Surveyor 6: The Third Time is the Charm

As the old saying goes, “the third time’s the charm”. This is likely an American variation of the British phrase “third time lucky” (which itself is probably derived from the sentiment expressed in a line from the first scene of the fifth act of Shakespeare’s 1602 play, The Merry Wives of Windsor – “…this is the third time; I hope that luck lies in odd numbers”). The saying refers to the casual observation that after trying something twice and failing, it frequently appears that luck will favor you on the third attempt which succeeds. While rational engineers and scientists do not really believe in this form of luck, it must have been in the back of some people’s minds a half century ago as NASA was preparing its sixth Surveyor robotic lunar landing mission formally designated “Surveyor F” before its launch.

Twice before in the Surveyor program, the intended target was nearly in the center of the Moon’s Earth-facing hemisphere in the small mare known as Sinus Medii (Latin for “Central Bay”). Located almost dead center of the equatorial zone where NASA’s initial Apollo Moon landings would occur, Apollo mission planners wanted to make a close up assessment of this area of the Moon which appeared rougher than many of the other proposed Apollo landing sites. NASA’s first attempt to land on Sinus Medii with Surveyor 2 launched on September 20, 1966 failed when the spacecraft tumbled out of control en route to the Moon due to a vernier engine failure during a planned mid-course correction the day after launch (see “Surveyor 2: Things Don’t Always Go As Planned”). NASA’s second attempt to land on Sinus Medii, Surveyor 4 launched on July 14, 1967, made it farther but failed when contact was lost during the final descent to the lunar surface and was never regained (see “Surveyor 4: The Impact of a Low Probability Event”). With only two approved Surveyor flights remaining, the Surveyor F mission would attempt a landing in Sinus Medii for the third time… and hopefully succeed.

 

The Surveyor Spacecraft

Work began on NASA’s Surveyor program in May 1960 under the responsibility of Caltech’s Jet Propulsion Laboratory (JPL) in Pasadena, California (for details on the early history and development of Surveyor, see “Surveyor 1: America’s First Lunar Landing”). Built by Hughes Aircraft Company (whose space division is now part of Boeing), Surveyor was arguably the most advanced lunar spacecraft of its day. The basic 2.4-meter tall structure consisted of a simple 27-kilogram tetrahedral frame made of tubular aluminum alloy members. In each of the three lower corners was a landing leg equipped with an aircraft-style shock absorber and a footpad of crushable honeycomb aluminum. The total span of the legs, once deployed, was 4.3 meters. Rising from the apex of the frame was a mast upon which was mounted a gimballed planar high-gain antenna and a solar panel supplying up to 85 watts of electrical power to the lander’s rechargeable silver-zinc batteries. From the footpads to the top of its mast, Surveyor stood three meters tall.

Buried inside the spacecraft’s frame was a Morton Thiokol-built 91-centimeter diameter TE-M-364 solid propellant rocket motor that would provide between 35.5 to 44.5 kilonewtons of thrust, depending on the motor’s temperature at ignition. This 656-kilogram motor, which would later be used as the third stage in various Delta launch vehicles models flown in the 1970s and as the final “kick stage” for the Pioneer and Voyager missions to the outer planets, would be used to negate most of Surveyor’s motion towards the Moon as the lander approached the lunar surface.

Surveyor also carried a second propulsion system for midcourse corrections and attitude control during the main retrorocket burn as well as for the final descent. This system consisted of three vernier engines fueled by monomethylhydrazine hydrate with MON-10 (a mixture of 90% nitrogen tetroxide and 10% nitric acid) serving as the oxidizer. These engines could be throttled by command of the spacecraft’s flight control subsystem producing between 130 and 460 newtons of thrust each. Yaw, pitch, and the descent rate were controlled by selective throttling of the engines while roll was controlled by swiveling a single gimballed vernier. During the trans-lunar coast, Surveyor’s attitude was controlled by a set of six nitrogen gas jets, each providing 270 millinewtons of thrust.

All the temperature sensitive electronics were carried in two thermal boxes mounted to the frame. These compartments were covered with 75 layers of aluminized Mylar insulation and the tops were covered by mirrored glass thermal regulators. Compartment A, which maintained it internal temperature between +4° and +52° C, carried a redundant set of receivers and ten-watt radio transmitters, the batteries, their charge regulators, and some auxiliary equipment. The second box, Compartment B, was designed to maintain the temperature between -15° and +52° C. This compartment carried the computer “brains” of the spacecraft which controlled all aspects of the lander’s operation using a total of just 256 commands. Mounted elsewhere on the frame were star sensors, a pair of radar systems for landing, low-gain antennas, propellant, and helium pressurization tanks.

The first “Block I” Surveyors were considered “engineering models” for the more advanced “Block II” versions of the lander which had been proposed. Most of its instrumentation were engineering sensors such as strain gauges, accelerometers, rate gyros, temperature sensors, and so on to be used to make more than two hundred measurements of the spacecraft’s performance and condition. While not specifically designed for investigating the lunar environment, many of these measurements could be used to determine some of its basic properties including surface mechanical properties and its temperature.

The only true scientific instrument carried by the initial batch of what would be seven Block I Surveyors was a slow-scan television camera. The camera was mounted in a 1.65-meter tall mast attached to the spacecraft’s framework. The camera pointed up into a movable mirror that allowed the camera to view 360° of azimuth and from 60° below to 50° above the normal plane of the camera. The 7.6-kilogram camera package was canted at a 16° angle to offer a clear view of the surface between two of the footpads out to the lunar horizon 2½ kilometers away. The camera was fitted with a 25 to 100 mm zoom lens that offered a field of view of between 25.3° and 6.4°.  The aperture could be set between f/4 and f/22 and the lens could be focused from 1.2 meters to infinity.  A shutter was also included so that various integration times could be used to obtain the ideal exposure.  While the nominal exposure time was 150 milliseconds, exposures as long as about thirty minutes could be accommodated.  The typical resolution of the camera was one millimeter at a distance of four meters. By combining a series of images taken in a stepwise fashion at various azimuth and elevation angles, panoramic mosaics of the spacecraft and the surrounding terrain could be created.

The camera and its pointing mirror assembly used on the Surveyor F mission were significantly upgraded to improve its performance and extend its life. Originally, the Surveyor camera was also fitted with a filter wheel containing clear and three color filters so that color images of the lunar surface could be reconstructed back on Earth with the aid of calibration targets mounted on key points on the spacecraft. For the Surveyor F mission, the color filters were replaced with polarization filters with angles of 0°, 45° and 90°. Combined with information on the lighting and viewing geometry, these polarization measurements could be used to help characterize the properties of the lunar surface. As with the cameras used on the earlier Surveyor missions, the camera could only operate in real time via remote control from Earth using a total of 25 commands. The primary means of transmitting images was through the high-gain antenna. Using this powerful antenna, an image would be broken up into 600 scan lines and transmitted back to Earth in 3.6 seconds. The use of the less powerful low-gain antennas, which served as a backup, would permit an image to be broken up into 200 lines and would require 61.8 seconds to transmit.

 

The Surveyor F Mission

With the cancellation of the more advanced Block II Surveyor missions on December 13, 1966 which would have explored the lunar surface with a wider array of instruments, program officials decided to incorporate some of the Block II science payloads already completing development on the remaining five approved engineering flights of the more limited Block I spacecraft. The third and fourth Block I Surveyor flights were selected to carry a remote controlled mechanical arm formally known as the Soil Mechanics Surface Sampler or SMSS (see “Surveyor 3: Touching the Face of the Moon”).

For the Surveyor 5 and upcoming F missions, the SMSS was replaced with a new instrument known as the Alpha Scattering Experiment designed to measure the concentration of various elements in the lunar soil (see “Surveyor 5: Pulling Success from the Jaws of Failure”). The 13-kilogram instrument consisted of three major components: a sensor head, a deployment mechanism and a thermally controlled electronics compartment. The sensor head consisted of a 17.1 by 16.5 by 13.3 centimeter high box on a plate with a diameter of 30.5 centimeters which would sit on the lunar surface once deployed. The sensor head was mounted on the spacecraft during flight and would be lowered to the lunar surface upon ground command by a nylon cord wrapped around a geared cylinder which would unwind under the action of lunar gravity in a series of controlled steps.

Once deployed on the lunar surface, a set of six small radiation sources composed of the artificial element, curium-242 with a half life of just 160 days, would bombard a small part of the lunar surface with alpha particles (essentially nuclei of helium-4 consisting of a pair of each of protons and neutrons) that would penetrate only about 25 micrometers into the lunar surface. A pair of alpha particle detectors located adjacent to the curium-242 sources would be used to detect the energy and number of alpha particles scattered by the lunar surface allowing the concentrations of elements heavier than lithium to be determined.

Four other detectors included inside of the sensor head would measure the energy and number of protons that were emitted by the lunar surface as a result of interactions with the alpha particles. These sensors would allow the concentration of boron, nitrogen, fluorine, sodium, magnesium, aluminum, silicon, phosphorus, sulfur, chlorine and potassium to be assessed. While stowed on the lander, the alpha particle and proton detectors in the sensor head were calibrated using small amounts of einsteinium-252 on the mounting platform. By measuring the concentration of elements present in the lunar surface, scientists could estimate what types of rocks made up the lunar surface.

The spacecraft for the Surveyor F mission, SC-6, also included a pair of metal bars (one magnetic and the other nonmagnetic) attached to one of the footpads just like Surveyor 4 and 5. Observations of these 5.1 by 1.3 by 0.3 centimeter thick bars by the television camera would allow scientists to determine the magnetic properties of the lunar soil. As in earlier missions, SC-6 included a pair of small mirrors mounted on one of the landing legs to provide a clear view of the underside of the Surveyor lander allowing an assessment of how the lander’s vernier engines disturbed the lunar soil. For this mission, a third mirror was also included to provided a better view of the Alpha Scattering Experiment sensor head when it was deployed on the lunar surface. The launch mass of SC-6 for the Surveyor F mission was 1,008 kilograms. After it had jettisoned its spent retrorocket and consumed its propellants during descent, the spacecraft would have a mass of about 298 kilograms upon landing, depending on how much vernier engine propellant was left after touchdown.

Surveyor’s launch vehicle was one of NASA’s most advanced rockets called the Atlas-Centaur. The Centaur upper stage used liquid hydrogen and liquid oxygen (LOX) as propellants – the first rocket stage to do so. This combination provided up to half again as much thrust than a like mass of more conventional propellants then in use. Unlike the earlier versions of the Atlas-Centaur used to launch the first four Surveyor missions which employed a modified Atlas D ICBM for its first stage, Atlas-Centaur 14 (AC-14) to be used to launch the Surveyor F mission would sport the improved SLV-3C model of the Atlas. In this version of the Atlas, the propellant tanks were stretched by 1.3 meters to increase the propellant load by 8% to 123 metric tons and lengthen the burn time by ten seconds. The thrust of the improved MA-5 propulsion system was also increased by about 2% to provide a total liftoff thrust of 1,756 kilonewtons. With a total height of 35.7 meters and a liftoff mass of 146 metric tons, the new Atlas-Centaur was now capable of launching up to about 1,135 kilograms towards the Moon on a Surveyor-type mission or about 68 kilograms more than the earlier Atlas-Centaur models. The first of these improved Atlas-Centaurs, AC-13, had flown successfully on September 8, 1967 to send Surveyor 5 to the Moon.

In order to maximize its payload and launch widow flexibility, the Atlas-Centaur was designed to first place its payload into a low parking orbit. The pair of RL-10 engines powering the Centaur would then reignite at the proper injection point to send the Surveyor lander on its way to the Moon. Due to problems with the development of the Centaur and its in-orbit restart capability, the initial two Surveyor missions were forced to use direct ascent trajectories to the Moon which required only a single burn of the RL-10 engines. With the successful test flight of AC-9 on October 26, 1966 where the Centaur finally demonstrated its in-orbit restart capability by sending a dynamic model of a Surveyor lander, designated SD-4, into a simulated lunar trajectory, the way was finally clear for the technique to be used in future Surveyor missions. While Surveyor 4 used the last direct-ascent variant of the Atlas-Centaur still left in NASA’s inventory, Surveyor 3 and 5 successfully used the parking orbit technique to reach the Moon.

Unlike the later Apollo lunar landing missions, Surveyor was designed to make a direct descent to the lunar surface from its translunar trajectory around 65 hours after launch with no intermediate stop in lunar orbit. Since Surveyor was designed with the capability of landing on the Moon with an approach trajectory substantially off of the local vertical, most of the lunar hemisphere facing Earth was accessible to Surveyor. Early flights, however, were limited to the equatorial mare regions of what was called the “Apollo landing zone” which appeared to be the safest landing sites based on orbital photography. It was intended that the initial Surveyor missions would provide ground truth data on these proposed sites to support the upcoming Apollo lunar landing missions.

Like the earlier five Surveyor missions, the objectives of the Surveyor F mission were chosen to support the upcoming Apollo lunar landings. The primary objectives were to land in Sinus Medii inside of a 60-kilometer wide target zone centered at 1.33° west longitude and 0.42° north latitude about 305 kilometers north of the large lunar crater, Ptolemaeus, and return television images of its surroundings. The initial approach to landing would be at least 25° to the local vertical necessitating a gravity turn during the descent to the lunar surface. Secondary objectives included using the Alpha Scattering Experiment to determine the composition of the lunar surface, conduct a vernier engine erosion experiment and obtain information on the bearing strength, radar reflectivity and thermal properties of the lunar surface.

Getting the Mission Underway

In order to meet the postlanding lighting requirements at the proposed Sinus Medii landing site as well as satisfy other mission and launch constraints, the Surveyor F mission had a six-day launch period running from November 7 to 12, 1967. The launch window on the first day ran from 2:22 AM to 3:17 AM EST. Landing was scheduled to occur about 65 hours after liftoff with the exact transit time dictated by the date and time of launch. Already scheduled for launch on November 7 was NASA’s Apollo 4 mission which was to perform the first test flight of the new Saturn V Moon rocket (see “Apollo 4: The First Flight of the Saturn V“). If NASA’s schedule for this high priority mission held, the launch of Surveyor F would take place later in the November launch period in order to avoid conflicts with key assets at the Cape and downrange required to support both missions.

Following a successful Combined Systems Test Stand Test at General Dynamics’ San Diego facility and the resolution of the minor issues it revealed, the Atlas 5902C booster for the Surveyor F mission was shipped by air to Cape Kennedy (which reverted to its original name Cape Canaveral in 1973) on August 25, 1967 followed the next day by the nose fairing and interstage adapter. The Centaur was shipped on August 30 with the components of the Surveyor SC-6 spacecraft arriving at the Cape on September 6. The Atlas and its Centaur upper stage were erected on Pad B of Launch Complex 36 (LC-36B), which had already been modified to handle the new SLV-3C, on September 11 – just three days after the successful launch of AC-13 carrying Surveyor 5.

Following inspection and the replacement of a faulty vernier engine on SC-6 found during routine preflight testing as well as hardware tests to help resolve issues with Surveyor 5 while en route to the Moon, the spacecraft was temporarily fitted with a dummy retrorocket as well as test instrumentation and mated to its adapter on September 23. Encapsulated in its nose fairing, SC-6 was mated to AC-14 at LC-36B on September 27 for the beginning of prelaunch testing. Following the completion of the AC-14/SC-6 Joint Flight Acceptance Composite Test on September 29, the spacecraft was removed from the launch vehicle to start final preparations for its flight. The encapsulated SC-6 was mated to AC-14 for the final time on November 1 after the launch vehicle had completed its Composite Readiness Test. With the two-day postponement of the launch of Apollo 4, Surveyor F could attempt its get underway during the first day of its launch period on November 7.

The countdown for the Surveyor F launch started at the T-680 minute mark at 12:52 PM on November 6. The spacecraft joined the countdown during a previously planned one-hour hold at T-90 minutes at 1:22 AM on November 7. The countdown proceeded without incident until another planned hold at T-5 minutes. This hold was extended by seven minutes from the originally planned ten minutes to allow better downrange tracking coverage. At 3:39:01.075 AM EST (07:39:01.075 GMT), Atlas-Centaur 14 successfully lifted off from LC-36B for the official start of the Surveyor F mission.

A near nominal performance from the Atlas 5902C rocket and the first burn of the Centaur placed the upper stage and its payload into a temporary 159.3 by 168.5 kilometer parking orbit with an inclination of 29.0°. After coasting in this orbit for 12 minutes, 51.7 seconds, the Centaur’s pair of RL-10 engines reignited for a second burn with a planned duration of 113 seconds. After burning for 2.7 seconds longer than originally expected based on real time data from the on board guidance system, the Centaur’s engines shutdown 24 minutes, 28.6 seconds after launch. One minute later, the payload, now officially designated Surveyor 6, separated from the spent Centaur to begin its independent flight in a geocentric 168.5 by 684,614 kilometer orbit with an inclination of 29.0° which would set the lander on an intercept course to the Moon. About five seconds later, the Centaur started turning to begin a retromaneuver first using its own hydrogen peroxide thrusters and then by venting an estimated 308 kilograms of residual cryogenic propellants still left in its tanks. The maneuver placed the spent stage into its own 168.5 by 401,718 kilometer geocentric orbit that would safely move it away from Surveyor 6. The Centaur would pass an estimated 28,000 kilometers from the Moon at about 15:00 GMT on November 10.

 

The Surveyor 6 Mission

After separating from the spent Centaur stage, Surveyor 6 started extending its appendages and searching for its celestial attitude references. A two-way communications lock with Earth was established about nine minutes after separation at 08:13:27 GMT on launch day followed by acquiring its first attitude reference, the Sun, at about 08:16 GMT. Surveyor 6 then started a slow roll so that its star sensor could map the stars in its view with final lock onto its second attitude reference, the bright star Canopus, coming at 16:28:30 GMT.

Tracking of the receding Surveyor 6 showed that its aim point was off by only 90 kilometers from the intended landing spot. At 02:02:59 GMT on November 9 (about 18 hours and 24 minutes after launch), Surveyor 6 began changing its attitude to align its vernier engines for a midcourse correction burn starting at 02:20:02 GMT. The 10.25-second burn changed the velocity of the spacecraft by 10.12 meters per second or just 0.06 meters per second more than desired – sufficient to bring Surveyor 6 down within an estimated 10.5 kilometers of its final aim point.

Surveyor 6 quickly reacquired its attitude references and continued its largely uneventful transit to the Moon. At 00:07:32 GMT on November 10, Ground controllers began to prepare Surveyor for its terminal maneuver and descent to the lunar surface. At 00:57:55.74 GMT, Surveyor’s radar detected that the quickly descending lander had hit the 96.5 kilometer altitude mark triggering the automatic sequence of events to follow. At 00:58:01.64 GMT the vernier engines ignited at an altitude of 72.99 kilometers and a velocity of 2,587 meters per second. This was followed 1.1 seconds later by the ignition of the Thiokol TE-M-364 retrorocket. After retrorocket burnout, the vernier engines were throttled to full thrust and the spent motor case was ejected at 00:58:54.6 GMT. Using data from its descent radar system, Surveyor’s computer guided the spacecraft to a soft landing at 01:01:05.467 GMT at a velocity of 3.4 meters per second with just a brief bounce on initial contact. Surveyor 6 had finally succeeded at landing on Sinus Medii coming down at 0.49° N, 1.40° W or about 3 kilometers northwest of its aim point.

After a routine set of post-landing systems checks and beginning to reconfigure various systems for surface operations, Surveyor 6 returned its first 200-line images of the Moon at 01:51 GMT. After aligning its solar panels and high gain antenna, the first high-resolution 600-line images were returned at 04:04 GMT on landing day. The images showed that Surveyor 6 had come down on a relatively level surface in a gently rolling, cratered landscape. Visible in the distance 2.5 kilometers away was a low mare wrinkle ridge which helped scientists determine the location of the landing site in the available Lunar Orbiter images. The overall impression was that the Surveyor 6 landing site was remarkably similar to the previous three Surveyor mare landing sites and would be safe enough to support a manned landing.

Four hours and 37 minutes after landing, the Alpha Scattering Experiment was powered up to begin its operations. After gathering about five hours of calibration data in the stowed position, the sensor head was partially deployed to gather six hours of background radiation measurements. At 12:07 GMT on landing day, the sensor head was finally lowered to the lunar surface to begin characterizing its composition. A total of 30½ hours of surface measurements indicated that the composition of the lunar soil at the Surveyor 6 site was essentially the same as that at the Surveyor 5 landing site about 745 kilometers to the east in Mare Tranquillatatis characterized just two months earlier. Once again, the mix of elements present in the lunar soil was dominated by oxygen, silicon and aluminum and was consistent with the composition of terrestrial basalt.

As part of its surface erosion investigation, Surveyor 6 was commanded at 03:22 GMT on November 11 to fire an attitude control jet mounted near the end of one of the landing legs for four seconds to see what effect it would have on the lunar soil. This was followed by a longer 60-second burst 24 minutes later. Television images showed that there was a slight disturbance in the surface material as a result of this erosion experiment.

After a week on the lunar surface, Surveyor 6 prepared to perform another, more extreme erosion experiment. The vernier engines would be fired for two seconds at a thrust level 667 newtons to make the lander hop briefly off the surface to see what effect the maneuver would have on the lunar soil. After using the solar panel and high gain antenna to selectively shade the vernier engines from the noontime Sun and get their temperatures below their 106° C maximum operating temperature, the three vernier engines were ignited at 10:32:02 GMT on November 17. With Surveyor rejecting the first shutdown command, a second command from ground controllers turned off the trio of vernier engines after a 2.5 second burn which consumed 0.7 kilograms of propellant. Surveyor 6 reached an estimated peak altitude of 3.0 to 4.6 meters before coming down about 2.4 meters from its original resting place after a 6.1-second hop.

The impressions left in the surface by the original touchdown could now be clearly seen as could the marks left by the vernier engines firing allowing the mechanical properties of the soil to be better characterized. The displacement caused by the hop also allowed stereoscopic imagery of the surrounding terrain. Unfortunately, the sensor head of the Alpha Scattering Experiment had come down on its side after the hop. Since the simple deployment mechanism was not designed to reel in the sensor head and redeploy it, the sensor was limited to gathering background radiation measurements for the rest of the mission. At very least, this short hop demonstrated that a spacecraft could be launched from the lunar surface (although there was no serious expectation among scientists and engineers that this would not be the case).

As local sunset approached, the spacecraft was configured to continue operations in the dark for up to 200 hours. At about 13:53 GMT on November 27, the Sun set on the Surveyor 6 landing site. A total of 44 television images of the Sun’s corona were obtained afterwards during a six hour period including, for the first time, through polarization filters. This sequence of images was followed by a series of dark frames to be used in calibrating the television images. Unfortunately, thermal switches on Surveyor’s pair of temperature controlled compartments used to house the spacecraft’s batteries and sensitive electronics failed to activate as expected allowing the battery temperature to fall faster than had been predicted. Surveyor 6 was placed into hibernation at 09:34 GMT on November 29 only 41 hours after sunset with the hope that it would survive the long lunar night. During its 18 days on the lunar surface, Surveyor 6 had returned a treasure trove of valuable data about the Moon including a total of 30,396 television images.

Some time after local sunrise, Surveyor 6 responded to ground commands to activate its systems at 16:45 GMT on December 14, 1967 to begin a second lunar day of surface operations. Unfortunately, the signal received from the lander was erratic but some engineering data were obtained. These data showed that the backup receiver was no longer operating and the power status of the battery could not be determined. Surveyor 6 was last heard from at 19:14 GMT after only 2½ hours of operation for the second day with no further response to ground commands. With the successful completion of the Surveyor 6 mission and with the Surveyor program having met all of its requirements to support the Apollo program, NASA had one last Surveyor lander left to send to a target of great scientific interest in the rugged lunar highlands near the rim of the famous 86-kilometer Tycho crater in January 1968.

 

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Related Reading

For more articles on NASA’s Surveyor program, check out Drew Ex Machina’s Surveyor Program page.

 

General References

Sixth Surveyor Mission, NASA Press Release 67-278, November 3, 1967

Surveyor VI Flight Performance Final Report, SSD 68189-6, Hughes Aircraft Company, January 1968

Surveyor VI Mission Report Part II. Science Results, Technical Report 32-1262, JPL, January 10, 1968

Surveyor VI A Preliminary Report, SP-166, NASA, March 1968

Surveyor VI Mission Report Part I. Mission Description and Performance, Technical Report 32-1262, JPL, September 15, 1968

Flight Performance of Atlas-Centaur AC-13, AC-14, AC-15 in Support of the Surveyor Lunar Landing Program, TM X-1844, NASA Lewis Research Center, February 1970