by In the 1990s, NASA designed an experimental space plane that was meant to be a cost-effective alternative to expensive rockets.
Called the X-33, it was based on a concept called SSTO – or “single stage in orbit.” SSTO does away with the rocket stages of traditional spaceflight – where rockets containing engines and fuel are dropped and fired as they lose weight – instead favoring a single, fully reusable spacecraft.
The X-33 was designed to launch vertically like a rocket, but land on a runway like an airplane, with the aim of reducing the cost of launching a payload into orbit from $10,000 to just $1,000.
However, the program was canceled in 2001 due to technical difficulties, adding to a list of similar projects that failed to come to fruition.
“I was in charge of the X-33 program, and we rejected it because our assessment was that it was going to cost more than what we were going to bargain for, and we were at the edge of the technological capability really remove it. ,” says Livingston Holder, an aerospace engineer, former USAF astronaut and X-33 program manager, and now CTO of Radian Aerospace—a Seattle-based company he co-founded in 2016 to revive the SSTO dream.
“Things have changed a lot since the X-33 – we have composite materials that are lighter, tougher and can take a greater thermal range than we once had. And it drives better than anything we’ve had, in terms of how efficiently it burns propellant and how big the systems are,” he says.
The product of this updated technology is the Radian One, a new space plane that will replace vertical launch with a very unusual system – a rocket-powered sled.
Wasted steps
To be able to escape Earth’s gravity and reach orbit, a rocket must reach a speed of about 17,500 miles per hour, according to Jeffrey Hoffman, professor of aeronautics and astronomy at the Massachusetts Institute of Technology and former -a NASA astronaut who. five Space Shuttle missions were flown. “The problem is, when you’re going up, not only do you have to lift the rocket and the payload, you also have to lift all the fuel you’re carrying,” he says.
A rocket that would be able to reach that velocity must spend 95% of its mass on fuel, according to Hoffman, which left very little room for everything else. “It was a dream to be able to go into orbit with one step,” he says. “But to do that, the structure of the rocket, the engines and the payload can’t be more than about 5% of the total mass of the entire system. And we don’t know how to build things like that.”
That’s why all the rockets ever used to reach orbit have been multi-stage, although current rockets like SpaceX’s Falcon 9 have fewer stages – two – than older ones like the Saturn V lunar mission Apollo, who had three.
“When you use all the propellant from the first stage, instead of carrying that structure all the way up to orbit, you just drop it. And that allows you to take a lot more payload for a given mass sitting on the launch pad,” explains Hoffman.
Traditionally, the spent stages of rockets fall back to Earth (usually in the ocean), burn up in the atmosphere or end up in orbit as space junk. SpaceX changed that paradigm by designing reusable boosters that can land independently on Earth. The essence of a single-stage space vehicle is to eliminate the rocket stages entirely, with the promise of reducing costs even further.
It is not easy to overcome 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 runs along a two-mile track and accelerates up to Mach 0.7 – 537 mph (864 kilometers per hour) – before launching the spaceplane, which then flies into orbit under the power of its own engine . .
“There have been various attempts to develop single-stage vehicles in orbit,” notes Hoffman. “NASA and the Air Force tried it back in the late 1980s and 1990s. They tried to solve the problem by having a scramjet engine, which would take the plane up through the atmosphere and burn oxygen from there instead of carrying it with you. It’s a great idea but technically it’s very difficult to build that kind of engine.”
“What Radian is doing with his rocket sled is the same as the scramjet,” Hoffman explains. “In other words, try to get the initial acceleration without burning your rocket propellant. That way, you get around some of the limitations of the rocket equation.”
Space pickup trucks
Radian is convinced that it can overcome the obstacles to a successful SSTO thanks to three key technologies.
The first is the sled launch system, which uses its fuel not only to power its own three engines, but also those on the spaceplane itself, leaving the spaceplane with a full tank just before takeoff. The second is the landing gear, which is designed for landing rather than taking off, making it much lighter. And the third is the wings, which are absent in a vertical rocket but to reduce the amount of thrust required by the system, by providing lift when it flies towards orbit.
“When we get to orbit, the Space Shuttle is probably the closest analogy,” Holder says. “We have a smaller bay, but we can do many of the same types of missions. When flying home, we have a stronger composite outer surface, allowing us to reuse the system over and over again with reduced inspection requirements and faster turnaround times.”
Radian says its spaceplane will be reusable up to 100 times, carrying a crew of two to five astronauts with a 48-hour turnaround time between missions. A scale model of the plane will be tested this year, according to Holder, with a full-scale version beginning flight tests – without reaching orbit – in 2028.
Like the Shuttle, Radian One would be able to deploy payloads such as satellites into orbit, or carry out missions using equipment sitting in the bay, such as observations of the Earth or surveillance and intelligence for defense entities or military. But the Holder says, the plane could also help with humanitarian aid in disaster areas when runways, for example, would be unusable – by releasing the payload from the bay in a controlled re-entry through the atmosphere.
He traces an analogy with a construction site, where the rockets are the 18-wheelers pulling up with big equipment, and the Radian One is the pick-up truck that brings smaller materials and the team. “I think there will always be a place for vertical launch rockets,” he says. “They’re going to take the really heavy stuff up.”
He knows the skepticism of another SSTO attempt. The last high-profile project to lose steam was Britain’s Skylon, a hydrogen-powered spaceplane intended to take off from a reinforced runway and land back on Earth. The company behind the project said last year that a two-stage-to-orbit system is now more likely.
“I don’t judge the people who scratch their heads and wonder if a single stage to orbit is viable,” says Holder. “It took me about a whole year on this program to convince myself again that it is. You have to be able to compare today’s technology with the technology of the past to see if it’s viable or not.”
The big question, according to Hoffman, is not only whether SSTO can be achieved technically, but whether it can be done at a cost that is economically competitive with other launch systems, such as SpaceX’s new starship, which can carry hundreds of payloads tiny take. in one address and doing it relatively cheap. “That’s the reason the single-stage dream has always been in orbit – in principle, it should be cheaper,” he said.
“I hope they succeed,” Hoffman added. “Because it would certainly be a first, technically – and we’ll see about the economics. You don’t know until they demonstrate the capability and see who signs up to use it.”
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