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Why did the Apollo spacecraft use both DC and AC power, requiring heavy inverters? Was it impractical to design everything to run on DC power only? Why? Are both AC and DC power still used in present day and future spacecraft designs?

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    $\begingroup$ The fuel cells in the SM and the batteries in the CM provided DC. But the electronics needed different DC voltages, not only one. To provide several different DC voltages the use of inverters was the best solution, in the times of Apollo and today. But there are some devices that needed AC instead of DC, for instance the gyros used for navigation. Radar needs HF AC, if magnetron tubes were used to generate the HF, a high DC voltage was necessary and genrated by an inverter. $\endgroup$ – Uwe Jun 25 '17 at 9:56
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    $\begingroup$ @Innovine Without using AC, several different small and larger DC voltages can't be generated efficently from one DC source only. To improve eletrical noise immunity it is better to use independent and isolated DC sources for sensible circuits even when the same nominal voltage is used. Using a lot of different batteries for different voltages is bad for reliability. You would need redundant batteries for every important DC voltage. The AC busses don't complicate the electrical system, they simplify it. $\endgroup$ – Uwe Jun 28 '17 at 15:00
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    $\begingroup$ Short answer: because spacecraft engineering derives/borrows heavily from aviation engineering because of numerous shared problems and constraints; aircraft have a long and established history using 400Hz AC and 28V DC for practical reasons. Higher voltage is just plain more practical/efficient as a way to deliver a lot of power over a longer distance - saves on conductor mass (which adds up). 400Hz is used rather than 50 or 60Hz because transformers can be smaller/lighter. $\endgroup$ – Anthony X Aug 19 '17 at 20:14
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    $\begingroup$ I do not know specifics about Apollo but some of the same considerations apply to shuttle. One of our new astronaut students became very interested in why the SSME controllers were AC powered - he felt that this made them unnecessarily prone to certain electrical problems. As with many original shuttle design decisions, the reasons for this were somewhat murky. But he kept digging and using a little astro-power managed to contact some of the original designers. Turns out there were several trades but maybe the most important was that the fuel cell output voltage wasn't well regulated. $\endgroup$ – Organic Marble May 23 '18 at 18:01
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    $\begingroup$ ...another was that the wiring for the DC design weighed several hundred pounds more. $\endgroup$ – Organic Marble May 23 '18 at 18:04
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There was no single DC voltage useful for the whole electronic system. A lot of different voltages were needed, for very noise sensitive systems dedicated DC sources were used to avoid interference.

Using three phase 400 Hz AC current was a well established method used in aircrafts before. Providing 400 Hz AC enabled the use of aircraft instruments within the spacecraft. A lot of weight could be saved at transformer cores by using 400 Hz instead of 50 or 60 Hz, about 87 % less. With three phase AC, very simple, robust and reliable arc free motors could be used. Instrument panel lights required AC current too. Adjusting brightness was done very simple using variacs.

To generate several DC voltages from three phase 400 Hz AC is easy and requires much less filtering than single phase 50/60 Hz AC. Filter condensators could be small if needed at all. Large mainframe computers from CDC and Cray of that time used 400 Hz three phase AC power supplies too, not only in the sixties, but also in the seventies and eighties.

The fuel cells of the Service module needed stabilisation anyway, the nominal voltage was 28 V DC, 27 to 31 V DC under normal load, but at peak power 20.5 V were possible, the inverters delivered 115 ± 2 V. For a reliable redundant design there were 3 fuel cell stacks switchable to 2 DC buses and 3 inverters switchable to 2 AC buses. A single inverter was capable to supply all AC current for the vehicle. But the inverters were not synchronized and could not be paralleled without damage. Therefore only two of the three inverters could be used simultaneously.

After separation from the SM the system used batteries within the Command Module. After Apollo 13 an extra 400 Ah battery was added to the SM.

The Shuttle also used 28 V DC from 3 fuel cell stacks, 3 DC buses and 3 inverters for 3 AC buses with 115 V 3 phase AC 400 Hz current. 3 single phase inverters were used for each 3 phase system, together 9 units.

Some NASA papers: 1, 2, 3, 4, 5, 6.

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