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
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.
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.
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.”
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.
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.
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.
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
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.
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.
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).
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.
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.
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
astronomy mission between NASA, Germany and United Kingdom, was launched
from the Cape Canaveral Air Force Station on June 1, 1990.
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.
The Boeing Delta II 7425
launch vehicle seen in this photo consists of three stages
stacked on top of each other, plus four small solid-fuel rockets strapped
to the outside of the first stage.