Launch sequence of an Atlas D ICBM test, April 22, 1960
Expendable launch system
|Country of origin||United States|
|Launch sites||LC-11, 12, 13 & 14, CCAFS
|First flight||April 14, 1959|
|Last flight||November 7, 1967|
The SM-65D Atlas, or Atlas D, was the first operational version of the U.S. Atlas missile. Atlas D was first used as an intercontinental ballistic missile (ICBM) to deliver a nuclear weapon payload on a suborbital trajectory. It was later developed as a launch vehicle to carry a payload to low Earth orbit on its own, and later to geosynchronous orbit, to the Moon, Venus, or Mars with the Agena or Centaur upper stage.
The fully operational D-series Atlas was similar to the R&D model Atlas B and C, but incorporated a number of design changes implemented as a result of lessons learned during test flights. In addition, the D-series had the full-up Rocketdyne MA-2 propulsion system with 360,000 pounds of thrust versus the 275,000 pounds of thrust in the Atlas B/C's engines. The B and C series had a common pump assembly feeding both booster engines, but the Atlas D had separate pumps on each engine, both still driven by a common gas generator. Operational Atlas D missiles retained radio ground guidance aside from a few R&D launches which tested the inertial guidance system designed for the Atlas E/F, and the Atlas D would be the basis for most space launcher variants of Atlas.
The Atlas D testing program began with the launch of Missile 3D from LC-13 on April 14, 1959. Engine startup proceeded normally, but it quickly became apparent that the LOX fill/drain valve had not closed properly. LOX spilled down the side of the missile, followed by leakage from the RP-1 fill/drain valve. The propellants mixed and exploded on the launch stand. Because of the open LOX fill/drain valve, the Atlas's propellant system suffered a loss of fuel flow and pressure that caused the B-2 engine to operate at only 65% thrust. Due to the imbalanced thrust, the Atlas lifted at a slanted angle, which also prevented one of the launcher hold-down arms from retracting properly, although subsequent review of film showed that the hold-down arm did not damage the missile, which took off with a trail of LOX streaming out of the fill/drain valve. Aside from the loss of B-2 thrust, LOX tank pressure steadily decreased during ascent. The flight control system managed to retain missile stability until T+26 seconds when unstable combustion from the imbalanced fuel mixture caused an explosion in the booster fuel feed system. The booster section ripped away from the Atlas, which then pitched over and sank backwards through its own trail of fire until the Range Safety destruct command was issued at T+35 seconds. The sustainer and vernier engines continued operating until destruction. The failure was attributed to electro-mechanical glitches in the propellant fill/drain valves, which differed from the ones used on the Atlas A/B/C and were designed to accommodate silo use. Space launcher versions of the Atlas D would use the Atlas C fill/drain valve in the interest of simplicity and reliability. On the positive side, all other hardware in Missile 3D had functioned well and there were no problems during the brief flight that were not directly caused by the LOX leak.
On May 18, Atlas 7D was prepared for a night launch of an RVX-2 reentry vehicle from LC-14, the second attempt to fly one after the launch of a C-series Atlas had miscarried two months earlier. The test was conducted with the Mercury astronauts in attendance in order to showcase the vehicle that would take them into orbit, but 60 seconds of flight ended in another explosion, prompting Gus Grissom to remark "Are we really going to get on top of one of those things?". This failure was traced to improper separation of the right launcher hold-down pin, which damaged the B-2 nacelle structure and caused helium pressurization gas to escape during ascent. At 62 seconds into the launch, the pressure in the LOX tank exceeded the pressure in the RP-1 tank, which reversed the intermediate bulkhead. Two seconds later, the missile exploded. Film review confirmed that the hold-down pin on the right launcher arm failed to retract at liftoff and was jerked from the missile. The resultant force caused a four-inch gap in the B-2 nacelle structure which also damaged low pressure helium lines. The hold-down pin had not retracted due to a sheared retaining bolt in the bell crank pulley system in the right launcher arm. Once again, all other systems in the Atlas functioned well and there were no problems not directly attributable to the launcher malfunction.
Atlas 5D (June 6) performed well until booster separation at which point the fuel staging disconnect valve malfunctioned, leading to loss of fuel tank pressure, reversal of the intermediate bulkhead, and missile self-destruction at T+157 seconds. On July 29, Missile 11D was launched and included a number of modifications designed to correct problems on the previous Atlas D tests. The flight was mostly successful, but some difficulties with the hydraulic system occurred due to low engine compartment temperatures caused by a probable LOX leak. After the flight of 14D in August, the Atlas D was declared operational. Subsequent tests in the fall and winter all performed well including the first Atlas launch from Vandenberg Air Force Base on September 9, although the launch of a boilerplate Mercury capsule on Atlas 10D (Big Joe) in September was a partial success because the booster engines failed to separate. Missile 26D on October 29 lost a vernier engine at booster jettison, the resultant loss of roll control causing it to impact only 600 miles downrange instead of the planned 1100. The high degree of success in late 1959-early 1960 boosted morale significantly after six failures in the first half of the year.
Because of growing confidence in the Atlas, it was decided to abandon PFRF (Pre-Flight Readiness Firing) tests except for the first handful of Atlas E flights as well as space launches. The final test of 1959, Missile 40D on December 19, utilized a "dry" start method (engine igniter activated before propellant injection). This experiment worked without any apparent problems.
On March 5, 1960, Missile 19D was undergoing a propellant loading exercise at 576-A2 at VAFB when a fuel leak started a fire on the pad that led to the explosion of the missile. The launch facility was written off due to the damage and not used again for almost 5 years.
On March 8, 1960, Missile 44D launched from LC-11 on the first test of the AIG (All Inertial Guidance System) and experienced a 90° roll transient at liftoff. The AIG managed to correct this problem and the missile completed a successful 3000 mile lob downrange.
With this string of successful Atlas tests, including twin launches from Cape Canaveral and VAFB within hours of each other on January 26–27, 1960, program officials were lulled into a sense of security that rudely ended on March 11, 1960 when Atlas 51D lifted from LC-13. The B-1 engine suffered combustion instability which caused loss of thrust within two seconds of liftoff. An explosion in the thrust section at T+4 seconds ruptured the propellant tanks and caused the missile to fall back onto the pad in a huge fireball. The Atlas went in for a repeat performance on April 8 when Missile 48D, launched from LC-11 and intended as the first closed-loop test of the AIG (All Inertial Guidance System), experienced combustion instability again, this time in the B-2 engine. The first indication of trouble was a pressure surge in the B-2 combustion chamber, followed by unstable thrust, engine shutdown, and an explosion that started a thrust section fire. The B-1 engine then shut down, followed by the sustainer and verniers. Since the propulsion system had not attained sufficient thrust, the launcher hold-down mechanism did not release the missile, which stayed in place and burned on the pad. The thrust section fire slowed down 15 seconds after the attempted launch, then resumed around 45 seconds. At 60 seconds, the Atlas was completely destroyed when the propellant tanks exploded.
Postflight analysis of the back-to-back failures found that in each case, the missile had fallen victim to rough combustion in one booster engine, which destroyed the LOX injector head and started a thrust section fire. In both missiles, the rough combustion cutoff sensor in the B-1 engine failed to operate. On 48D, the rough combustion did not occur in that engine and the lack of RCC cutoff was not a problem (B-1 thrust was terminated instead by the turbopump overspeed sensor). The B-2 RCC sensor operated correctly and terminated thrust before liftoff could be achieved. On 51D, it resulted in the B-1 continuing to operate until the missile lifted, resulting in a destructive pad fallback. The exact reason for the rough combustion was unclear, although it had occurred over a dozen times in static firing tests of the MA-2 engines. However, it was noted that the separate exhaust duct for the gas generator vent pipe had been removed from both LC-11 and LC-13 after engineers decided that it was unnecessary and impeded removal and installation of protective covers on the pipe during ground testing. It could not be determined with certainty if the lack of an exhaust duct had anything to do with the failures, and in any case, camera coverage did not offer any evidence in support of this theory. However, it was decided to put the exhaust duct back on the Atlas pads at CCAS in order to comply with the configuration of operational Atlas missile silos, and as a "just in case" measure. Adjustments to the insulation boots on both missile was also ruled out as a probable cause of the failures. Aside from re-installing the exhaust duct, camera coverage of the flame deflector pit at ignition would also be increased. Two launch facilities were now in need of repair. LC-13 was severely damaged by the fallback of 51D and would not be used again for six months, while damage to LC-11 was less extensive and repairs were completed in only two months. Attention shifted to LC-12 where Atlas 56D flew over 9000 miles with an instrumented nose cone, impacting the Indian Ocean.
After the back-to-back pad explosions, it was decided to go back to using a wet start (propellants injected into the combustion chamber prior to ignition) on the Atlas rather than the failed experiment of a dry start to ensure smoother engine startup. Missile 60D (July 2) returned to the dry start method, this time with a hold down period to check for combustion instability rather than immediately releasing the missile upon full thrust. While no on-pad explosion resulted on this attempt, the Atlas failed to complete all of its test objectives when the vernier start tanks were inadvertently vented and refilled several times during the flight. This resulted in depletion of control helium and loss of sustainer and vernier thrust near the end of powered flight, and so the Mark III Mod 1B reentry vehicle landed some 40 miles short of its intended target point. An electrical glitch was believed to have caused the unplanned start tank venting, although the specifics of could not be determined.
Atlas D tests on the West Coast hit a series of snags in the following months as well. Atlas 23D (May 6) lifted from 576B-1, a coffin silo, at VAFB and began experiencing abnormal pitch gyrations within 10 seconds of launch. After about 20 seconds, the missile started tumbling out of control upon which the RSO sent the destruct command. The next flight, 74D (July 22) broke up 70 seconds into launch due to a failure of the pitch gyro. Missile 33D (September 29) failed to stage its booster section when the staging electrical disconnect plug pulled out at T+125 seconds. 81D (October 13) failed when the LOX pressure sensor malfunctioned, causing the pneumatic system to overpressurize the LOX tank with helium until it ruptured the intermediate bulkhead and destroyed the missile at T+72 seconds. The problem was eventually traced to incorrectly installed umbilicals in the 576-B silos, which pulled out prematurely during the launches of the four failed Atlases listed above plus Missile 99D (the final Atlas launch of 1960) and shorted out components in the missiles. Repair work was performed on the 576-B silos which resumed launches in May 1961.
Atlas 71D on October 13 carried three mice and other experiments in a biological nose cone which successfully completed a 5000 mile lob downrange from LC-11 at the Cape. This missile utilized a dry start method without any hold-down time at liftoff with no apparent ill effects and all airborne systems performed well aside from an unexplained decrease in B-1 and sustainer thrust a few seconds before BECO. Cameras mounted on the nose cone photographed the spent Atlas after capsule separation.
Most failures after this point were high altitude or partial, and Atlas 90D's successful launch on January 23, 1961 concluded the R&D phase of the Atlas D program.
Most Atlas D launches were sub-orbital missile tests; however several were used for other missions, including orbital launches of manned Mercury, and unmanned OV1 spacecraft. Two were also used as sounding rockets as part of Project FIRE. A number were also used with upper stages, such as the RM-81 Agena, to launch satellites.
The Atlas D was deployed in limited numbers as an ICBM due to its radio guidance while the fully operational E and F-series missiles had inertial guidance packages and a different ignition system that allowed faster engine starts.
Atlas Ds used for space launches were custom-built for the needs of the mission they were performing, but when the Atlas was retired from missile service in 1965, Convair introduced a standardized Atlas vehicle (the SLV-3) for all space missions. Remaining D-series missiles were flown until 1967 for suborbital tests of reentry vehicles and a few space launches.
A total of 116 D-series missiles (not including vehicles used for space launches) were flown from 1959-67 with 26 failures.
The warhead of the Atlas D was originally the G.E. Mk 2 "heat sink" re-entry vehicle (RV) with a W49 thermonuclear weapon, combined weight 3,700 lb (1,680 kg) and yield of 1.44 megatons (Mt). The W-49 was later placed in a Mk 3 ablative RV, combined weight 2,420 lb (1,100 kg) The Atlas E and F had an AVCO Mk 4 RV containing a W-38 thermonuclear bomb with a yield of 3.75 Mt which was fuzed for either air burst or contact burst. The Mk 4 RV also deployed penetration aids in the form of mylar balloons which replicated the radar signature of the Mk 4 RV. The Mk 4 plus W-38 had a combined weight of 4,050 lb (1,840 kg).
- "Flight Test Evaluation Report, Missile 3D." Convair, 29 April 1959
- "Flight Test Evaluation Report, Missile 7D." Convair, 3 June 1959
- "Flight Test Evaluation Report, Missile 44D." Convair, 29 March 1960
- "Flight Test Evaluation Report, Missile 48D." Convair, 28 April 1960
- "Flight Test Evaluation Report, Missile 60D. Convair, 15 July 1960"
- "Flight Test Evaluation Report, Missile 71D." Convair, 29 October 1960
- Encyclopedia Astronautica - Atlas