(CNN) –– In the 1990s, NASA designed an experimental space plane that was intended to be a cost-effective alternative to expensive rockets.
Called the X-33, it was based on a concept called SSTO (single stage to orbit), which dispenses with the rocket stages of conventional spaceflight (in which rockets contain engines and fuel and are dropped during ascent to reduce weight) in favor of 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 goal of reduce the cost from sending half a kilo of payload into orbit from US$10,000 to just US$1,000.
However, the program was cancelled in 2001 due to technical difficulties, and ended up on a list with other similar projects that have not materialized.
“I ran the X-33 program, and we decided to get out of it because our assessment was that it was going to cost more than we expected and we were right on the edge of the technological capability to actually pull it off,” said Livingston Holder, an aerospace engineer, former USAF astronaut and X-33 program manager, and now chief technology officer of Radian Aerospace, a Seattle-based 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 back then. And the propulsion is better than anything we’ve ever had, in terms of the efficiency with which the fuel is burned and the weight of the systems,” he said.
The product of this updated technology is the Radian onea new space plane that will replace vertical launch with a very unusual system: a rocket-powered aircraft.
In order to escape Earth’s gravity and reach orbit, a rocket needs to reach a speed of about 17,500 miles per hour, according to Jeffrey Hoffman, a professor of aeronautics and astronautics at the Massachusetts Institute of Technology and a former NASA astronaut who has flown five space shuttle missions. “The problem is that as you go up, you not only have to lift the rocket and the payload, but you also have to lift all the fuel that’s on it,” he said.
A rocket capable of reaching that speed would have to devote 95 percent of its mass to fuel, Hoffman said, leaving very little room for anything else. “It would be a dream to be able to get to orbit with a single stage,” he added. “But to do that, the rocket structure, engines and payload can’t account for more than 5 percent of the total mass of the entire system. And we just don’t know how to build things like that.”
That's why all rockets used to reach orbit have been multistage, although current rockets like SpaceX's Falcon 9 have fewer stages (two) than older ones, like the Saturn V on the Apollo lunar mission, which had three.
“Once you run out of fuel on the first stage, instead of carrying that structure with you to orbit, you just drop it. And that allows you to carry a lot more payload for a given mass on the launch pad,” Hoffman explained.
Traditionally, spent rocket stages fall 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 booster rockets that can autonomously land on Earth. The premise of a single-stage space vehicle is to dispense with rocket stages altogether, with the promise of further reducing costs.
It’s not easy to get around what Hoffman calls the “tyranny of the rocket equation,” or to solve the problem of having to carry the weight of fuel into space. Radian’s solution is a rocket-powered aircraft that runs along a 3-kilometer rail and accelerates to Mach 0.7 (864 kilometers per hour) before releasing the space plane, which then flies into orbit under the power of its own engines.
“There have been several attempts to develop single-stage vehicles to orbit,” Hoffman said. “NASA and the Air Force tried it in the late 1980s and 1990s. They tried to solve the problem with what they call a ramjet engine, which would take the aircraft through the atmosphere and burn oxygen from there instead of having to carry it with them. It’s a great idea, but it’s technically very difficult to build that kind of engine.”
“What Radian is doing with its rocket ship is sort of the equivalent of a ramjet,” Hoffman explained. “In other words, it’s trying to get the initial acceleration without burning the rocket propellant. In that way, you overcome some of the limitations of the rocket equation.”
Radian is convinced that it can overcome the obstacles to successful SSTO thanks to three key technologies.
The first is the aircraft's launch system, which uses its fuel not only to power its three engines but also those of the space plane itself, leaving the plane's fuel tank full just before takeoff. The second is the landing gear, which is designed only for landing and not takeoff, making it significantly lighter. And the third is the wings, which are not present on a vertical rocket but reduce the amount of thrust the system needs when providing lift as it flies into orbit.
“Once we get to orbit, the closest analogy is probably the space shuttle,” Holder said. “We have a smaller bay, but we can do a lot of the same types of missions. And when we come home, we’ll have a more robust exterior surface, and that will allow us to reuse the system over and over again with lower inspection requirements and faster turnaround times.”
Radian said its space plane will be reusable up to 100 times and will carry 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, Holder said, and a full-scale version will begin flight testing (without reaching orbit) in 2028.
Like the shuttle, Radian One could deploy payloads such as satellites into orbit or perform missions using equipment located in the bay, such as Earth observations, surveillance and intelligence for military or defense entities. But, Holder added, the aircraft could also help with humanitarian aid in disaster areas when runways, for example, have become unusable, by dropping the payload out of the bay in a controlled reentry through the atmosphere.
He drew an analogy to a construction site, where the rockets are the 18-wheelers that arrive with large equipment and the Radian One is the pickup truck that carries smaller materials along with the crew. “I think there will always be a place for vertical launch rockets,” he added. “They’re going to carry the really heavy stuff up.”
He is aware of the skepticism that another attempt at SSTO will provoke. The latest high-profile project that lost strength was Britain's Skylon, a hydrogen-powered space plane that would take off from a reinforced runway and land back on Earth. The company behind the project said last year that a two-stage orbiting system was most likely at this point.
“I don’t criticize people who scratch their heads and wonder if single-stage orbit is feasible,” Holder said. “It took me almost a full year on this program to convince myself again that it is. You just have to be able to compare today’s technology to that of the past to see if it’s feasible or not.”
The big question, Hoffman said, is not just whether SSTO can be technically achievable, 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 small payloads on a launch and do so relatively cheaply. “That’s always been the reason for the single-stage-to-orbit dream — in principle, it should be less expensive,” he said.
“I hope they succeed,” Hoffman added. “Because it would definitely be a novelty, technically speaking, and we’ll see what happens with the economics. You never know until they prove the capability and see who signs up to use it.”
