How the Delta IV Heavy Works

So if rockets are a means of transportation, just what are they transporting? Predominantly, a rocket's cargo (or payload) is a satellite (see How Satellites Work). Since they don't have the means of launching themselves, satellites use rockets to get off the ground and make it to the correct altitude above the Earth.

Satellites must also get to the correct orbit above the Earth. An orbit is a circular path that the satellite follows as it rotates around the Earth, in the same way that the Earth and the other planets in our solar system orbit the sun. Different orbits circle the Earth at different altitudes and at different speeds. A satellite's functions determine which orbit it must follow. Rockets both lift a satellite to the right altitude and insert it into the correct orbit.

But rockets need to be more than just a means of transport. Satellites are great tools; they have revolutionized communications and shown us more about our planet and the universe in which we live than we could ever have discovered without them. The one thing satellites aren't, though, is cheap. All those specialized components and their highly complex software, to say nothing of the huge quantities of fuel necessary for launch, represent major investments in time and money. This puts pressure on rocket engineers to create rockets that can deliver larger and heavier cargo in a single flight and do so with lower cost and higher reliability and accuracy. It's a lot cheaper to use one rocket to put two or more satellites into orbit. Another challenge is to accurately deliver a satellite to a specific location in space where it can most efficiently enter its orbit. Satellites are engineered to function in a precise way at a precise location -- if it's delivered too far away from the optimal place, the satellite's thrusters must expend precious fuel to make up the difference. The rocket needs to be reliable enough to deliver its cargo precisely where it needs to be.

Now let's take a closer look at the Delta IV family of rockets.

Currently, the Delta IV family has three main configurations or styles:

Each configuration has a first stage (the bottom two-thirds of the rocket) containing fuel tanks and main engines and a second stage (the top third of the rocket) that houses the secondary engine and fuel tanks along with the payload and various electronics. The Medium capacity's first stage consists of a single common booster core (CBC) powered by an RS-68 engine. Its second stage is powered by an RL10B-2 engine and includes various maneuvering and altitude-control electronics such as the Redundant Inertial Flight Control Assembly (RIFCA) used on the Delta II, as well as fuel and oxidizer tanks.

The Medium-Plus capacity has the same first stage components as the Medium capacity but also includes either two or four 60-inch-diameter (1.5-m), solid rocket, strap-on graphite epoxy motors (GEMs). All Medium-Plus versions use the RL10B-2 engine to power the second stage, but versions 5.2 and 5.4 have larger-diameter fuel tanks and longer oxidizer tanks than the Medium and Medium-Plus 4.2 versions.

The Heavy capacity looks like a rocket on steroids. Not only does it have the main common booster core, but it also contains two additional strap-on boosters.

Each of the three boosters contains its own RS-68 engine. The Heavy capacity also has in its second stage a 5-meter-diameter fuel tank and 5-meter-diameter, payload-accommodations hardware.

Now that you know the basic structure of the Delta IV family of rockets, let's see how all the different components work together to get the Heavy capacity off the ground and into the skies. As mentioned before, the rocket has two stages. The first stage has one goal: getting the rocket off the ground.

The bottom section of each common booster core (CBC) contains an RS-68 engine. The middle section contains the fuel tanks, in this case liquid hydrogen and liquid oxygen. For the two strap-on boosters, that's all there is. They exist exclusively to provide the extra fuel and engines necessary to lift heavier payloads into orbit.

New to the Delta IV family, the RS-68 is 30 percent more efficient than the liquid-oxygen/kerosene engines it replaces. It has fewer parts, making it more reliable and less expensive, and is environmentally friendly, producing steam as its only byproduct. It also produces 650,000 pounds (2,891 kN) of thrust at liftoff. Combining the three booster cores, the Delta IV Heavy rocket is capable of lifting 50,800 pounds (23,040 kg) to low-Earth orbit. Its closest brother, the Delta IV Medium-Plus (version 5.4), can lift 25,300 lbs (11,475 kg) to the same orbit. (To learn about satellite orbits, see How Satellites Work.)

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.

On December 21, 2004, the newest member of the Delta IV family lifted off from Cape Canaveral Air Force Station in Florida for its maiden flight. Close to six hours later, the rocket had delivered its payload and completed the mission. Unfortunately, the rocket was unable to reach the proper orbit. When scientists looked at the data, they determined that the first stage burn was not as long as they had expected it to be. However, with so much new and upgraded technology, to have only one thing go wrong caused a relatively minor blip on the radar screen. The Delta IV Heavy rocket's first test flight met all of its major test objectives and was considered a success.

Boeing is already making plans for improvements to the Delta IV Heavy rockets and for the creation of the next generation of Deltas. Some of the changes in the works are modifications to the RS-68 main engine, the addition of GEMs to the three CBCs and improvements in fuel density and pathways.

For more information on the Delta family of rockets, the Delta IV Heavy and related topics, check out the links on the next page.