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Almost all spacecrafts launched by humanity consist of electrical components which require electricity to run. Usually, spacecrafts are either powered by a Solar Panel or using Radioisotope Thermoelectric Generator (RTG). We use batteries to store this power. We use this energy either directly from the source or from the battery.

We know that there are two forms of Electric Current - Alternating Current (AC) and Direct Current (DC). Alternating current (AC) is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that we typically use when we plug electrical appliances into a wall socket. Direct Current (DC) has many uses, from the charging of batteries to large power supplies for electronic systems, motors, and more. These two forms of electric current have their own advantages and disadvantages.

What form of Electric Current do we use to power Spacecrafts (Satellites, Landers, Rovers) - Alternating Current (AC) or Direct Current (DC)? Does this preference vary whether it is a crewed or un-crewed spacecraft? Or does it depend on whether it is an orbiter or a lander or rover? Why have we chosen this over the other form? Are there any exceptions to this, or is this fact mission-specific?

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Batteries, fuel cells, radioisotope thermoelectric generators and solar panels provide DC. To get several different DC voltages AC and a transformer with several secondary windings is needed followed by a recifier and smoothing filter. A switching circuit converts DC to AC. A higher switching frequency allows the use of very small and lightweight transformers. A switching converter may provide voltage stabilization too.

To produce AC directly, a rotating electric generator is needed as used by many small and large aircrafts but not by present spacecrafts.

There is some research using stirling engines and generators to produce AC for future spacecrafts.

The Apollo spacecrafts used three phase 400 Hz AC, see my answer to another question here. 20 to 31 V DC from the fuel cells or batteries were converted to 115 ± 2 V AC.

All manned and unmanned spacecrafts as well as landers and rovers are using DC voltage as primary source. But to generate the necessary different DC voltages for the analog and digital electronics, conversion to AC and back to DC is needed. The electronics for communication with ground stations also need several DC supply voltages.

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    $\begingroup$ Many DC to DC (Boost and Buck) voltage converters do not use transformers. They do still have inductors though. (also see Analysis of Four DC-DC Converters in Equilibrium) $\endgroup$ – uhoh Oct 27 '19 at 12:48
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    $\begingroup$ 'thermonuclear' is generally used to refer to fusion. The term you're looking for is 'radiothermal'. $\endgroup$ – Hobbes Oct 27 '19 at 19:40
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    $\begingroup$ @Intellex even in craft that have AC available like Apollo and Shuttle most devices are DC powered. Only motors and a few other devices (SSMECs, TACANs, etc.) were AC powered. $\endgroup$ – Organic Marble Oct 28 '19 at 12:10
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    $\begingroup$ @uhoh I'm pretty sure all of the voltage conversion on ISS uses boost/buck converters. (There may be some point-of-use AC conversion, but for transmission, everything is DC) $\endgroup$ – Tristan Oct 28 '19 at 15:09
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    $\begingroup$ Using transformers floating DC voltages may be generated. The voltages produced by boost/buck converters are all referenced to the same common ground. But you need sometimes a bipolar pair of supply voltages (+15 V/- 15 V) for analog circuits. $\endgroup$ – Uwe Oct 28 '19 at 15:16
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Shuttle used both AC and DC power.

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DC, because that is what the fuel cell power plants generated, and AC, to enable the use of compact three-phase electrical motors. The AC was generated from the DC using inverters.

For details on the shuttle electrical power system, read section 2.8 of the Shuttle Crew Operations Manual and/or the Electrical Power System section of the 1982 Press Manual.

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    $\begingroup$ Not only the Shuttle but also the Apollo spacecraft. $\endgroup$ – Uwe Oct 27 '19 at 12:44
  • $\begingroup$ Thanks for the answer. Does the same apply to landers, rovers and other non-shuttle spacecrafts? $\endgroup$ – Guru Vishnu Oct 27 '19 at 16:16
  • $\begingroup$ I am unable to answer for the hundreds of other craft out there. $\endgroup$ – Organic Marble Oct 27 '19 at 16:24

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