At first the dream of riding a rocket into space was laughed off the stage by critics who said that 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, they said: To put a kilogram of payload into orbit we just need 98 kg of rocket and rocket fuel.
What a ratio, what a cost. To transport a kilogram of cargo, commercial air freight services typically charge about US $10; space flight reaches $10,000. Sure, you can save money by re-using 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 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 it in a gun, it would sling it from the end of of a carbon-fiber tether that spins around in a vacuum chamber for as long as an hour before reaching terminal speed. It lets go milliseconds before gates in the chamber open up to allow it 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, a suborbital altitude. Such a system might bear a small rocket high enough to ignite and finish the job of reaching orbit. But the ultimate goal is a much larger launcher that would do that job all by itself, lofting payloads of up to 200 kilograms.
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,” he 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.”
SpinLaunch says it will announce the site for its full-scale orbital launcher within the next five months. It will likely be 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 slinged into orbit sometime around 2025.