Question about the Delta IV Heavy?

[SlabGizor117]

Hey, I just wondered how the staging works on the Delta IV Heavy. From what I can tell, it would seem that fuel crossfeed is the most efficient staging technique for shedding weight fast and regularly. Why is the Falcon Heavy the only rocket that does it? Wouldn't fuel crossfeed serve the Delta IV Heavy much better than what they use now?

[Taki117]

the Delta IV heavy throttles the center engine so that it uses less fuel on ascent. Then throttles up once the boosters have dropped away. I'm fairly certain the Falcon Heavy goes full throttle the entire time, necessating the fuel cross feed, but providing more weight to orbit.

[AngusJimiKeith]

Fairly simple reason, actually: in reality, crossfeed is pretty difficult, and costs quite a bit of money to develop. Since no customers for the Delta IV have required the extra performance crossfeed provides, it hasn't been developed. There's no reason to.

And, for the record, Falcon Heavy is the same way: last I heard, it was not going to use crossfeed unless some customer needs it, which gives a (monetary) incentive to develop it.

[SlabGizor117]

Huh, thanks for the answers, guys!

[Red Iron Crown]

Falcon Heavy is also not a real rocket yet, just a concept and some parts prototypes. Whether they get crossfeed to work properly on it remains to be seen.

The only real life crossfeed I know of is the space shuttle drawing fuel from its external tank.

[NathanKell]

Making pumps powerful enough and robust enough to handle crossfeed, and making piping big enough and strong enough to handle the massive volume--yet able to be detached on staging--is hard.

[Pecan]

... and yet long-range fuel drop-tanks have been used on aircraft for decades. This is not an insurmountable problem, just not an economic one.

[sta7ic_matt]

That's an apples-to-oranges kind of comparison, since jet fuel doesn't mix in oxidizer, nor involves cryogenic engine fuel.

It's also very much a reliability and complexity problem. It's possible to do crossfeeds, but it's another set of moving parts that can fail, in an environment where failures are often catastrophic and _very_ expensive.