I note that the launch of the Delta IV Heavy rocket from Vandenberg AFB in California last week produced some rather spectacular images of gas igniting before the launch. Aviation Week notes:
Aside from the size of the vehicle - 253 ft. tall and 1.5 million lb. - a spectacular feature of the initial climb-out of a Delta IV Heavy is the intentional fire that swirls around the CCBs. Seconds before liftoff, thousands of pounds of hydrogen are dumped through the three RS-68 engines to optimize their hydrogen/oxygen mix for ignition.
The excess of this hydrogen forms a cloud around the vehicle ...
... that is deliberately burned off with spark generators - called radial outwardly firing igniters - just prior to ignition.
Mostly, the excess hydrogen dissipates in a swirl of yellow flame that does little more than blacken the exterior of the first stage. But some of the hydrogen clings so tenaciously to the CCBs that pockets of it are the source of flame shooting out from the sides of the vehicle. ULA says these are harmless and disappear as the vehicle slowly rises.
Photographic evidence gives no indication that any of the hydrogen spectacle reaches the second-stage P&W Rocketdyne RL-10 or the payload fairing, much less the satellite inside.
There's more at the link.
The photographs above are screenshots taken from a video report of the launch, below. The gas can be seen forming and igniting within the first 30 seconds of the clip, just before liftoff.
Why do I get the impression that it'd be a really, really bad idea for a smoker to light up around all those thousands of pounds of hydrogen?
Peter
I saw that launch, and the appearance of the boosters burning until they were black with obvious fire around their tops as the stack cleared the tower was so strikingly unusual, that I had to swap emails with a friend of a friend on that program. He said, "they all do that".
ReplyDeleteSince I've seen every Delta IV heavy launch from the KSC and never seen them with black boosters, or boosters on fire, I was a little surprised. But it appears to be by design, and we can expect them to look like that from now on.
What that Aviation Leak writer gets wrong is that it isn't the swirls of hydrogen that burn yellow; that's from the booster's surface. Hydrogen burns invisibly. The old pad-technicians kept a straw broom to push into places where hydrogen might be burning. The H2 is invisible, but the straw burns yellow.