An Ode to Delta IV Heavy


The final Delta IV Heavy launch from Vandenburg just happened, and it kind of makes me sad to see it wind down – especially when comparing it to SLS. There are plenty of things to complain about relating to SLS, but two stand out when comparing to Delta IV Heavy: the engines and the boosters.

Its main RS-68 engines are derived from the Space Shuttle's main engines, but are intended for a single use, and are simpler and less expensive as a result. The heavy variant has three nearly-identical cores in the first stage, with the outer two serving as boosters. The RS-68 sheds much of the complexity of the RS-25's by cooling the nozzle ablatively, which you can do when you only need to use it once.

Compare that to SLS. If it ever gets off the ground, it will result in four flight-proven, re-usable RS-25 SSME's being thrown into the ocean as garbage. An earlier iteration of the Shuttle-derived booster concept did use a few upgraded RS-68's, but they decided the engineering required to manage the interactions of the RS-68's ablative nozzles with the solid boosters was too difficult to overcome.

And yeah, the boosters. SLS uses Shuttle-derived solid rocket boosters. I think this is really bad for everyone that doesn't work for Northrop-Grumman. Humans should never fly on a rocket that uses solids as part of its primary propulsion. Obviously, there's Challenger. That failure was predicted and ignored by managers suffering from go-fever. But in the leadup to SLS's predecessor program, the Air Force determined that in some phases of flight, a solid rocket failing would have a 100 percent chance of killing the crew due to the escaping capsule's flight through a shower of burning fuel. Watching a capsule full of crew successfully escape a failing rocket only to lose its parachutes to burning debris would be heartbreaking.

Delta IV Heavy doesn't use solids for primary propulsion. A failing side- or core booster would be really bad, of course. But it would probably result in an explosion that isn't followed by a persistent fire. By the time an escaping crew came back down through the altitude where the failure happened, the propellant would already be done burning.

Okay, so how about the upper stages? I'd compare them, but SLS uses Delta IV's second stage in an almost unchanged state. Of course there are plans to upgrade SLS's upper stages in the future. But that requires it to fly more than once or twice. We'll see.

Crew-rating the Delta IV Heavy would take some effort. NASA did a study (pdf) early in the Constellation days and said it could be done. It would require some new software, avionics, and process changes in manufacturing. The biggest change would be to modify the second stage. Here's the thing – that modified second stage could be done, because that's what the SLS second stage is. Although I'm not sure how I feel about Boeing's ability to safely implement hardware and software changes without cutting corners.

In the end, I'd feel a lot better about a crew-rated Delta IV Heavy than SLS. Sure, it's expensive, something like $250 million a pop. But SLS is like eight times that. And you have the solid failure debris problem, and the bad feeling of throwing SSME's into the ocean. And while I think the safest crew-rated rocket on the planet right now is Falcon 9, I'd much rather climb onto a Delta IV Heavy than SLS.