Mass Budget

Mass Budget (vehicle-design/mass-properties/mass-budget)

Use when the task is building the vehicle mass budget at the conceptual level: allocating subsystem masses, applying the growth allowance and contingency margin policy, rolling up the estimated total mass, and checking the margin-backed total against the MTOW target.

Domain quick reference

• The mass budget is the mass breakdown of the vehicle by subsystem (wing, fuselage, empennage, systems, payload, and so on); the rollup is the sum of the subsystem masses in kg.
• Growth allowance is the percentage added to an estimated mass to cover later design refinement; common practice tightens it as the design matures: about 10 percent in the conceptual phase, about 6 percent in the preliminary phase, and about 3 percent in the detailed phase.
• Contingency margin is an extra percentage on top of the growth-allowance total to cover estimation uncertainty.
• Margin-backed total mass: estimated total multiplied by (1 + growth allowance) and then by (1 + contingency margin).
• The MTOW target check subtracts the margin-backed total from the target takeoff mass; a non-negative margin is within target, a negative margin is over target, and the margin percent is relative to the target.
• Mass budget practice sits in the FAR-25 / CS-25 weight and balance context (CG limits and operating weights rely on the budgeted mass).

Workflow

1. Collect the subsystem mass estimates in kg, one entry per subsystem.
2. Roll up the budget with rollup_mass_budget.
3. Select the design phase and get its growth allowance with phase_growth_allowance.
4. Apply the growth allowance with apply_growth_allowance and the contingency margin with contingency_mass.
5. Check the margin-backed total against the MTOW target with mtow_check and read the verdict before gating the weight control plan.

Pitfalls

• Applying the growth allowance and the contingency margin only to selected subsystems: both apply to the rolled-up total, not per item, unless the item has its own agreed margin.
• Confusing the two margins: growth allowance covers design refinement, contingency covers estimation uncertainty; they are separate percentages.
• Reporting the raw rollup as the budget total: the margin-backed total is what the weight control plan tracks.
• Reading margin percent with the wrong reference: margin percent is relative to the target mass, not to the estimated mass.
• Passing an empty subsystems dict or a negative fraction; the module raises ValueError instead of guessing.

Behavior contract (gate 3)

The rollup, growth allowance, contingency margin, and MTOW target check logic are exercised by the gate 3 contract test: scripts/test_mass_budget.py against scripts/mass_budget_logic.py (stdlib unittest, offline). Run: python3 scripts/test_mass_budget.py

Compliance

• Standards referenced, not reproduced: FAR-25 is US government work (public domain) and CS-25 is a free EASA download; growth allowance and contingency margin values are common weight engineering heuristics, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.