The second M-69 spacecraft was identical to its predecessor (launched six days before) and was intended to enter orbit around Mars on Sept. 15, 1969. Like its twin, it also never reached intermediate Earth orbit.
At launch, at T+0.02 seconds, one of the six first-stage engines of the Proton malfunctioned. Although the booster lifted off using the remaining five engines, it began veering off course and eventually turned with its nose toward the ground (at about 30 degrees to the horizontal). At T+41 seconds, the booster impacted about 2 miles (3 kilometers) from the launch site and exploded into a massive fireball. The launch complex was not affected, although windows shattered in the Proton assembly building.
Engineers believed that even if either, or both, of these M-69 Mars spacecraft had gotten off the ground, they probably had very little chance of success in their primary missions. Both the M-69 spacecraft were brought to launch in a period of immense stress and hurry for engineers at Lavochkin. In remembering the M-69 series, one leading designer, Vladimir Dolgopolov, later remembered that, “these were examples of how not to make a spacecraft.”
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.