Since its debut in 1960, the Delta rocket has been a mainstay of the American space program. Over the course of six decades and more than 380 missions, the Delta has launched numerous significant payloads including communication, weather, and scientific research satellites, as well as robotic space probes to explore the solar system. The Delta rocket’s longevity and track record of success make it one of the most important launch vehicles in spaceflight history.
Origins and Early History
The Delta rocket traces its origins back to the Thor intermediate-range ballistic missile developed by the US Air Force in the 1950s. The Thor was designed to deliver nuclear warheads, but was adapted by NASA to serve as the basis for a space launch vehicle. The first Delta rocket, designated Delta-DM19, blasted off from Cape Canaveral on May 13, 1960 carrying the Echo 1 communications satellite.
The original Delta rockets used a Thor first stage with liquid oxygen and kerosene propellant, plus an Aerojet AJ10 second stage engine burning hypergolic propellants. A small solid-fueled third stage could be added for high-altitude satellites. These early Deltas could place around 100 kg into low Earth orbit.
Over the 1960s, the Delta rapidly evolved with more powerful engines, lengthened fuel tanks, and strap-on solid rocket boosters. The naming scheme changed to a four digit number indicating the features and capability. For example, Delta 0900 series used the Thor core but with the more powerful MB-3 engine. Delta 1000 series added three Castor solid boosters. By the end of the decade, the Delta 1900 could carry 200 kg to geostationary transfer orbit.
Emergence as a Reliable Workhorse
The Delta became NASA’s primary medium-lift workhorse for the 1970s and 1980s. Continued upgrades expanded its capabilities significantly. The Delta 2000 series replaced the Thor core with the more powerful Thor-ELT first stage. The Delta 3000 series enlarged the tanks further and added 6 or 9 Castor-2 strap-on boosters. A new PAM-D (Payload Assist Module) solid motor spinning upper stage provided the final boost to geosynchronous orbit.
With these improvements, the Delta emerged as a highly reliable and versatile rocket. It launched most of the early GOES weather satellites, Landsat Earth observation satellites, and Intelsat and Satcom communication satellites. The Delta was also used for many important scientific missions such as Explorer 55, IUE, and IRAS. By the end of the 1980s, the Delta had chalked up an impressive record of 178 successful launches out of 186 attempts, a reliability rate exceeding 95%.
Delta II
In 1989, the Delta II was introduced, featuring an upgraded first stage designated Extra-Extended Long Tank Thor (XELT), more powerful strap-on graphite-epoxy motors (GEM), and the new Aerojet AJ10-118K restartable second stage engine. The Delta II increased low Earth orbit payload capacity to 5,000 kg and GTO capacity to 1,800 kg.
The Delta II became the rocket of choice for NASA’s most high-profile missions of the 1990s and 2000s. It launched the Mars Global Surveyor, Mars Pathfinder, Mars Odyssey, Spirit and Opportunity rovers. The Delta II was also used for Earth science missions like UARS and TRMM, and commercial imaging satellites like Ikonos and QuickBird.
The Delta II completed 155 missions from 1989-2018, with only two failures. It was praised for its reliability and was nicknamed the “workhorse” of the US space program. A Delta II now stands on display at the Kennedy Space Center Visitor Complex.
Delta III and Delta IV
In the late 1990s, the Delta product line was split in two. The Delta III was developed by Boeing as a lower-cost commercial alternative to the Delta II, using a single first stage engine, while the Delta IV was developed by McDonnell Douglas as a next-generation heavy-lift vehicle.
The Delta III had an inauspicious debut, with its maiden flight in 1998 failing to reach orbit. A second launch in 1999 also failed. The rocket was quickly retired with no successful flights. In contrast, the Delta IV had a successful first launch in 2002 and went on to become a mainstay of the US Air Force’s Evolved Expendable Launch Vehicle (EELV) program.
The Delta IV comes in five variants:
- Delta IV Medium (5,000 kg to LEO)
- Delta IV Medium+ (4,2) (6,390 kg to LEO)
- Delta IV Medium+ (5,2) (7,300 kg to LEO)
- Delta IV Medium+ (5,4) (8,510 kg to LEO)
- Delta IV Heavy (25,980 kg to LEO)
The Delta IV Heavy uses three Common Booster Cores strapped together as a first stage, making it one of the most powerful rockets currently operational. It has launched several classified National Reconnaissance Office payloads as well as NASA’s Orion EFT-1 test flight and the Parker Solar Probe.
As of 2021, the Delta IV has launched 41 times with a nearly flawless success record. However, it is being phased out in favor of the lower-cost SpaceX Falcon rockets and ULA’s new Vulcan rocket. The final Delta IV Heavy launch is expected no earlier than 2023.
Legacy and Significance
Over its more than 60 year history, the Delta rocket has left an indelible mark on the history of spaceflight. It enabled the buildup of worldwide communication satellite networks, provided data to improve weather forecasting, and launched groundbreaking scientific missions to study the Earth, Sun and solar system. The Delta played a crucial role in establishing the United States’ leadership in space.
The longevity of the Delta family is a testament to the soundness of its fundamental design. The Delta’s evolution also exemplifies the aerospace industry’s ability to continually improve and adapt rocket technology to meet new challenges and mission requirements. With ongoing upgrades, the Delta went from being able to lift just 100 kg to low Earth orbit in 1960, to being able to lift more than 25,000 kg by the 2000s, a 250-fold increase in capability.
The Delta rocket’s legacy of reliability is perhaps its most significant attribute. Across all variants, the Delta has achieved a 95.7% success rate over more than 380 total launches. The Delta II in particular was the gold standard for dependability, entrusted with NASA’s most valuable scientific probes and enabling the new era of Mars exploration in the 1990s and 2000s.
While the Delta rocket program is now complete, with the final Delta IV Heavy launch in 2024, its place in history is firmly secured. For six decades, the Delta was a pillar of the US space program and helped shape the modern space age. It will be remembered as one of the great rockets that opened the frontier of space.
