Mass Budget
Overview
The mass budget captures every component’s dry mass, growth allowance, and contingency. It protects against late-stage growth that can threaten performance or launch-vehicle capability. ECSS-E-ST-10C calls for design margins and growth factors to be defined early and tracked under configuration control.
Typical Allocation Table
| Sub-system | CBE (kg) | Growth % | Contingency (kg) | Allocated (kg) |
|---|---|---|---|---|
| Structure | 180 | 5 | 9 | 189 |
| EPS | 60 | 8 | 4.8 | 64.8 |
| ADCS | 45 | 10 | 4.5 | 49.5 |
| TT&C | 32 | 7 | 2.2 | 34.2 |
| Thermal | 20 | 5 | 1.0 | 21.0 |
| Payload | 95 | 10 | 9.5 | 104.5 |
| Dry total | 432 | — | 31.0 | 463 |
(CBE = Current Best Estimate)
Margins & Growth Allowances
| Phase | Typical System-level Growth Allowance |
|---|---|
| Pre-PDR | 20–25 % |
| PDR-to-CDR | 10–15 % |
| CDR-to-Flight | 5 % |
CubeSat missions often adopt 10 % system margin and freeze the mechanical layout by CDR.
ECSS recommends allocating additional qualification margin for dynamic environments (loads, vibro-acoustics).
Workflow
- Initial allocations based on analogues and top-down constraints
- Mass property database under CM control (tool, spreadsheet, PDM)
- Periodic audits – delta vs. allocation; identify mass drivers
- Mitigation – lightweighting, COTS swap-outs, descopes
- Launch-vehicle integrated analysis – verify CG, inertia, margins
Case Studies
JWST (good practice)
Launch-vehicle mass cap (≈ 6 600 kg to L2) forced aggressive lightweighting: the 6.5 m primary mirror is 625 kg – meeting the < 1 000 kg allocation and 20 kg m-2 areal-density goal. [^1]
BepiColombo (healthy margin)
Split-launch Soyuz design closed with 19–23 % mass margin after SEP stage optimisation, protecting the Mercury cruise trajectory. [^2]
CubeSat Overruns (cautionary)
SE01 2U CubeSat baseline was 1.9 kg vs. 2.66 kg limit, leaving only ~28 % margin for late adds; careful part swaps kept final mass within requirement.
Diagrams & Resources
CubeSat Mass Breakdown Example
References
[1^]: ECSS-E-ST-10C — System Engineering (Annex K).
[2^]: H.P. Stahl, “Large Space Optics: From HST to JWST,” NASA MSFC.
[3^]: ESA, BepiColombo Mission Brochure BR-165, 2000.
[4^]: NASA “CubeSat 101” (Basic Concepts & Processes).
[5^]: Serrano et al., “3U CubeSat Mass Budget,” 2021.