No Automation, One Shot at Landing: It’s Really Hard to Fly SpaceShipTwo

SpaceShipTwo cockpit
Virgin Galactic’s spaceship, meant to eventually take wealthy passengers on brief rides into space, crashed during a test flight over southern California on Friday morning. One of its two pilots was killed, the other suffered serious injuries and remains hospitalized.
 
It’s a major setback for the space tourism effort, and a loss for the aerospace community. The deceased pilot, who has not been named, was like all his colleagues among the best on the planet. Virgin Galactic only hires the best, guys who tested planes for the Air Force or flew missions for NASA. The talent pool is limited because it has to be: Flying SpaceShipTwo is damn difficult.
 

A Simple Approach

Virgin Galactic uses two aircraft to get customers into space. WhiteKnightTwo is a double-hulled plane that resembles a catamaran, optimized for high altitudes and load-bearing capacity. It carries SpaceShipTwo, which itself carries six passengers. When the planes reach about 50,000 feet, WhiteKnightTwo drops the smaller craft, which fires a rocket that sends it to the very edge of the atmosphere. Those aboard experience four minutes of weightlessness before SpaceShipTwo glides back to Earth.
 
The design philosophy behind the two aircraft was to keep everything as basic as possible. “A simple system is less likely to fail,” chief pilot Dave Mackay told us during a recent visit to Virgin Galactic’s HQ in Mojave, California. Mackay, a former RAF test pilot, has flown 130 kinds of aircraft. He tested the Harrier hover jet and flies a 1909 Bleriot for fun. The man has skills, and they’re put to use on WhiteKnightTwo and SpaceShipTwo, which have no flight control computers. All the systems are manually operated. No autopilot here: When all your pilots are among the best in the world, you don’t need one. Better to let them just fly on their own.
 
The key innovation in SpaceShipTwo, designed by aerospace master Burt Rutan, is a solution to the issue of reentering the atmosphere. There are two conventional ways to transition from space back to Earth: One is to come in as a winged airplane, like NASA’s space shuttle. That gives you the freedom to land where you want, but hitting the perfect angle of attack is tricky. The second method is as a capsule, which is essentially dummy-proof, but has no control over where it lands and subjects its occupants to high G-forces. SpaceShipTwo is a bit of both. Its tail folds up and down, changing the vehicle’s shape from something that falls like a capsule to one that flies like an airplane once it’s back in the atmosphere.
 
SpaceShipTwo is a “small, agile, really high-performance vehicle,” says Mackay. “It’s the most remarkable thing I’ve ever flown.” That’s because its characteristics change drastically as it moves through stages of flight. “It behaves like an enormous variety of aircraft,” Mackay says. He and his fellow pilots see this as a fun challenge, and because they’ve flown so many aircraft, the changing nature of the spaceship doesn’t bother them.
 

The Flight Plan

As planned, a Virgin Galactic trip to space lasts about 90 minutes. For the first 45 to 60 minutes, WhiteKnightTwo flies in a corkscrew pattern, gaining altitude. At about 50,000 feet, WhiteKnight2 unhooks SpaceShipTwo, naturally pulls up because of the dropped weight, and gets out of the way. After free falling for three seconds, SpaceShipTwo fires its engine and turns straight up. “You’re forced back in your seat and there’s this howling noise behind your head, you get a little bit of vibration,” Mackay says. “It’s actually smoother than I expected it to be. But it’s a tremendous acceleration.” This is the only new territory for the Virgin pilots, most of whom don’t have experience in space. To make sure they’re ready for the heavy G forces, they get weekly flights in an Extra 300 acrobatic plane that does spins and tumbles.
 
After a minute of rocket-powered flight, which delivers 3Gs to the chest, the rocket cuts off. The pilots use the ship’s feathering system to flip it upside down, for a simple reason: The windows are mostly near the top of the cabin, and they believe the passengers will be most interested in looking down at the planet they’re leaving behind. At this point, the passengers and crew are 150,000 feet up, nearly out of the atmosphere, and SpaceShipTwo coasts upward a few more hundred thousand feet, hitting three times the speed of sound.
 
Like any object thrown into the air, SpaceShipTwo follows a parabolic curve and falls back to Earth. Passengers get about four minutes to float weightlessly around the cabin. That time would be longer if SpaceShipTwo were flying faster, but Virgin compromised to minimize the G forces on its customers. The plan for getting them back in their seats is pretty simple, says Beth Moses, Virgin Galactic’s astronaut trainer: “You hope they have enough survival instinct” and follow pilots’ instructions to strap on their five-point harnesses before gravity kicks in. The pilots are strapped in the whole time, but “we might look out the window and admire the view as well,” Mackay says.
 
While SpaceShipTwo is out of the atmosphere, it can be controlled with small jets on the nose and wings that fire compressed air. The goal, however, is to use them as little as possible, to avoid interfering with the passenger experience. No matter the angle at which the ship reenters the atmosphere, the feathering system will flip it into the correct position with the belly parallel with the Earth’s surface, but the pilots plan to get it properly oriented beforehand using those jets, to make things more comfortable for the passengers. So on top of all the aerospace challenges the Virgin pilots deal with, they have to worry about the comfort of the wealthy tagalongs in the back of the ship.
 
Then it’s up to the pilots to get back onto the runway. They exactly one shot at getting it right, since SpaceShipTwo at this point is unpowered—there’s no pulling up and circling around if they screw up the approach angle. Again, that’s where hiring the planet’s best pilots is a big help: These guys have spent their lives flying high-performance aircraft. But they also practice. A lot.
 

Practice

Actually flying the spaceship is a terribly inefficient way to learn to land it: You have to hook it up to WhiteKnightTwo, fly both up into the sky, and drop it, just for one landing. So, like pilots everywhere, the Virgin crew spends time in the simulator, usually two three-hour sessions each week, Mackay says. They train for both WhiteKnightTwo and SpaceShipTwo, with helpfully sadistic operators who throw everything they can think of at them, from failed engines to horror show weather. The pilots also spend time in the classroom, studying text books some of them helped write.
 
But the simulator isn’t an adequate substitute for real life flying. So they have another way to practice: flying WhiteKnightTwo. The control systems in the two aircraft are nearly identical (remember the whole simplicity idea), and by adjusting the air brakes, they can make the double-hulled plane’s flight characteristics exactly replicate those of the spaceship. The pilots can glide back down to the runway as if they were in SpaceshipTwo, but then fire up the four Pratt & Whitney engines, get back in the air, and circle around for another landing. And they can do it all day. By the time pilot Mike Marsucci tried landing SpaceShipTwo for the first time (successfully), he had done it hundreds of times in the simulator, and dozens in WhiteKnightTwo.
 
Of course, today’s crash proves that no matter the skill of the pilots involved, disasters happen. But the loss of a pilot can’t be expected to hold Virgin back for long: It’s already working on its next spaceship.
 
(Source: wired.com)
 

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20 Apr 2018 - 16:38

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