SM-65A Atlas

Atlas A (SM-65A)

Launch of Atlas 10A from LC-12 at CCAS
Function	Prototype ICBM
Manufacturer	Convair
Country of origin	United States
Launch history
Status	Retired
Launch sites	LC-12 & LC-14, CCAFS
Total launches	8
Successes	4
Failures	4
First flight	11 June 1957
Last flight	3 June 1958

The Convair SM-65A Atlas, or Atlas A, also designated X-11^[1] 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 produced a mere 135,000 pounds of thrust compared with the 360,000 pounds of the fully operational Atlas D.

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. 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 9800 feet (2900 meters) and tumbled end-over-end through its own exhaust trail until T+50 seconds when the Range Safety officer sent the destruct command.

Analysis of telemetry data confirmed that the Atlas had malfunctioned due to hot exhaust gases being recirculated into the thrust section, which burned through wiring and caused loss of thrust. 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.

On September 25, Missile 6A was launched. Aside from more instrumentation in the thrust section, it was identical to 4A and predictably met the same fate as once again, the thrust section overheated, causing wiring to burn through. 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 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 also changed to point outward and away from the missile instead of directly underneath it. The engine nozzles were covered with fiberglass insulation boots. Finally, the Atlas was modified so that helium from the propellant tanks could vent down into the thrust section and fill it with an inert gas, reducing the risk of fire.

The overheating problems had not shown up on the static firing tests of Missiles 1A-3A, however the FRF (Flight Readiness Firing) performed on the pad during prelaunch checks was a different situation in that the missile sat upright on the pad instead of on its side like it did in static tests. This would have caused exhaust gases to drift upward into the thrust section during FRFs, which meant that Missiles 4A and 6A probably already had damage to their internals at launch, the flight itself being the final coup-de-grace.

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. Failure of the guidance system tracking beacon at T+75 seconds was the only abnormality encountered on 12A's flight.

On January 10, 1958, Missile 10A launched and flew successfully, with no significant abnormalities occurring in the flight. This was also the first Atlas with functioning vernier engines, although they were not attached to the autopilot loop. On February 7, Missile 13A was launched, the first Atlas with full-up engines supplying 150,000 pounds of thrust and the vernier engines added to the autopilot loop. At T+108 seconds, the engines started oscillating in all three axes and at T+117 seconds, the B-1 engine shut down. One second later, the B-2 engine shut down. The Atlas broke up at T+163 seconds. The failure was attributed to a short in the vernier engine feedback transducer which caused the unexpected engine oscillation, but the actual loss of thrust was believed to be due to a turbopump failure. Two weeks later, Missile 11A was launched. This was the first flight where a roll program was added to the autopilot. Once again, the vernier feedback transducer shorted, leading to complete loss of control at T+109 seconds. The engines shut down starting at T+120 seconds and missile breakup occurred at T+126 seconds. Film and photographic evidence suggested that the loss of thrust was caused by a ruptured LOX duct.

Missile 15A was launched on April 5. Because aerodynamic heating was believed to have caused the electrical malfunctions on 13A and 11A, more insulation and resistors were added around the vernier wiring (the verniers were also added to the roll program for the first time). The flight was uneventful until T+96 seconds when a momentary drop in B-1 thrust occurred. Total engine shutdown occurred at T+105 seconds and the Atlas fell into the Atlantic Ocean, remaining structurally intact until impact. Postflight analysis concluded that a bearing in the LOX turbopump gearbox had come loose, resulting in shutdown of the pump and loss of thrust. The Atlas A program concluded with the flight of 16A on June 3. There were a few difficulties with the pneumatic system and several telemetry measurements malfunctioned, returning none or spurious data, however the missile completed its objectives satisfactorily. The pressure level in the turbopump gearbox was also modified slightly to prevent a recurrence of the malfunctions on 13A and 15A.^[2]

The Atlas A conducted eight test flights, of which four were successful. All launches were conducted from Cape Canaveral Air Force Station, at either Launch Complex 12 or Launch Complex 14.^[1]

Launch history

Date	Time (GMT)	Pad	Serial	Apogee	Outcome	Remarks
1957-06-11	19:37	LC-14	4A	2 km (1.2 mi)	Failure	First launch attempt of an Atlas vehicle. Performance was normal until T+30 seconds when the missile lost thrust due to exhaust gases being sucked back into the boattail and burning through wiring. Range Safety issued the destruct command at T+50 seconds. The flight was considered a "partial success" because the Atlas's balloon skin had maintained its structural integrity until vehicle destruction.
1957-09-25	19:57	LC-14	6A	3 km (1.9 mi)	Failure	Loss of thrust followed by vehicle tumbling and RSO destruct at T+74 seconds due to overheating that led to gas generator failure.
1957-12-17	17:39	LC-14	12A	120 km (75 mi)	Success	First successful flight of an Atlas missile.
1958-01-10	15:48	LC-12	10A	120 km (75 mi)	Success
1958-02-07	19:37	LC-14	13A	120 km (75 mi)	Failure	Short circuit in the guidance system caused engine shutdown and vehicle breakup at T+167 seconds.
1958-02-20	17:46	LC-12	11A	90 km (56 mi)	Failure	Short circuit in the guidance system caused engine shutdown and vehicle breakup at T+126 seconds.
1958-04-05	17:01	LC-14	15A	100 km (62 mi)	Failure	Turbopump failure caused loss of thrust at T+105 seconds. Vehicle remained structurally intact until impacting the Atlantic Ocean 200 miles downrange.
1958-06-03	21:28	LC-12	16A	120 km (75 mi)	Success

See also

• SM-65 Atlas

References

1. Encyclopedia Astronautica - Atlas A
2. http://www.drewexmachina.com/2015/06/11/first-atlas-tests/