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Able rocket stage was a rocket stage manufactured in the United States by Aerojet for the Vanguard rockets used in the Vanguard project from 1957 to 1959. The rocket engine stage use as a rocket propellant Nitric acid and UDMH. Able rocket stage was the second of three stages on the multistage rocket Vanguard. The Able rocket stage was discontinued in 1960. A further improved versions were used in the upper stage in the Thor rocket family (Thor-Able). An upgrade to the Able Stage was the Thor-Ablestar rocket. The Ablestar second stage was an enlarged version of the Able rocket stage, which gave the Thor-Ablestar a greater payload capacity compared to the Thor-Able. It also incorporated restart capabilities, allowing a multiple-burn trajectory to be flown, further increasing payload, or allowing the rocket to reach different orbits. It was the first rocket to be developed with such a capability and development of the stage took a mere eight months.
Some Able rocket stage parts were also used as the engine of Apollo Service Module. The Able stage name represents its place as the first in the series, from the Joint Army/Navy Phonetic Alphabet.
To save weight Aerojet use an aluminum tube for the engine thrust chamber. The engine used on Vanguard was an AJ10-37. On later Able models rockets the AJ10-41 and AJ10-42 engines were used. Aerojet made 21 Able rocket stage for use on rockets. Vanguard project launched 11 of the Able stages, three were able to put satellites into orbit: Vanguard 1, Vanguard 2 and Vanguard 3. Able rocket stage and the Vanguard project were an important part of the space race between the USA and Soviet union. While the Able rocket stage ended in 1960, the AJ10 engines used in the Able rocket stage continue to be used on later rockets and are still in use. In honor of the rocket stage during an animals in space test, a rhesus monkey name Miss Able, flew with Miss Baker on May 28, 1959.