The Nedelin catastrophe or Nedelin disaster was a launch pad accident that occurred on 24 October 1960, at Baikonur Cosmodrome during the development of the Soviet R-16 ICBM. As a prototype of the missile was being prepared for a test flight, it exploded on the launch pad when its second stage motors ignited prematurely, killing many military personnel, engineers, and technicians working on the project. The official death toll was 90, but estimates are as high as 200, with 120 being the generally accepted figure. Despite the magnitude of the disaster, news of it was covered up for many years by the Soviet government and did not emerge until the 1990s. Strategic Rocket Forces Marshal Mitrofan Nedelin, the commander of the R-16 development program, was among those killed in the explosion and fire.
Chief Marshal of Artillery Mitrofan NedelinDesigned by experienced rocket scientist Mikhail Yangel, the R-16 development program was commanded by Strategic Rocket Forces Marshal Mitrofan Ivanovich Nedelin. In October 1960 the rocket was nearing completion, and Yangel and Nedelin hoped to produce a successful launch before the 7 November anniversary of the Bolshevik Revolution. A prototype of the rocket was ready on the launchpad at Site-41 at Baikonur Cosmodrome, and the numerous tests that had to be undertaken before launch were commenced without delay.
On 23 October, the R-16 rocket prototype was on the launching pad awaiting final tests before its firing. The rocket was fueled with Devil's Venom — hypergolic UDMH-nitric acid — which is used in rocketry despite the fact that it is an extremely corrosive and toxic binary fuel that produces poisonous gas when burned. These risks were accounted for in the safety procedures in preparing the rocket for launching, but, late that day, technicians accidentally ruptured the pyrotechnic membranes of the first-stage fuel lines and allowed the fuel into the combustion chamber. Although that was not immediately dangerous, the fuel's nitric acid component was so corrosive it could not be in the fuel lines for more than two days without seriously damaging the R-16 rocket. Thus, the rocket team had either to launch the next day or drain the fuel from the rocket and then rebuild the engine, and so delay the program several weeks. The rocketeers decided to fire the rocket and accelerated preparations. Several other rocket components were tested that day and either replaced or adjusted per procedure. Nedelin notified military dignitaries of the launch so they could go to the site and see it.
24 October :
Site 41 after the catastrophic R16 rocket's explosion.On 24 October launching preparations continued. So much work remained that some procedures were performed simultaneously. Nedelin, impatient with the delay, left the military dignitaries in the observation post and returned to the launching pad to oversee the preparations of the rocket; he set a chair beside it.
In the course of the pre-launching operations, a Programmable Current Distributor (PCD) was left set to the post-launch setting; it should have been re-set to the pre-launch setting — from which it would issue timed electrical commands to the rocket to rupture the appropriate pyrotechnic membranes and coordinate the engine firing and stage separation. Later, an engineer noticed the PCD had not been re-set to zero and so he did it. However, the rocket’s on-board batteries had been powered and connected, and the safety blocks had been disabled in the course of testing. The re-setting of the PCD opened the pyrotechnic valves and fired the second stage engines of the rocket.
The second stage engines fired immediately. The flames cut into the first-stage fuel tanks below and they exploded. Automatically-activated cinema cameras set around the launching pad filmed the explosion. People near the rocket were instantly incinerated; those farther away were burned to death or poisoned by the resulting toxic gases. Andrei Sakharov described many details—as soon as the engines were fired, most of the personnel there ran to the perimeter but were trapped in it by the security fence and then engulfed in the fireball of burning fuel. Nedelin and 125 other rocket personnel were killed, but Yangel survived. He had left the area to smoke a cigarette, where he and the chief technicians discussed possibly abandoning the rocket launching. Eighty-four soldiers and officers were buried in a common grave in the Leninsk town park.
The remains of the R-16 rocket prototypeComplete secrecy was immediately imposed on the events of 24 October by Nikita Khrushchev. A news release stated that Nedelin had died in a plane crash, and the families of the other engineers were advised to say their loved ones had died of the same cause. Khrushchev also ordered Leonid Brezhnev to assemble a commission and head to the launch site to investigate. Among other things, the commission found that many more people were present on the launch pad than should have been — most were supposed to be safely offsite in bunkers.
According to Sergei Khrushchev, Brezhnev had insisted that the commission did not intend to punish anyone, explaining that "The guilty have already been punished".
After the question to Yangel asked by Nikita Khrushchev "But why have you survived?" (А ты почему остался жив?) Yangel answered in a trembling voice - "Walked away for a smoke. It's all my fault" («Отошел покурить. Во всем виноват я»). Later he suffered an infarction and was out of work for months. 
After the committee presented their report, the R-16 rocket program was resumed in January 1961 with its first successful flight that November. The delay to the R-16 spurred the USSR on toward the development of more effective ICBMs and sparked Khruschev's decision to install IRBMs in Cuba. Before the disaster Yangel had ambitions to challenge Sergei Korolev as leader of the Manned Space program, but he was directed to focus on the R-16.
A memorial to the dead was erected near Baikonur and is still visited by RKA officials before any manned launch.
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