PLANETARY SPACECRAFT

There were essentially three general design series of Russian planetary spacecraft. None of them resembled their American counterparts because, unlike the latter, the Russian spacecraft required pressurized containers for most of their electronics. The Venus and Mars flights in 1960-61 used the first generation spacecraft, which were simple pressurized canisters with attached solar panels and high gain antennas. Their payloads were specific to the target planet, but in general the spacecraft were the same. Of the four launched, only Venera 1 was successfully dispatched and it failed early in its cruise through interplanetary space.

The second generation introduced the first modular spacecraft, with a pressurized carrier that had the propulsion system at one end and a module for the payload at the other. They were individually outfitted for missions to Mars or Venus, with either an entry probe or a flyby module for remote sensing. (This same modular approach was adopted for the second generation Ye-6 lunar spacecraft series.) There were two sub-types of this spacecraft, 2MV and 3MV. Six 2MV spacecraft were launched in 1962, three for Venus and three for Mars, but only one, Mars 1. survived its launch vehicle. The flight of Mars 1 was plagued with problems and it succumbed half way to its target, but the lessons learned were applied in developing the 3MV. Seventeen 3MV spacecraft were launched between 1963 and 1972, five of which. Venera 4 to 8. achieved their planetary objectives. One of the Mars types, Zond 3. did achieve significant results by imaging the far side of the Moon as it departed and subsequently testing the communications system by transmitting the pictures from deep space.

The third generation planetary spacecraft were a major design change, enabled by the powerful Proton launcher with the Block D upper stage. These much larger and

figure 5.5 Representative Soviet planetary spacecraft to scale: first generation Venera 1 (upper left); second generation Venera 4 to 8 (upper right); and third generation Venera 9 to 14 at lower left; and Mars 2. 3, 6 and 7 at lower right (from Space Travel Encyclopedia).

more complex spacecraft were meant to provide planetary orbiters and soft-landers, starting with Mars in 1969 and Venus in 1975. Of twenty-two launched, Venera 9 to 16 and Vega 1 and 2 ran up a string of straight successes at Venus. The other twelve experienced a more difficult challenge at Mars, where only five can be deemed even partial successes, Mars 2 and 3, Mars 5 and 6, and Phobos 2. The Phobos missions of 1988 and the Mars-96 spacecraft were derivatives of this class, but with upgrades sufficiently significant for them perhaps to be regarded as another generation.

In normal flight, Russian spacecraft were flown in uniaxial orientation in which their static solar panels were oriented constantly towards the Sun and the craft spun at 6 revolutions per hour on the axis perpendicular to the plane of the solar panels. The command uplink was at 768.6 MHz through semi-directional conically-shaped spiral antennas which were also used for low-rate data transmission. Because these antennas generate funnel-shaped radiation patterns, several were placed around the spacecraft pointing at the Sun, and at any point in the mission the one with the best funnel angle for Earth was used. For high data rate transmissions, a parabolic high – gain antenna was affixed to the spacecraft. This had to be aimed directly at Earth by disabling the uniaxial control mode, reorienting the spacecraft appropriately, and switching to the three-axis orientation control mode. Circularly polarized decimeter (~920 MHz) and centimeter (~5.8 GHz) band transmitters shared the dish antenna.

In 2MV and 3MV missions, planetary probes and landers were designed for direct transmission to Earth by small spiral antennas with pear-shaped radiation patterns.

The heavier Proton-launched Mars and Venera landers were designed to relay their transmission through flyhy or orbiter spacecraft using large meter band (186 MHz) helical antennas mounted on the rear of the solar panels. The Mars 3 class of entry vehicle carried small wire antennas on the entry stage and another set on the lander. The Venera 9 class of entry vehicle had another large helical antenna installed on top of the lander. Data from the Mars and Venus entry systems was stored for later transmission, but in the case of the Venus landers it was also relayed in real-time as a precaution. The entry system data link operated at 72,000 bits/s for Mars and at 6,144 bits/s for Venus.