In the 1990s, NASA designed an experimental space plane that was intended to be a cost-effective alternative to expensive rockets.
This was called the X-33 and was based on a concept called SSTO, which stands for "single stage to orbit." SSTO does away with the rocket stages of conventional spaceflight, where rockets contain engines and fuel and are dropped during ascent to shed weight. Instead, it favors a fully reusable single-stage spacecraft.
The X-33 was designed to launch vertically like a rocket, but to land on a runway like an airplane. The goal was to reduce the cost of launching a pound of payload into orbit from $10,000 to just $1,000.
However, the program was canceled in 2001 due to technical problems, which only extended the list of similar projects that never got off the ground.
"I was leading the X-33 program, and we elected to exit because we thought it was going to cost more than we anticipated, and we were at the edge of the technological capability to actually make it happen," said Livingston Holder, an aerospace engineer, former USAF astronaut and X-33 program manager, and now CTO of Radian Aerospace, a Seattle company he co-founded in 2016 to revive the SSTO dream.
"Things have changed dramatically since the X-33 - we have composite materials that are lighter, stronger, and can handle a greater thermal range than we had then. And the propulsion is better than anything we had before, in terms of how efficiently it burns fuel and how much the systems weigh," he says.
The product of this updated technology is Radian One, a new spacecraft that will replace vertical launch with a very unusual system: a rocket-powered sled.
Wasteful stages
To escape Earth's gravity and reach orbit, a rocket must reach speeds of about 17,500 miles per hour, says Jeffrey Hoffman, a professor of aeronautics and astronautics at the Massachusetts Institute of Technology and a former NASA astronaut who flew five Space Shuttle missions. "The problem is that as you go up, you have to lift not only the rocket and the payload, but also all the fuel that you're carrying," he says.
A rocket capable of reaching that speed would have to devote 95 percent of its mass to fuel, Hoffman says, leaving little room for anything else. "It's a dream to get to orbit with a single stage," he adds. "But to do that, the structure of the rocket, the engines, and the payload can't be more than about 5 percent of the total mass of the entire system. And we just don't know how to build something like that."
That's why all rockets ever used to reach Earth orbit have been multi-stage, although current rockets like SpaceX's Falcon 9 have fewer stages (two) than older rockets like the Apollo moon mission's Saturn V, which had three.
"Once you use up all the propellant in the first stage, you just jettison that structure instead of taking it all the way up to space. And that allows you to carry a lot more payload for a given mass that's sitting on the launch pad," Hoffman explained.
Traditionally, spent rocket stages fall back to Earth (usually in the ocean), burn up in the atmosphere, or end up in orbit as space junk. SpaceX has changed that paradigm by designing reusable boosters that can land back on Earth autonomously. The premise of a single-stage spacecraft is to eliminate the rocket stages altogether, promising to reduce costs even further.
It's not easy to circumvent what Hoffman calls "the tyranny of the rocket equation," or to solve the problem of carrying the weight of fuel into space. Radian's solution is a rocket-powered sled that travels along a two-mile-long track, accelerating to Mach 0.7 - 537 mph (864 kilometers per hour) - before releasing the spaceplane, which then flies into orbit under the power of its own engines.
"There have been several attempts to develop a single-stage vehicle that can fly into orbit," Hoffman notes. "NASA and the Air Force tried it in the late '80s and '90s. They tried to get around the problem by using what's called a scramjet engine, which would take the plane through the atmosphere and burn oxygen there instead of having to carry it around with you. It's a great idea, but it's very difficult technically to build that engine."
"What Radian is doing with their rocket sled is kind of the equivalent of the scramjet," Hoffman explained. "In other words, trying to get the initial acceleration without burning your rocket fuel. That way you get around some of the limitations of the rocket equation."
Space pick up truck
Radian believes it can overcome the obstacles to successful SSTO through three key technologies.
The first is the sled launch system, which uses its fuel not only to power its own three engines, but also the spaceplane itself, allowing the spaceplane to have a full tank of fuel just before takeoff. The second is the landing gear, which is designed only for landing rather than takeoff, making it considerably lighter. And the third is the wings, which are absent from a vertical rocket, but reduce the amount of thrust the system needs by providing lift as it flies toward orbit.
"Once we get to orbit, the best comparison is probably the Space Shuttle," Holder said. "We have a smaller bay, but we can do many of the same types of missions. And when we fly home, we have a more robust composite exterior, and that allows us to reuse the system over and over again with reduced inspection requirements and faster turnaround times."
Radian says the spaceplane will be reusable up to 100 times, carrying a crew of two to five astronauts with a 48-hour turnaround between missions. A scale model of the plane will be tested this year, Holder said, with a full-scale version beginning flight tests in 2028 - without reaching orbit.
Like the Shuttle, Radian One could launch payloads such as satellites into orbit, or conduct missions using equipment housed in the bay, such as Earth observation or surveillance and intelligence for defense or military entities. But, Holder added, the aircraft could also help with humanitarian relief in disaster areas when, for example, runways are unusable, by dropping its payload from the bay in a controlled reentry through the atmosphere.
He draws an analogy to a construction site, where the rockets are the 18-wheelers that pull in with big equipment, and the Radian One is the pickup truck that brings in smaller materials and crew. "I think there's always going to be a place for vertical launch rockets," he adds. "They're going to be the ones that lift the really heavy stuff."
He is aware of the skepticism that another SSTO attempt will face. The last such high-profile project to lose steam was Britain's Skylon, a hydrogen-powered spaceplane that would take off from a reinforced runway and land on Earth. The company behind the project said last year that a two-stage-to-orbit system is now more likely.
"I'm not criticizing people who question whether a single stage to space is viable," Holder says. "It took me about a whole year to convince myself that it is. You just have to be able to put yourself in the shoes of today's technology versus the technology of the past to see whether it's viable or not."
The big question, Hoffman said, is not just whether SSTO is technically feasible, but whether it can do so at a price that is economically competitive with other launch systems, such as SpaceX's new spacecraft, which can carry hundreds of small payloads in a single launch and do so relatively cheaply. "That's always been the reason to pursue the dream of a single Earth orbit - in principle, it should be cheaper," he said.
"I hope they're successful," Hoffman added. "Because it would absolutely be a first technically - and we'll see how the economics work out. You never know until they demonstrate the capability and see who signs up to use it."
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