With three CBC (Common Booster Core) modules firing their RS-68A engines simultaneously, a Delta 4 Heavy launch is an impressive sight.

Charred D4 Heavy

The RS-68 is a LOX/LH2 engine. As part of the launch sequence, a fair bit of hydrogen is run through the engine, prior to actual ignition to cool down pumps, components, etc and condition the engine. The SSME (Space Shuttle Main Engine, RS-25) does the same thing.

With the D4H however, there is sufficient hydrogen run through, that at ignition, it ignites, and actually starts an awesome to see fire, running up the sides of the core modules, and the insulation on the outside gets quite charred.

In order to minimize this they have tried to start one engine first, then the other two, so as to burn off some of the hydrogen first.

Thus my question. On the Space Shuttle, there are sparklers that ignite as the SSME's go through their conditioning cycle to burn off the excess hydrogen gas. Why not use the same on the D4H launches?


1 Answer 1


They do use sparklers: the Radially Outward Firing Initiators (ROFI), derived from the Shuttle system, are started at T-15 seconds. Note that a hydrogen fire is the expected result of this system; they just didn't expect it to be this extensive.
On the NasaSpaceflight forum, someone suggested that the scorching of the insulation is caused by the hydrogen not mixing with air immediately, so the ROFI doesn't burn all of it until the oxygen turbopumps are spooled up just prior to ignition. The flame duct is enclosed which allows hydrogen to pool, that doesn't help either.
On the Space Shuttle pad, the water deluge system helps keep down any fires. The Delta IV is launched dry. The launch photo you show, with the insulation extensively blackened, may have been an anomaly due to some part of the timing being off (i.e. more hydrogen present than expected when the ROFI started).


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.