Traveling-Wave Tubes Travel Far
Electronic components designed for NASA see use in satellite systems and ground applications
Forty-five years ago, the Voyager 2 spacecraft launched on a mission to visit the outer planets. One vital component of the craft that still works is the key to getting data as it leaves the solar system. But this piece of the now-interstellar spacecraft, the traveling-wave tube (TWT), has also become a necessary component for utilizing microwaves in several applications back on Earth. For example, satellite radio spacecraft use the amplification power of TWTs, and thanks to NASA’s help, listeners have coverage over all of North America and receive better sounding audio.
Wherever you see a specialized microwave radio transmitter, there’s usually a traveling-wave tube somewhere within. Traveling wave tubes were initially created during the 1940s and were instrumental to the development of technologies like radar. Like the cathode-ray tube in an old television, the traveling-wave tube works due to the movement of electrons within it: Inside a vacuum tube, a stream of electrons is fired from one end of the tube, which causes movement back and forth between electrodes on either side of the tube. A spiral of wire wraps around the path of the beam, and a radio signal is passed through it. The movement of electrons synchronizes with the radio frequency, boosting the signal. All-in-one traveling-wave tube units that include a power source gain the additional moniker of “amplifier.”
In the early 1960s, Hughes Space and Communications Group received contracts from NASA’s Jet Propulsion Laboratory in Pasadena to build spacecraft for NASA’s Surveyor program, which successfully landed five robotic Moon missions before astronauts arrived during Apollo. To maintain communications 238,000 miles from Earth, these landers’ transmitters needed massive amounts of amplification, with the additional constraint of having to fit on a small spacecraft. When Surveyor 1 landed on June 2, 1966, it was able to send signals carrying television images of the Moon’s surface back to Earth, paving the way for astronauts to land there a few years later.
Throughout the 1970s and 1980s, Hughes supplied the traveling wave tubes for every deep space mission, working to the exacting standards that NASA set, including the two Voyager spacecraft, as well as the Galileo and Cassini missions to Jupiter and Saturn respectively. This work pushed the tube’s capabilities further. Thanks to NASA’s need for better ways to transmit data, modern traveling-wave tubes are smaller and able to amplify signals in a wider range of frequencies, such as the Ka band that space telescopes use to send back high-resolution imagery of distant stars and galaxies.
“If it wasn’t for NASA, the technology wouldn’t be anywhere near how advanced it is today,” said Nick Gritti, executive vice president of strategy and business development at Stellant Systems of Torrance, California, a successor company to Hughes.
In a Solid-State World, Vacuum Tubes Endure
It might seem strange that a component most associated with retro aesthetics is still relevant in modern technology, but there are several reasons why the venerable tube is better than alternatives.
“This is the last field where vacuum tubes thrive,” said Wayne Harvey, an engineer at JPL who’s worked on numerous spacecraft missions.
Much as some high-quality audio amplifiers still use vacuum tubes, microwave amplifiers made from tubes can have much better performance than other signal-strengthening methods developed with more conventional solid-state electronics.
“Solid-state electronics can’t keep up in the ranges we need,” said Rainee Simons, a microwave electronics engineer who specializes in traveling wave tubes at NASA’s Glenn Research Center in Cleveland. “The TWTA has much higher efficiency at the same frequencies.”
Keep on Traveling
After splitting from the larger Hughes Aircraft Company, Hughes Microwave Tubes continued to make these specialized components. From the 1990s to the early 2000s, the company existed as part of Boeing before being sold to L3 Corporation, where the company was combined with another vacuum electronics manufacturer under the company’s portfolio, Litton Electron Devices. In 2021, the company split from the L3Harris conglomerate and became Stellant Systems, still operating at the same facility that Hughes did in the 1960s.
“I like to compare us to the island of Sicily,” said Gritti. “We’re the most conquered company in the world.”
Today, Stellant Systems is the only company in the United States that makes space-rated traveling wave tube amplifiers. In addition to being present in NASA spacecraft like the Lunar Reconnaissance Orbiter and the Kepler Space Telescope, Stellant amplifiers are also used in the National Oceanic and Atmospheric Administration’s Earth-observation satellites and in military applications like onboard radar for planes. Compared to the enormous TWTs used in applications on the ground like Doppler weather radar, the space-based TWTs are miniature but still powerful enough to boost signals enough to transmit massive amounts of data from orbit.
Privately funded operations are also benefiting from Stellant’s decades of experience with miniaturizing traveling wave tubes. In late 2021, Stellant sold traveling wave tubes to SiriusXM for its next generation of satellite radio spacecraft. And machines that provide Lasik eye surgery use these tubes to ensure their beams are properly amplified.
“I think for TWTAs, it’s a sort of a leapfrog process,” Harvey said. “Sometimes we’re taking advantage of developments in the commercial world, sometimes those come back around to NASA missions. There’s a synergy there.”
In 2020, the traveling wave tube amplifier was added to the Space Foundation’s Space Technology Hall of Fame. And as of 2022, the TWT on Voyager 2 is still plugging away, transmitting data as it continues to make the journey through interstellar space.
“Nobody can explain why the Voyager TWT is still working,” said Harvey. “There’s going to be life for this technology for some time.”