If at first you don’t succeed… (part 1)
by Andrew J. LePage
Sergei Korolev was the Chief Designer at OKB-1. This design bureau was responsible for many of the early Soviet space programs including the first generation of probes to Venus and Mars. (credit: NASA)
In order to meet the demanding schedule as well as incorporate the lessons learned from the development and flight problems of his first quartet of Soviet planetary probes, Korolev envisioned a common multipurpose interplanetary spacecraft design to be used for both the Mars and Venus missions, designated 2MV. While sharing a common configuration and most key systems, slight variations would exist in this basic design to better tailor the spacecraft for their specific targets and missions. With a launch mass of just under a metric ton (2,200 pounds), these spacecraft would be far larger and more capable than the American Mariner spacecraft also under development at that time.
|Chance and celestial mechanics meant that the next launch windows to Venus and Mars would be back to back—August/September 1962 to Venus and October/November 1962 to Mars—thus increasing the workload on Korolev and his team.|
On July 30, 1961, the 2MV series was officially approved for development by OKB-1. The 2MV spacecraft were about 3.6 meters (12 feet) tall and consisted of two sections. The first, known as the orbital compartment, was a cylinder with a diameter of 1.1 meters (3.6 feet) that was about as tall. As was the usual Soviet practice, the interior of this compartment was pressurized with dry nitrogen to 113 hectopascals (16.4 pounds per square inch) in order to simulate an Earth-like laboratory environment for the internal equipment. This approach increased the mass of the spacecraft but greatly simplified the thermal design and testing of the spacecraft’s systems. The equipment inside the orbital compartment included communications gear, power supplies and their associated batteries, automated control systems, data recorders, and some experiment electronics. Because of the problems encountered with the astro-orientation system during the brief flight of Venera 1, various sun and star sensors were now mounted inside the orbital compartment and looked through a window to acquire and track their targets in order to provide better thermal control for this vital system.
Mounted on top of the orbital compartment was a course correction system that employed a KDU-414 engine designed and built at OKB-2, led by Aleksei Isayev. This same engine had been included in the previous quartet of Soviet planetary probes but had not had a chance to be used. The pressure-fed KDU-414 burned UMDH (unsymmetrical dimethylhydrazine) and nitric acid to generate two kilonewtons (440 pounds-force) of thrust. Normally this engine would be employed twice during a typical mission: once a few days after leaving the Earth to correct the 2MV trajectory for launch errors, and a second time a few days before encountering its target to refine its approach trajectory to meet the mission’s objectives. Also located here was the attitude control system, which used pressurized nitrogen stored in a pair of tanks mounted on the orbital compartment.
Mounted either side of the orbital compartment were a pair of solar panels with a total span of about 4 meters (16 feet) that provided power for the spacecraft. Attached to the ends of the solar panels were hemispherical radiators designed to provide thermal control for the spacecraft’s interior systems. Water pumped through heat exchangers in the interior would circulated out through black- or white-painted sections of the radiators to heat or cool the spacecraft systems as needed to maintain the interior’s temperature between 20°C and 30°C (68°F and 86°F). A complicated louver system controlled by electric motors was employed on the earlier Soviet planetary probes but it proved to be unreliable and insufficient. Engineers hoped that this new system would be more capable and robust.
A two-meter (6.6-foot) in diameter high-gain directional antenna mounted on the anti-sun side of the orbital compartment was used for long distance communications. Various low gain antennae were also mounted on the exterior of the orbital compartment to provide an omni-directional communications capability, using transmitters operating in three different frequency bands. Instrument sensors to measure magnetic fields, various types of radiation, and micrometeoroids that were mounted on the exterior rounded out the orbital compartment.
The second pressurized section of the 2MV spacecraft was called the planetary compartment. Mounted on the bottom of the orbital compartment, the planetary compartment held instruments to be used to study the target planet. The first type of planetary compartment contained cameras as well as other optical instruments that looked through a porthole in the compartment’s base to study the target planet during a close flyby. There were two variants of 2MV spacecraft that carried such a payload: the 2MV-2 designed for a Venus flyby and the 2MV-4 for Mars. At the heart of the 2MV planetary compartment was a 32-kilogram (71-pound) camera system. The system was designed to take a total of 112 images on 70 mm film through 35 mm wide angle and 750 mm telephoto lenses. Film-based imaging systems like this had better performance than the early vidicon-based imaging systems of the day like that employed by the American Ranger lunar probe (see “Ranger: America’s first successful lunar program”, The Space Review, February 3, 2014) and could more efficiently store larger amounts of data than any analog or digital data storage system of the time. After all of the images were acquired, the film would be automatically developed and then scanned for transmission back to Earth as the Soviets had done with Luna 3 when it photographed the Moon’s unexplored far side in October 1959. The developed images could be scanned at 1440, 720, or 96 lines, and the film could be rewound and rescanned to retransmit images if desired.
The approximately one-meter (three-foot) tall planetary compartment also contained its own high-power impulse transmitter to send images back to the Earth after the encounter with the target planet was completed. The planetary compartment’s transmitter fed directly through to the high gain antenna to transmit pictures at a rate of 90 pixels per second. It would take over six hours to transmit a full-resolution, 1440-line image. The lower-resolution preview images could be transmitted much more quickly through the planetary compartment’s dedicated transmitter or more slowly through a less capable transmitter in the orbital compartment as a backup. At full resolution, the system was capable of returning images with a pixel footprint as small as 650 meters (2,000 feet) from a range of 10,000 kilometers (6,200 miles).
|Since the 2MV landers were intended for targets with very different conditions (some of which were only very poorly understood at the time), there were notable differences between the two lander variants.|
Also included in the planetary compartment was an ultraviolet spectrograph designed to make measurements of the target planet’s atmospheric composition. It made periodic exposures on the same film used by the camera. The Mars-bound 2MV-4 variant also carried an infrared spectro-reflexometer that was coaligned with the camera and ultraviolet spectrograph. It was designed to make measurements in the 3 to 4 micron wavelength range of what were then called “Sinton bands” named after American astronomer William M. Sinton of the Lowell Observatory. He had found a trio of absorption features near 3.5 microns in the late 1950s that were thought by some to be potential evidence of plant life on Mars. Similar instruments were to have been carried by the pair of ill-fated 1M Mars probes launched in October 1960 but were removed to save weight. The 2MV-2 also carried infrared instrumentation to observe Venus and remotely measure its temperature.
Russian diagram of the 2MV-4 Mars flyby probes launched in 1962 (credit: RSC Energia)
The second type of planetary compartment carried by the 2MV had a much more ambitious mission: the 2MV-1 and 2MV-3 planetary compartments were designed to separate from the orbital compartment shortly before their planetary encounter and land on Venus and Mars, respectively, years ahead of what NASA was hoping to achieve with their proposed Mariner B spacecraft then under study. Both landers were spheroids about one meter (three feet) in diameter with their center of gravity offset from their center of figure so that they would naturally orient themselves blunt side down during entry without the need of an active attitude control system. Starting in the summer of 1960, lander models were lofted to altitudes as high as 50 kilometers (165,000 feet) on R-11A rockets (the sounding rocket version of the infamous SS-1 Scud missile) to test the lander designs at high altitude.
Both lander variants carried instruments to measure the temperature, pressure, density and composition of the atmosphere during descent and on the surface. Also carried was an instrument to measure gamma rays so that the quantities of radioactive elements like potassium-40, thorium, and uranium present in the surface could be measured, allowing geologists to identify the types of rocks present. The landers carried no cameras since their data volume requirements exceeded the limited capacity of the direct radio communications link from the lander to the Earth.
Since the 2MV landers were intended for targets with very different conditions (some of which were only very poorly understood at the time), there were notable differences between the two lander variants. The 2MV-1 Venus lander was expected to survive an entry velocity of about 11.7 kilometers (7.3 miles) per second, compared to the 2MV-3 Mars lander’s typical 6.1 kilometers (3.8 miles) per second entry velocity. Engineers at OKB-1 expected Venus to have a dense atmosphere of carbon dioxide with a surface pressure of 1.5 to 5 times that of Earth’s and temperatures up to 77°C (171°F). While there were those in the astronomical community who thought the atmosphere of Venus could be much denser with surface temperatures as high as 324°C (615°F), it was still anyone’s guess at this point in time. Based on the best information then available, Soviet engineers expected Mars to have a thin atmosphere composed primarily of nitrogen with a surface pressure of around 10% to 30% that of Earth’s with temperatures typically well below freezing. As a result, the Venus-bound 2MV-1 was more heavily built with a more robust heat shield and a smaller parachute while the 305-kilogram (672-pound) Mars-bound 2MV-3 sported a lighter heat shield but a much larger and heavier parachute system to ensure a safe landing.
|Despite its problems, the 8K78 was still the most powerful rocket in the world at the time and had about four times the lifting capability of the American Atlas-Agena, which had been experiencing its own problems.|
The nature of the surface of Venus was almost completely unknown at this time because of the unbroken layer of clouds that shroud the planet from our view. As a result, the 2MV-1 lander was designed to not only survive a touchdown on dry land but also float in any Venusian ocean that could exist with a motion detector providing information on any wave action. While scientists at this time did not believe that any oceans existed on Mars, the possible presence of smaller bodies of water could not be excluded and the 2MV-3 Martian lander could likewise float in the unlikely event of a water landing. The 2MV-3 was also equipped with a simple experiment to search for signs of Martian life that was widely believed to exist at that time. In order to minimize the chances of contamination, the Mars and Venus landers were sterilized. The orbital compartments that carried the landers were expected to burn up on entry.
The launch vehicle for the 2MV spacecraft was the four-stage 8K78 (soon to be known as “Molniya” after the communication satellite series that started regularly using this rocket in 1964.) Designed and built at OKB-1, all four stages of this rocket used kerosene and liquid oxygen (LOX) as propellants. The first two stages of the 8K78 consisted of a core and four tapered boosters based on the Soviet’s R-7A ICBM (also known as the 8K74 by the Soviets or the SS-6 “Sapwood” in the West). Fitted with the Blok I third stage, this rocket would serve as the basis of the Soyuz launch vehicle whose distant descendants are still in use today after half a century of service. In order to provide the final boost to propel the 2MV from low Earth parking orbit and towards Venus or Mars, a Blok L escape stage topped the launch vehicle.
The 8K78 had performed poorly during its first three flights. The two 1M Mars probes launched in October 1960 never made it to orbit because of failures involving the Blok I third stage. The first 1VA Venus probe made it into its parking orbit but was stranded by a Blok L escape stage malfunction. Only the fourth flight successfully sent Venera 1 on it way to Venus February 12, 1961. For the 2MV flights, engineers made a number of improvements to enhance the performance and reliability of the 8K78 and lengthened the payload shroud by 2.3 meters (7.5 feet) to accommodate the larger payload. Despite its problems, the 8K78 was still the most powerful rocket in the world at the time and had about four times the lifting capability of the American Atlas-Agena, which had been experiencing its own problems.
The story of the 2MV missions continues in part 2 next week.
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