SM-65F Atlas

The SM-65F Atlas, or Atlas-F, was the final operational variant of the Atlas missile, only differing from the Atlas E in the launch facility and guidance package used. It first flew on 8 August 1961, and was deployed as an operational ICBM between 1961 and 1966. Following retirement as an ICBM, the Atlas-F, along with the Atlas-E, was refurbished for orbital launches as the Atlas E/F.^[1]

The Atlas E and F also differed in their launch facilities; Atlas E utilized the same coffin silos as Atlas D missiles, with the missile stored horizontally and raised upright for launch. Atlas F for comparison used a vertical silo with an elevator similar to the Titan I. The Atlas F was originally conceived when the Air Force decided that the coffin silos used for the Atlas D and E were too exposed and vulnerable to enemy attack.

Atlas E and F used the MA-3 propulsion system which had a separate gas generator for all three engines, unlike the Atlas D where one gas generator drove both booster turbopumps. In addition, they used pyrotechnic cartridges for rapid starting and the gas generators and pumps were mounted on each booster engine instead of being placed in the central thrust section. Total thrust of the MA-3 was 375,000 pounds.

Atlas E/F used the ARMA inertial guidance system rather than Atlas D's radio ground guidance; some IOC (Initial Operational Capability) Atlas D flights tested the ARMA system instead of the standard GE Mod II radio guidance. Aside from minor differences in the ARMA on the Atlas E and F, the major difference between the two consisted of differently-placed quick fill lines for the two silo systems used.

Most refurbished Atlas F space launches used solid-fueled upper stages, a notable exception being Missile 23F which launched Seasat, a NASA oceanography satellite, on June 27, 1978 using the last-ever Agena stage flown. The final Atlas F launch took place on June 23, 1981 when Missile 87F successfully placed a NOAA weather satellite into orbit. Atlas E/F space launchers were given an extensive tear-down and rebuild by Convair and the ARMA system replaced with the GE Mod II guidance system.

The first two Atlas F flights were Missile 2F on August 8, 1961 and Missile 4F on November 22, both from LC-11 at CCAS. Both tests were entirely successful, and on December 12, Missile 5F launched from LC-11. This test was a partial failure; an electrical short in the guidance system caused an early SECO command to be sent and the missile fell short of its planned range. On the 21st, Atlas 6F lifted off of LC-11 carrying a rhesus monkey named Scatback. This was the third launch of a primate on an Atlas in the past month - the first was Mercury-Atlas 5 which successfully lofted a chimpanzee on a three-orbit mission while the second, an Atlas E test, malfunctioned shortly after liftoff and had to be destroyed, claiming the life of the squirrel monkey it was carrying. Atlas 6F performed acceptably until after booster jettison when a hydraulic failure caused loss of sustainer thrust. The capsule separated and splashed down near Ascension Island in the South Atlantic, however the tracking beacon failed to operate and recovery crews could not locate it. They gave up the search and Scatback was officially lost at sea. The failure was traced to combustion of spent propellant following booster jettison which was damaging valves and/or ducting; it was solved by the installation of cutoff valves in the booster section.^[2]

So far, the Atlas F test program had gone well and nobody was prepared for the upcoming disaster on April 9, 1962 when Missile 11F exploded only one second after liftoff from LC-11. Subsequent investigation found that the sustainer LOX turbopump exploded due to the impeller blades rubbing against the metal casing and causing a spark that ignited the LOX. This failure mode also occurred in two static firing tests of the MA-3 engines, and it led to a plastic liner being added to the turbopump casing.^[3] Testing now began at Vandenberg AFB and Missile 15F flew successfully from 576-E on August 1. Nine days later, Missile 57F launched from OSTF-1 and failed to perform its roll maneuver. The pitch sequence was executed properly, but the lack of the roll program prevented the missile from attaining the correct trajectory and it was blown up by Range Safety 67 seconds into the launch. An electrical short in the programmer was found to be the cause, although the specific failure point was not determined. The roll signal had been extremely weak and barely enough to have any effect, as a consequence the missile's flight path was about 53° to the right of the planned trajectory.^[4] The next test launch took place from the now-repaired LC-11 at the Cape two days later and all subsequent R&D flights went without a hitch except for 13F on November 14 which experienced a thrust section fire at liftoff that led to sustainer shutdown. The booster engines retained attitude control until BECO, at which point the absence of sustainer thrust resulted in immediate missile tumbling and loss of the mission. The cause of the thrust section fire was unknown, however a fuel leak was suspected. The next launch, Missile 21F, successfully concluded the R&D phase of the Atlas F program.

Atlas F tests achieved a high degree of success during 1963, and seven flights took place through the year. Missile 52F (March 24) exploded 91 seconds after launch, however the reason for the failure was not determined because the vehicle was intended to demonstrate flying an Atlas ICBM to the maximum possible range and thus it did not carry a telemetry system.

Operational tests of the Atlas F were mostly successful, but two flights from Vandenberg (Atlas 45F in October 1963 and Atlas 3F in April 1964) suffered stuck engine valves and tipped over at liftoff. Both failed launches were from OSTF-2, which was repaired in only two months following Atlas 45F. Atlas 3F put the silo out of use for six months. In both cases, the failure was found to be accumulated hypergol residue in the main fuel valve from previous engine firings. On January 8, 1965, the final Atlas ICBM test, Missile 106F, successfully launched from OSTF-576G. That year, the Atlas was retired from service as an ICBM and remaining missiles used for suborbital and orbital launches over the next few decades. This included tests of ABRES, RMP, and BMRS reentry vehicles at Vandenberg. The OSTF silos were converted into dedicated launch pads for these and Nike/Zeus target missile testing (the latter were never launched with Atlas Fs). Sixty-five such tests took place between 1965 and 1974, roughly half of which used Atlas Fs. Some had a Trident solid upper stage and there were several failures, the most notable being a 1967 test where the Atlas's RSO charges were ignited by static electricity at RV separation, after which cork insulation was added to the RV to prevent a recurrence of this failure mode.

There were two major failures during the 20 years as a satellite launcher. One was an attempted launch of an unknown military test payload in April 1975 on Atlas 71F. A glob of LOX/kerosene gel fell into the flame pit at ignition, exploded, and damaged the Atlas's sustainer engine. The launch vehicle continued to operate for over two minutes, but the sustainer and verniers eventually shut down and the range safety destruct command was issued at T+303 seconds. Since the LOX gel explosion also damaged wiring in the base of the booster, the telemetry system lost power and no data was returned during launch, making it difficult to determine the exact cause of the engine failure. It was believed that damage to a piece of plumbing was the most likely explanation.

The other was a launch of a NOAA weather satellite on May 29, 1980 using Atlas 19F. Performance appeared normal until booster jettison when the trajectory was discovered to be off-course. The sustainer engine kept burning 50 seconds past its normal cutoff time. When the satellite finally separated, it was inserted into a useless elliptical orbit.

The failure was caused by a seal jostled loose by the rapid-fire pyrotechnic cartridge ignition system used in the Atlas E/F missiles, flooding the B-1 turbopump with fuel and slowing down its rotation speed, cutting thrust levels in the engine to 80% and considerably reducing booster velocity and fuel consumption. The Atlas's onboard computer tried to compensate first by extending the booster engine burn time, but eventually a backup command forced booster shutdown and jettison. After BECO, the computer still tried to compensate by extending sustainer burn time until it went nearly a minute past what would have been normal cutoff. Following SECO, the booster also had to execute vernier solo mode for another 11 seconds. However, for simplicity reasons, the NOAA satellite had no electrical interface with the Atlas which caused it to fire is posigrade rockets at a preset time. Since it was still attached to the Atlas, it was impossible for the small thrusters to align the spacecraft correctly and thanks to the extended burn time, SECO would now take place at T+377 seconds, but at T+370 seconds, the timer on the NOAA automatically activated the solid fueled kick motor with the Atlas's sustainer engine still operating. The LOX tank dome was then blown off the Atlas, which registered on telemetry readouts as an instant loss of tank pressure. The NOAA separated, but ended up in a useless orbit and had to be abandoned.

Atlas F launches were conducted from Cape Canaveral Air Force Station, at Launch Complexes 11 and 13, and Vandenberg Air Force Base at OSTF-2, LC-576 and SLC-3.^[1] A total of 55 Atlas F missile tests were conducted between 1961 and 1974, the post-1965 launches all being tests of reentry vehicles as Atlas had been retired from ICBM service, along with 39 space launches during 1967 to 1981. There were 16 failures.

See also

• SM-65 Atlas

References