In the 1990s, NASA designed an experimental space plane intended to be a cost-effective alternative to expensive rockets.
Called the X-33, it was based on a concept called SSTO — or “single stage to orbit”. The SSTO eliminates rocket stages in conventional spaceflight — in which the rockets containing engines and fuel are discarded during the climb to reduce weight —, favoring a single fully reusable spacecraft .
The X-33 is designed to launch vertically like a rocket but land on a runway like an airplane, with the goal of reducing the cost of sending a pound of payload into orbit from $10,000 to just $1,000.
However, the program was canceled in 2001 due to technical difficulties, joining a list of similar projects that did not come to fruition.
“I was leading the X-33 program, and we gave it up because our assessment was that it was going to cost more than we were willing to spend, and we were at the limit of technological capability to actually do 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 dramatically since the X-33 — we have composite materials that are lighter, stronger, and can handle a wider thermal range than we had back then. And the propulsion is better than anything we had, in terms of how efficiently it burns propellant and how much the systems weigh,” he says.
The product of this updated technology is Radian One, a new spaceplane that will replace vertical launch with a very unusual system — a rocket-powered sled.
Wasted Internships
To escape Earth’s gravity and reach orbit, a rocket needs to reach a speed of about 17,000 mph, according to Jeffrey Hoffman, a professor of aeronautics and astronautics at the Massachusetts Institute of Technology and a former NASA astronaut who flew on five space shuttle missions.
“The problem is that as you go up, you not only have to lift the rocket and the payload, but also all the fuel it’s carrying,” he says.
A rocket capable of reaching that speed would need to devote 95 percent of its mass to fuel, Hoffman says, leaving very little room for anything else. “It’s a dream to get to orbit with one stage,” he adds. “But to do that, the rocket structure, engines, and 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 things like that.”
That’s why every rocket ever used to reach orbit has multiple stages, although current rockets, like SpaceX’s Falcon 9, have fewer stages—two—than older ones, like the Apollo moon mission’s Saturn V, which had three.
“Once you use up all the propellant in the first stage, instead of carrying that structure with you to orbit, you just jettison it. And that effectively allows you to carry a lot more payload for a given mass on the launch pad,” Hoffman explains.
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 this paradigm by designing reusable boosters that can autonomously land back on Earth. The premise of a single-stage space vehicle is to eliminate 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 sled that runs along a two-mile track and accelerates to Mach 0.7 — 540 mph (864 km/h) — before releasing the spaceplane, which then flies into orbit under its own engines.
“There have been several attempts to develop single-stage-to-orbit vehicles,” Hoffman notes. “NASA and the Air Force tried this in the late 1980s and 1990s. They tried to get around the problem by having what they call a scramjet engine, which would take the aircraft through the atmosphere and burn oxygen there instead of having to carry 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 its rocket sled is something equivalent to the scramjet,” Hoffman explains. “In other words, trying to get the initial acceleration without burning your rocket propellant. That way, you get around some of the limitations of the rocket equation.”
Space truck
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 of the spaceplane itself, leaving the spaceplane with a full tank just before liftoff. The second is the landing gear, which is designed only for landing, not takeoff, making it significantly lighter. And the third is the wings, which are absent on a vertical rocket but reduce the amount of thrust needed by the system, providing lift as it flies toward orbit.
“When we get to orbit, the closest analogy is probably the space shuttle,” says Holder. “We have a smaller compartment, but we can do many of the same types of missions. And when we fly back home, we have a more robust composite outer skin, which allows us to reuse the system multiple times, with fewer 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 space shuttle, Radian One would be able to deploy payloads, such as satellites, into orbit, or perform missions using equipment housed in the bay, such as Earth observations or surveillance and intelligence for defense or military entities.
But, Holder adds, the plane could also help with humanitarian relief operations in disaster areas when, for example, runways have been rendered unusable — by releasing the payload from the compartment in a controlled reentry into the atmosphere.
He draws an analogy to a construction site, where rockets are the 18-wheelers carrying big equipment, and Radian One is the pickup truck bringing in smaller materials and crew. “I think there will always be a place for vertical-launch rockets,” he adds. “They’re going to carry the really heavy stuff.”
He is aware of the skepticism that another SSTO attempt will face. The last high-profile project of this kind to lose momentum 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 was now more likely.
“I don’t criticize those who scratch their heads and wonder if single stage to orbit is feasible,” says Holder. “It took me about a full year into this program to convince myself again that it is. You just have to be able to put yourself in today’s technology versus the technology of the past to see whether or not it is feasible.”
The big question, Hoffman said, is not just whether SSTO can be technically feasible, 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 in a single launch and do so at relatively low cost. “That’s always been the reason for pursuing the single-stage-to-orbit dream — in principle, it should be less expensive,” he said.
“I hope they’re successful,” Hoffman added. “Because it would definitely be a first, technically — and we’ll see about the economics. You never know until they demonstrate the capability and see who signs up to use it.”
This content was originally published in Meet the company that wants to replace rockets with space shuttles on the CNN Brasil website.
Source: CNN Brasil
Charles Grill is a tech-savvy writer with over 3 years of experience in the field. He writes on a variety of technology-related topics and has a strong focus on the latest advancements in the industry. He is connected with several online news websites and is currently contributing to a technology-focused platform.
