Ranger: America’s first successful lunar program
by Andrew J. LePage
|Only 17 months after President John F. Kennedy committed the United States to landing a man on the Moon with Project Apollo, it was beginning to look as though the Americans would never make it.|
NASA’s earliest Pioneer lunar probes, a program started by the military and inherited by the agency after it was founded in October 1958, were plagued by a series of launch vehicle failures (see “The Pioneer lunar orbiters: a forgotten failure”, The Space Review, December 13, 2010). Out of all of NASA’s initial attempts to launch probes towards the Moon, only the tiny six-kilogram (13-pound) Pioneer 4 built by the Jet Propulsion Laboratory (JPL) and launched on March 3, 1959, by a team at the Army Ballistic Missile Agency (ABMA) headed by Wernher von Braun (which would become the basis of NASA’s Marshall Space Flight Center) managed to escape Earth’s gravitational grasp to make a very distant flyby of the Moon.
The initial flights of NASA’s first in-house lunar program, Ranger, which was built and managed by JPL, fared little better than the Pioneers. The two flights of the Block I Ranger, which were designed to test the innovative Ranger design in extended Earth orbit, were stranded in short-lived low Earth orbits due to failures of the upper stage of the Atlas-Agena B launch vehicle (see “Ranger: Voyage to the Moon and beyond”, The Space Review, August 22, 2011). The three Block II Ranger flights, which were designed to hard-land a small probe on the lunar surface, fared little better. While most of the launch vehicle issues were resolved, fatal malfunctions of key spacecraft components resulted in complete failure of all of these missions (see “The Difficult Road to the Moon”, The Space Review, January 23, 2012).
As 1962 was drawing to a close, the situation with the American Moon program looked bleak. The failure of the last Block II Ranger, Ranger 5 launched on October 16, 1962, was NASA’s sixth consecutive lunar mission failure in three years. Only 17 months after President John F. Kennedy committed the United States to landing a man on the Moon with Project Apollo, it was beginning to look as though the Americans would never make it. If NASA could not get a simple unmanned probe to the Moon in working order, how could they hope to pull off the much more complicated mission of a manned lunar landing?
Lunar Reconnaissance Orbiter image from October 29, 2009 of the crater made by the impact of the unsuccessful Ranger 6 over 45 years earlier. (credit: LRO/NASA)
The mission of the Block III Ranger was to perform high resolution lunar imaging before crashing on the Moon about 65 hours after launch. But before the Block III could fly, the problems with the spacecraft had to be addressed. Formal investigations into the failures of the Ranger program started on October 30, 1962. Over the course of the next month, several groups inside NASA and out examined every aspect of the project in an attempt to pin down the causes of the failures and recommend changes. On November 30, NASA Headquarters released the findings of its inquiry: In brief, the report recommended streamlining management and changing the mission goals to be more in line with supporting the needs of the Apollo lunar missions. This meant concentrating on lunar imaging and dropping all other experiments on the new Block III Rangers.
The report also called for a thorough reevaluation of the Ranger design, modifying vulnerable systems and the inclusion of more backup systems. More extensive testing of systems and better quality control for components were recommended. Most of all, the report urged the immediate abandonment of prelaunch sterilization of the spacecraft. Sterilization was pinpointed as the cause of many of Ranger’s system failures and it was now felt to be unnecessary, given that the hostile lunar environment was unlikely to harbor any indigenous life forms. Unless these changes were made, the Block III Rangers were likely to suffer the same fate as their predecessors. With these recommendations in hand, JPL set about redesigning and rebuilding the Block III Rangers.
|The report urged the immediate abandonment of prelaunch sterilization of the spacecraft. Sterilization was pinpointed as the cause of many of Ranger’s system failures and it was now felt to be unnecessary, given that the hostile lunar environment was unlikely to harbor any indigenous life forms.|
The first improved Block III Ranger, designated Ranger A, was finally ready by the end of 1963. Much had been changed from the previous design. The Ranger hexagon-shaped bus was similar to previous models with some notable exceptions. First, the framework of the bus was now made of aluminum due to its better thermal characteristics. A second battery to provide additional backup power was added. The course correction system was enlarged to provide a 60-meter per second (135 mile-per-hour) velocity change capability; a one-third increase over the earlier Block II Ranger. The sequencer which controlled spacecraft functions was redesigned to incorporate components which were not heat sterilized. This included features that increased the chances of a successful mission in case of equipment failure. A second, independent attitude control system was added for redundancy.
The bus was also fitted with new rectangular-shaped solar panels similar to the ones carried by Ranger’s successful cousin, the Mariner 2 Venus probe launched in 1962. This design had portions of the solar panels electrically isolated from each other to avoid a repeat of the total solar panel failure experienced by Ranger 5. All of these changes increased the weight of the Block III Ranger. This prompted the deletion of every instrument except for the large television camera package to keep the probe’s mass under 368 kilograms (810 pounds)—the limit for the Atlas-Agena B launch vehicle for Ranger’s mission profile.
Two independent chains of slow-scan vidicon cameras developed and built by RCA (a leading electronics manufacturer of the day, bought by GE in 1986 and subsequently dismantled) were enclosed in a 1.5-meter (five-foot) tall tower mounted on top of the bus. Clad in polished aluminum for thermal control, the 173-kilogram (380-pound) cylindrical tower tapered from 69 centimeters (27 inches) at its base to 41 centimeters (16 inches) at the top, where the low-gain antenna was mounted. The six cameras viewed the approaching lunar surface through a 33-centimeter (13-inch) square opening on the side of the tower. Their optical axes were canted at a 28-degree angle from the spacecraft’s long axis. Also enclosed inside the tower were two independent power supplies, camera sequencers, and batteries; one set for each chain of cameras. Each chain also possessed its own sixty-watt transmitter to independently transmit images in real time back to Earth. The bus still carried its own three-watt transmitter which would now only carry engineering telemetry.
The first camera chain was the full-scan or F chain, which consisted of two cameras. One camera was fitted with a 35-millimeter lens, providing a 25-degree field of view, while the other used a 76-millimeter lens with an 8.4-degree field of view. Each camera would scan the entire 1,152-line vidicon once the exposure had been taken: over twice the resolution of conventional television of the day and comparable to today’s high definition format. As a set, the F chain returned one image every 2.56 seconds. Normally the cameras would be turned on by commands sent from Earth. If this failed, the bus’s onboard sequencer would activate the package at a preset time. If this failed, the F chain had its own timer that was activated by the spacecraft’s separation from the Agena B escape stage. After 67 hours and 45 minutes of flight, the F chain would automatically turn on and start transmitting images. In this way, even if both primary systems were to fail, at least a few hundred full scan images would be returned.
Independent of the F chain was a second set of four partial-scan vidicon cameras called the P chain. Like the F chain, 35 and 76-millimeter lenses were used, but only three hundred partial lines—about seven percent of the vidicon’s face—was read and transmitted back to Earth. This resulted in images with the same effective resolution as the F chain but covered a smaller area. This was done so that images could be returned at a rate of five images per second in hopes of capturing at least a partial image a couple of tenths of a second before impact. At this altitude of only 300 to 600 meters (one or two thousand feet), a resolution of 0.3 meters (one foot) or better was possible. If the F chain were to malfunction, the P chain could independently return thousands of images after receiving a command either directly from Earth or from Ranger’s central sequencer and timer.
With all these hardware changes, including redundant and more fault tolerant systems as well as five hundred to eight hundred hours of prelaunch testing, the new Block III Ranger was much more likely to reach its target in working order.
|The launch and injection into a translunar trajectory of Ranger 6 went perfectly except for a telemetry channel that inexplicably switched into an unscheduled mode for 67 seconds when the booster engines separated from the ascending Atlas.|
The Block III mission profile was very similar to the Block II up until the encounter with the Moon. Since the Block III probe did not have to be concerned with the site and trajectory constraints of a hard lander, the impact point could be over a much larger range of longitude near the lunar equator. Typically the most easterly aim point was targeted at the beginning of the multiday launch window. The aim point then drifted westward by about thirteen degrees of longitude per day, so that the impact point would have the optimum lighting conditions.
About one hour before impact, the spacecraft would begin its terminal maneuver and reorient itself. This maneuver would aim the cameras along Ranger’s flight path towards approaching lunar surface with the high gain antenna turned to point towards Earth. Some seventeen minutes before impact, the F chain of cameras would be commanded to warm up for ninety seconds. The P chain then would take its turn and warm up. Finally, fourteen minutes before impact at an altitude of about 1,900 kilometers (1,200 miles), the F chain’s sixty-watt transmitter would start beaming images back to Earth, followed by the P chain typically 150 seconds later. Transmission would continue until the spacecraft impacted the lunar surface at 2,600 meters per second (5,800 miles per hour). If everything worked perfectly, over 4,200 close-up television images of the lunar surface would be transmitted back to the Earth.
Because of various minor schedule slips, the first modified Block III spacecraft, designated Ranger A, was ready for launch by the beginning of 1964. Its primary targets were in the smooth equatorial mare regions, which were considered likely Apollo landing sites. On the first day of the launch window, the site would be a point at 8.5 degrees north and 21.0 degrees east in Mare Tranquillitatis, the Sea of Tranquility. After several short holds, Ranger 6 lifted off on its first attempt on January 30, 1964—fifty years ago last week. The launch and injection into a translunar trajectory went perfectly except for a telemetry channel that inexplicably switched into an unscheduled mode for 67 seconds when the booster engines separated from the ascending Atlas.
Initial tracking of Ranger 6 indicated that it would miss the Moon by about 965 kilometers (600 miles). More refined calculations later indicated a miss of only 796 kilometers (495 miles) that was corrected by a 67-second burn of the course correction engine about 16 hours and 41 minutes after launch. This 41.2-meter per second (92.2-mile per hour) change of velocity placed Ranger 6 on course for an impact on the western edge of Mare Tranquillitatis 65 kilometers (40 miles) south of the crater called Ross.
|Ranger 6 was definitely a very successful engineering test. Still, from the point of view of the public as well as the scientific community, this was NASA’s seventh consecutive lunar mission failure.|
On February 2, as Ranger 6 passed the 2,076-kilometer (1,290-mile) altitude mark moving at 1,998 meters per second (4,471 miles per hour), the television cameras were switched into warm up mode with all systems functioning normally. When the time came for the cameras to switch to full power and start returning images, however, only static was received. Quickly a series of emergency commands were sent from Earth, but to no avail. Ranger 6 crashed into the lunar surface at 9.39 degrees north, 21.51 degrees east at a speed of 2,658 meters per second (5,946 miles per hour) without returning a single picture.
Ranger 6 was definitely a very successful engineering test. With the exception of the cameras, all systems worked perfectly. In addition, the navigation accuracy was the best ever attained; the spacecraft impacted the Moon only 31 kilometers (19 miles) from its aim point and only 0.3 seconds before its post-mid-course maneuver predicted impact time. Still, from the point of view of the public as well as the scientific community, this was NASA’s seventh consecutive lunar mission failure. NASA Headquarters formed another board of inquiry to investigate this mishap. The March launch of Ranger B was postponed pending the outcome of this new investigation. The pressure was on NASA, and JPL was fighting for its life.
The NASA investigation into the failure of the Ranger 6 camera package was released on March 17, 1964. The 75-page report pinned the problem squarely on the RCA camera package itself. The completely redundant camera system was found not to be perfectly so. There was a single line that carried commands to both camera chains. Somehow a command was sent to the camera package during ascent that turned it on, hence the anomalous telemetry reading during launch. The cameras were turned on and, in the relatively dense atmosphere, both camera power supplies arced and shorted out.
While the source of the errant command was not known at the time, several changes in the RCA camera package were suggested. These included changes to simplify ground testing and in-flight operation, telemetry system modifications to increase failure mode coverage, inclusion of additional noise suppression in the camera command circuitry, and a more rigorous prelaunch inspection of the television circuitry. These changes also included an interlock that would prevent the cameras from being turned on during launch. In addition, the tower temperature would be lowered by 11°C (20°F).
While these changes would further increase Ranger’s chances of success, the blame did not totally lie with Ranger. It was later discovered that the jettisoning of the Atlas booster engines caused Ranger’s cameras to turn on. When the Atlas 199D dropped its booster engines, about 180 kilograms (400 pounds) of unburned propellant were expelled and subsequently ignited by the sustainer. This small detonation had caused some problems during the development of the Atlas E/F ICBM but was never a problem for the Atlas D. The detonation wave produced during the flight of Ranger 6 worked its way into a mechanically sealed umbilical door on the Agena. The umbilical pin that controlled the camera package was 6 millimeters (0.25 inches) from another pin carrying twenty volts. The burning fuel vapor was conductive enough to short the two pins briefly, cause the camera package to turn on prematurely and, as a result, burn out.