Bypass Ratio Trade

Bypass Ratio Trade (propulsion/turbofan/bypass-ratio-trade)

Use when the task is the bypass ratio design trade for a turbofan: the thrust split between the fan and core streams, the specific thrust, and the TSFC across candidate bypass ratios at fixed core conditions.

Domain quick reference

Units are SI, with TSFC in g/(kN*s):

• Bypass ratio B = mdot_fan / mdot_core, the fan stream mass flow divided by the core stream mass flow; at fixed total mass flow the split is mdot_fan = B/(1+B) * mdot_total and mdot_core = mdot_total/(1+B).
• Net thrust per stream F = mdot*(Vj - V0), so total thrust is the fan share plus the core share; the fan jet velocity runs well below the core jet velocity.
• Specific thrust F/mdot_total in m/s (N per kg/s), a fan-size loading measure: high specific thrust means a small, highly loaded fan.
• Propulsive efficiency eta_p = 2/(1 + Vj/V0) for a stream at jet velocity Vj in flight at V0; a lower average jet velocity raises eta_p.
• TSFC = mdot_fuel/F with mdot_fuel = f * mdot_core (f = core fuel/air ratio), reported in g/(kN*s); at fixed core conditions a higher BPR lowers TSFC.
• The gain has a price: a larger fan, weight, and drag, and the fan pressure ratio rise with fan jet velocity cuts the efficiency gain. The trade sits in the FAR-33 engine design context.

Workflow

1. Fix the core conditions: total mass flow, core and fan jet velocities, flight velocity, and the core fuel/air ratio.
2. Compute the stream split with thrust_split (mdot_fan, mdot_core, F_core, F_fan, F_total).
3. Normalize the thrust with specific_thrust (F_total / mdot_total).
4. Compute the fuel burn per unit thrust with tsfc in g/(kN*s).
5. Sweep the candidate bypass ratios with bpr_trend and read the thrust split, specific thrust, and TSFC trend.
6. Assess the fan with fan_pressure_ratio_note and fold the fan loading penalty into the trade verdict.

Pitfalls

• Treating bypass ratio as a mass fraction: B is a ratio of two mass flows, and the fan share of total flow is B/(1+B), not B.
• Mixing units in TSFC: fuel flow in kg/s and thrust in N must be converted before quoting g/(kN*s).
• Forgetting the fixed-core caveat: the TSFC gain with BPR assumes constant core and fan jet velocities, while real fans grow diameter, weight, and drag as BPR rises.
• Ignoring the fan pressure ratio: a higher FPR raises fan jet velocity, which erodes the propulsive efficiency gain.
• Comparing specific thrust across engines with different total mass flows.

Behavior contract (gate 3)

The thrust split, specific thrust, TSFC, and fan pressure ratio logic is exercised by the gate 3 contract test: scripts/test_bypass_ratio_trade_logic.py against scripts/bypass_ratio_trade_logic.py (stdlib unittest, offline). Run: python3 scripts/test_bypass_ratio_trade_logic.py

Compliance

• FAR-33 is referenced, not reproduced: US government work (public domain); the trade model is common propulsion methodology, summary only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.