Reading the flight crew operating manual of the Space Shuttle I discovered that:

  • The three fuel cells could have been stopped and restarted (unlike previous Apollo missions)
  • The fuel cell start procedure required the turning on of heaters, pumps, and open (electrical?) valves for oxygen and hydrogen
  • The orbiter had no batteries or other electrical sources (when not connected to the ground pad)
  • The control logic of each fuel cell was powered by one of the ESS DC buses

So, my question is: what happened if a total electrical failure occurred (let's say for simplicity that astronauts momentarily close all the fuel cells)? Could they have been restarted or would the spacecraft have been lost if no DC power was available?

I found this sentence:

The orbiter’s three FCs operate as independent electrical power sources, each supplying its own isolated, simultaneously operating DC bus.

Does this mean that they could also start autonomously? If yes, how could the FC move the reactant valves without an external power source?


1 Answer 1


The spacecraft is lost if no DC power is available.

That was indeed true.

A subset of the shuttle electrical busses were redundantly powered so that loss of one or, in some cases, even two fuel cells did not mean that the bus lost power. An example of the triply redundant busses were the "essential busses".

These busses were required to switch key equipment in the electrical system and power the control box of the fuel cells. Another function they powered was the "bus tie" switches. These switches allowed the Orbiter main busses to be interconnected, so that if a fuel cell was lost, the loads that were powered only by it, could be repowered. Depending on why the fuel cell was lost, it could potentially be brought back online.

There was a large amount of redundancy built into the Orbiter's electrical system, but you had to have at least one fuel cell producing power for any of it to work.

Does this mean that they can also start autonomously?

No. A fuel cell required DC power (to run its control box) and AC power (to run its pumps) to function. (You mentioned the reactant valves - they latched open or closed - required power only to move.) The DC and AC power to run the fuel cell had to come from somewhere. If no other fuel cells were working to provide this power, you could not start it.

Note: quotes are from original version of question

Source: Former shuttle EPS instructor

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    $\begingroup$ That's pretty scary. One might think that they would have considered a hand-cranked generator for that kind of emergency. I'm not sure how much juice they'd need to generate, so maybe that wasn't feasible, anyway. $\endgroup$
    – Wastrel
    Commented Jan 17, 2022 at 15:47
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    $\begingroup$ @Wastrel the flight rules were pretty conservative about cutting the mission short when fuel cell problems cropped up. I can think of 2 missions that were drastically shortened when a single fuel cell failed - STS-2 and STS-83. $\endgroup$ Commented Jan 17, 2022 at 15:58
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    $\begingroup$ @Uwe yes. You could perform a bus tie to repower that fuel cell's main bus, and that would allow you to power the AC inverters for that fuel cell's pumps. Once the fuel cell was restarted, you could break the bus tie and the fuel cell would be powering its own main bus. The procedure is found in EPS SSR-6 FC RESTART in this procedural book nasa.gov/centers/johnson/pdf/359891main_125_MAL_G_J_2_E1.pdf (procedure assumes bus is tied as initial condition) The bus tie is broken in box 38. $\endgroup$ Commented Jan 17, 2022 at 18:15
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    $\begingroup$ It is apparently a not entirely uncommon pilot error to accidentally switch off the remaining good engine in case of an engine failure. The stress reduces us to great apes, basically. That should really not happen with those fuel cells. $\endgroup$ Commented Jan 17, 2022 at 19:49
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    $\begingroup$ @Peter-ReinstateMonica and the (very important) fuel cell reactant valve switches were laid out on the panel in 1-3-2 order! No new shuttle pilot ever grabbed the wrong switch here in a sim....yeah right i.imgur.com/aTKpMoL.png $\endgroup$ Commented Jan 17, 2022 at 20:08

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