At first, the dream of riding a rocket into space was
laughed off the stage by critics who said you’d have to carry along fuel that weighed more than the rocket itself. But the advent of booster rockets and better fuels let the dreamers have the last laugh.
Hah, the critics said: To put a kilogram of payload into orbit we just need 98 kilograms of rocket plus rocket fuel.
What a ratio, what a cost. To transport a kilogram of cargo, commercial air freight services typically charge about US $10; spaceflight costs reach $10,000. Sure, you can save money by reusing the booster, as Elon Musk and Jeff Bezos are trying to do, but it would be so much better if you could dispense with the booster and shoot the payload straight into space.
The first people to think along these lines used cannon launchers, such as those in Project HARP (High Altitude Research Project), in the 1960s. Research support dried up after booster rockets showed their mettle. Another idea was to shoot payloads into orbit along a gigantic electrified ramp, called a railgun, but that technology still faces hurdles of a basic scientific nature, not least the need for massive banks of capacitors to provide the jolt of energy.
Imagine a satellite spinning in a vacuum chamber at many times the speed of sound. The gates of that chamber open up, and the satellite shoots out faster than the air outside can rush back in—creating a sonic boom when it hits the wall of air.
Now SpinLaunch, a company founded in 2015 in Long Beach, Calif., proposes a gentler way to heave satellites into orbit. Rather than shoot the satellite in a gun, SpinLaunch would sling it from the end of a carbon-fiber tether that spins around in a vacuum chamber for as long as an hour before reaching terminal speed. The tether lets go milliseconds before gates in the chamber open up to allow the satellite out.
“Because we’re slowly accelerating the system, we can keep the power demands relatively low,” David Wrenn, vice president for technology, tells IEEE Spectrum. “And as there’s a certain amount of energy stored in the tether itself, you can recapture that through regenerative braking.”
SpinLaunch began with a lab centrifuge that measures about 12 meters in diameter. In November, a 33-meter version at Space Port America test-launched a payload thousands of meters up. Such a system could loft a small rocket, which would finish the job of reaching orbit. A 100-meter version, now in the planning stage, should be able to handle a 200-kg payload.
Wrenn answers all the obvious questions. How can the tether withstand the g-force when spinning at hypersonic speed? “A carbon-fiber cable with a cross-sectional area of one square inch (6.5 square centimeters) can suspend a mass of 300,000 pounds (136,000 kg),” he says.
How much preparation do you need between shots? Not much, because the chamber doesn’t have to be superclean. If the customer wants to loft a lot of satellites—a likely desideratum, given the trend toward massive constellations of small satellites–the setup could include motors powerful enough to spin up in 30 minutes. “Upwards of 10 launches per day are possible,” Wrenn says.
How tight must the vacuum be? A “rough” vacuum suffices, he says. SpinLaunch maintains the vacuum with a system of airlocks operated by those millisecond-fast gates.
Most parts, including the steel for the vacuum chamber and carbon fiber, are off-the-shelf, but those gates are proprietary. All Wrenn will say is that they’re not made of steel.
So imagine a highly intricate communications satellite, housed in some structure, spinning at many times the speed of sound. The gates open up, the satellite shoots out far faster than the air outside can rush back in. Then the satellite hits the wall of air, creating a sonic boom.
No problem, says Wrenn. Electronic systems have been hurtling from vacuums into air ever since the cannon-launching days of HARP, some 60 years ago. SpinLaunch has done work already on engineering certain satellite components to withstand the ordeal—“deployable solar panels, for example,” he says.
After the online version of this article appeared, several readers objected to the SpinLaunch system, above all to the stress it would put on the liquid-fueled rocket at the end of that carbon-fiber tether.
“The system has to support up to 8,000 gs; most payloads at launch are rated at 6 or 10 gs,” said John Bucknell, a rocket scientist who heads the startup Virtus Solis Technologies, which aims to collect solar energy in space and beam it to earth.
Keith Lostrom, a chip engineer, went even further. “Drop a brick onto an egg—that is a tiny fraction of the damage that SpinLaunch’s centripedal acceleration would do to a liquid-fuel orbital launch rocket,” he wrote, in an emailed message.
Wrenn denies that the g-force is a dealbreaker. For one thing, he argues, the turbopumps in liquid-fuel rockets spin at over 30,000 rotations per minute, subjecting the liquid oxygen and fuel to “much more aggressive conditions than the uniform g-force that SpinLaunch has.”
Besides, he says, finite element analysis and high-g testing in the company’s 12-meter accelerator “has led to confidence it’s not a fundamental issue for us. We’ve already hot-fired our SpinLaunch-compatible upper-stage engine on the test stand.”
SpinLaunch says it will announce the site for its full-scale orbital launcher within the next five months. It will likely be built on a coastline, far from populated areas and regular airplane service. Construction costs would be held down if the machine can be built up the side of a hill. If all goes well, expect to see the first satellite slung into orbit sometime around 2025.
This article was updated on 24 Feb. 2022 to include additional perspectives on the technology.