Black Fox knowledge base · 0 g / Moon / Mars

Habitat Fire Spread

A side view of a 6 m × 3 m, two-deck habitat with six rooms. The model solves the ventilation flow, then moves heat, smoke and oxygen through the rooms. Flames spread over the furnishings using flammability limits fitted to NASA BASS, BASS-II, Saffire and partial-gravity tests in your knowledge folder. Open or close doors and hatches to see how the fire moves from room to room.

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Starting
Click a furnishing to start a fire. Click a door or hatch to close it (shown blue) or open it again. Air enters the Crew quarters on the upper left and leaves through the Lab grille on the lower right.

Room tracker

laid out as in the habitat

Fire area by room

cm² burning

Event log

    Flammability map

    The wall lining burns above its limit line. Very slow flow starves the flame of oxygen (quenching, left). Fast flow blows it off (right). The dot is the first ignition point, using its local effective flow (ventilation plus buoyancy).

    concurrent limit (downstream spread) opposed limit (upstream / downward) 0 g flight-data envelope at ~1 atm

    Evidence

    Outside the measured envelope
      Calculator check · measured limits and PSI envelopes
      Nearest records in the knowledge base
        Scenario and result as schema.json objects 
        
      
        
    
        
  
        

  
        
    
        The mathematical model
        
    
        Each time step of 0.1 s solves these equations on a 120 × 60 grid of 5 cm cells. Equations 1–3 describe the cabin air. Equations 4–7 describe the burning solids.
        
    
          
      
        1. Ventilation flow (potential flow).
        ∇²φ = 0 in cabin air;  φ = 1 at supply, φ = 0 at return, ∂φ/∂n = 0 on walls and closed hatches;  u = −c ∇φ
        
          c is set so the air leaves the supply diffuser at the chosen speed. Solved by SOR (ω = 1.9). It is re-solved whenever a door or hatch changes.
          2. 
      
        2. Buoyant rise of hot gas.
        wb = Ub · min(1, θ / 900 K),   Ub = 30 cm/s · √(g / gE) · (P / 101.3 kPa)
        
          The 30 cm/s value is the order-of-magnitude buoyant flow near a flame on Earth (curated fact). The pressure term follows Gr ∝ g·P² (low pressure as a proxy for low g, pmc-6907022). At 0 g it vanishes, so smoke and heat move only with the fans.
          3. 
      
        3. Heat, smoke and oxygen transport. θ = gas temperature rise, d = O₂ deficit, s = smoke.
        ∂χ/∂t + ∇·[(u + wb ĵ) χ] = D ∇²χ + Sχ − λχ χ,   χ ∈ {θ, d, s},  D = 15 cm²/s
        
          Each burning cell adds Sθ = 250 K/s × hm × Y/21, Sd = 1.2 %/s × Y/21 and smoke. Heat is lost at λ = 0.01 s⁻¹ (radiation), rising to 0.06 s⁻¹ next to walls. The scheme is explicit finite-volume upwind with CFL ≤ 0.4.
          4. 
      
        4. Flammability limit (fitted to knowledge-base data).
        Ylim(U) = a + b/U + c·U + 4·(1 − P/101.3),   U = √(|u|² + Ub²)
        
          b/U is the quenching side, from BASS-II quench speeds of 1–5 cm/s. c·U is the linear blowoff side (Marcum 2016). There are separate (a, b, c) for concurrent and opposed spread.
          5. 
      
        5. Flame spread over a surface. ψ = angle between the spread direction and the local flow.
        V(ψ) = Vopp + (G·Vcon − Vopp)·cos⁴ψ  (cos ψ ≥ 0),   Vcon = kc·U·Φ·(Y/21)·√(P/P₀),   Vopp = ko·Φ·(Y/21)²·(P/P₀)0.3·h(U)
        
          Φ = 1 − exp(−(Y − Ylim)/3). G = 1 + (Ub/U)·min(L/5 cm, 8) makes buoyant upward fires speed up as they grow. h(U) peaks near 5 cm/s, matching the non-monotonic opposed spread measured on the ISS (pmc-5760683).
          6. 
      
        6. Ignition by hot gas. This is how fire jumps a doorway.
        dξ/dt = max(0, T − 600 K) / (300 K · 8 s),   ignite when ξ ≥ 1
          7. 
      
        7. Extinction and burnout. A burning cell goes out when its local Y < Ylim(Ulocal). This is quenching below the flow optimum √(b/c) and blowoff above it. Otherwise it burns out after tb·21/Y, unless it is still heating the next cell.
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        This is a teaching and design-exploration model, not a validated fire code like FDS. The flow is potential flow, not Navier–Stokes. The limit lines are hand-fitted to published trends, and spread rates are order-of-magnitude. Before using a number, open the cited NTRS record and check it against the PSI experiment tables.