Delta IV Heavy in Action
A launch starts with the ignition of the three RS-68 main engines and then liftoff. Within a few minutes, the strap-on CBCs are jettisoned (dropped off the main rocket), having used up their fuel and served their purpose of getting the rocket off the ground. After that, the main central engine (the one attached to the central CBC) is turned off and the bottom two-thirds of the main CBC, consisting of the main engine, the lower fuel tanks and the interstage, which connects the first stage to the second stage, is also jettisoned. What is left is the second stage, consisting mainly of fuel tanks, RL10B-2 engine, guidance electronics and payload, all encased in a protective cone called a fairing.
Compared to the first stage, the second stage is like a ballerina sitting on the shoulders of a linebacker. It may not have the massive power of the three booster engines, but it has the strength, balance and precision to handle the more delicate task of putting a satellite into a sustainable and correct orbit. Once the first stage components have fallen away, the second stage fires up its engine and jettisons the protective fairing. Next is the second-stage engine cutoff (SECO)-1, where the RL10B-2 engine is shut off and the second stage maneuvers with its thrusters through a coast period. Guidance is provided throughout the second stage by avionics and attitude control systems. The Redundant Inertial Flight Control Assembly helps make sure the rocket inserts the payloads into the proper orbit.
For its first flight on December 21, 2004, Delta IV Heavy contained three satellites, the primary DemoSat and two auxiliary, student-built satellites, referred to collectively as NanoSat-2. During the coast period of the first flight, the NanoSat-2 satellites were activated and released.
Two engine restarts and cutoffs (SECO-2, SECO-3) followed the release of NanoSat-2. These allowed the second stage to conserve energy.
Because the Delta IV Heavy is so efficient, it has the fuel necessary to enable it to deploy to almost any altitude and orbit. In addition, because the second stage engines are doing most of the positioning and are capable of inserting their payloads into orbit with great accuracy, satellites expend much less energy and can use that extra fuel to power their own functions longer. When the second stage reached the required orbit, the DemoSat payload, now capable of maintaining its own orbit, was activated and separated from its carrier.