SM-65C Atlas

The SM-65C Atlas, or Atlas C was a prototype of the Atlas missile. First flown on 24 December 1958, the Atlas C was the final development version of the Atlas rocket, prior to the operational Atlas D. It was originally planned to be used as the first stage of the Atlas-Able rocket, but following an explosion during a static test on 24 September 1959, this was abandoned in favor of the Atlas D. Atlas C was similar to Atlas B, but had a larger LOX tank and smaller RP-1 tank due to technical changes to the Rocketdyne engines. Improvements in materials and manufacturing processes also resulted in lighter-weight components than the Atlas A and B. Booster burn time was much longer than the A/B series, up to 151 seconds. All launches took place from LC-12 at CCAS.

The Alas C test program began with the successful flight of 3C on December 24, 1958. All systems performed well and the extended booster burn time was carried out with no ill effects. On January 27, Missile 4C experienced a complete Mod III GE guidance system failure at T+80 seconds. The propellant utilization system operated fuel rich, resulting in low sustainer thrust following BECO, and LOX depletion caused simultaneous sustainer/vernier cutoff 5 seconds earlier than the planned SECO. In addition, a malfunction of the pneumatic system caused decay of fuel tank pressure starting at T+120 seconds. Tank pressure remained high enough to maintain structural integrity through powered flight, the intermediate bulkhead possibly ruptured at T+320 seconds, at which point tank pressures had dropped below a safe limit. Because of the guidance system failure, no separation signal was received by the reeentry vehicle. Impact occurred about 40 miles short of the target point in the South Atlantic.^[1]

Missile 5C (February 20) performed well until BECO, at which point the fuel staging disconnect valve failed, causing a gradual fuel leak and decay in tank pressures. When propellant levels in the tank dropped low enough, the open staging disconnect valve allowed helium pressure gas to escape, causing a more rapid pressure loss. At approximately T+168 seconds, the intermediate bulkhead reversed, followed by explosion of the missile at T+172 seconds.^[2]

Missile 7C (March 19) suffered a guidance system failure at T+85 seconds, followed by premature BECO at T+129 seconds. A backup command from the missile programmer jettisoned the booster section at T+151 seconds. After the premature booster cutoff, the missile became unstable because it was impossible for the autopilot to gimbal the sustainer engine with the booster section still attached. Missile stability was partially regained after booster jettison, then completely lost after SECO. No cutoff command was issued to the sustainer or verniers because of the guidance system failure, sustainer cutoff occurred at T+282 seconds, but the exact reason for it was not determined. During the vernier solo phase, the missile started tumbling. VECO took place at T+311 seconds when the vernier start tanks became depleted. Reentry vehicle separation also did not occur.^[3]

The final two C-series flights (8C on July 21 and 11C on August 24) were successful. Missile 8C was the third attempt to fly an RVX-2 reentry vehicle (the second attempt on a D-series Atlas had failed three months earlier) and the first successful one. All missile systems performed well aside from high thrust section temperatures starting at T+85 seconds. Sixty-three minutes after launch, the RV was successfully recovered.^[4] Missile 11C carried a movie camera in the nose cone which filmed missile separation and a large portion of the Earth's surface on a 250-mile (402 km) lob, taking it to an apogee of 700 miles (1126 km). Recovery of the film capsule was successful. Overall performance of 11C was quite good, the sustainer HS valve malfunctioned and resulted in reduced LOX flow to the engine, this resulted in low sustainer thrust and complete fuel depletion by SECO, and it was suspected that a leak in a LOX duct had affected the HS valve. The high thrust section temperatures on 8C also recurred, in addition tracking film showed debris falling off the missile between T+46 and 54 seconds, the debris was not identified and did not appear to have any adverse effect on vehicle performance.^[5]

One of the more significant upgrades to the Atlas C was the addition of motion detectors in the gyroscope package to ensure proper operation. This was implemented after the first B-series Atlas had failed in flight due to launch crews neglecting to power on the gyroscopes and would soon become a standard part of all ballistic missile guidance systems.

Missile 9C was designated for the first Atlas-Able lunar probe launch, which was scheduled to launch on October 2, 1959 from LC-12 at Cape Canaveral. The vehicle was similar to Missiles 10B and 13B in that it had several weight saving modifications as well as modified vernier start tanks.

NASA had wanted to use the operational Atlas D rather than the developmental Atlas C, but there were no available Atlas D missiles for the Able program, so instead they were forced to use an Atlas C.

On September 24, the Atlas was prepared for a planned 12-minute PFRT (Pre-Flight Readiness Test). The satellite was not on the launch vehicle, the third stage was a dummy, and the second stage was unfueled. After 2.5 seconds of engine operation, a fire erupted in the Atlas's thrust section. Fed by leaking liquid oxygen, it proved impossible to put out with pad extinguishers and soon a huge inferno was raging on LC-12. Thirty-seven seconds after the fire began, the Atlas toppled over in a massive fireball, the biggest explosion seen at Cape Canaveral up to that point. The pad was severely damaged, with both umbilical towers totally destroyed, the concrete launch stand caved in, and the service tower knocked over. It would take months of repairs to get it back online. The final explosion was believed to be the result of loss of tank pressure. Engine shutdown caused the propellant valves to snap shut, causing overpressurization of the propellant tanks. Ground operators in the blockhouse flipped manual override switches to lower RP-1 pressure and increase LOX pressure, but the missile did not respond, possibly due to fire-induced damage to the control wiring on the pad. The pneumatic control system automatically threw open the LOX boil-off valve to vent tank pressure, which apparently continued until the tanks lost support pressure to the point where their structural integrity failed. At around 35 seconds, the intermediate bulkhead collapsed, causing the LOX to fall into the RP-1 tank and mix. The propellants turned into a gel that detonated with the force of 20,000 pounds (9070 kg) of TNT and resulted in an extremely powerful explosion that caused extensive pad damage.

Investigators concluded that the disaster was due to the above-mentioned configuration change on the Atlas C, in addition to several weight-saving modifications unique to Missile 9C. When the Atlas was assembled at Convair, workmen attached a helium vent line to a port near the bottom of the RP-1 tank, below the anti-slosh baffles. On the Atlas B missiles, the vernier helium tank was mounted in a higher location resulting in a different fuel tank attachment point above the baffles. Helium pressure gas from the vernier propellant tanks leaked into the sustainer RP-1 turbopump, leading to cavitation which caused propellant unloading, pump overspeed, and rupture of the propellant lines. This then caused the fire that led to vehicle destruction. It was not clear whether the fire had been started by rupture of a LOX duct due to sudden pressure change from the helium venting or rubbing of the turbopump blades against the metal casing due to overspeed caused by the pump running without propellant in it. The accident was ultimately ruled to be the result of poor engineering judgement in attaching the vernier helium vent line to the bottom of the RP-1 tank.

Six flights were made. These were all sub-orbital test flights of the Atlas as an Intercontinental Ballistic Missile, with three tests succeeding, and three failing.

All Atlas C launches were conducted from Cape Canaveral Air Force Station, at Launch Complex 12.

Launch history

See also

• SM-65 Atlas
• Able (rocket stage)

References

• Encyclopedia Astronautica - Atlas C

1. ↑ "Flight Test Evaluation Report, Missile 4C February 15, 1959"
2. ↑ "Flight Test Evaluation Report, Missile 5C March 6, 1959"
3. ↑ "Flight Test Evaluation Report, Missile 7C April 3, 1959"
4. ↑ "Flight Test Evaluation Report, Missile 8C August 3, 1959"
5. ↑ "Flight Test Evaluation Report, Missile 11C September 5, 1959"