Reading Twitter comments about the recent Falcon 9 pad explosion, some postulated the problem was that a static discharge ignited the fuel as it was being loaded.

Since TEA-TEB is required to ignite the Falcon's fuel (something you'd assume would be easy, but it's not!), I am skeptical that static discharge is even a plausible cause of the explosion. However, since I am not a rocket scientist, I thought I'd get some experts' opinions.

Is it even possible to ignite the Falcon's fuel with a static discharge?

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    $\begingroup$ Note that spark-igniting a proper mix of kerosene vapor with gaseous oxygen is relatively easy. However, inside the engine there's a large flow of liquid propellants, so TEA-TEB is used for reliable ignition at a particular time. $\endgroup$ – Russell Borogove Sep 11 '16 at 20:01
  • $\begingroup$ Yes, makes sense. Any idea if the conditions at fueling time are different enough from liftoff to allow electrostatic discharge as an ignition method? $\endgroup$ – Thane Brimhall Sep 11 '16 at 23:42
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    $\begingroup$ Presence of LOX boil-off vapor seems unavoidable during fueling (and there's oxygen in the air anyway), so I assume the basic safety goal is to avoid release of significant amounts of kerosene vapor at any point. Under those conditions, a spark can't cause much trouble. $\endgroup$ – Russell Borogove Sep 11 '16 at 23:58
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    $\begingroup$ That said, it seems like any small amount of vapor from e.g. kerosene spill, if ignited, would have produced visible fire for some time before the second stage fireball-rupture. $\endgroup$ – Russell Borogove Sep 12 '16 at 0:03
  • $\begingroup$ @RussellBorogve Very interesting insights. Perhaps all together they make an answer? :) $\endgroup$ – Thane Brimhall Sep 12 '16 at 0:07

The Falcon 9 uses RP-1, which is refined kerosene. Kerosene is routinely ignited via sparks in e.g. gas turbine engines, but that's with finely misted kerosene in hot air. The 25 kV of an ignition system is readily attainable by static discharges.

Gasoline fires due to static discharge happen occasionally at petrol stations. Diesel and kerosene are a bit harder to light, but the MSDS for kerosene warns to take precautions to prevent static discharge when handling kerosene.

  • $\begingroup$ Any idea whether flammability conditions would be satisfied during the fueling sequence? $\endgroup$ – Thane Brimhall Sep 12 '16 at 1:26
  • $\begingroup$ It gets even easier if there is more than the usual atmospheric 21% oxygen concentration - for example if there were any vapor from the LOX boil-off, or an actual leak. $\endgroup$ – uhoh Sep 12 '16 at 5:42
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    $\begingroup$ Kerosene is not used in aviation piston engines, but in gas turbines. It is possible to drop a lighted match into a pool of kerosene and the match will extinguish. (Except on a very hot day, when the flashpoint (~38C) can be exceeded), If the fuel is in a fine mist, such as in a high pressure leak, the situation is different. When the fuel is ignited during start-up of a gas turbine, the fuel is sprayed through nozzles to form a mist, and the surrounding air in the combustion chamber is hot and pressurised. And even then, spark igniters are used. $\endgroup$ – Mike H Sep 12 '16 at 6:03
  • $\begingroup$ Anti-static precautions are always taken before refueling aircraft with kerosene. The aircraft is attached to the tanker with a bonding wire, and the tanker attaches to ground with another wire before the fueling hose is connected and remains so until after disconnection. Also fueling operations are suspended during thunderstorms. $\endgroup$ – Mike H Sep 12 '16 at 14:00
  • $\begingroup$ @MikeH that's for a standard 21% O2 atmosphere only. What is kerosene like with 50% O2 ambient atmosphere? What about 100%? $\endgroup$ – uhoh Sep 12 '16 at 14:15

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