A half-century ago, NASA was preparing feverishly for a moon landing in a race against the former Soviet Union. The non-stop campaign of testing and launches was also a race against time—specifically to honor slain president John F. Kennedy’s 1961 pledge for the country to land a spacecraft on the moon (and return safely) before the end of the decade.

America would meet that challenge on July 20, 1969, but the effort would be built on sacrifice and tragedy. Some eight astronauts and astronaut candidates died in airplane crashes or vehicle tests, many other NASA ground crew and workers perished during accidents, while dozens of test pilots died in the decades leading up to Apollo.

“The Apollo experience was unique,” says space historian John Logsdon, professor emeritus at George Washington University. “It was closer to a war in a military-like experience of a great battle or an invasion than today’s space activities.”

To get a sense of how the Apollo program was different than today’s human spaceflight efforts, it’s important to go back to the post-World War II era. During this time, test pilots were breaking the sound barrier—and often their new jet aircraft—to reach supersonic speeds.

In the late 1940s, these pilots came from either the Navy or Air Force, or the National Advisory Committee for Aeronautics (NACA), the forerunner to NASA (which was created in 1958).

Howard C. “Tick” Lilly was the first NACA engineering pilot and the fourth person to break the sound barrier in the skies over California’s Mojave Desert. But on May 3, 1948, Lilly’s Douglas D-558-1’s engine compressor failed, severing control cables, and the airplane crashed. He was the first NACA pilot to die in the line of duty.

A month later, Capt. Glen W. Edwards and four crew members were killed in their experimental “Flying Wing” aircraft, and the California flight facility was renamed Edwards Air Force Base. During a stretch of 1952, seven test pilots died each month at Edwards, according to James Hansen’s biography of Neil Armstrong, First Man.

By the time the space program was up and running in the early 1960s, many of the surviving test pilots had entered NASA’s astronaut corps. Others combined piloting experience and a science background, such as college-educated engineers Neil Armstrong and Buzz Aldrin.

“These were people who were used to accepting risks,” says former NASA historian Roger Launius about the test pilots who later became astronaut candidates. “But their families were not used to it. It was always devastating for the wives and the children.”

Launius says that not only were the astronauts more accepting of risk, they also knew that their aircraft and possibly spacecraft might fail. Armstrong himself encountered near-disaster during his first space mission, Gemini 8. After a critical onboard failure, Armstrong and pilot David Scott began spinning out of control in space. After struggling to resist blacking out, Armstrong eventually regained control and landed safely. On the ground, these former pilots also flew aircraft from base to base to undergo astronaut training.

Theodore Freeman, a member of the first group of 14 Apollo astronauts, died in October 1964 when a flock of geese was sucked into the engine of his T-38 training aircraft near Houston. In February 1966, astronauts Eliot See and Charles Bassett crashed during bad weather on approach to Lambert Field in St. Louis. Their T-38 ended up not 500 feet from the Gemini 9 simulator they were preparing to use for training.

Perhaps the most horrific disaster occurred when astronauts hadn’t even left the ground. Apollo 1’s Gus Grissom, Ed White and Roger Chaffee died in a cockpit fire on January 27, 1967 while strapped into their command module during launch testing at the Kennedy Space Center.

“There was real concern after the loss of the three astronauts that they weren’t going to be able to make that deadline,” Launius says.

There were doubters from both Congress and the American public over whether a moon mission was worth both the cost in lives and in money. Congress focused on the causes of the Apollo 1 fire, while civil rights leader Rev. James Abernathy led a protest over allocating spending to the space program while poverty persisted in the country.

“People [at NASA] are looking at the clock ticking and wondering if they were going to make it,” says Launius. There was even internal debate at NASA over whether the decade of the 1960s ended at the end of 1969 or 1970.

By the time Armstrong, Aldrin and Michael Collins suited up for Apollo 11 in July 1969, both the astronauts and the NASA mission controllers were confident that the mission would be a success. That was borne out by the countless hours of human testing and engineering work invested to ensure the men came home safely, according to Teasel Muir-Harmony, curator of the Apollo collection at the Smithsonian Air and Space Museum.

“They wanted to make sure they were trained for any scenario,” Muir-Harmony says. “Sometimes they would train seven or eight hours for every hour of the mission.”

The level of intensity and sacrifice poured into the moon landing effort was unique—and, many historians argue, is unlikely to be repeated, even if humans decide to reach beyond Earth to return to the moon or perhaps Mars.

As Muir-Harmony says, the pressure to fulfill Kennedy’s promise meant that, “There was this built-in expectation there were going to be lots of risks and you just keep on doing your job.”