The Virgin Galactic SpaceShipTwo rocket separates from a carrier aircraft before its midair explosion during a test flight on Oct. 31. The explosion killed a pilot aboard and seriously injured another. (Kenneth Brown/AP)

For test pilots on Virgin Galactic’s SpaceShipTwo flying to the edge of space aboard an experimental plane, the first seconds after firing the rocket engine could be cripplingly disorienting. Gravitational forces hammered them back and down into their seats. The cockpit convulsed. Freezing air blasted over the wings with a deafening shriek.

Amid this chaos, they were expected to perform “housekeeping”: following their flight test cards, checking speed and altitude, mastering a shaking console of several dozen onboard gauges, switches and displays. Death at this height, flying faster than sound, was always only one split-second mistake away.

The midair disintegration Friday of Virgin Galactic’s suborbital spacecraft has underscored the enormous dangers still facing the fledgling business of commercial space. But it has also cast new light on the industry’s elite cadre of test pilots, heavily trained aviators for whom death-defying flight is a part of life.

Unlike the computer-guided flight and systematic training regimen of sophisticated military aircraft, civilian test-pilot training often depends on a homemade mix of precision exercises, quick reflexes and self-confidence, former test pilots said. But it is no less rigorous, demanding years of experience and hundreds of hours of flight to qualify for Federal Aviation Administration ratings on wide varieties of craft.

“How much [education] they’ve had or whether they’re an engineer doesn’t really matter,” said Chuck Coleman, one of the world’s top aerobatic pilots, who worked as a Scaled Composites test pilot until 2010. “You have to stay alert and focused and think clearly in a time of crisis, and some people do that really well, and some people don’t.”

An undated photo released by Scaled Composites shows Michael Alsbury, who was killed while co-piloting the test flight of Virgin Galactic SpaceShipTwo on Oct. 31. (AP)

Investigators could not say what caused the SpaceShipTwo, a showcase of Virgin Galactic’s $500 million space tourism campaign, to shatter shortly after launching more than eight miles above the Mojave Desert. But early reports have focused on the premature unlocking of the craft’s rotating tail “feather,” which could have led the craft to rip apart.

The body of co-pilot Mike Alsbury, 39, was found strapped to his seat in the rocket plane’s wreckage, which was scattered across more than 35 miles of desert floor. Pilot Pete Siebold, 43, apparently managed to free himself from his seat as the craft disintegrated around him, before freefalling through numbingly thin air colder than 50 degrees below zero and parachuting to safety with serious injuries.

Both pilots worked for Scaled Composites, the Mojave-based aerospace company developing and testing the craft for Virgin Galactic. The “spaceline” had hoped to begin flying passengers to the edge of earth’s atmosphere as early as next year, and more than 700 people have paid as much as $250,000 each for a ride.

Not quite pilots, not quite astronauts, test pilots are tasked with mastering aircraft built to withstand far more than the rigors of typical flight. The SpaceShipTwo, for instance, could break the sound barrier in eight seconds and soar past 2,500 mph, and only pilots’ flight jumpsuits and a carbon-composite cockpit as thin as a closet shielded them from instant death.

Yet the rocket plane was flown the old-fashioned way, via stick and rudder. Scaled’s founder, legendary aircraft designer Burt Rutan, believed the system lent itself more comfortably to pilots used to flying simple hand-controlled systems.

Becoming a test pilot at the highest echelons takes an enormous investment of time, but flying is often a small fraction of their work. Siebold, a trained computer engineer, coded the simulator, ground-control system and advanced navigation software for the rocket craft’s predecessor, SpaceShipOne.

Alsbury was a certified flight instructor with 75 full days of flight time, mostly aboard experimental aircraft. Last year, he helped win the Society of Experimental Test Pilots’ Ray E. Tenhoff Award for an intricately detailed technical paper on “The Road to Rocket Powered Flight.”

An undated photo released by Scaled Composites shows Peter Siebold, who was co-piloting SpaceShipTwo when it exploded Oct. 31 and suffered serious injuries. (AP)

Siebold, who got his pilot’s license at 16 and has flown three dozen different aircraft, twice won the Oscar of flight testing, the Iven C. Kincheloe Award, for his work with Scaled in 2004 and 2009. (To underline the dangers of the job: The Kincheloe and Tenhoff awards, like Edwards Air Force Base, are named for test pilots killed in flight.)

Some of the private sector’s test pilots come from the specialized academies and units of military training, aviation experts said. Others begin as commercial pilots or air racers, seeking the excitement and risk of more adventurous flight.

Many test-pilot hopefuls train at the National Test Pilot School, the only civilian test pilot school in the country. Located at the Mojave Air and Space Port, the school is near Edwards Air Force Base, where military research pilots were profiled for the 1979 book “The Right Stuff.”

Test-pilot training often demands a mastery of different styles of flight, as well as passing a battery of stomach-churning tests, including high-G training, experts said. Before stepping into a cockpit, pilots spend hundreds of hours running simulations in virtual cockpits, learning the intricacies of different planes’ navigation and control.

Training for the SpaceShipTwo was even more specialized largely because of the prototype craft’s competing identities, a mix of glider, rocket and jet. Pilots ran long simulations and practiced on the similar console of the WhiteKnightTwo, the carrier jet that tows the spacecraft to its launch altitude. But the feel of the weighty jet was far removed from that of the SpaceShipTwo’s twitchy, tail-heavy glider.

Strapped at the front of their rocket planes without pressurized space suits, Scaled’s test pilots separated from the carrier jet and fired their engines only once they had reached an altitude thousands of feet above where commercial jetliners fly.

At that height, eight miles up, the curvature of the Earth becomes startlingly clear. But they are still 50 miles away from the true boundary of space, and for the rest of the trip, aside from the “third eye” of mission control, the rocketing pilots are on their own.

Scaled already has other test pilots who can step in for the next iteration of suborbital craft. The next SpaceShipTwo­, Virgin Galactic said, is 65 percent complete. Those pilots will face demands like those suggested by Rutan when he described retired Scaled test pilot Mike Melvill in 2010: “He flew by the seat of his pants, and by that I mean he flew by feeling the aircraft, by being one with it. And he couldn’t be rattled, no matter what might go wrong.”