Value chain · CLM-0888
In plain terms
Cable mass rises with the square of distance and falls with the square of voltage. The Chinese design says it can use a low five thousand volts because it is built as fifty modules, each sending its power only about 310 metres. The 1978 design sends all five gigawatts across the satellite as one system, and its report says it chose forty thousand volts to save conductor and equipment mass.
The claim as recorded
Conductor mass for a spacecraft power bus scales as the square of length and the inverse square of voltage, m = 2 x rho_m x rho_e x L^2 x P / (f V^2), and by our arithmetic, with an assumed 2 per cent resistive loss and assumed aluminium conductor, it is consistent with the reason CAST's own paper gives for its 5 kV bus, that each of its fifty modules sends power only 310 m: a single bus carrying 1 GW at 5 kV over an assumed 5 km run would need about 7,600 tonnes of aluminium, 5.6 times CAST's entire stated 1,360 tonne power transmission and management subsystem, whereas each module's stated 48 MW at 5 kV over its stated 310 m cables needs about 1.4 tonnes, or about 70 tonnes across all fifty. The 1978 reference design, routing 5 GW from array to antenna as one system, states that it prefers 40 kV DC on power conductor and conditioning equipment mass grounds, which the same relation is consistent with.
Evidence behind this: Arithmetic performed here over held figures, using a relation derived from first principles rather than quoted from a source.
Scope and limits
THE CONCLUSION IS SENSITIVE TO AN ASSUMED INPUT AND THE RELATION IS QUADRATIC IN IT. Doubling the assumed run to 10 km raises the CAST single-bus figure from 7,614 t to 30,456 t; halving it quarters it. The finding that a monolithic 5 kV bus does not fit CAST's mass budget survives any run length above about 2 km, but the specific tonnages do not stand on their own. The 595 t against 5,863 t comparison is a CONSISTENCY CHECK, not a validation: the stated mass covers switchgear, conditioning and structure as well as conductor, so agreement to an order of magnitude is all it can show. The CAST paper gives its own reason, that the 310 m transmission distance "allows for the use of relatively shorter cables and lower voltage (5 kV)", but no conductor mass, so this relation shows that reason is consistent with its stated mass budget, not that the paper used this relation. The single-bus case uses 1 GW; the paper's solar panel produces 2.4 GW DC, so a single bus carrying that would be heavier still.
What it assumes
FRACTIONAL LOSS IS ASSUMED FOR BOTH DESIGNS, AND RUN LENGTH IS ASSUMED FOR THE SINGLE-BUS CASES. 5 km is chosen as characteristic of a single-bus layout; the 310 m module cable run is stated by the CAST paper. f = 2 per cent is a representative distribution loss; the CAST paper states 89 per cent efficiency for its whole power transmission and management system, converters included, and no cable loss on its own. Aluminium conductor is assumed; no source states the material. That the 5 kV main bus and the 40 kV bus are the same kind of thing, which CLM-0879 establishes from the sources' own labels.
Where it comes from
This is the project's own working, shown in full:
Derivation: I = P/V, loss = I^2 R = fP, so R = f V^2 / P; with R = rho_e L / A the area is A = rho_e L P / (f V^2) and the mass of two conductors is m = 2 rho_m rho_e L^2 P / (f V^2). Aluminium at rho_m 2700 kg/m3 and rho_e 2.82e-8 ohm-m gives 2 rho_m rho_e = 1.5228e-4. At L 5 km and f 0.02: 5 GW at 40 kV gives 595 t; 1 GW at 5 kV gives 7,614 t. At L 310 m: 48 MW at 5 kV gives 1,405 kg, times 50 modules is 70 t. CAST inputs, SRC-169 PDF p5 (printed p111): "The 48 MW output power is transmitted to the antenna subpanel bus via the 310-m-long cables along the two vertical trusses". The 32 MW per antenna subpanel is microwave power radiated, "(42.7 MW x 76%) = 32 MW", not power on the 5 kV cables.
Basis: kg of conductor
Related records
- Answers question
- QST-043 not published in this release
Cite this
energyaperture.com/c/clm-0888
CLM-0888, Energy Aperture, as of 2026-09.
Connected records
Other published records linked to this one. A list, not a judgement.
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