One of the glaring reasons for so many failures in Luna, Zond, Venera and Mars
missions in the late 60s and early 70s was poor performance of the Proton vehicle.
Succeeding in its initial launch and in two of its next three launches in 1965-66, its
initial performance appeared promising. But its record in the 3 years from March
1967 to February 1970 was abysmal. Ten of nineteen spacecraft were lost when the
Proton failed to deliver the Block D to Earth orbit. Another three achieved orbit but
were stranded when the second burn of the Block D failed. Only six of the nineteen
launches were fully successful. Sixteen were interplanetary, and the Proton failed in
eleven cases- four failures out of eight Zond launches to the Moon, five failures out
of six Luna launches, and the failure of both Mars launches in 1969. Unfortunately,
the failures were distributed throughout the vehicle including all stages, so it was
very difficult to make the vehicle reliable.
NPO-Lavochkin was so concerned at the Proton failures that General Designer
Georgi Babakin met with the Minister of General Machine Building in March 1970
to demand action. After the rocket underwent a full engineering review a number of
improvements were made, and the vehicle was re-qualified in a successful test flight
in August 1970. After this, the success record improved dramatically and eventually
the Proton became one of the most reliable workhorses in the Soviet launcher fleet.
Indeed, it today enjoys an excellent reputation and a large share of the commercial
launch market.