The Atlas, first tested in 1959, was the United States' first successful ICBM (Intercontinental Ballistic Missile). It was a "1.5 stage", liquid-fueled (LOX and RP-1) rocket, with three engines producing 1,590 kN of thrust.
Though never used in combat, it was used as the expendable launch system for the Mariner space probes used to study Mercury, Venus, and Mars (1962–1973); and to launch ten of the Mercury program missions (1962–1963). The Mercury-Atlas missions resulted in the first American to orbit the earth (Lt. Col. John H. Glenn Jr) in February of 1962. (Major Yuri A. Gagarin, a Soviet cosmonaut, was the first human in orbit on April 12, 1961.) Atlas launched the Agena Target Vehicles used during the Gemini program. Direct Atlas descendants continue to be used as satellite launch vehicles into the 21st century.
Atlas was suggested for use by the United States Air Force in what became known as Project Vanguard. This suggestion was ultimately turned down, however, as Atlas would not be operational in time and was seen by many as being too heavily connected to the military for use in the U.S.' IGY satellite attempt.
Atlas, named for the Atlas of Greek mythology, got its start in 1946 with the award of a Army Air Forces research contract to Convair for the study of a 1,500 to 5,000 mi. (2,400 to 8,000 km) range nuclear armed missile. This was the MX-774 or Hiroc project. The contract was canceled in 1947 but the Army Air Forces allowed Convair to launch the three almost-completed research vehicles using the remaining contract funds. The three flights were only partially successful. However they did show that balloon tanks, and gimballed rocket engines were valid concepts.
Atlas is almost unique in its use of balloon tanks for fuel, made of very thin stainless steel with minimal or no rigid support structures. Pressure in the tanks provides the structural rigidity required for flight. An Atlas rocket will collapse under its own weight if not kept pressurized. The only other known use of balloon tanks at the time of writing is the Centaur high-energy upper stage.
Atlas also has a unique and somewhat odd staging system. Most rockets stage by dropping both engines and fuel tanks. Atlas drops just two of its three engines and no fuel tanks. Rockets using this technique are sometimes called stage and a half boosters. This technique is made possible by the extreme light weight of the balloon tanks. The tanks make up such a small percentage of the total booster weight that the weight penalty of lifting them to orbit is not offset by the technical and weight penalty required to throw half of them away mid-flight. Depending on how you look at it, this makes Atlas a single-stage-to-orbit booster (though most call it a 1.5 stage to orbit).
The newest version of Atlas, the Atlas V, is an Atlas in name alone as it contains little Atlas technology. It no longer uses balloon tanks nor 1.5 staging, but incorporates a rigid framework for its first stage booster much like the Titan family of vehicles. Ironically, given Atlas's origin as a military weapon, the Atlas III and Atlas V use Russian-designed engines.
Posigrade rockets fire for 1 s giving 15 ft/s (4.6 m/s) separation.
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5 Second Rate Damping
ASCS damps capsule rates for 5 seconds in preparation for turnaround maneuver.
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Turnaround Maneuver
Capsule (ASCS) system rotates capsule 180 degrees, to heat shield forward attitude. Nose is pitched down 34 degrees to retro fire position.
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Orbital Operations
Orbital operations and experiments for 3 orbits.
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Retro Sequence Start
Retrofire in 30 s. (ASCS) checks for proper retro attitude -34 deg pitch, 0 deg yaw, 0 deg roll.
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Retrofire
Three retro rockets fire for 10 seconds each. They are started at 5 second intervals, firing overlaps for a total of 20 s. Delta V of 550 ft/s (168 m/s) is taken off forward velocity.
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Retract Periscope
Periscope is automatically retracted in preparation for reentry.
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Retro Pack Jettison
One minute after retrofire retro pack is jettisoned, leaving heatshield clear.
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Retro Attitude Maneuver
(ASCS) orients capsule in 34 degress nose down pitch, 0 degrees roll, 0 degrees yaw.
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.05 G Maneuver
(ASCS) detects beginning of reentry and rolls capsule at 10 deg/s to stabilize capsule during reentry.
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Drogue Parachute Deploy
Drogue parachute deployed at 22,000 ft (6.7 km) slowing descent to 365 ft/s (111 m/s) and stabilizing capsule.
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Snorkel Deploy
Fresh air snorkel deploys at 20,000 ft (6 km). (ECS) switches to emergency oxygen rate to cool cabin.
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Main Parachute Deploy
Main parachute deploys at 10,000 ft (3 km). Descent rate slows to 30 ft/s (9 m/s).
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Landing Bag Deploy
Landing Bag Deploys, dropping heat shield down 4 ft (1.2 m).
