Arc Skills / Architecture and engineering budgets
Build a system mass budget
A mass budget needs a defined configuration and inclusion rule before its total means anything. Arc Skills turns a bill of materials and mass evidence into a line-item rollup with unresolved mass visible.
Use this skill
Use Arc Skills to build a mass budget for my configured System.
Inputs:
- Product tree, included and excluded hardware, quantities, and mass basis
- Component masses with units, source revisions, and evidence maturity
- Governing mass limit, missing items, and any approved growth-allowance rule
Return line-item extended masses, non-overlapping rollups, the known subtotal, and signed headroom equation. Keep unmeasured installed items as TBD rather than zero and do not claim compliance from an incomplete total.What you provide and what you get
| What you have | How it is used | What you get |
|---|---|---|
| Configured bill of materials | Sets quantities and prevents assembly/child double counting | Extended mass by component |
| Mass evidence | Separates measured, supplier and estimated values | Provenance and missing-value flags |
| 3.0 kg allocation and inclusion basis | Defines the comparison boundary | Subtotal and unresolved final headroom |
Two batteries, bracket and missing wiring
Illustrative engineering example.
Assume a fictional installed dry assembly with two batteries, one bracket and a harness. A spare battery held on the ground is excluded by the stated configuration; the harness remains part of the installed assembly. The two battery masses are assumed to include their own packaging but not the separate bracket.
Flight hardware: 2 batteries + 1 bracket + 1 wiring harness
Ground-only spare: 1 battery, excluded
Installed dry-mass allocation: ≤3.0 kg| Item | Quantity × unit mass | Extended mass | Evidence / treatment |
|---|---|---|---|
| Battery | 2 × 1.2 kg | 2.4 kg | Illustrative supplier mass; packaging included as specified |
| Bracket | 1 × 0.4 kg | 0.4 kg | Illustrative measured mass |
| Wiring harness | 1 × TBD | TBD | Installed; cannot enter zero |
| Known subtotal | 2.4 + 0.4 kg | 2.8 kg | Excludes unknown harness; not final installed total |
| Ground spare | 1 × 1.2 kg | Excluded | Explicitly outside flight configuration |
The known subtotal is 2.8 kg. Raw headroom against the 3.0 kg allocation is 3.0 − 2.8 = 0.2 kg before harness and any authorized growth allowance. A harness above 0.2 kg would exceed the limit even before adding allowance; a harness exactly 0.2 kg would leave zero raw headroom. A lighter harness still would not settle an unstated allowance policy.
If the battery line already represented a battery assembly including brackets, adding the bracket again would double count. The source list should say what each unit mass includes. No percentage reserve is inserted here because the example supplies no program rule.
How to make the rollup defensible
- Name the installed configuration and mass basis, including consumables, harnesses and exclusions.
- Expand component quantities once and keep measured, estimated and supplier values distinct.
- Sum only non-overlapping rows in one unit and keep TBD components in the total expression.
- Compare the current subtotal with the limit and show how unknown mass and allowance affect the decision.
Questions about this task
What should I do with an unweighed harness?
Keep a TBD line and request an estimate or measurement. The subtotal can still guide work, but it cannot close the system limit.
Should ground spares be included?
Only if the specified mass basis includes them. Record the configuration decision beside the budget.
Sources and further reading
- NASA Systems Engineering Handbook: appendix: Terminology for budgets, margin and technical resources.
- NASA Systems Engineering Handbook: system design processes: Stakeholder expectations, logical decomposition, functional allocation and design decisions.