In May 2020, the Crew Dragon became the first space vehicle to launch humans from American soil in nine years. Built by Elon Musk's SpaceX, it's part of Nasa's plan to hand over space station flights to private companies. Here's our guide to the vehicle.
Elon Musk says human spaceflight had always been the fundamental goal for his pioneering company SpaceX.
The entrepreneur achieved that ambition on Saturday 30 May 2020, when the Crew Dragon spacecraft carried Nasa astronauts Doug Hurley and Bob Behnken into orbit for a rendezvous with the International Space Station (ISS). It was the first crewed vehicle to fly from US soil since the retirement of the space shuttle.
But the company had to walk before it could run. Crew Dragon evolved from an earlier design, called Dragon 1, which launched 20 times on missions to deliver cargo to the ISS between May 2012 and March 2020.
In May 2014, Musk unveiled the seven-seat Crew Dragon concept during an event at SpaceX's headquarters in Hawthorne, California.
Key questions about the mission
What's the mission plan?
Who are the astronauts?
How we got to this point
Like Dragon 1, the crewed version is a capsule design, making it more similar to the Apollo command modules that carried astronauts to the Moon than the winged space shuttle concept, which was conceived to carry both a crew and a large payload.
From launch up until shortly before re-entry, the capsule is attached to a section called the trunk which has solar panels, heat-removal radiators and fins to provide stability during emergency aborts. Together, the capsule and trunk stand around 8.1m (26.7ft) tall, with a diameter of 4m (13ft).
The Crew Dragon is equipped with 16 Draco thrusters that are used to manoeuvre the vehicle in orbit. Each Draco is capable of producing 90 pounds of force in the vacuum of space.
SpaceX engineer John Federspiel, explains: "When we wanted to take Dragon and make it human-rated, I think we took a different approach to spaceship design than has previously been done, because we wanted this to feel like a 21st Century spaceship.
"Probably one of the biggest features of Dragon are the touchscreens on the inside. We designed them not just to be very functional, but with a user experience in mind."
The three large displays that allow Hurley and Behnken to monitor systems and control the spacecraft are a world away from the analogue buttons, dials and control stick that featured in the cockpit of the space shuttle, which both astronauts trained on.
The pair worked with SpaceX to get the vehicle ready for its first crewed flight. Hurley admits that the "glass cockpit" took a bit of getting used to.
"As far as actual physical feedback, you certainly don't get that from the touchscreen," he says.
"But what you do get is an indication of where you touched," Hurley adds, before describing how it works: "I touched that button and that made the vehicle go up and I got the return flash that that's what the vehicle recognised as my input."
For the types of scenarios where astronauts might need to assume manual control of the normally autonomous craft, such as finishing off a docking sequence with the space station, the touchscreen controls are "much more than adequate", Hurley adds.
"It just might not be the same thing you'd want to use if you were suited up and trying to fly an entry or descent, for example, like we could do with the space shuttle," says Behnken.
But tailoring the spacecraft to the user experience involved more than just the design of the controls. "When I think of comfort for the astronauts, it's really every aspect of how you could interact with the spaceship that comes to mind," says John Federspiel.
"We have three different seat sizes, we even go so far as moulding the foam around the astronaut's body so there's not any pressure points and it's just generally a pleasurable journey into space."
When the astronaut gets ready to strap in, they plug an "umbilical" line from their seat into a port on the right thigh of their space suit. The umbilical provides the suit with life support systems, including air and power connections.
"The suit is really one part of the bigger Dragon system... we think of it as a suit-seat system," says Chris Trigg, space suits and crew equipment manager at SpaceX.
But in case anything goes wrong on the pad or during the climb to orbit, SpaceX has designed an innovative abort system for the capsule. The launch escape system (LES) consists of a set of SpaceX-designed SuperDraco engines that fire in the event of an emergency to propel the capsule and its crew safely away from the rocket.
Commenting on the LES, something the space shuttle lacked, Doug Hurley says: "That perspective for me is huge compared to shuttle, where there were what we call 'black zones'... scenarios where it didn't really matter if you had the right combination of failures, you were likely not going to survive."
Hurley adds that the capsule design is safer than a winged vehicle under most circumstances.
Crew Dragon is also designed to be "two-fault tolerant". This means that any two things can fail, such as a flight computer and a thruster, and the spacecraft can still bring the crew home safely.
The vehicle is designed to dock with the space station autonomously - that is, without having to be guided in by a human. Jessica Jensen, director of Starship mission hardware and operation at SpaceX, says: "We have GPS sensors on Dragon, but also cameras and imaging sensors such as Lidar on the nosecone as it's approaching the space station.
"All these sensors are feeding data back to our flight computer to say: 'Hey, how far away am I from the space station? What's my relative velocity to the space station?'"
The flight computer then uses algorithms that determine - based on this information - how to fire the thrusters to most effectively get to the docking target.
The vehicle's lifetime in orbit is limited to a few months because of its solar panels, which degrade in the harsh environment of space.
When it returns to Earth, the Crew Dragon can't simply land on a runway like the space shuttle. "I think there's an argument that the return is more dangerous in some ways than the ascent," says Elon Musk.
During re-entry, the heat shield - made of a material called Pica-X - must survive temperatures hotter than the surface of the Sun, as the Crew Dragon screams through the atmosphere at up to 25 times the speed of sound.
There's a minor chance that the spacecraft's asymmetric design - driven by the placement of its emergency escape system - could cause it to roll too much. Musk has said in the past that the issue, known as roll instability, has been extensively studied, but that it still worries him.
Then, after the fiery re-entry phase, the spacecraft needs to deploy four parachutes to slow its descent.
Finally, the Crew Dragon splashes down in the Atlantic Ocean, 450km off the coast of Florida, where recovery ships will take the astronauts to safety and retrieve the capsule.
Hurley says that while he and Behnken - along with others at Nasa - provided input, "this spacecraft - Crew Dragon - is SpaceX's design, from start to finish. Make no mistake about that".
He adds: "Just to see the vehicle come from not very much, a preliminary design, to where it is today, the operability of the vehicle, the clean lines, how it is inside the vehicle, how it is for a crew... we're just excited to put it through its paces."
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