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In a recent question, an answer showed a diagram of a Soviet S5.2/9D21 rocket engine. According to the diagram, there are four pyrocharges (3, 7, 24, and 27) in the engine. What is the purpose/function of these pyrocharges? Are these part of the Flight Termination System or something else?

S5.2/9D21 rocket engine engine

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    $\begingroup$ Those four pyrocharges are all associated with valves (purge, ox shutoff, fuel shutoff, shutoff fuel drain valve) so I would guess they're designed to explosively and rapidly close the shutoff valves (maybe allowing venting of the remaining propellants at the same time?) when the rocket is on its intended trajectory. For a tactical ballistic missile, prompt cutoff of thrust at a particular point would be desirable. $\endgroup$ – Russell Borogove Jan 21 at 17:10
  • $\begingroup$ Agreed, pyrotechnics are typically used when you want something irreversible to happen reliably. $\endgroup$ – Organic Marble Jan 21 at 17:17
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Each of those pyro charges is associated with a valve; according to this page on Norbert Brügge's site, the pyros actuate the valves to decisively start and stop propellant flows:

Ignition is accomplished by a hypergolic start fuel designated TG-02 "Samin" filled into the fuel line at the main fuel valve. TG-02 consists of 50 percent Tri-ethyl Amine and 50 percent Xylidiene. The propellant fed system is a turbo pump driven by a bipropellant gas generator using the main propellants. The start and shut down valves are one shot devices, actuated by pyrotechnic charges.

Prompt shutoff of engine thrust is nice to have in a tactical ballistic missile in order to accurately control range, although the Scud does have active inertial guidance, so it may not be totally critical.

Here's a representative pyro-valve (obviously not the same one that's used in the Scud series) that's available in normally-open (i.e. explosively-closed) or normally-closed form; it works with a variety of propellants. I know "IPA" stands for isopropyl alcohol, but I think it's hilarious to imagine they tested it with India Pale Ale.

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