Single-stage-to-orbit: How the holy grail of spaceflight could soon become reality
It's the holy grail of spaceflight, and it could come sooner than many expect.
Spaceflight’s long-held dream could soon come roaring back.
This month, Washington-based Radian Aerospace announced that it’s building a single-stage-to-orbit spaceplane that takes off and lands horizontally. The reveal sparked excitement about what could be considered the holy grail of the decades-old industry.
Christie Maddock, a lecturer in mechanical and aerospace engineering at the University of Strathclyde, says that the “alluring concept” has been around for nearly a century.
“The announcement by Radian Aerospace is exciting, mainly in that it means work in this field, towards this goal, is ongoing,” Maddock tells Inverse. “What exactly that work is, and how successful they may be, is impossible to tell without knowing the technical details.”
If Radian can pull it off, it could mean rockets that work more like planes. That means better reusability, rapid re-use, and in the case of Radian, the ability to land on a wide variety of runways.
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Here’s what you need to know about the technology:
Single-stage-to-orbit: Why do we have stages?
First, a quick primer. Space rockets don’t ignite their engines and head straight for their destination like a car: they are actually comprised of multiple mini-rockets that fall away after they’re used.
Everyday Astronaut notes that the German V2 rocket was the first to go to space, when in 1944 it reached an altitude of 109 miles. That set it soaring past the 62-mile Kármán line, generally agreed as the boundary of space.
A replica of a V2 rocket.
That’s fine for a short demonstration, but rockets need to go faster to reach orbit. The goal is to break away from Earth’s gravity. The speed at which an object can escape Earth’s gravitational forces is its escape velocity, and for Earth it’s around 25,000 mph.
The problem with reaching that speed is that rockets are heavy. More fuel gives more power, but it makes the rocket heavier. You end up in a cat-and-mouse game trying to add enough fuel to power your increasingly-heavy rocket.
The Soviet Union’s R-7 rocket family, which sent the first satellite into space, got around this by dumping engines and fuel tanks after the fuel was depleted. The rocket would then use another smaller set of engines and fuel tanks to continue moving upwards.
“Same thrust, lighter mass, means higher acceleration,” Maddock says.
This process of shedding weight, known as “staging,” powers the space industry as it stands today. NASA’s Saturn V, which sent the first humans to the Moon, used three stages. SpaceX’s Falcon 9 uses two stages.
NASA’s Saturn V.
What is single-stage-to-orbit?
The problem with staging is that you’re dumping components as you launch. That makes it harder to re-use a rocket.
SpaceX has focused on making the first stage of its Falcon 9 rocket fully reusable, but the second stage is still expendable. SpaceX CEO Elon Musk has pointed to airplanes as an ideal for why spaceflight firms should reuse rockets — it would be unreasonable to crash the plane or shed components during every flight.
“It’s really crazy we’re building these sophisticated rockets and then crash them every time they fly,” Musk said in 2017. SpaceX is currently developing a fully-reusable Starship, where the booster and the ship both land safely.
Airplanes, in a sense, are the ultimate example of why a single-stage-to-orbit would be ideal — more sustainable, cheaper, less manufacturing involved.
Radian wants to make use of a concept known as a rocket sled launch, which starts out horizontally, going down a track to gain speed before firing off a ramp at the end, starting the rocket off at an angle as it lifts off as opposed to a vertical launch that gradually rounds out its trajectory to reach orbit.
How can we make a single-stage-to-orbit rocket?
Maddock explains that there are a couple of techniques researchers have explored:
- Lifting surfaces like wings could help generate lift and reduce the need for more propellant — assuming the mass of the wings is less than the mass of the propellant
- Using air from the atmosphere as part of the propulsion system, replacing the oxidizer used in a propulsion system. Unfortunately, it would still have to switch back to a more traditional rocket engine as the vehicle moved through the thinner atmosphere at higher altitudes.
- Use of lighter materials to make the mass smaller
What is Radian Aerospace promising?
We know little so far, but Radian is making big promises with the Radian One vehicle:
- A rocket that takes off horizontally with a sled-assist
- A comfortable ascent with lower forces against the crew
- Smooth landings on any 10,000-foot runway
- The ability to land and re-fly in just 48 hours
Radian plans to use it for sending crew and light cargo to low-Earth orbit. It claims to already have launch service agreements with “commercial space stations, in-space manufacturers, satellite, and cargo companies,” as well as the U.S. and foreign governments.
In its most recent reveal, Radian announced that it has closed $27.5 million in seed funding. Its advisory team includes Michael López-Alegría, who will fly as one of the first crew members on Axiom Space’s Ax-1 mission in late March.
Who else has tried to make a single-stage-to-orbit?
Radian is not the first to consider a sled. Maddock notes that in 1933, Eugen Sänger unveiled plans for the Silbervogel. This would have used a Radian-like rocket sled to accelerate the vehicle to 1,200 mph using a series of V2 rockets.
Jalopnik reports that the idea was dropped by the German government, but the U.S. Air Force explored a similar idea. Boeing’s X-20 Dyna-Soar swapped V2s for Titan IIIs, but it was also dropped in the 1960s. The concepts ultimately helped build NASA’s multi-stage Space Shuttle.
In the 1986 State of the Union speech, president Ronald Reagan voiced his support for a new single-stage-to-orbit project:
“We are going forward with research on a new Orient Express that could, by the end of the next decade, take off from Dulles Airport, accelerate up to 25 times the speed of sound, attaining low Earth orbit or flying to Tokyo within two hours.”
The Rockwell X-30 formed part of NASA's SSTO program.
The National Aero-Space Plane, a project at NASA, promised to reach orbit from practically any airport. The New York Times reports that it promised to reach speeds of up to 17,000 mph and encircle the globe in 90 minutes.
Unfortunately, the technical hurdles proved too much. In 1994, after spending $1.6 billion, the project was cancelled.
Also in the 1980s, Rolls-Royce and British Aerospace explored the HOTOL space vehicle. This vehicle, with the initialism HOrizontal TakeOff and Landing, promised to get from the U.K. to Australia in around an hour. It would have carried 15,400 pounds to low-Earth orbit.
Whether Radian can succeed where others failed remains to be seen.
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