Sixty years ago on this day, July 21, 1961, Virgil “Gus” Grissom became the second U.S. astronaut to fly into space. What was learned from this mission resulted in changes in spacecraft design to improve safety, but the changes would instead contribute to Grissom’s death almost six years later.

The flight of Liberty Bell 7 atop a Mercury-Redstone rocket was textbook from its 7:20 a.m. launch to splashdown in the Atlantic 15 minutes and 30 seconds later. It was while floating in the ocean that things went wrong.

One of the big differences between Grissom’s Liberty Bell 7 capsule and first American in space Alan Shepard’s Freedom 7 capsule was the hatch. Mercury astronauts’ capsule hatches were bolted shut. Grissom’s was the first to feature explosive bolts to quickly open the door. The plan was for a helicopter to hook to the capsule and lift it upright. Then Grissom would fire the charge that would blow the hatch and he would climb out, get into a horse collar and be lifted into the hovering helicopter without ever touching the water.

While Grissom was securing Liberty Bell 7 after splashdown and talking with helicopter pilot Jim Lewis hovering above, he got ahead on his checklist and removed the cap from the detonator and pulled the safety pin. But he never pushed the plunger.

Grissom recalled what happened shortly afterwards. “I was just lying there minding my own business when pow!” he said. “The hatch went and I looked up and saw nothing but blue sky, and water started coming in over the sill. So I tossed my helmet off. The only two moves I remember making were tossing my helmet off and grabbing the instrument panel and pulling myself out.”

As the capsule quickly began to sink, Lewis saw Grissom in the water giving a thumbs-up signal. Thinking the astronaut OK, he flew in and his crew attempted to hook to and lift Liberty Bell 7 from the ocean. With the wheels of the helicopter touching the water, they were able to hook to the capsule just as it was about to go under.

The helicopter lifted the capsule back to the surface and the pilot was allowing water to drain out to lighten the load before lifting the craft clear. Lewis recalled later that he almost got the capsule out as far as the heat shield when a wave came along and swamped it again.

With a “chip detector” alarm indication of metal flakes in the oil, a sign that the engine could fail at any moment, Lewis made the decision to cut Liberty Bell 7 loose. The craft quickly sank in three miles of water.

As all this was going on, no one realized that Grissom was in trouble. In his haste to get out of the sinking capsule he had forgotten to close the oxygen valve on his spacesuit. Normally, with the valve closed the suit is water tight and he could float. Now he was fighting to keep his head above water as his suit filled with water.

With the capsule lost, a second helicopter came in and lifted Grissom from the water.

NASA’s investigation never uncovered any mechanical reason for the hatch to blow prematurely. Grissom was also vindicated when future Mercury astronauts who blew the hatch suffered bruising from the recoil of the plunger. There was no bruising on Grissom, showing that he never touched the plunger, therefore no pilot error.

Grissom remained in the flight rotation and became the first American to fly in space twice when he flew again in project Gemini. He named his Gemini 3 spacecraft Unsinkable Molly Brown in reference to what happened to Liberty Bell 7. NASA turned down his first choice of a name, Titanic.

Because of what happened to Grissom on his Mercury flight, the hatch for the Apollo program was redesigned. The outward opening hatch with explosive bolts was replaced with an inward opening three-part hatch with more than 20 latches that were opened by pumping a handle that operated a latch drive. In theory it was to take about five seconds to work the latches before opening the hatch and the crew should be out in under 90 seconds.

Just as Grissom was the first to use the explosive hatch in Mercury, he would be the first to use the redesigned hatch as a member of the crew of Apollo 1.

On Jan. 27, Grissom, along with astronauts Ed White and Roger Chaffee were conducting a “plugs out” test of Apollo 1. Part of this test required raising the pressure in the command module to test for leaks. During this part of the test a fire broke out in the pure oxygen environment, racing through the cabin. The heat of the fire increased the pressure in the spacecraft. The astronauts followed the emergency exit procedures, but were unsuccessful in opening the hatch and died in the fire.

The investigation that followed determined that the extreme pressure generated by the fire held the inward opening hatch shut, greatly diminishing any chance of escape.

North American, the designer of the command module, originally recommended outward opening hatches and the use of explosive bolts to blow the hatch but was overruled by NASA based on what happened to Grissom on his Mercury flight.

Now because of what happened to Grissom and the other astronauts on Apollo 1, the hatches would be redesigned again. They would open out and would use a pressurized nitrogen cartridge to quickly drive the release mechanism in an emergency.

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