CALC-079

Specific transfer power: what JPL assumed against what has flown

Feeds CLM-0701. Answers QST-004. Basis: watts of electrical power per kilogram of transferred mass. No currency.

Why it was needed. synthesis/orbit-transfer.md carried an open gap: whether an SBSP could sustain comsat-class power-to-mass at its own scale. JPL's 1980 study produces trip times for a 2,000-tonne payload that sit close to those measured for 2-tonne comsats, which is only possible if something scales. Finding out what scales is the whole of this record.

Inputs.

Working, transcribed.

JPL:      150e6 W / 2e6 kg                =  75 W/kg

Flown comsat practice, ABS-3A at 1,975 kg:
   1 thruster    4,500 W / 1,975 kg       =  2.28 W/kg
   2 thrusters   9,000 W / 1,975 kg       =  4.56 W/kg
   4 thrusters  18,000 W / 1,975 kg       =  9.11 W/kg

Ratios of JPL's assumption to flown practice:
   against 2.28   32.9 x
   against 4.56   16.5 x
   against 9.11    8.2 x

Result: JPL assumes between about 8 and 33 times the power-to-mass ratio that flown missions achieve.

State the two denominators, because they are not the same quantity. JPL's 75 W/kg is per kilogram of payload, the mass being transferred. The comsat figures are per kilogram of launch mass, which includes propellant and everything else the vehicle carries. These are not the same basis, and a like-for-like comparison would need either JPL's wet mass or the comsat's dry payload mass, neither of which is in the sources held. The direction of the mismatch is worth stating: launch mass is the larger denominator, so using it makes the comsat figure smaller and the 8-33x ratio larger than a fully matched comparison would give. Recorded as a basis note rather than as an error, because the claim's assumes and limits fields already restrict this to a comparison of a 1980 assumption against 2015 practice.

Conclusion, and the finding is about where the gap now sits. JPL did not assume comsat-class power-to-mass; it assumed 8-33x better. That is coherent for a vehicle whose payload is a power station, and it is nowhere demonstrated. It also explains the puzzle the record started from: transfer time scales as mass over power, and JPL scaled the power with the mass, which is why 160-213 days at 2,000 tonnes sits so close to the approximately 200 days measured for 2-tonne comsats. The gap moves from "nobody modelled it" to "somebody modelled it on an assumption nobody has tested", which is a more useful place for it to sit.

Where this fails. - The comsat comparison is a range, not a figure, and the claim says so: the simultaneous thruster count is not established in any source held. The 8-33x band is wide for that reason and must not be quoted as a single number. - One comsat, one thruster type, one launch mass. Not a survey of flown practice. - A 1980 assumption against 2015 practice - 35 years apart - and neither is shown to be right. - VER-566 PARTIAL 2026-09-04.

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