An Atlas D LV-3B launching Mercury-Atlas 6
|Function||Manned expendable launch system|
|Country of origin||United States|
|Height||28.7 metres (94.3 ft)|
|Diameter||3.0 metres (10.0 ft)
width over boost fairing 4.9 metres (16 ft)
|Mass||120,000 kilograms (260,000 lb)|
|Payload to LEO||1,360 kilograms (3,000 lb)|
|Launch sites||CCAFS LC-14|
|First flight||29 July 1960|
|Last flight||15 May 1963|
|Thrust||1,300 kilonewtons (300,000 lbf)|
|Burn time||134 seconds|
|Diameter||3.0 metres (10.0 ft)|
|Thrust||300 kilonewtons (67,000 lbf)|
|Burn time||5 minutes|
The Atlas LV-3B, Atlas D Mercury Launch Vehicle or Mercury-Atlas Launch Vehicle, was a man-rated expendable launch system used as part of the United States Project Mercury to send astronauts into low Earth orbit. It was derived from the SM-65D Atlas missile, and was a member of the Atlas family of rockets.
The Atlas D missile was the natural choice for Project Mercury since it was the only launch vehicle in the US arsenal that could put the spacecraft into orbit and also had a large number of flights to gather data from. In addition, the Atlas's stage-and-a-half design with all engines firing at liftoff and no upper stages made it easier to verify and test that all systems were operating correctly prior to launch. But its reliability was far from perfect and Atlas launches ending in explosions were an all-too common sight at Cape Canaveral. Thus, significant steps had to be taken to man-rate the missile and make it safe and reliable unless NASA wished to spend several years developing a dedicated launch vehicle for manned programs or else wait for the next-generation Titan II ICBM to become operational.
Shortly after being chosen for the program in the spring of 1959, the Mercury astronauts were taken to watch the second D-series Atlas test, which exploded a minute into launch. This was the fifth straight complete or partial Atlas failure and the booster was at this point nowhere near reliable enough to carry a nuclear warhead or an unmanned satellite, let alone a human passenger. Plans to man-rate Atlas were effectively still on the drawing board and Convair estimated that 75% reliability would be achieved by early 1961 and 85% reliability by the end of the year.
Aside from the modifications described below, Convair set aside a separate assembly line dedicated to Mercury-Atlas vehicles which was staffed by personnel who received special orientation and training on the importance of the manned space program and the need for as high quality workmanship as possible. Components used in the Mercury-Atlas vehicles were given thorough testing to ensure proper manufacturing quality and operating condition, in addition components and subsystems with excessive operating hours, out-of-specification performance, and questionable inspection records would be rejected. All components approved for the Mercury program were earmarked and stored separately from hardware intended for other Atlas programs and special handling procedures were done to protect them from damage.
Propulsion systems used for the Mercury vehicles would be limited to standard D-series Atlas models of the Rocketdyne MA-2 engines which had been tested and found to have performance parameters closely matching NASA's specifications.
All launch vehicles would have to be complete and fully flight-ready at delivery to Cape Canaveral with no missing components or unscheduled modifications/upgrades. After delivery, a comprehensive inspection of the booster would be undertaken and prior to launch, a flight review board would convene to approve each booster as flight-ready. The review board would conduct an overview of all prelaunch checks, hardware repairs/modifications, and reviews of Atlas launches over the previous few months, both in the Mercury program and unmanned NASA/Air Force programs, to see if any launch failures had occurred that involved components or procedures relevant to Project Mercury.
The NASA Quality Assurance Program meant that each Mercury-Atlas vehicle took twice as long to manufacture and assemble as an Atlas designed for unmanned missions and three times as long to test and verify for flight.
Central to these efforts was the development of the Abort Sensing and Implementation System (ASIS), which would detect malfunctions in the Atlas's various components and trigger a launch abort if necessary. Added redundancy was built in; if ASIS itself failed, the loss of power would also trigger an abort. The system was tested on a few Atlas ICBM flights prior to Mercury-Atlas 1 in July 1960, where it was operated open-loop (MA-3 in April 1961 would be the first closed-loop flight).
The Mercury LES used on Redstone and Atlas launches was identical, but the ASIS system varied considerably between the two boosters as Atlas was a much larger, more complex vehicle with five engines, two of which were jettisoned during flight, a more sophisticated guidance system, and inflated balloon tanks that required constant pressure to not collapse.
Atlas flight test data was used to draw up a list of the most likely failure modes for the D-series vehicles, however simplicity reasons dictated that only a limited number of booster parameters could be monitored. An abort could be triggered by the following conditions, all of which could be indicative of a catastrophic failure:
- The booster flight path deviated too far from the planned trajectory
- Engine thrust or hydraulic pressure dropped below a certain level
- Propellant tank pressure dropped below a certain level
- The intermediate tank bulkhead showed signs of losing structural integrity
- The booster electrical system ceased operating
- The ASIS system ceased operating
Some failure modes such as an erroneous flight path did not necessarily pose an immediate danger to the astronaut's safety and the flight could be terminated via a manual command from the ground (e.g. Mercury-Atlas 3). Other failure modes such as loss of engine thrust in the first few moments of liftoff required an immediate abort signal as there would be little or no time to command a manual abort.
An overview of failed Atlas test flights showed that there were only a few times that malfunctions occurred suddenly and without prior warning, for instance on Missile 6B when the turbopump quit working 80 seconds into launch. Otherwise, most of the failures were preceded by obvious deviations from the booster's normal operating parameters. Automatic abort was only necessary in a situation like Atlas 6B where the failure happened so fast that there would be no time for a manual abort and most failure modes left enough time for the astronaut or ground controllers to manually activate the LES. A bigger concern was setting up the abort system so as to not go off when normal, minor performance deviations occurred.
The rate gyro package was placed much closer to the forward section of the LOX tank due to the Mercury/LES combination being considerably longer than a warhead and thus producing different aerodynamic characteristics (the standard Atlas D gyro package was still retained on the vehicle for the use of the ASIS).
The range safety system was also modified for the Mercury program. There would be a three-second delay between engine cutoff and activation of the destruct charges so as to give the Launch escape system (LES) time to pull the capsule to safety. The ASIS system could not terminate engine thrust for the first 30 seconds of flight in order to prevent a malfunctioning launch vehicle from coming down on or around the pad area; during this time only the Range Safety Officer could send a manual cutoff command.
The old-fashioned electromechanical autopilot on the Atlas was replaced by a solid-state model that was more compact and easier to service.
The guidance antenna was modified to reduce signal interference.
Combustion instability was an important problem that needed to be fixed. Although it mostly occurred in static firing tests, two consecutive missile launches (Atlas 51D and 48D) had exploded on the pad due to unstable thrust in one engine. Thus, it was decided to install extra sensors in the engines to monitor combustion levels and the booster would also be held down on the pad for a few moments after ignition to ensure smooth thrust. The engines would also use a "wet start" (meaning that the propellants were injected into the combustion chamber prior to igniter activation) which would eliminate rough ignition. If the booster failed the check, it would be automatically shut down. Once again, these upgrades required testing on Atlas R&D flights. By late 1961, Convair had developed a significantly upgraded propulsion system that featured baffled fuel injectors and a hypergolic igniter in place of the pyrotechnic method, but NASA was unwilling to jeopardize John Glenn's upcoming flight with these untested modifications and so declined to have them installed in Mercury-Atlas 6's booster. As such, that and Scott Carpenter's flight on MA-7 used the old-style Atlas propulsion system and the new variant was not employed until Wally Schirra's flight late in 1962.
In early 1962, two static engine tests and one launch (Missile 11F) fell victim to LOX turbopump explosions caused by the impeller blades rubbing against the metal casing of the pump and creating a friction spark. This happened after over three years of Atlas flights without any turbopump issues and it was not clear why the rubbing occurred, but all episodes of this happened when the sustainer inlet valve was moving to the flight-ready "open" position and while running untested hardware modifications. A plastic liner was added to the LOX turbopump to prevent friction rubbing, in addition Atlas 108D, the booster used for Wally Schirra's flight, was given a PFRT (Pre-Flight Readiness Test) to verify proper functionality of the propulsion system.
After MA-1 was destroyed in-flight due to a structural failure, NASA began requesting that Convair deliver Atlases with thicker skin. Atlas 10D (as well as its backup vehicle 20D which was later used for the first Atlas-Able flight), the booster used for the Big Joe test in September 1959, had sported thick skin and verified that this was needed for the heavy Mercury capsule. Atlas 100D would be the first thick-skinned booster delivered while in the meantime, MA-2's booster (67D) which was still a thin-skinned model, had to be equipped with a steel reinforcement band at the interface between the capsule and the booster.
The vernier solo phase, which would be used on ICBMs to fine-tune the missile velocity after sustainer cutoff, was eliminated from the guidance program in the interest of simplicity as well as improved performance and lift capacity. Since orbital flights required an extremely different flight path from missiles, the guidance antennas had to be completely redesigned to ensure maximum signal strength. The posigrade rocket motors on the top of the Atlas, designed to push the spent missile away from the warhead, were moved to the Mercury capsule itself. This also necessitated adding a fiberglass insulation shield to the LOX tank dome so it wouldn't be ruptured by the rocket motors.
A common and normally harmless phenomenon on Atlas vehicles was the tendency of the booster to develop a slight roll in the first few seconds following liftoff due to the autopilot not kicking in yet. On a few flights however, the booster developed enough rolling motion to potentially trigger an abort condition if it had been a manned launch. Although some roll was naturally imparted by the Atlas's turbine exhaust, this could not account for the entire problem which instead had more to do with engine alignment. Acceptance data from the engine supplier (Rocketdyne) showed that a group of 81 engines had an average roll movement in the same direction of approximately the same magnitude as that experienced in flight. Although the acceptance test-stand and flight-experience data on individual engines did not correlate, it was determined that offsetting the alignment of the booster engines could counteract this roll motion and minimize the roll tendency at liftoff. After Schirra's Mercury flight did experience momentary roll problems early in the launch, the change was incorporated into Gordon Cooper's booster on MA-9.
Nine LV-3Bs were launched, two on unmanned suborbital test flights, three on unmanned orbital test flights, and four with manned Mercury spacecraft.[full citation needed] Atlas LV-3B launches were conducted from Launch Complex 14 at Cape Canaveral Air Force Station, Florida.
It first flew on 29 July 1960, conducting the suborbital Mercury-Atlas 1 test flight. The rocket suffered a structural failure shortly after launch, and as a result failed to place the spacecraft onto its intended trajectory. In addition to the maiden flight, the first orbital launch, Mercury-Atlas 3 also failed. This failure was due to a problem with the guidance system failing to execute pitch and roll commands, necessitating that the Range Safety Officer destroy the vehicle. The spacecraft separated by means of its launch escape system and was recovered 1.8 kilometres (1.1 mi) from the launch pad.
A further series of Mercury launches was planned, which would have used additional LV-3Bs; however these flights were canceled after the success of the initial Mercury missions. The last LV-3B launch was conducted on 15 May 1963, for the launch of Mercury-Atlas 9.