The Convair SM-65A Atlas, or Atlas A, was the first full-scale prototype of the Atlas missile, which first flew on 11 June 1957. Unlike later versions of the Atlas missile, the Atlas A did not feature the stage and a half design. Instead, the booster engines were fixed in place, and the sustainer engine was omitted. The propulsion system used on the initial Atlas As was an early version of the Rocketdyne MA-1 engines with conical thrust chambers that produced a mere 135,000 pounds of thrust, compared with the 360,000 pounds of the fully operational Atlas D. Several pieces of hardware found on the operational Atlas were either missing on the A-series or only partially implemented. Powered flight on the A-series would last about two minutes and compared to later Atlases, long pad hold-down times, with up to 11 seconds between engine start and launcher release. The first three Atlases built were used merely for static firing tests with Missile 4A being the first flight article. It was delivered to Cape Canaveral in December 1956 and erected on LC-14 in March 1957, where it sat until the following summer.
Test history
On June 11, 1957, the Atlas made its maiden voyage. Engine start proceeded normally and the launcher release system also functioned properly. All went well until T+26 seconds when the B-2 engine lost thrust, followed two seconds later by the B-1 engine. The Atlas reached a peak altitude of 9,800 feet (3,000 meters) and tumbled end-over-end through its own exhaust trail until T+50 seconds when the range safety officer sent the destruct command. During 4A's launch, thousands of spectators lined the beaches around Cape Canaveral to watch, although the Air Force did not confirm that the new missile was in fact an Atlas. The sensational press reports of the missile cartwheeling and exploding in mid-air belied the fact that program officials did not really consider the test a failure, and that all things considered, the Atlas had performed far better than expected. Analysis of telemetry data confirmed that the Atlas had malfunctioned due to hot exhaust gases being recirculated into the thrust section, which apparently caused failure of propellant ducting and engine shutdown due to LOX starvation. The pneumatic system also malfunctioned as tank pressure never properly transitioned to in-flight levels and along with propellant flow and pressure steadily decreased during ascent. The flight was considered a partial success because the missile had otherwise performed well. In particular, the Atlas's inflated balloon structure, which engineers doubted would even fly at all, had held together as the missile tumbled. The flight control system also worked well as it tried in vain to correct the missile's flight path. Convair engineers decided that the Atlas needed a heat shield in the thrust section more substantial than the thin fiberglass one included on the missile. They proposed a modified heat sink made from steel and fiberglass, but the Air Force rejected that idea as the shield would be extremely heavy and also complicate booster section staging on operational Atlases. As one small modification, the pneumatic system was modified to vent inert helium gas down into the thrust section to reduce the risk of fire. On September 25, Missile 6A was launched. Aside from more instrumentation in the thrust section and the above-mentioned helium vent modification, it was identical to 4A and predictably met the same fate as once again, the thrust section overheated. Thrust levels in both engines dropped to only 35% at T+32 seconds and two seconds later, the propulsion system completely shut down. The Range Safety destruct command was sent at T+63 seconds. This time, overheating and high vibration levels had caused a LOX regulator to fail, resulting in gas generator flameout. After this debacle, the Air Force relented and accepted the need for an improved heat shield. Other modifications were made as well, including removal of the long skirt covering the boattail and engine nozzles. The gas generator vent pipe was changed to point outward and away from the missile instead of directly underneath it. The engine nozzles were covered with fiberglass insulation boots and aluminum plumbing in the Atlas changed to steel plumbing which had a greater heat tolerance. The autopilot received additional filters to dampen vibration levels. The overheating problems had not shown up on the static firing tests of Missiles 1A-3A, but it was later revealed that the engineering crews at the Sycamore test stand had had the plumbing changed to steel because it reduced the risk of overheating compared with the aluminum plumbing on flight article missiles. The PFRF (Pre Flight Readiness Firing) tests conducted on 4A and 6A also would have caused exhaust gases to go up into the boattail, and thus they probably already had internal damage at launch. On December 17, Missile 12A lifted from LC-14. The modified boattail worked; the Atlas performed well on its first successful launch, an event that raised morale after the devastating blow of two Soviet space launches and the failure of Vanguard a week earlier. This was the first Atlas with a functional guidance system (although open loop—the guidance system was included merely for evaluation purposes and not actually added to the flight program until the B-series tests), as Missiles 4A and 6A merely had a dummy guidance receiver that test patterns were transmitted to. At T+75 seconds, the guidance system tracking beacon shorted, which caused a momentary large drain on the batteries but did not otherwise affect missile performance. The missile's flight trajectory in the yaw axis by the time of Booster Engine Cutoff (BECO) deviated by about 10,000 feet (3,000 meters) from the planned program, studies of trajectory data showed this anomaly to be nearly constant during powered flight. It was concluded that there had been an incorrect offset in the yaw axis, either from a misaligned engine, yaw gyro, or some combination of both. After the success of the launch, the Air Force acknowledged for the first time that the missile was in fact an Atlas.
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