Is It Safe?

The first company with a plan—and a rocket—to send humans to orbit answers the existential question.

The Falcon 9 shown during ground tests at Cape Canaveral, Florida, last January. (NASA)
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The next step is Falcon 9, a 180-foot-tall, two-stage rocket, 17 feet in diameter at its widest point, with nine of SpaceX's regeneratively cooled engines instead of one. Falcon 9 is set for its first launch from Florida this spring.

NASA is investing in Falcon 9 through its Commercial Orbital Transportation Services, or COTS, program, to help develop private space vehicles the agency might someday hire. The seed money will help SpaceX fund the expensive process of engineering and certifying Dragon and Falcon 9 to carry cargo and, eventually, humans to and from the space station.

SpaceX has so far met all of NASA's milestones and is ahead of Orbital Sciences Corporation, the other company receiving COTS funding. SpaceX designed Falcon 9 and the Dragon capsule to be human-rated from the start, without any assurance NASA would ask for this. As it will dock with the manned space station, Dragon must meet about 80 percent of the human-rating standards anyway. 

Human-rating requirements fall into three main areas: structural elements, such as fuel tank walls; redundancy, such as backup power and control systems; and mission design, such as launch trajectory, which determines G force—cargo can withstand a lot more of it than the human body can. Following the two shuttle disasters, NASA's Astronaut Office insisted that any new launch system be an order of magnitude safer than the shuttle. "If we wish to send explorers into space on increasingly ambitious missions, we must first solve the problem of putting humans into orbit more safely than is possible with our current launch systems," the office wrote in a May 2004 memo. The shuttle is statistically likely to suffer nine fatal accidents per 1,000 launches; the Astronaut Office wanted no more than one.

Unlike the Russians' Soyuz, the shuttle has no means of escape if something goes disastrously wrong during ascent. So NASA's human-rating standards now require an automated abort-and-escape system that works all the way to orbit.

In fact, a vehicle less dependable than the shuttle's boosters could be made safer with one modification: an Apollo-style abort system, which bundled powerful rockets in a small tower atop the stack to lift the capsule away from a failing booster. A similar unit on the Soyuz has twice saved cosmonauts, once on the launch pad and once in flight. NASA plans to equip Orion with such a system.

The rocket trajectory, though, must be designed so that astronauts would survive an abort. Unmanned rockets such as the Delta IV and Atlas V, which have relatively underpowered second stages, fly a "lofted trajectory," where the first stage shoots them very high and they actually start falling before the second stage lifts them again. If astronauts abort near the high point, their capsule could plummet straight down and belly flop on the atmosphere at extreme G force. "Structural safety margins will be blown to hell, and you'll almost certainly kill people," Musk says flatly. "This was one of the main reasons given by NASA for not using those vehicles for manned spaceflight."

So SpaceX designed Falcon 9 with a second stage about four times as powerful as that of an Atlas or a Delta, allowing for a more slanted, softer trajectory into space. The fuel's weight adds cost, but if astronauts abort, their flight path will catapult Dragon horizontally, slicing more gradually into the atmosphere.

Falcon 9 will be the first rocket since Saturn that can lose an engine without compromising the mission. The vehicle's main structure will be built to withstand flight loads 40 percent higher than what engineers expect it to encounter. The safety margin for unmanned rockets is 25 percent above expected loads.

Riding a rocket is sort of like sitting atop a controlled, sustained explosion. Falcon 9's engines exert nearly a million pounds of force, consuming 3,200 pounds of propellant each second. The rocket must control the explosion all the way to space, while also doing battle with sound. The most intense stresses occur at liftoff, when sound energy from the engines bounces off the ground and slams back into the rocket. Sound levels reach 140 decibels, louder than an up-close ambulance siren and enough to immediately injure human eardrums and damage components mounted near a rocket's outer skin. The most intense pressure after launch accumulates as the rocket goes supersonic, when shock waves and buffeting come close to what the rocket faces at liftoff.

About Michael Milstein

Michael Milstein is a freelance writer who specializes in science. He lives in Portland, Oregon.

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