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I was watching the (eventually scrubbed) launch of Delta IV Heavy with the Parker Solar Probe. I noticed a fairly large blue/red flare near the launch pad. I'm guessing it's LH/LOX, but I thought hydrogen burns more yellowish.

This Reddit question may address the same topic, but I couldn't find the flare in their screenshot.

Below is a screenshot (literally a photo of TV screen... sorry) with the flare in the bottom left.

Delta IV Heavy on the launch pad with blue/red flare at bottom left

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  • $\begingroup$ Hydrogen flame is faint bluish, almost invisible. It is possible that a colorant is added to the waste gas for safety reasons (visibility of the flame, or visible deflagration in case of leak and fire). See h2tools.org/bestpractices/hydrogen-flames $\endgroup$ – kkm Aug 11 '18 at 12:27
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The answers on the reddit thread you linked are essentially correct. The camera angle is the only difference between the two images, making the flare appear further away.

As you mention, Delta IV and Heavy use cryogenic hydrogen and oxygen as propellants at approximately -255°C and -185°C respectively. As these are loaded into the tanks in the lead up to launch they continuously boil off, producing their gaseous equivalents. To mitigate pressure build-up inside the tanks, they are vented:

  • The venting oxygen can be seen as white clouds around the vehicle before launch.

  • The venting hydrogen is piped a short distance away from the launchpad and burnt in a controlled manner.

Once the loading is complete, a small amount of propellant is continually fed into the tanks to keep them topped up.

See the User Guide for more detail, here for a thorough description of the vehicle and this short description of the venting process.

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  • $\begingroup$ The red color of the flame is then probably due to the lighting conditions. The blue /red almost made it look like acetylene, but that didn't make sense. $\endgroup$ – Ludo Aug 11 '18 at 10:38
  • $\begingroup$ @Ludo probably yes - see here for an example. I've also heard that some other gas it burnt at the vent to keep the flame going, but I'm not sure what - presumably something cheaper to produce like propane. $\endgroup$ – Jack Aug 11 '18 at 10:40
  • $\begingroup$ The picture there, burning at night, looks quite similar! A bit smaller. $\endgroup$ – Ludo Aug 11 '18 at 10:47

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