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28 VDC is a super-common spacecraft bus voltage, and the linked article claims (unsourced) this has aircraft heritage. So is this a true claim and if so why was 28 V an aircraft standard?

And regardless of the veracity of the aircraft claim, what are the sound engineering reasons 28 V has remained a standard. I mean humanity has been building spacecraft for over 50 years, so it must have some inherent advantages over legacy at this point. I’m assuming it’s a combination of solar cell and battery chemistry and the inherent efficiencies of power conversion circuits in electronics.

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    $\begingroup$ "humanity has been building spacecraft for over 50 years, so [28 VDC] must have some inherent advantages over legacy at this point" -- the opposite is true. The longer a standard is in use, the more likely it is that legacy issues will override optimality issues. $\endgroup$
    – Russell Borogove
    Commented Apr 10, 2015 at 16:21
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    $\begingroup$ To illustrate @RussellBorogove's point, look at the keyboard you used to type your question. The top row of letters most likely starts with QWERTY. This ad hoc standard is a layout that supposedly was quite optimal for early mechanical typewriters. Even though I haven't seen a mechanical typewriter for decades, keyboards to this day almost exclusively have that QWERY layout. $\endgroup$
    – David Hammen
    Commented Apr 10, 2015 at 21:43
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    $\begingroup$ Yep, QWERTY was designed to avoid mechanical jamming by separating the letters in common digraphs like "t-h" and "s-t" so the type-bars attached with those letters wouldn't collide when typing quickly. More than 50 years after IBM's Selectric "golf-ball" print head eliminated type-bars, we're still using it. $\endgroup$
    – Russell Borogove
    Commented Apr 10, 2015 at 22:02
  • $\begingroup$ I appreciate the QWERTY analogy, but don't think it has sufficient explanatory power in this context. QWERTY, compared to the alternative schemes like Dvorak or laying the keys out alphabetically, is the choice between teaching all of English-typing humanity to switch while a continuum of people exist trained in the previous method. It is pretty easy to design electronics to run off a higher supply voltage and down-convert. That's done anyway as many chips run at lower voltages like 5 or 3.3. It makes sense the 120 VAC standard doesn't change, but spacecraft are not all on one electrical grid. $\endgroup$
    – Adam Wuerl
    Commented Apr 11, 2015 at 2:04

2 Answers 2

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From an forum titled "The Wings of the Web" on Airliners.net

I found a conversation asking the same thing and read this:

Larger aircraft adopted 24/28 (battery/generator) reduce the weight of the wiring system and components. Since work relates to the Watts it consumes (electrical power) and Watts are a product of voltage and current, the higher the voltage you use the lower the current requirments and the smaller the components (to an extent) and the wiring.

Some aircraft (like the SAAB 340) have a 48 vdc starter that uses the 2 on-board 24 volt batteries and connects them in series during a start. So even more power is available with relatively small starter wires.

Most larger aircraft use the battery for starting the APU only. So there is no need to go higher in voltage. But since AC power is plentiful, they use a battery charger that is powered from AC. It still only puts out 28 VDC though.

Here is a link to the orginal forum post.

I do realize that your question was directed towards spacecraft specifically, but since they are still in the world of aviation, there tend to be a lot of shared components. Because of this, I assume that the space industry's reasoning would be very similar to the aviation.

I hope that this quote, helps,

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  • $\begingroup$ That does provide some sourcing behind the claim that spacecraft use 28 V because aviation does. It also provides some rationale as to why 28 V is better than something lower, but it doesn't do much to explain why it’s not higher. Was there a practical upper limit in battery voltage in that time period? $\endgroup$
    – Adam Wuerl
    Commented Apr 11, 2015 at 2:01
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    $\begingroup$ The 24 V is likely derived from the chemistry of lead-acid batteries (2 V per cell), where 12 cells is a reasonable compromise between high-enough voltage and few-enough cells. $\endgroup$
    – Hobbes
    Commented Apr 11, 2015 at 7:50
  • $\begingroup$ @AdamWuerl Higher voltage, even in a low voltage range, comes with its own problems; that's why ICs have gone from running on 5 V, to 3.3 V, to 1.5 V, to 0.7 V (or so I recall; don't take the exact numbers as gospel, but IC supply voltage has dropped over the years, let alone how vacuum tubes commonly needed a few hundred volts). Voltage conversion isn't perfect, and especially in the vacuum of space, getting rid of waste heat is a big problem. Changing the voltage either way would need some sort of rationale where the benefits outweigh the downsides. $\endgroup$
    – user
    Commented Dec 8, 2015 at 12:22
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One reason a technical standard can persist is the availability of off-the-shelf components designed with a given standard in mind, and the economics of developing and manufacturing components around a new standard. Having a catalog of components with established reliability records can be a big time and cost saver over having to accumulate test experience with new designs based around work new standards. This is especially true where reliability matters - such as on a space probe a billion miles from Earth where there's no opportunity to swap out a failed part, or when a component failure can put human life in jeopardy.

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