The space shuttle SRB's Hydraulic Power Unit (HPU) provided hydraulic power to the booster's TVC system. This HPU controller schematic shows "One Shot" blocks that I'm suddenly curious about. Any idea what these might be? I want to guess they are one-time switches to pass and permanently hold a signal once some condition is met? Many thanks if you know and can clarify!

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1 Answer 1


A one-shot is an electronic circuit to generate single pulses of constant duration even when triggered several times during the one-shot output is set (the not retriggerable version) . It is called a monostable flipflop too.

See Wikipedia https://en.wikipedia.org/wiki/Monostable_multivibrator

The SRB APU controller regulates the turbine speed using a pulse controlled valve. This pulse controlled valve may only open or closed. To achieve 75 % flow, it is open for 75 % of the time and closed for 25 % of the time. The one shot is used to control the pulse duration used to open the valve.

If we assume 120 ms for the one shot pulse duration, 75 % flow may be achieved by a gap of 40 ms between the pulses of 120 ms.

$120 / (120 + 40) = 120 / 160 = 75 $ %

  • $\begingroup$ Curious, by the way: what’s the purpose of the monostables in the space shuttle HPU controller, and how are they used elsewhere? What kinds of things can you do with a fixed-duration triggered signal, I guess I’m wondering? $\endgroup$
    – user39728
    May 19, 2021 at 23:24

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