In late October of 1960, about 150 families around the Soviet Union got letters notifying them that a loved one had died in a plane crash. It took thirty years for the public to be informed of what actually happened. It's known as the Nedelin Catastrophe, and it's one of the most chilling accidents of the nuclear age.
One of the most horrific aspects of the Nedelin Catastrophe was that it was so easily preventable. Any one of a dozen safety measures could have been taken to stop the explosion - starting with keeping the rocket on schedule. The R-16 rocket was an ICBM meant to improve on the existing model. It included a fuel that would allow the missile to be launched more quickly, and kept ready longer, than the outdated R-7 rocket. The fuel included an oxidizer, making it far more volatile than fuels that needed a supply of oxygen imported to burn quickly. The fact that the fuel was so corrosive, and when burned, so poisonous, that it was called "the devil's venom," was a problem, but wouldn't have done so much damage if the rocket had stayed on its original schedule. Designed by Mikhail Yangel, it was undergoing a lot of electrical problems but coming along very well. Unfortunately, the project was headed by Air Marshal Mitrofan Nedelin, who wanted it to be ready in time for the anniversary of the Bolshevik Revolution. He forced back the launch date giving the engineers ten fewer months to work out the bugs in the design.
When the R-16 got to the launch pad, and was discovered to be leaking this volatile fuel, it was suggested that, schedule or no, the fuel should be drained. Nedelin said no. The fuel, if let out of the rocket, was so corrosive that the entire rocket would be compromised. They ordered people to fix the leak. Meanwhile, the rocket itself had some systematic problems. Many of the valves that allowed the fuel to flow from one place to another were single-use. They weren't maneuvered opened, they were blown open, which meant that they couldn't be tested. If they worked, it was only going to be once. A small electrical problem had already sprung up during the testing of the rocket. Either through wiring errors or human error, the wrong valves were blown open. The rocket had to be repaired on the fly, with the clock counting down.
This left the launch pad, on October 24th, crowded with people. Repairs were being made, and leaks being plugged, and electrical systems were being rewired. The pad, and the road up to it, were paved with fresh tar. The boundary fence, meant to mark the edge of the safe zone around the launch pad, was covered with barbed wire. Nedelin repeatedly refused to drain the leaking fuel, and at one point called a person asking him to move away from the rocket a "coward." After a series of tests, a switch was accidentally left down, keeping one of the sets of valves between the fuel and the combustion chamber of the rocket open. The engineers also flipped a switch on what was meant to be a back-up system which was meant to allow the fuel to flow if the regular system somehow shut off.
At about 6:45 in the evening, the engine came to life, and the combustion hit the fuel. The entire rocket exploded in a fireball that killed many of the 250 people on the launch pad. The wall of flame melted the tar on the road, sticking some people in place as the fire moved toward them. Some people made it to the perimeter fence, only to get caught up in the barbed wire. As a last resort, some people flung themselves into ditches, out of the way of the flames, only to be poisoned by the fumes from the ignited fuel. About 150 people died - some of them immediately, some of them after days of suffering in the hospital.