The beginnings of planetary exploration
by Andrew J. LePage
|Once the first Soviet Luna and American Pioneer probes had reached the Moon in 1959, the next logical targets were the planets.
Studies finished in early 1958 showed that there would be a favorable launch opportunity to Mars in August 1958 and to Venus in June 1959. Probes could be launched on direct ascent escape trajectories using the same 8K72 rocket that launched the Luna probes, although more powerful variants then under development were preferred. Almost immediately it was recognized that it would not be possible to develop a probe in time to launch to Mars in 1958, but the next launch window in September-October 1960 and the Venus launch window in June 1959 were thought to be achievable. By the summer of 1958 work began in earnest at OKB-1 on the Object 1V to land on Venus and Object 1M to fly past Mars.
In parallel with the effort to the design and build the planetary probes, work on more powerful launch vehicles needed for these and other missions was also progressing at OKB-1. All Soviet space launch vehicles at this time were based on the R-7 ICBM or 8K71 (also known by the NATO designation SS-6 or Sapwood) developed by Korolev and his talented team at OKB-1. The R-7 consisted of a cylindrical core called Blok A surrounded by four tapered boosters designated Bloks B, V, G, and D. The R-7 was designed so that the core and all four boosters would ignite on the pad. After the boosters had exhausted their propellants about 120 seconds after liftoff, they would be jettisoned leaving the core to continue alone using its larger propellant supply.
Each booster of the R-7 was powered by an RD-107 engine that used kerosene and liquid oxygen (LOX) as propellants. The core used an engine of similar design called the RD-108. These engines were designed and built by OKB-456 under the direction of Valentin Glushko. The RD-107/108 consisted of a single turbopump assembly feeding a cluster of four combustion chambers. The major external difference between the RD-107 and 108 was that they incorporated two and four small gimbaled vernier engines, respectively, to steer the R-7 and trim its velocity. The RD-108 also ran at a lower thrust level so that it could operate up to 210 seconds longer than the boosters’ RD-107 engines and its nozzles were optimized for operation at high altitudes. The rocket that launched the first Sputniks, the 8K71PS (known as the SL-1 in the West), was simply a stripped down version of this ICBM. The 8K72 (or SL-3) that was used to launch the first Luna probes was an R-7 with a small upper stage added called Blok E. An upgraded version of this rocket called the 8K72K was also under development to launch the Soviet Union’s first manned spacecraft, Vostok.
During 1958 plans were developed for a much-improved launch vehicle designated the 8K78. The first stages of the 8K78 would be based on the improved R-7A ICBM or 8K74 then under development. In addition to a laundry list of improvements to the avionics, structure and operations, the 8K74 sported upgraded RD-107/108 engines which generated 4,020 kilonewtons (902,000 pounds-force) of thrust at liftoff. In the original studies, a Blok Zh upper stage would be fitted to the R-7A. The Blok Zh was a modified second stage from an R-9A ICBM or 8K75 (eventually known as the SS-8 Sasin) then under development at OKB-1. This tapered upper stage used a four-chamber 8D715 engine developed by OKB-154 burning kerosene and LOX to generate nearly 295 kilonewtons (66,000 pounds-force) of thrust. This much improved launch vehicle could send probes weighing as much as 400 to 500 kilograms (880 to 1,100 pounds) to Venus or Mars via a direct ascent trajectory of the type used by the first Luna probes.
A simulated image of Mars as it might have appeared using the 1M camera system at a range of 30,000 km (top) compared to a simulated view with an Earth-based telescope (bottom). (credit: A.J. LePage)
But by the end of 1958, theoretical studies had discovered a better method for reaching the Moon and planets beyond. Instead of a continuous powered ascent where the payload is hurled directly into an escape trajectory, an escape stage with the payload attached would first be placed into a temporary parking orbit around the Earth. After coasting for a time, the escape stage would ignite at the optimum ejection point to send the payload on its escape trajectory. This technique has a number of advantages over the direct ascent technique: launch windows are longer and more flexible, the accuracy requirements for the launch vehicle are relaxed, and gravity losses are reduced, allowing more payload to be sent beyond the Earth.
|With a more capable launch vehicle under development, Korolev’s plans became even more ambitious.
With this new information in hand, the direct ascent technique was abandoned in early 1959 and a new, larger variant of the 8K78 (eventually known as the SL-6 Molniya) was hurriedly developed. The original Blok Zh was heavily modified and stretched to increase the propellant load by about half for an upgraded 8D715K engine. Designated Blok I, the new stage was now a cylinder with a diameter 2.66 meters (8.7 feet) and an overall length of 8.1 meters (26.6 feet). Combined with the R-7A, this three-stage rocket would eventually evolve into the Soyuz (or SL-4) launch vehicle whose descendants are still used today.
The new escape stage, designated Blok L, was 7.1 meters (23 feet) long and used a S1.5400 engine developed by OKB-1 that generated 67 kilonewtons (15,000 pounds-force) of thrust burning kerosene and LOX like the other stages of the 8K78. The technical challenges during the development of this stage proved especially difficult since it had to ignite after spending tens of minutes weightless in orbit—a technique that had yet to be developed anywhere. All together, the new 8K78 with its payload fairing for the 1V or 1M probe stood 41.7 meters (137 feet) tall and had a launch mass of 305.5 metric tons (674,000 pounds).
With no launch vehicle immediately available and with development of the 1V probe taking much longer than expected, it was quickly realized that the June 1959 launch opportunity to Venus would be missed. But since American plans to launch a Pioneer probe to Venus had also been cancelled and the first NASA planetary mission was not scheduled until 1962, Korolev and his team still had an opportunity to beat the Americans. Development efforts on the Mars probe for the September-October 1960 launch window continued and work shifted to a Venus launch in January-February 1961. Since the launch windows were now just months apart, the two planetary probes would share a unified design called 1MV. The probe outfitted specifically for Mars would keep its 1M designation while the modified Venus probe would now be designated 1VA. By August 1959 theoretical work on the missions to Mars and Venus was completed. On December 10, 1959, the Soviet government officially approved the 1MV mission.
As design work on the 1MV probes progressed through 1959, work on the new 8K78 launch vehicle proceeded steadily. The R-7A in its ICBM configuration was successfully tested on it first launch on December 24, 1959. The prototype 8K78 with a live Blok I third stage but an inert Blok L escape stage was successfully launched on a suborbital test flight that ended in the Pacific Ocean on January 20, 1960. A subsequent test of the R-7A with the third stage took place ten days later.
By early 1960 the design of the 1M probe was completed and its instruments selected. The 1M probe was a cylinder 105 centimeters (41 inches) in diameter with a roughly hemispherical cap giving it a height of around two meters (6.5 feet) and a mass in excess of 650 kilograms (1,430 pounds). The interior was pressurized and the temperature controlled to provide a laboratory-like environment for the internal equipment. A pair of solar panels with a total area of two square meters (22 square feet) was used to recharge batteries that supplied power to the probe’s equipment. It was also fitted with a KDU-414 engine generating 2 kilonewtons (440 pounds-force) of thrust to be used for midcourse maneuvers to ensure the probe flew between 5,000 and 30,000 kilometers (3,000 and 19,000 miles) from Mars. The module containing the engine and its propellants would be jettisoned around March 1961 after it was no longer needed.
The 1M carried instruments to measure magnetic fields, cosmic rays, radiation, the solar wind, and micrometeorites. It also carried spectrometers, an instrument designed to detect organic molecules on the Martian surface, and a camera similar to the one carried by Luna 3, which photographed the far side of the Moon in October 1959. This imaging system used photographic film that was developed and scanned electronically onboard after the photographs were taken during the flyby. The images, which were expected to reveal features as small as 3 to 6 kilometers (2 to 4 miles), would be transmitted back to Earth using an umbrella-like antenna with a diameter of 2.33 meters (7.64 feet) operating at a wavelength of 8 centimeters. Other data would be transmitted back to Earth periodically using a transmitter operating at decimeter wavelengths.
|Korolev’s tight schedule was coming unraveled in the summer of 1960 as final preparations for the launch of the 1M probes and the new 8K78 proceeded.
With a more capable launch vehicle under development, Korolev’s plans became even more ambitious. In February 1960 he unveiled a plan to not only fly a pair of probes by Mars and Venus but also probes to land on their surfaces. Described as being the size of a television set and weighing no more than 285 kilograms (630 pounds), the lander would detach from the 1MV bus and use parachutes to make the final descent to the surface. Through the summer of 1960 models of the landers were carried to altitudes of 50 kilometers (165,000 feet) by R-11A rockets (the sounding rocket version of an early variant of the infamous SS-1 Scud missile). But in the end Korolev’s goal proved too ambitious and the landers were abandoned, leaving only the pair of Mars flyby probes. Ironically, even if the landers did make it to their targets they were doomed to fail. The assumed atmosphere of Venus was too thin and the lander would have been crushed kilometers above the surface while still descending on its parachute. Moreover, the assumed atmosphere of Mars was too thick, so that the lander would have crashed into Mars having been barely slowed from its interplanetary approach speed.
Despite the loss of the landers, celestial mechanics gave the Soviet Union a shot at a potentially huge space spectacular. Final trajectory calculations showed that the Mars launch window would open on September 20 and be optimum on the 27th for the 1M probes to reach their destination in mid-May 1961. By chance, the 1VA probes to be launched four months later on a shorter course would reach Venus around the same time. In order to spread out the critical flyby tracking tasks (as well as maximize the propaganda value), it was decided to schedule the first 1VA probe to flyby Venus on May 11 followed by the two 1M probes reaching Mars on May 13 and 15 with the second Venus flyby taking place on May 19. Coming on the heels of the launch of the first manned Vostok in April 1961, this achievement would have had a devastating impact on the American image of technological preeminence—if it had worked.
Korolev’s tight schedule was coming unraveled in the summer of 1960 as final preparations for the launch of the 1M probes and the new 8K78 proceeded. In August, three Soviet tracking ships left their Black Sea port to take up stations around the globe to monitor key events of the Mars probe launch. But as the launch window opened on September 20, neither the rocket nor the probe was ready. The Blok L escape stage still had not been certified while the final assembly and testing of the 1M probe had yet to be completed as problems with the radio system were being diagnosed. Interference between the radio and camera package ultimately forced the removal of the camera. With the earliest possible launch now past the optimum date, mass now needed to be removed from the probes anyway.
But with every passing day as the launch energy requirements increased, additional equipment was removed from the 1M, including the spectrometers. The 1M now had a mass of 650 kilograms (1,430 pounds) with only 10 kilograms (22 pounds) allotted for scientific instruments. Finally by the first week of October, the Blok L escape stage was certified. Skipping the final pressure testing to save time, the first 1M was delivered to the Baikonur Cosmodrome in Soviet Kazakhstan on October 8 and final preparations for launch began.
|Coming on the heels of the launch of the first manned Vostok in April 1961, this achievement would have had a devastating impact on the American image of technological preeminence—if it had worked.
On October 10, 1960 8K78 number L1-4 lifted off its pad carrying the first 1M probe. Unfortunately resonant vibrations in the Blok I while under the power of the Blok A core stage became so severe that it caused a malfunction in the rocket’s pitch control. After the Blok I stage ignited, the rocket veered off course and was shut down early at 309 seconds after launch. The 8K78 reached a peak altitude of 120 kilometers (75 miles) before falling back to Earth and crashing in eastern Siberia.
With only days left, the next launch vehicle and payload were hurriedly prepared. On October 14 the second 1M probe was launched on 8K78 number L1-5 but it was doomed from the start. The 8D715K engine in the Blok I stage failed to ignite after separating from the core at 290 seconds after launch. The kerosene fuel in the engine inlet had been frozen solid while the rocket was still on the pad due to a cryogenic LOX leak. The unpowered stages, with the 1M probe, broke up in the atmosphere as they fell back to Earth. Any hopes for a Mars mission were now pushed to 1962.
These launch failures were never acknowledged by the Soviet authorities even after American intelligence agencies made them public in June 1963. All Soviet effort now turned towards launching the pair of 1VA probes to Venus in just three months in hope they would be the first successful planetary missions.