Fire Protection Sizing

Fire Protection Extinguishing Agent Sizing (vehicle-design/sizing/fire-protection-sizing)

Use when you must size the aircraft fire protection extinguishing agent at the conceptual level for a protected compartment: the total-flooding agent mass from the zone free volume and the design agent concentration by volume, the concentration closure check, the installed agent rollup from the bottle and shot count, and the discharge nozzle count from the zone coverage. This leaf implements the standard total-flooding model (pure Python, stdlib only). It pairs with vehicle-design/sizing/nacelle-sizing for the nacelle geometry that bounds the engine fire zone and with vehicle-design/sizing/ice-protection-sizing for the other nacelle systems. The zone identification itself belongs to the zonal hazard analysis leaf, which never sizes agent.

Domain quick reference

• Total-flooding closure: the agent vapor volume at the discharge temperature is W * S, where W is the agent mass in kg and S the agent vapor specific volume in m3/kg. The concentration closure requires W * S / (V + W * S) = C / 100, with V the zone free volume in m3 and C the design concentration in percent by volume.
• Required agent mass per shot: W = (V / S) * C / (100 - C), solved from the closure relation. S_AGENT_DEFAULT = 0.158 m3/kg (Halon-1301-class agent vapor specific volume at about 20 C, design value).
• Design concentrations: C_CARGO_DEFAULT = 5.0% by volume for a Class C cargo compartment (FAR 25.855 context) and C_POWERPLANT_DEFAULT = 6.0% for a powerplant fire zone (FAR 25.1191 context).
• Closure check: closure_fraction = W * S / (V + W * S) must return the target C / 100 for the computed mass (identity to 1e-4).
• Installed agent rollup: installed = mass_per_shot * n_bottles * shots_per_bottle; one bottle of one shot carries exactly one required discharge.
• Discharge nozzles: one nozzle per NOZZLE_M3_PER_NOZZLE = 4.0 m3 of free volume, rounded up, with a floor of MIN_ENGINE_ZONE_NOZZLES = 2 nozzles for an engine/APU fire zone.
• Coverage verdict: PASS when the installed agent mass meets the required agent mass per shot; an installation that carries less than the requirement FAILS and the layout is reworked (more bottles or shots, or a higher-capacity bottle).
• Units are SI: m3 free volume, percent by volume concentration, kg agent mass. FAR 25.851/25.855/25.1191 give compartment and fire zone context only; the relations above are standard engineering methodology, summary-only.

Workflow

1. Fix the protected zone: Class C cargo compartment (25.855 context, C_CARGO_DEFAULT) or powerplant/APU fire zone (25.1191 context, C_POWERPLANT_DEFAULT), and its free volume in m3.
2. Compute the required agent mass per shot with agent_mass, which also returns the agent vapor volume and the achieved closure fraction.
3. Check the closure: the returned closure_fraction must equal C / 100 within 1e-4; concentration_closure cross-checks any proposed mass directly.
4. Roll up the installation: installed_agent with the per-shot mass, the bottle count and the shots per bottle gives the installed agent mass.
5. Set the discharge layout: nozzle_count from the free volume (and the powerplant zone flag for the two-nozzle floor).
6. Gate the layout: coverage_verdict compares the installed mass with the required mass, and fire_protection_summary returns the whole sizing in one dict with the coverage verdict.
7. Confirm the deterministic checks with the contract test scripts/test_fire_protection_sizing.py.

Worked example

Reference installation: a Class C cargo compartment of 40 m3 free volume at 5% concentration, and an engine nacelle core fire zone of 1.8 m3 at 6% with two bottles of two shots each.

• Cargo agent_mass(40, 5.0): mass_kg = 13.32445 kg (bound 13.32 kg), vapor_volume_m3 = 2.10526 m3 (bound 2.105 m3), closure_fraction = 0.05000 (5.00%).
• Engine zone agent_mass(1.8, 6.0): mass_kg = 0.72717 kg per shot (bound 0.727 kg), vapor_volume_m3 = 0.11489 m3, closure_fraction = 0.06000 (6.00%).
• Installed rollup installed_agent(0.72717, 2, 2): installed_kg = 2.90870 kg (bound 2.91 kg), mass_per_shot_kg = 0.72717 kg.
• Nozzles: nozzle_count(40, False) = 10; nozzle_count(1.8, True) = 2 (ceiling of 0.45 raised to the floor of 2).
• Summary fire_protection_summary(1.8, 6.0, True, n_bottles = 2, shots_per_bottle = 2): required_mass_kg = 0.72717 kg, closure_fraction = 0.06000, installed_kg = 2.90870 kg, nozzle_count = 2, coverage_verdict = PASS.

Pitfalls

• Using the wrong concentration for the zone: a Class C cargo compartment defaults to 5.0% and a powerplant fire zone to 6.0% by volume; the required agent mass W = (V/S) * C/(100 - C) is nonlinear in C, so the zone type must match the concentration.
• Skipping the closure check: the computed mass must satisfy W * S / (V + W * S) = C / 100 to 1e-4 (13.32445 kg in 40 m3 at 5% closes to 0.05000); a mass that does not close the identity does not meet the concentration.
• Rolling up installed agent below the requirement: the coverage verdict FAILs when the installed mass (bottles * shots * per-shot mass) is less than the required mass per shot - an installation that looks plumbed but under-carries fails the gate.
• Forgetting the nozzle floor on a small powerplant zone: an engine/ APU fire zone takes at least 2 nozzles even when the volume suggests one (1.8 m3 gives a ceiling of 0.45 raised to 2), while a 1 m3 cargo compartment legitimately takes 1.
• Sizing agent for a zone that was never identified: the protected zone and its class come from the zonal hazard analysis leaf; this leaf sizes the agent, it does not identify the zones.
• Feeding non-physical inputs: a non-positive free volume, out-of-range concentration, non-positive specific volume, negative mass, or bottle and shot counts below one all raise ValueError.

Verification

• Confirm agent_mass(40, 5.0) returns 13.32445 kg with closure 0.05000 within 1e-4, and that doubling the free volume to 80 m3 doubles the mass to 26.64890 kg at the same concentration.
• Confirm the closure identity: concentration_closure(40, 13.32445, 0.158) returns 0.05000, the target concentration fraction.
• Confirm the engine rollup: installed_agent(0.72717, 2, 2) returns 2.90870 kg, and one bottle of one shot equals the per-shot mass.
• Confirm nozzles: 40 m3 cargo gives 10, a 1.8 m3 engine zone gives 2 (floor), and a 1 m3 cargo compartment gives 1.
• Confirm coverage: 10 kg installed against the 13.32445 kg cargo requirement returns FAIL; installed equal to required returns PASS.
• Confirm every non-positive free volume, out-of-range concentration, non-positive specific volume, negative mass, and bottle or shot count below one raises ValueError.
• Run the contract test offline: python3 scripts/test_fire_protection_sizing.py (35 tests, deterministic).

Related leaves

• vehicle-design/sizing/nacelle-sizing: nacelle aerodynamic geometry that bounds the engine bay; it never sizes fire protection agent.
• vehicle-design/sizing/ice-protection-sizing: nacelle surface thermal anti-icing, a separate nacelle system from zone fire protection.
• systems-engineering-safety/arp4761a/zonal-safety-analysis: fire zone identification and zonal hazard analysis, the upstream source of the protected zones this leaf sizes.
• avionics/do160/lightning-protection: equipment electrical transient protection on the avionics side, unrelated to extinguishing agent sizing.

Behavior contract (gate 3)

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

python3 scripts/test_fire_protection_sizing.py

The test covers the two worked examples (cargo 13.32445 kg at 5%, engine 0.72717 kg per shot at 6%), the closure identity and linear volume scaling, the installed rollup from bottles and shots, the nozzle count with the powerplant floor, the coverage PASS/FAIL comparison, the fire_protection_summary convenience dict with its key convention, determinism, and ValueError rejection of non-physical inputs.

Compliance

• Standards referenced, not reproduced: FAR 25.851 (cargo compartment class context), FAR 25.855 (compartment context) and FAR 25.1191 (powerplant fire zone context) are regulatory framing only; the total-flooding relations above are standard engineering methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.