-control-sizing: produces the governing cabin pressure altitude this leaf consumes as the demand driver.
• vehicle-design/sizing/fuselage-sizing: seat count and cabin layout source for the occupant split.
• vehicle-design/sizing/ice-protection-sizing: surface thermal protection, distinct from the breathing oxygen demand sized here.

Behavior contract (gate 3)

Run the deterministic contract test (stdlib unittest, offline):

python3 scripts/test_aircraft_oxygen_system_sizing.py

The test covers the 150 passenger / 6 crew / 1800 psi worked example within the spec magnitude bounds, the doubling identities (passengers, pressure, mass), the exact standard-litre mass round trip, the oxygen_summary convenience keys and values, and ValueError rejection of every non-physical input class (34 test methods).

Compliance

• Standards referenced, not reproduced: FAR 25.1443 frames the supplemental oxygen context (framing only); the flow, density and protection-duration values above are standard engineering design practice per the summary-only convention of standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.