Ethan Wong

March 3, 2023

After the end of the space shuttle, many payloads were given to recently developed rockets such as the Falcon 9, while the Atlas V rocket retired. However, the Delta rockets have still been launching since the 1950s, carrying the first Orion capsule test into orbit, as well as other probes and vehicles like the Parker Solar Probe. The Delta IV Heavy officially retired last year as its final flight of 3 remaining rockets launched from Vandenburg, California on September 24th.

After the Soviet Union launched its Sputnik rocket in 1957, the U.S. planned to bounce back with a rocket of their own called Delta; based on the PGM-17 Thor, a ballistic missile designed by the U.S. Air Force during the 1950s and 60s. This 65-foot tall IRBM was soon used to develop the rocket. The rocket had a combined 4 stages, with the last stage being named Thor “delta”, after its greek alphabetical name, yet the entire Thor-Delta rocket became better known as “Delta”

From 1960 to 1962, NASA launched twelve Thor DM-19 Delta rockets, with eleven of them being successful. These rockets provided many payloads into orbit, such as the first communications satellite: Echo 1A, the first British-developed satellite: Ariel-1, as well as the very first active direct-relay communications satellite. Thor 144 Delta 1 was the maiden flight for the Thor-Delta rockets and remained the only unsuccessful launch throughout its 12 launches. There remained a problem in the upper stage of the rocket–including a malfunction with the altitude control systems. However, the next 11 launches would be successful and mark the end of the Thor-Delta rocket. Many more Delta rockets would be launched over the years and carry on the legacy left behind by the Thor-Delta.

Delta A would only launch a total of 2 times before Delta B launched from 1962-1964. The Delta B had an improved AJ-10 engine and allowed Delta to finally send the first spacecraft into geostationary orbit. Delta C was then introduced in 1963 and flew eleven times until 1967, with some minor changes added onto the third stage of the rocket. However, Delta D released a new type of feature: three Castor strap-on motors that were equipped for the first stage of the rocket. The Delta G rocket was essentially the same as the Delta E but they decided to remove the third stage entirely from the rocket, which flew twice in 1966-67. NASA also brought back the “Long Thor” Delta rockets. Delta M carried this attribute along with a Star 37 solid rocket motor, which allowed around 350 kg to be transported up to geostationary orbit. The Delta N also used the Long Thor variant but with the Delta E second stage, and flew half a dozen successful times, while Delta M successfully flew twice that. Additionally, the “Super Six” was developed on either the Delta M or N, which allowed for 6 Castor side-rocket boosters instead of the typical 3. 

Even though the Delta rockets were extremely reliable and useful, they were put to a stop in the 1980s when the space shuttle was introduced, which seemed more practical at the time. Yet, in 1986, the Air Force ordered the production of twenty more Delta 6000-series rockets  (Delta II) after the Challenger disaster. Although the Delta II could only transport small payloads into orbit, it still was able to put in work as it launched the Mars Exploration Rovers: Spirit and Opportunity in 2003.

The Delta III came along next, following more strap-on boosters and the capability for heavier lifts to geostationary orbit. Sadly, Delta III only launched a total of three times, and two of them were not successful. Starting in 2002, Delta IV rockets were introduced along with the RS-68 engines powering the main core stage of the rocket, and in different models depending on the types of payloads needed to be made. Delta IV Heavy rockets had 3 core stages which allowed them to have a thrust capable of bringing weights of 23000 kg to low-earth orbit. Delta IV Heavy was the only rocket to ever reach orbit by using hydrogen and oxygen as fuel and has been used as one of the primary rockets for transporting large payloads since its retirement in 2022.