Delta rocket
| Delta Family | |
|---|---|
 |
|
| The Delta rocket family. | |
| Role | Expendable launch system with various applications |
| Manufacturer | United Launch Alliance |
| Introduction | 1960 |
| Status | active |
Delta is an American versatile family of expendable launch systems that has provided space launch capability in the United States since 1960. There have been more than 300 Delta rockets launched, with a 95% success rate. Two Delta launch systems – Delta II and Delta IV – are still in use, though the Delta II will soon be retired. Delta rockets are currently manufactured and launched by the United Launch Alliance.
The original Delta rockets used a modified version of the PGM-17 Thor, the first ballistic missile deployed by the United States Air Force, as their first stage. The Thor had been designed in the mid-1950s to reach Moscow from bases in Britain or similar allied nations, and the first wholly successful Thor launch had occurred in September 1957. Subsequent satellite and space probe flights soon followed, using a Thor first stage with several different upper stages. The fourth upper stage used on the Thor was the Thor "Delta," delta being the fourth letter of the Greek alphabet. Eventually the entire Thor-Delta launch vehicle came to be called simply, "Delta."
The Delta name stems from its position as the fourth, or D version, of the Thor based rocket combination. The vehicle has been known both as Thor-Delta and simply Delta.
NASA intended Delta as "an interim general purpose vehicle" to be "used for communication, meteorological, and scientific satellites and lunar probes during '60 and '61". The plan was to replace Delta with other rocket designs when they came on-line. From this point onward, the launch vehicle family was split into civilian variants flown from Cape Canaveral which bore the Delta name and military variants flown from Vandenberg Air Force Base which used the more warlike Thor name. The Delta design emphasized reliability rather than performance by replacing components which had caused problems on earlier Thor flights, in particular the trouble-prone AC Spark Plug inertial guidance package which was replaced by a radio ground guidance system mounted to the second stage rather than the Thor itself. NASA let the original Delta contract to the Douglas Aircraft Company in April 1959 for 12 vehicles of this design:
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