The first flight of a Delta IV rocket in November 2002 successfully delivered a commercial telecommunications satellite into orbit. The Delta IV uses the first liquid-fueled rocket engine (the RS-68) designed, built, and flown in the United States in more than 20 years. (U.S. Air Force)

EELV: The Next Stage of Space Launch

Randy Kendall

U.S. launch capabilities continue to evolve to meet increasingly demanding space asset requirements. Aerospace is helping to ensure that the latest generation of advanced launch vehicles will lead a long and productive life.

The end of the Cold War forced a retrenchment of many defense programs in the 1990s. Congress asked the Department of Defense to generate a plan for ensuring access to space despite increasing budgetary constraints. The resulting Space Launch Modernization Plan of 1994, developed with the participation of The Aerospace Corporation, presented various alternatives ranging from no change at all to a complete overhaul of the space-launch acquisition strategy. The Evolved Expendable Launch Vehicle (EELV) concept was ultimately chosen, as it offered the best approach for managing cost and risk.

The EELV program was designed to reduce the cost of government space launches through greater vehicle modularity, component standardization, and contractor competition (see A New Approach to Launch Acquisition). Aerospace helped develop system requirements that emphasized simplicity, commonality, standardization, new applications of existing technology, streamlined manufacturing capabilities, and more efficient launch-site processing. In fact, the EELV System Performance Requirements Document listed only three "key performance parameters." These stipulated specific mass-to-orbit requirements for each class of vehicle, design reliability of 98 percent at 50 percent confidence level, and standardization of the launchpads and payload interface.

The first Atlas V lifted off from Cape Canaveral on August 21, 2002. This marked the first operational use of a rocket designed under the EELV's joint Air Force/industry partnership. (U.S. Air Force)

The program includes two families of launch vehicles—the Atlas V and the Delta IV—along with their associated infrastructure and support systems. Each is based on a two-stage medium-lift vehicle, augmented by solid rockets as needed to increase payload capability, and a three-core heavy-lift variant. Both have achieved notable successes in their first launches, but the EELV program is still in its infancy, and will need continued scrutiny to ensure that the anticipated gains in cost and reliability will be realized over the long term. In fact, Aerospace involvement in the program was initially limited, as the government sought to position itself more like a commercial customer; however, as the date approached for the first national security launch (for the Defense Satellite Communications System in March 2003), an increased emphasis on mission assurance prompted a return of Aerospace's traditional role in independent launch verification.

Atlas V Evolution

The Atlas V traces its roots to the Atlas ICBMs developed in the late 1950s, although its modern evolution begins with the Atlas IIA, introduced in 1992. The Atlas IIA featured a 3-meter-diameter pressure-stabilized booster tank powered by three liquid-oxygen/kerosene booster and sustainer engines producing 2.1 meganewtons of thrust at sea level. The rocket's upper stage—the Centaur II—was also 3 meters in diameter and featured a dual RL10A-4 engine. The avionics that control the Atlas were located on the Centaur, with booster-specific components residing in an avionics pod attached to the outside of the first stage. In this configuration, the Atlas IIA could lift 3066 kilograms to a geosynchronous transfer orbit. The Atlas IIAS, introduced in 1993, used four solid rocket boosters to increase performance to 3720 kilograms to geosynchronous transfer orbit.

The next major Atlas variant, the IIIA, successfully flew on its first attempt in May 2000. This vehicle included the Russian-built RD-180 engine, which is also featured on Atlas V. Use of the RD-180 presented significant challenges for the government and Aerospace in conducting flight verification activities because access to the engine's design and test data was restricted. (A U.S. coproduction capability is now being developed as a risk-reduction effort.) Fueled by liquid oxygen and kerosene, the RD-180 has two chambers fed by a common turbopump using a staged combustion cycle to deliver 3.8 meganewtons of thrust at sea level. To accommodate a higher mixture ratio, the liquid-oxygen tank was lengthened approximately 4 meters. The Atlas IIIA was also the first to use the Centaur III upper stage. In this configuration, the Atlas IIIA can lift 4060 kilograms to geosynchronous transfer orbit.

The Atlas evolution continued with the IIIB, first flown in February 2002. This vehicle introduced the Common Centaur upper stage, which can be flown with either single or dual RL10A-4-2 engines. The Atlas IIIB can lift 4500 kilograms to geosynchronous transfer orbit. The Centaur tanks on the Atlas IIIB were lengthened by approximately1.7 meters more than the IIAS; as a result, Aerospace recommended additional structural qualification testing, which is scheduled to be completed in spring 2004.

The Atlas V Family

The final step in the Atlas evolution was the introduction of the 3.8-meter-diameter Common Core Booster, which forms the basic building block of all Atlas V vehicles. Upgrades to avionics and redundant systems were also incorporated. The Atlas V core vehicles can be equipped with payload fairings measuring 4 or 5 meters in diameter; the 4-meter version can carry up to three solid motors, and the 5-meter version can carry up to five. A heavy-lift version, still in development, will consist of three Common Core Boosters strapped together. All variants use the same main engine, core booster, Common Centaur, and avionics. This commonality enables the Atlas V to support a wide range of missions and facilitates upgrade from one variant to the next if performance requirements increase. In fact, the Atlas V is the first Atlas that can support direct injection into geosynchronous orbit. The 4-meter vehicles can lift 4950–7620 kilograms to geosynchronous transfer orbit, the 5-meter series can lift 3950–8665 kilograms, and the heavy-lift vehicle will lift 12,650 kilograms.

Atlas V vehicles carry a three-digit designation indicating the diameter of the payload fairing, the number of solid rocket boosters, and the number of Centaur engines. Thus, the most basic vehicle—the 401—would have a 4-meter fairing, no solid motors, and a single-engine Centaur. A 552 vehicle would have a 5-meter fairing, five solid rocket boosters, and a dual-engine Centaur. The heavy-lift vehicle consists of three cores strapped together. (view larger image).

Launch processing for the Atlas V centers on the "clean pad" concept at Cape Canaveral. The benefits of this approach include the ability to launch several Atlas V configurations from the same pad. The vehicle is fully integrated off-pad in a vertical position, including payload stacking and integrated testing. On the day of launch, the rocket is rolled to the pad on the mobile launch platform, where the propellants are loaded. The vehicle is then ready for countdown. There is no spacecraft or launch vehicle access at the pad, so any hardware problems require rolling the rocket back to the vertical integration facility. In fact, the second Atlas V flight had to do just that to allow replacement of avionics components; a successful launch followed within 24 hours.

Fewer Atlas V launches are scheduled for the West Coast, so the clean pad concept will not be used there. Rather, the Atlas V team is upgrading an existing Atlas III pad and will use a more traditional processing approach. This pad will accommodate the largest 5-meter vehicles, but not the heavy-lift version. Aerospace personnel who were involved with previous launch-pad upgrades at Vandenberg are helping to support this activity.

Delta IV Evolution

The Delta IV lineage also traces back to the late 1950s and has its origin in the Thor ballistic missile. The modern evolution stems from the Delta II, which completed its first mission—a GPS satellite launch—in 1989. Subsequent configurations have included the RS-27A liquid-oxygen/kerosene main engine on a core vehicle measuring 2.4 meters in diameter. The RS-27A provides only 0.9 meganewtons of thrust at sea level, so with a minimum gross liftoff mass greater than 100,000 kilograms (without solids), the Delta II requires strap-on solid rocket motors for liftoff. The second stage is powered by an engine running on N2O4 and Aerozine 50. For high-energy missions, such as a GPS transfer orbit or Earth escape trajectory, a third stage can be added with a solid rocket motor.

The next major development was the introduction of the Delta III with a 4-meter-diameter upper stage powered by an RL10B-2 engine. Fueled by liquid oxygen and liquid hydrogen, the RL10B-2 is similar to the RL10A-4 flown on the Centaur and includes an extendable nozzle. The Delta III uses a shorter and wider fuel tank than the Delta II to accommodate the larger upper stage and payload fairing; this design keeps the overall length roughly the same and allows the Delta III to maintain control authority and to maintain compatibility with existing facilities. In addition, slightly larger graphite-epoxy solid rocket motors are employed.

The Delta IV family includes three classes of vehicles. The medium-class vehicle has a Common Booster Core and a 4-meter-diameter upper stage and payload fairing. The medium-plus has two basic versions: one with a 4-meter-diameter upper stage and payload fairing and two solid motors, and one with a 5-meter-diameter upper stage and fairing and two or four solid motors. The heavy-lift vehicle consists of three cores strapped together (view larger image).

The heart of all Delta avionics is the redundant inertial flight control assembly; introduced in 1995, this assembly uses six ring-laser gyros and six accelerometers to provide complete redundancy in each axis. Capable of lifting 3810 kilograms to geosynchronous transfer orbit, the Delta III doubled the performance of the Delta II, allowing it to fly a much larger class of payloads. While its success record was not stellar, the Delta III was a critical step forward, enabling Delta to compete in the intermediate and heavy launch market. Although Delta III was an entirely commercial development, Aerospace participated in the anomaly resolution that followed the first Delta III failure in 1998 and performed independent validation of the modifications to the flight control software that was determined to be the root cause. Aerospace was also actively engaged in the anomaly resolution following the second Delta III failure that involved the RL10B-2 engine. Prior to the successful third flight, Aerospace personnel provided hardware review and software validation expertise.

The Delta IV Family

The final step in the evolution of the Delta IV brought the Delta III 4-meter-diameter upper stage to a new 5-meter-diameter Common Booster Core. The core's RS-68 main engine is the first liquid-oxygen/liquid-hydrogen main engine developed and flown in the United States since the space shuttle. It uses a gas generator cycle with a relatively low chamber pressure. Although it has significantly lower specific impulse than the space shuttle main engine, it produces almost twice the thrust and is much simpler and cheaper to produce. Aerospace provided significant support during the development and testing of this engine, including the resolution of several turbomachinery vibration issues.

A Delta IV rocket lifts off from here Aug. 29. A Defense Satellite Communication System (DSCS) was placed into orbit by the rocket. It was the last of the DSCS satellites to be launched. (U.S. Air Force photo by Carleton Bailie)

The Delta IV Common Booster Core appears on all vehicles in the Delta IV family, with some tailoring of skin thickness to optimize weight as appropriate. The complete Delta IV family includes three classes of vehicles—medium, medium plus, and heavy. The medium vehicle comprises a Common Booster Core and a 4-meter-diameter upper stage and payload fairing. The medium-plus vehicle includes a version with a 4-meter-diameter payload fairing and two solid motors and a version with a 5-meter-diameter upper stage and fairing and two or four solid motors. The heavy-lift vehicle, similar to Atlas V, consists of three cores strapped together. The Delta IV medium can lift 4210 kilograms to geosynchronous transfer orbit, while the medium-plus variants can lift 4640–6565 kilograms and the heavy-lift vehicle can carry up to 13,130 kilograms.

The Delta IV system launches from two pads on the East and West coasts. The launchpads themselves are fairly conventional, with mobile service towers to provide protection from the environment and access to the vehicle and payload. The launch vehicle is processed off-pad in a horizontal position. The first stage is mated to the upper stage in the processing facility, and the vehicle is then rolled out to the pad and hydraulically rotated to vertical on the launch table. The encapsulated payload can then be hoisted and mated to the launch vehicle, followed by integrated system testing. On the day of launch, the mobile service tower is rolled back prior to propellant loading approximately 8 hours before launch.

Standard Payload Interfaces

Along with the improvements in performance, reliability, and operability, one of the most significant achievements of the EELV program was the development of a standard interface for all EELV payloads. The Standard Interface Specification was developed by a joint government-industry team with representatives from launch vehicle and space vehicle programs; Aerospace served as the technical coordinator. The document includes more than 100 requirements for all aspects of the launch vehicle/spacecraft interface, including not only mechanical and electrical interfaces but also mission design requirements, flight environments, and ground interfaces and services.

While rigorous mission integration is still required, spacecraft that adhere closely to the specification can greatly simplify the process. The specification facilitates the dual integration of payloads to fly on both the Delta IV and Atlas V and also eases the transition of a spacecraft from one payload class to another. This is because all but 12 of the interface requirements are common across all medium, intermediate, and heavy-lift variants. The fact that both Delta IV and Atlas V provide the same standard interface is a significant improvement over the heritage systems, where moving from a Delta II to an Atlas II or from an Atlas II to a Titan IV was highly complex, if at all possible.

The Next Steps

A still frame from the onboard video camera carried by the Atlas V during its inaugural launch. A jettisoned booster section can be seen falling away from the rocket toward Earth. (International Launch Services)

Both the Atlas V and Delta IV have successfully completed three out of three launches. Atlas V has flown three commercial communications satellites on the 4- and 5-meter configurations. Delta IV has launched two Defense Satellite Communications System spacecraft on medium vehicles and a commercial communications satellite on a medium-plus vehicle. On the day of launch, Aerospace personnel supported the government mission director by monitoring prelaunch and flight data from specialized facilities at the launch site and in El Segundo.

Although the commercial market remains weak, the EELV contractors have already been awarded 26 more government launch contracts, with up to 20 more expected to be awarded in summer 2004. While the expected cost efficiencies (based upon large numbers of commercial launches) have not yet materialized, the program is still meeting its cost-reduction goals—even with expected price increases in the next procurement round. The primary reason is that many of the payloads that can fly on an EELV intermediate variant would have required a much more expensive Titan IV vehicle in the past.

The program's next major challenge will be the Delta IV heavy-lift demonstration flight, scheduled for July 2004. The unprecedented flight of three 5-meter liquid-fueled cores through the atmosphere presents a number of structural dynamics and flight controls challenges, and Aerospace is working hand-in-hand with the Air Force and the contractor to ensure a successful mission.

Acknowledgement

The author thanks Pete Portanova for his contributions to this article.

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