February 1981:Voyager preparations

Introduction

On the morning of 20 August 1977 the palmetto scrub round Cape Canaveral's launch complex 41 was shaken by the departure of the first of two NASA Voyager spacecraft on a journey to the outer planets and beyond. Tracing complex paths across the Solar System,the Voyager project is the most ambitious planetary mission to date and as is to be expected for such a complicated flight, the route to pad 41 was as convoluted as the journey to the edge of the Solar System will be. In this article the evolution of the Voyager spacecraft is traced from its beginnings in the Grand Tour missions of the Sixties to the 808 kg spacecraft nestling on top of a Titan Centaur rocket at Cape Canaveral.

The Grand Tour

In 1925 Hohmann advanced the idea of using the gravitational field of a planet to alter the trajectory of a passing spacecraft but it was not until the mid-Sixties that developments in interplanetary spacecraft focused attention on the science of astrodynamics. In 1963 Minovich presented the detailed analysis of a gravity assisted mission trajectory to the inner planets from which developed the 1973 Mariner 10 flight and a year later Hunter suggested the gravitational field of Jupiter could be used to reduce the flight time to the outer planets.

At first it appeared that new guidance technology would be required to make such missions possible but studies by Sturms and Cutting showed that existing techniques were adequate. Garry Flandro while carrying out post-graduate studies at JPL, discovered that a mission using the gravitational field of Jupiter to accelerate a spacecraft to Saturn and then to use Saturn's gravity to go on to Uranus and Neptune was possible during the period of 1976-1978. The title given to this daring proposal was the ‘Grand Tour'. The original concept involved a 1977 launch arriving at Jupiter in 1979; from here the spacecraft would be accelerated by 11 km/sec and its course deflected by 97 degrees towards Saturn, where it would arrive in 1980. From Saturn the spacecraft would proceed to Uranus in 1984 and then on to a 1986 encounter with Neptune. However the celestial mechanics of this flight would require passage very close to Saturn where considerable danger to the spacecraft could be expected from impacts with the particles which constitute Saturn's rings.

A more conservative approach was detailed in a 1969 paper by James Long of JPL's advanced projects office, using two spacecraft both on courses which kept them clear of Saturn's rings.

The first flight, GT 1, would be launched in August 1977 to fly past Jupiter 17 months later, this spacecraft would arrive at Saturn in August 1980 for a final deflection onto a five year four month coast to Pluto in December 1986. The role of Saturn in this mission is not to change the velocity of the spacecraft appreciably but rather to bend the trajectory up by almost 25 degrees, out of the ecliptic, to meet Pluto which in 1986 would be over 1 billion km above the plane of the Solar System and closer to the Sun than the planet Neptune. Grand Tour 2 would be launched later, in November 1979 to encounter Jupiter in 1981, Uranus in 1985 and Neptune in 1988. Long identified 1977-1979 as the vintage years for launching Grand Tour missions, pointing out that this period allowed flight times to the outer planets to be reduced considerably without the necessity for launch vehicles beyond existing capabilities. It is interesting to note that he proposed the use of Titan Centaur rockets which were not then available. Although the Grand Tour itself had not received funding at this time work was underway on the spacecraft and computers which would be required for such a mission.

The TOPS Spacecraft

By 1967 NASA had flown three successful interplanetary missions. Mariners 2, 4 and 5, and no spacecraft had flown beyond the orbit of Mars so it was obvious that much engineering development would be needed to fulfill the demands of a 10 year Grand Tour mission. Some consideration of the advanced type of spacecraft required for such a mission had been given by Long in his original proposal and from these the staff of the JPL carried out a detailed study of the type of vehicle required for such a flight. The name given to the study was TOPS, reflecting both the nature of the proposed mission and the technology underlying it, TOPS was a contraction of Thermo-electric Outer Planets Spacecraft. The TOPS study was not a final design for a specific space mission but was organised as an advanced systems development exercise to define the limits of existing technology and to advance the state of the art where necessary.

The project was to consist of:

(1) The design of the complete outer planet spacecraft.

(2) The design, fabrication and test of certain hardware elements for subsystem capability demonstrations.

(3) The fabrication of a Feasibility Model spacecraft to demonstrate the required advanced technical capability.

(4) A test programme to explore interactions between subsystems and demonstrate the design concepts.

(5) A Reliability and Quality Assurance effort to provide a basis for future project assurance activities.

At the completion of a six month project definition in December 1968 the R & D funding was estimated to be 17.5 million dollars. In October 1969 when the concept was better understood the cost of hardware development was estimated to be 7M in FY70 and 10M in FY71 so a less ambitious plan deleting the feasibility model was selected. Some of the funds for the reduced project did not materialise on time so a further 4M was granted to allow an extension into FY72. This permitted a number of efforts to reach fruition and brought the final cost to about 21 million dollars.

The accomplishments of the project varied in nature some being only paper studies and others being major working models, known as breadboards. Although the feasibility model was dropped some system interactions were investigated separately and developments in microelectronic components served as vehicles to advance reliability technology. At the end of this major design effort it was possible to describe in some detail the type of spacecraft envisioned but the description which follows represents only one of the possible configurations of the TOPS.

The requirement for a 3657 msec -1 (12.000 ftsec -1) departure velocity limited the spacecraft weight to about 680 kg (15001 lbs) if existing launch vehicles were to be used and the final configuration of the TOPS weighed 656 kg (1446 lbs) of which 117 kg represented the weight of four Plutonium 238 Radioisotope Thermoelectric Generators (RTG’s) used to provide electric power during the mission. The power-to-weight ratio of RTG's becomes more favourable than that of solar cells for deep space missions because the diminishing amount of sunlight beyond the orbit of Mars requires the use of increasingly large collecting areas. These nuclear power sources were to be positioned at least 1.5 metres from the main spacecraft bus on deployable booms and were expected to provide 439 watts at the end of the planned mission. The remainder of the power supply system, consisting of voltage regulators, distributors, etc., weighed 10.4 kg (23 lbs).

The use of RTGs for power generation required the protection of the scientific instruments and spacecraft electronics from the radiation emitted by these devices. Although partly solved by positioning the RTGs on the booms additional protection was achieved by shielding and by the hardening of the components to the twelve year integrated dose from RTGs at a distance of 1.5 million km. Radiation protection allowing a close flypast of Jupiter was included to permit flexibility in the type of missions able to be flown. It was estimated that to be able to detect the transition from solar to interstellar magnetic fields the magnetometer would need to be removed at least 8.5 m (25 ft) from the rest of the spacecraft and so this and the plasma wave detector were mounted on a boom 9.2 m (30 ft) long. To maximise their field of view other scientific instruments, including the cameras,were also carried on booms. Studies were made to confirm that these booms would not render the vehicle dynamically unstable.

The spacecraft was to be stabilised in three axes using the type of Sun and Canopus sensors developed for earlier Mariner missions, but unlike the Mariners the TOPS attitude control system was to be based on reaction wheels. By storing momentum in a rapidly spinning flywheel and tapping it off again when required a considerable reduction in the number of thruster firings needed during the long mission could be achieved. This reduction in the propellant mass required was sufficient to compensate for the additional weight and complexity of the reaction wheels. Only when these became saturated would a monopropellant hydrazine thruster be used to unload surplus angular momentum.

For midcourse trajectory corrections a 110 Newton thrust hydrazine engine would provide a total AV of 220 m/sec,one and a half times the expected total impulse requirement for a Grand Tour mission.

For communication purposes a 4.3 metre diameter high gain antenna was considered necessary to support a data rate of 2048 bits per sec (bps) from the distance of Neptune. The antenna was carried into orbit in a stowed configuration and deployed when the spacecraft had separated from the launch vehicle's final stage. The design of this antenna proved to be a major challenge since to ensure maximum efficiency at long range it should accurately deploy to maintain the correct shape. Throughout the mission the antenna would be subjected to temperature gradients and micrometeor impacts both of which would tend to cause distortion. Studies indicated the need for approximately 48 ribs and suggested the use of a tricot knit of 0.017 mm (0.0007 in) Chromel-R alloy, gold plated for RF reflectivity. This was similar to the RCA developed high gain antenna used for the Apollo ALSEP.

The data rate was chosen to allow 400 pictures, each of five million bits, to be returned from Neptune over a period of 11 days. This set a correspondingly higher rate of data return for the nearer planets because of the reduced communication distances. Two redundant 10 bit tape recorders were included in the payload for use during encounters with the planet Saturn and beyond. At Jupiter, except during the period of Earth occultation, the 131,072 bps data rate available would allow realtime transmission of all science data. In the event of both tape recorders failing a 10.8 bit buffer could be used either to maintain the imaging experiments or for storage of most of the data from the other instruments.

The TOPS was intended to carry five S-band and two X-band transmitters for data return, although the X-band is only usable if the ground station has good clear weather. To provide extra redundancy two of the S-band transmitters used solid state amplifiers, the other three having the travelling wave type. A refinement to reduce tracking and data acquisition costs for this mission was the storage of science and engineering data on board for transmission at weekly intervals during the interplanetary cruise. This technique had been used by earlier Soviet spacecraft but not by any American probes so far flown.

The Grand Tour is Cancelled

The estimated cost of the project grew rapidly towards a figure of 900 million dollars. Consequently when NASA requested funds for a start on the mission in FY72 the proposal was not popular with budgetery officials and the U.S. President's 1972 budget request did not include funds for the mission. The official announcement of the cancellation was made on January 24,1972. However, the work of the planners was not to be wasted completely for John E. Naugles. Associate Administrator for Space Science, announced before a sub-committee of the House of Representatives committee on Science and Astronautics the creation of a new project, a “mini Grand Tour" based on Mariner type spacecraft and costing only 360 million dollars.

Work on a reduced programme had begun in January 1972 but the new project was not officially approved by NASA until the Contractual Task order was signed on 18 May 1972. The Project approval document was signed on June 3rd 1972 and finally the project plan was signed on 8 December of that year. Although some of the scientific community were disappointed by the cancellation of the Grand Tour the U.S. National Academy of Science declared itself completely behind the new project, which was to be named Mariner Jupiter/Saturn.

Mariner Jupiter/Saturn

To keep the MJS-77 within its budget, vital since NASA was already suffering financial cutbacks and more appeared likely in the near future, it was decided to use Mariner design and experience wherever possible and to supplement this with subsystems designed for the Viking orbiter to provide the required performance and reliability. The detailed design would be modified by information on the deep space and Jovian environments received from Pioneers 10 and 11 as they penetrated further from Earth than any previous spacecraft.

With the project now approved work commenced on specific planning of the missions and the scientific objectives of the flight were defined as comparative studies of Jupiter and Saturn through measurements of the environments, atmospheres and body characteristics of the planets, their satellites and of Saturn's rings. Specific objectives included study of the physical properties,surface features, periods of rotation, energy balances and thermal regimes of the planets and moons and investigation of electromagnetic and gravitational fields throughout the mission. Items of special scientific interest included Jupiter's great red spot,Saturn's rings and the Saturnian satellites Iapetus,Titan and low density Rhea.

The formal request for experiment proposals was made in April 1972 and over 200 replies were received; from these ninety scientists were chosen, mainly from the U.S. but including representatives of four European countries, namely France,Sweden,West Germany and the United Kingdom.

Other Outer Planets Missions

Although budgetary cutbacks had cancelled the Grand Tour nothing could change the planetary alignments and NASA still hoped to make limited use of them for other missions. One such proposal was Mariner Jupiter-Uranus. launching a backup MJS-77 spacecraft in November 1979 to flyby Jupiter in April 1981 and proceed to Uranus arriving in mid-1985. The 1979 launch was particularly favourable because of the unique approach geometries possible on arrival at Uranus. Since the planet would be pole on to the approaching spacecraft there would be an opportunity to study the high latitude regions under good lighting conditions for a prolonged period. In addition the orbits of the planet's five moons would be seen not as beads on a wire but rather as a target as Mariner sped towards rendezvous. Typical of the scientific results expected would be improved values of radii and masses for the planet and its satellites, unique photographs and infra-red studies impossible from Earth.

Other proposals based on Mariner and Pioneer type spacecraft included a Mariner Jupiter Orbiter,now developed into Project Galileo, and Pioneer missions carrying atmospheric entry probes either directly to Saturn or to Uranus via Jupiter Although only the Jupiter orbiter was chosen for development Uranus was not forgotten and plans to send one of the MJS-77 spacecraft on to Uranus were discussed by Robert S. Kramer, Director of Planetary Programmes, in November 1972. He described the Uranus and Neptune options and added that although not designed for this extended flight specifically he believed the spacecraft to "have a chance”. Perhaps the Grand Tour was not quite lost after all.

Project Voyager

Early in 1977 work on the project was well advanced for a late summer launch but the mission that had undergone so many changes was to undergo one more. It had been felt that since the MJS-77 spacecraft had departed so much from the original Mariner family that a new name would be appropriate. A competition was held to choose a new name and the winning nomination “Voyager", approved on 4 March 1977, was the name which has already earned a place in scientific history.

Prelaunch Operations

The Preshipment Review for the spacecraft was held at the JPL on April 12,1977 and the two flight spacecraft were shipped to the Cape on 21 April and 19 May 1977 respectively. The shipment required a caravan of trucks and took about four days for the trip from California to Florida. Each truck had two drivers so that only meal and fuel stops were required. The vehicles were special air-ride trucks and were instrumented to assure that no equipment damage occurred during shipment.

After the twe Voyagers were delivered to Cape Canaveral they underwent extensive checking and faults were discovered in the Attitude and Articulation Control (AACS) and Flight Data Subsystems (FDS) of the VGR77-2 spacecraft. This vehicle was to be launched first and to avoid a delay the decision was made to interchange the two spacecraft. Since the first launch trajectory included an option to extend the mission to Uranus the VGR77-2 spacecraft had RTGs of a higher output and it was necessary to exchange these with the ones aboard VGR77-3.

This was completed successfully and VGR77-3 was encapsulated on 9 August 1977. Post-encapsulation checks detected a fault in the low energy charged particle experiment and the spacecraft was removed from the shroud for repairs. Re-encapsulation took place the next day and after satisfactory checks VGR77-3 was moved to launch complex 41 and mated with the Titan-Centaur TC-7 launch vehicle.

Work went ahead to identify the failures within the VGR77-2 spacecraft. The AACS problem was traced to a thermally intermittent resistor in its computer and the AACS proof test model was installed in the Voyager in its place. The anomaly in the FDS was never duplicated in tests and remains a mystery to this day.

Departure

At 10:29:45 a.m. EDT on 20 August 1977. two years to the day after the launch of the first Viking Spacecraft. VGR77-3 - now known as Voyager 2 - was launched from Cape Canaveral. At the JPL employees and contractors were able to see the launch broadcast live from pad 41.