VC03 Thermal management

Thermal management: where it stands

As of 2026-10-01

In the 1978 US design, waste heat set one of two limits on the transmitter, the large antenna that sends the beam down (CLM-0204). The design set a limit of 22 kW/m2 at the centre of the antenna, from the waste heat of its microwave tubes (CLM-0204). It assumed 2,457 t of antenna thermal control, 8 per cent of dry mass (CLM-0206), and a 322.8 m2 radiator for each of 228 DC-DC power converters (CLM-0430). A 1975 study concluded that tubes of 70 per cent efficiency on an aluminium frame needed near-uniform spacing across 1 km. The beam power behind this result is not stated, and the result depends on it (CLM-0968). This site's own calculation, at medium confidence, finds that for 6.72 GW radiated and the 1975 limit of 3.6 kW/m2, the antenna face sheds its own waste heat only above 70.4 per cent tube efficiency (CLM-0969). A 1993 report on building and testing one 'space ready' rectenna contains no vacuum or thermal test. That does not mean no test was done (CLM-0948).

Scope. Where waste heat arises on the satellite and in rectennas (receiving antennas that turn the beam back into DC), how much radiator area and mass it needs, and what temperatures parts can stand. In vacuum, a satellite sheds heat by radiating it.

What is settled. Waste heat was one of two limits that sized the 1978 US design's transmit antenna. The design capped power per square metre at the centre of the antenna because of waste heat from its microwave tubes. The report states the cap as 22 kW/m2, in one place as microwave power (CLM-0204), and elsewhere as about 21 kW/m2 of heat (CLM-0023). Together with a limit on power density in the ionosphere, this sized a 1 km array for about 5 GW of DC delivered on the ground (CLM-0204). The antenna section carried its own thermal control, assumed at 2.457 million kg. That is about 8 per cent of the 30.47 million kg dry mass (mass without propellant), before a 25 per cent growth margin (CLM-0206). Each of 228 DC-DC converters (units that change DC power from one voltage to another) was to take 5.443 MW at 96 per cent efficiency and shed 218 kW. Each had its own 322.8 m2 radiator, about 1.5 m2 per kW, and was to run between 0 and 70 degrees C (CLM-0430). All are 1978 design assumptions, not measurements.

What is uncertain. The main question is whether the transmitter face can shed the heat of its own tubes. This site's own calculation finds that it can only if the tubes are more than about 70 per cent efficient (CLM-0969).

That calculation has medium confidence, because it uses a 1975 judgement of 1975 materials and assumes even heat flux. It takes 6.72 GW of microwave power radiated by the 1 km face and a limit of 3.6 kW/m2 of waste heat. The face can then shed at most 2.83 GW (CLM-0969). Tube waste heat is 2.88 GW at 70 per cent efficiency and 1.19 GW at 85 per cent. The break-even is 70.4 per cent, close to the 70 per cent the 1975 study gave for its tubes (CLM-0969). A hotter radiator, or one not confined to the face, changes the result with the fourth power of temperature.

The 3.6 kW/m2 comes from a 1975 Raytheon study. It is the waste heat per square metre that an aluminium or graphite/epoxy frame can stand, against 8.1 kW/m2 for polyimide (CLM-0968). It applies to the 1975 study's frame materials, not to the 1978 design's cap above. The study gave a formula for the heat flux at the antenna centre and took 70 per cent as a typical tube efficiency. It concluded that with an aluminium frame the tubes had to be spread so evenly that their density barely falls from centre to edge (a spacing scale of at least 1,100 m on a 1 km face). The study's plot of this relation does not state the beam power, and the conclusion depends on it (CLM-0968). At 1,100 m the 70 per cent curve still reads 3.7 to 4.1 kW/m2, above the limit. The calculation above solves the formula at 7 GW, a figure from another volume of the same study, with 87.5 per cent of the heat radiated toward the frame, and gives about 1,820 m, which is a uniform spread. At about 5.5 GW the study's 1,100 m holds (CLM-0968).

Two other studies show where else heat matters. In a 1992 model of a thin-film rectenna, heat from each diode (the rectifier in each element) leaves almost only through its base into the copper circuit. The two thin gold wires on the lid carry very little. This holds for one ceramic-packaged diode, and another package would change the path (CLM-0971). In Grumman's 1978 estimate for a 5.4 km aluminium module of an early Boeing design, cantilevered from its construction base, a 200 degF (about 110 K) difference between top and bottom members bends the free end (CLM-0998). The report's text gives 8 m and its plots about 80 m. The expected difference was about 120 degF. The two have not been reconciled (CLM-0998).

On the test side, a 1993 report titled for a "space ready" rectenna describes readiness for space only as survival of launch loads and fit in a package of 27 by 10 by 1.6 cm. It reports no vacuum or temperature testing. The report does not say whether such tests were done elsewhere, and its silence does not predict failure (CLM-0948).

What is missing. None of the sources above gives a measured heat result for a transmit tube or rectenna in vacuum, or a thermal design for a current concept. A vacuum thermal test would help, as would measured converter efficiency at space solar power levels and a current thermal model that states its radiator sink temperature.

Read next. Collector area and waste heat: what the designs state. Efficiency of power beaming at range. Phase coherence across a flexible array. Holding the structure steady. From direct current to beam. The receiving field.