Delta-IV stages

[gre8]

Sorry if there is a topic about it, I didn't really find what I was looking for.

So I was thinking about the Delta-IV CBC (common booster core) design, and I didn't really get why exactly the boosters run out before the central rocket.

Certainly I overlooked something, but what exactly amounts to this difference of burn time if they are virtually the same rockets?

[TheCanadianVendingMachine]

Slightly less thrust from the center, making less fuel go through the engine

[astropapi1]

Slightly less thrust from the center, making less fuel go through the engine

This.

They throttle down the core while the side boosters are burning, and then throttle it up after booster separation.

[sgt_flyer]

Yup - Delta IV heavy (with two CBC as boosters) liftoff with all 3 cores with their throttle at 102%. 50s into the flight, the central core thrust is lowered to 58%.

At 235s the side boosters thrust is lowered to 58% too to limit acceleration - shortly before separation. On separation, the core stage engine is throttled to full thrust once again

Some time ago, i made a delta-iv lookalike in ksp, and played with clipped engines to approximately simulate the throttling down of the central core

http://forum.kerbalspaceprogram.com/threads/75183-0-23-5-some-replicas-of-heavy-lifters

[magnemoe]

if you launch a rocket with SRB using mechjeb you will see this behavior too.