Mass budget
Spacecraft Mass Budget
Will it fit the launch vehicle? Roll up subsystem dry masses, fold in maturity-based contingency, add propellant, and check the wet mass against the cap.
// SMAD Ch.14 subsystem mass roll-up with maturity-based contingency (mass-growth allowance). per-line contingency, propellant, launch-mass cap, ~20% early-design margin rule. engineering trade study only — not a verification-grade mass-properties report.
This calculator rolls up subsystem dry masses with maturity-based contingency — mass-growth allowance (MGA) of roughly 5% for flight-proven hardware, 10% nominal, 20% for new development — adds propellant (a sized quantity that carries no growth allowance), and checks the wet mass against a launch-vehicle cap and the ~20% early-design margin rule (Space Mission Analysis and Design (SMAD) ch. 14; AIAA (American Institute of Aeronautics and Astronautics) S-120). The presets scale from a 3U CubeSat mass budget to a rideshare-class 180 kg microsat.
How a mass budget works & what it omits
Mass is the hardest currency in spacecraft design. Every kilogram has to be lifted to orbit, and the launch vehicle (or the rideshare port on it) sets a hard cap. A mass budget is the running ledger that tracks where every kilogram goes — subsystem by subsystem — so the team knows, at any point in the design, whether the spacecraft still fits.
This tool follows the standard first-order treatment from SMAD (Space Mission Analysis and Design, Ch. 14) and NASA / AIAA S-120 (Mass Properties Control for Space Systems). Each subsystem — structure, power, ADCS, propulsion, comms, payload, thermal, harness, avionics — carries its own dry-mass estimate plus its own contingency (also called mass-growth allowance, MGA). Contingency is a maturity-based reserve: a flight-proven box that has been built and weighed carries roughly 5%, an off-the-shelf or modestly-modified item carries roughly 10% nominal, and a new-development item whose design has not matured must reserve roughly 20%. The contingency-loaded subsystem mass is m × (1 + contingency / 100).
Summing the contingency-loaded subsystem masses gives the dry mass with contingency. Adding propellant — a sized quantity carrying no growth allowance, because it is not an estimate that can creep — gives the wet mass. The wet mass is what is checked against the launch-vehicle mass cap. The difference is the margin: margin = cap − wet mass. SMAD / AIAA S-120 margin philosophy holds that early in design — before Preliminary Design Review — a spacecraft should carry roughly 20% margin against its allocated mass, burning that reserve down as the design matures and estimates firm up. This tool flags whether the budget clears both the hard cap and that recommended reserve.
What this tool does not capture: mass properties beyond total mass (centre of gravity, moments of inertia, balance), the allocation flow-down from system to subsystem to component, separate dry / burnout / injected mass accounting against multiple launch milestones, or the formal mass-properties control board process. It is a fast trade-study roll-up for sizing and feasibility — not a substitute for a verification-grade mass-properties report.
// load a class, then edit the subsystem line items.
Subsystem dry masses
// per-line contingency: ~5% flight-proven, ~10% nominal, ~20% new development.
Propellant & cap
// propellant carries no contingency; cap is the launch-vehicle allocation.
Mass budget roll-up
// 9 subsystems · contingency 13.6 kg
119.0 kg
Total dry mass
132.6 kg
Dry + contingency
147.6 kg
Wet mass
52.4 kg
Margin vs cap
26.2%
Margin percent
PASS
Cap check
// dry mass with contingency, by subsystem
- structure26.4 kg · 20%
- power19.8 kg · 15%
- adcs12.6 kg · 10%
- propulsion9.90 kg · 7%
- comms6.30 kg · 5%
- payload36.0 kg · 27%
- thermal5.50 kg · 4%
- harness7.70 kg · 6%
- avionics8.40 kg · 6%
// shareable URL encodes every line item. no backend.
// ai-generated breakdown of what these numbers mean — with diagrams.
Common questions
How much mass contingency should each subsystem carry?
It follows maturity, not preference: roughly 5% for a flight-proven box that has been built and weighed before, roughly 10% for an off-the-shelf or modestly-modified item, and roughly 20% for a new development whose design is still moving — the maturity-based mass-growth-allowance ladder this tool applies per line. Propellant is the exception: it carries no contingency, because it is a sized quantity computed from the mission's delta-v budget (see the Delta-V Maneuver Budget), not an estimate that can creep.
What mass margin should a spacecraft hold early in design?
The SMAD / AIAA S-120 philosophy: before Preliminary Design Review (PDR), hold roughly 20% margin against the allocated mass — on top of the per-line contingency — and burn that reserve down deliberately as estimates become weighed hardware. The tool flags both checks separately: whether the wet mass clears the hard cap at all, and whether it still holds the recommended early-design reserve.
How much mass does contingency actually add?
More than intuition suggests. In the 3U CubeSat preset, 3.5 kg of subsystem estimates grow by ~0.34 kg once each line's allowance lands — leaving only ~4% of the 4.0 kg cap, which the tool flags as under-reserve even though the budget technically fits. In the 100 kg smallsat preset, contingency alone adds 8.1 kg — about the mass of an entire subsystem. Budgeting that growth up front is the whole point of a CubeSat mass budget: discovering it at the weigh-in is how payloads get descoped.
What is the difference between dry mass and wet mass?
Dry mass is everything except propellant — structure through avionics, summed with each line's contingency. Wet mass adds the propellant load, and it is the number the launch cap constrains: the launch vehicle lifts propellant kilograms exactly like payload kilograms. This tool tracks total mass only; centre of gravity, moments of inertia, and the formal mass-properties control process are among the listed omissions.
References
- // Wertz, J. R., Everett, D. F., Puschell, J. J. (eds.) (2011). Space Mission Engineering: The New SMAD, ch. 14 (Spacecraft Design and Sizing).
- // AIAA S-120A-2015. Mass Properties Control for Space Systems. American Institute of Aeronautics and Astronautics.
- // NASA-HDBK-8739.21 / NASA mass-properties control practice — maturity-based mass-growth allowance and design-margin philosophy.
- // Larson, W. J., Wertz, J. R. (eds.) (1999). Space Mission Analysis and Design, 3rd ed., ch. 10 & 14.
