Why do rocket engines use nitrogen actuators to operate the fuel/oxidiser valves instead of using electric servos?

  • $\begingroup$ Not all do. Best to call out the one(s) you are interested in. $\endgroup$ Aug 4, 2019 at 23:36
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    $\begingroup$ The Apollo CSM SPS for example. $\endgroup$
    – Tom C
    Aug 4, 2019 at 23:44
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    $\begingroup$ @TomC can you add that back into your question and if possible find a supporting link? Thanks! $\endgroup$
    – uhoh
    Aug 5, 2019 at 0:07
  • $\begingroup$ To save some weight? $\endgroup$
    – Uwe
    Aug 5, 2019 at 7:58

2 Answers 2


To produce a valve operating electric motor involves a very large number of parts to turn high speed low torque continuous rotary motion into short limited throw high torque motion. Every one of these parts is a potential failure that needs to have time and money spent to investigate and mitigate. They are also hard to test once fitted since once tank is loaded the motor and related sensors cannot be operated/tested so can seize, bind, short circuit or otherwise misbehave.

Gas driven systems can have as few as one moving part, two if you count the working fluid. Supporting hardware generally consists of a pressure vessel of some kind, and as long as this still indicates pressure valve can be assumed to be ready to operate, simplifying go-nogo checks before launch and during flight. It also also relatively easy to supply redundancy to everything but the final valve assembly.

There are problems in terms of weight and flexibility but where 100% reliability is a goal using gas driven systems, like open loop hydraulics can make sense.

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    $\begingroup$ Wouldn't you still need an electric valve to release the gas? $\endgroup$ Aug 5, 2019 at 11:47
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    $\begingroup$ @DiegoSánchez A solenoid valve and a piston vs. an electric motor, a gear train, an electronics control box.... $\endgroup$ Aug 5, 2019 at 12:11
  • $\begingroup$ I think it's easy to make an argument against using servos for valve actuation but electromagnetic dual poppet valves are a great fit. (With the added benefit of simply having to add a second coil to add redundancy) $\endgroup$
    – GittingGud
    Aug 6, 2019 at 5:53

I'm not convinced that they (all) do. This other answer mentions a use case were open-loop hydraulics can be useful. I can also imagine some cases where you don't want to use electronic signals due to potential sparks with nasty consequences.

However, I found this report evaluating pneumatic actuators versus electric torque motors for the new shuttle Orbital Maneuvering System. They reference specifically the problems encountered in the Apollo program (see table 4-1 on page 4-2, which is too big to paste here). The following table summarises the findings.

summary of valve concept evaluation

(Excerpt from "OMS engine shutoff valve and actuation system design and evaluation", Wichmann, H. (Marquardt Corp., Van Nuys, CA, United States), 1974)

They conclude that

Based on the data presented in Table 4-VII it was concluded that the two electric torque operated concepts (the dual poppet valve and the ball valve with a retracted seat) where the most promising concepts for the OME valve and actuation system application. Not only were they more than 30% lighter than the nearest pneumatically operated concept but they were also considered substantially superior from the point of view of reliability and maintenance requirements.

It is possible that pneumatically operated valves have some advantage that is not evaluated here, and I'm still searching for other analyses for other uses cases to support that.

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    $\begingroup$ There's another two-part report from Parker Hannifin on ntrs that comes to the same conclusion re: the OMS. The reason that the OMS ended up with a pneumatic actuator nonetheless...that I leave as an exercise for the student. $\endgroup$ Aug 5, 2019 at 10:28
  • $\begingroup$ It's important to note that technology in all concerned fields has significantly progressed since 1974. You need a more recent evaluation. $\endgroup$ Aug 6, 2019 at 5:22
  • $\begingroup$ @DrunkenCodeMonkey sure, but the opening post (at least the comment) was specifically asking for Apollo CSM technology, hence I started there. $\endgroup$
    – Ludo
    Aug 6, 2019 at 7:25
  • $\begingroup$ @OrganicMarble I got some homework to do then. $\endgroup$
    – Ludo
    Aug 6, 2019 at 7:25
  • $\begingroup$ I think it's just that the decision maker(s) was not buying the argument that a solenoid valve and a piston is more likely to fail than an electric motor-gear train-electronic control box arrangement. Especially with Apollo SPS experience fresh in their mind. $\endgroup$ Aug 6, 2019 at 11:18

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