Just after Thanksgiving, a teletype message clattered into his office: “Program approved, proceed at maximum pace.” A month later, the firm of Douglas Aircraft won the contract to build it. The Air Force stood ready to provide engines, guidance systems, and nose cones that would protect the warhead when re-entering the atmosphere at high speed. Douglas' task then was to assemble these components and manufacture the missile on a production line.

Jack Bromberg, a hard-driving Douglas manager known as “Thorhead,” set a tight schedule. The design of the missile was nearly complete in July 1956, just seven months into the program. The first Thor flew to the launch centre at Cape Canaveral, Florida, in October, aboard an Air Force transport plane. Launched in January 1957, it rose nine inches (23 centimetres) into the air. Then it lost thrust, fell back, and exploded. Engineers found the cause of the failure and tried again, but the second and third Thors also blew up for different reasons. The fourth one went out of control and broke up in flight, but the fifth, in September, was successful. Other successes followed. In September 1958 the first Thors went to England, as operational missiles ready for war.

 
The first successful launch of the Thrust Augmented Thor with an Agena D upper stage, 18 March 1963.

By then Thor was taking on a new role, mounting upper stages and launching spacecraft. The first such space vehicle, the Thor-Able, used a second stage developed from the existing Aerobee rocket made by the Martin Company. Its guidance system was designed in an electronics lab at the firm of Ramo-Wooldridge, which provided General Schriever with technical support. Initial flights of Thor-Ables tested nose cones, flying successfully during July 1958. Next on the agenda was a particularly demanding goal: a flight to the Moon.

During the first attempt, in August, the Thor first stage blew up in midair. Two months later, the effort came so close to success that everyone could taste it. Launched well before sunrise, the Thor traced a bright curving streak across the night sky as it rose and turned to fly downrange. The third stage, which burned solid propellant, failed to separate properly and flew off at an improper angle, falling short of the necessary launch velocity. Still the spacecraft was on its way, reaching an altitude of 70,745 miles (113,853 kilometres) and touching the fringes of interplanetary space. It fell back into the atmosphere and burned up like a meteor, but the project leader Simon Ramo offered this perspective: “What we gained this weekend was a few seconds on infinity.”

In a separate effort, the Air Force fitted Thor with a different second stage called Agena. Built by Lockheed, with a rocket engine from Bell Aircraft, the Thor-Agena would launch Corona reconnaissance satellites. It flew from Vandenberg Air Force Base on the California coast, where the launch crew included veterans who called themselves “broomlighters.” They claimed that when a rocket failed to fire, one of them would rush out with a flaming broom that had been soaked in kerosene to make the engine ignite.

The first Corona mission thundered into space in February 1959. It too fell short; an analyst from the Central Intelligence Agency later wrote that “most people believe it landed somewhere near the South Pole.” However, the next one reached orbit successfully, as did five of the next ten and eight of the ten that followed. In this fashion, the Thor-Agena began to build a record of reliability.

The Thor-Able and Thor-Agena both were Air Force rockets, with the latter continuing to orbit Corona spacecraft until that program ended in 1972. In addition, both launch vehicles saw service with the National Aeronautics and Space Administration (NASA). In 1959, that agency's Goddard Space Flight Centre ordered 12 of the Thor-Able type from Douglas Aircraft, with the first of them flying in August 1960. They used a modified second stage called Delta. In time, as new Thor-based launch vehicles proliferated, the name “Delta” came to apply to all space launchers of this general type.

During the 1960s, designers added several improvements that enabled these rockets to carry heavier payloads. Increases in the thrust of the Thor engine were particularly important. The engines came from the Rocketdyne company; the earliest version gave 135,000 pounds (600,510 newtons) of thrust. This increased to 150,000 (667,233 newtons) and then topped 200,000 (998,644 newtons). In 1963, Thor first stages began flying with three small solid boosters for extra thrust following lift-off. In 1966, the increasing power of the main Thor engine brought the “long tank” series, which carried more propellant.

Agena saw its own improvements. The earliest version carried propellant for only 120 seconds of burn time and could not be restarted in space. The Agena B, which first flew in 1960, doubled the size of the propellant tanks and introduced the ability to restart. This enabled it to lift heavier payloads and broadened the range of possible orbits. Lockheed then introduced the Agena D, a standardized design. NASA purchased its own Thor-Agenas, complete with long tank and strap-on solid boosters, and used them to launch weather satellites along with Orbiting Geophysical Observatories for scientific studies.

There was strong interest in using Delta vehicles to orbit communications satellites, but the preferred orbit flew at an altitude of 22,300 miles (35,900 kilometres) and was nearly as difficult to reach as the Moon. Even with its long tank and three strap-ons, the Delta of the mid-1960s could boost only 82 kilograms of payload to that orbit. NASA, therefore, introduced several improvements.

It increased the number of strap-on solids to as many as nine, while enlarging their size and raising their thrust and burn time. The Thor main engine also saw its own thrust rise, from 195,000 pounds to 207,000 (867,403 to 965,264 newtons). The second stage changed to a large-diameter version that carried more propellant, while the third stage received its own improvements. These developments proceeded step by step through 1975 and raised the high-orbit payload limit eleven fold, from 82 kilograms to 907 (180 to 2,000 pounds).

Through such improvements, the Delta became NASA's workhorse. It continued to launch communications satellites as they grew in weight, complementing them with weather satellites and scientific spacecraft. But the advent of the Space Shuttle, which first flew in 1981, seemed to spell doom for Delta. NASA stopped placing orders, expecting that the Shuttle would launch its payloads. The number of Deltas diminished as the agency flew off its existing ones. In 1986 NASA had only three left in its inventory.

In January of that year, the destruction of the Space Shuttle Challenger showed that NASA was pushing its Shuttles too hard. A new policy from Washington de-emphasized the Shuttle and placed new emphasis on vehicles such as Delta. Its production line had been shut down, but in January 1987, Air Force Secretary Edward Aldridge awarded a contract for up to 20 new Delta II launchers. Three months later, its builders announced that nine paying customers had booked flights of communications satellites on this same rocket.

The Delta family of expendable launch vehicles.

Delta II continued the trend of earlier Deltas, mounting larger strap-ons with more thrust, an enlarged second stage that used a more powerful engine, and a longer-burning third stage. The user could fly it with three, four, or nine strap-ons, delivering up to 2,064 kilograms (4,550 pounds) to high orbit. It first flew in February 1989. Delta IIs have launched the entire Global Positioning System satellite fleet, which provides accurate guidance for the smart bombs for the war in Afghanistan.

The Delta II expendable launch vehicle with the ROSAT (Roentgen Satellite), cooperative space X-ray astronomy mission between NASA, Germany and United Kingdom, was launched from the Cape Canaveral Air Force Station on June 1, 1990.

Recent developments have emphasized the use of liquid hydrogen, the most powerful rocket fuel available. The Delta III, which first flew in 1998, uses a new second stage with a hydrogen-burning engine from Pratt & Whitney. It can deliver 3,810 kilograms (8,400 pounds) to high orbit. The Delta IV introduced an entirely new first stage that also burns hydrogen, with Rocketdyne providing an engine of 650,000 pounds (2.9 million newtons) of thrust. It too comes in a family of versions, carrying 4,210 kilograms (9,280 pounds) when flying alone and as much as 13,130 kilograms (28,950 pounds) when launched with two such stages as boosters. The latter model stands 225 feet (69 meters) tall, nearly four times the 61-foot (19-meter) length of the ancestral Thor missile that started it all. Taken together, the Delta family of launch vehicles accounts for some 34 percent of the commercial launches in the entire world. Launch services today are highly competitive, with the United States, Russia, Europe, and China all offering rockets in a range of sizes. Yet in the face of this competition, the Deltas remain what the world needs and uses.