“They are absolutely not interchangeable with some of the other bands,” says Jerry Ozovek, president of L-3 Communications Electron Technologies. “You can do training, obviously, but there is a certain knack to the Ka-band. It is a difficult product to build.”
Once a wave tube is assembled, a lengthy “burn-in” cycle ensues to confirm the stability of its cathode, focusing, power and gain. Thermal tests then monitor the device's performance across temperature cycles that simulate the vacuum of space, a test and verification process that can take months.
Most time-consuming, however, is the build, where individual piece parts and their precise configuration can differ depending on frequency and power needs. In Ka-band, “you're talking about literally what could be an infinite number of combinations and permutations in terms of configuration that are all driven by the platform designers,” Ozovek says. “It's not as though you can buy the parts, put them on the shelf, wait to see who wins and then build their configuration.”
Although a TWT is comprised of roughly 1,000 parts, only the vacuum tube is standard, leaving suppliers unable to stock parts in anticipation of demand. David Bair, technical director for satellite fleet operator Eutelsat, says component standardization at the prime level could remove a major obstacle for suppliers that is causing “choke points” in the timely delivery of Ka-band satellites.
“Ka-band tubes came into play very recently and put a real strain on the system,” Bair says. “It slows things down and some [manufacturers] have bought tubes on speculation.”
Martin Halliwell, chief technical officer at SES S.A., cautions that satellite manufacturers and suppliers need to react in unison for standardization to work. “It's the correct way to go, but we shouldn't underestimate how difficult this is going to be to get the entire industry to sign up for it,” Halliwell says.
David Bernstein, senior vice president of program management at Space Systems/Loral, says harmonizing designs at the prime level could prove daunting, though some TWT components offer the potential for standardization. One example is the TWT base plate that serves as a mechanical interface between tube and spacecraft.
Bernstein says TWT base-plate designs vary in size and configuration and are determined largely by a spacecraft's mode of heat dissipation. Because heat dissipation is integral to the spacecraft architecture, a fundamental change in its thermal design would be costly, given the time needed to develop, test and requalify a satellite.
“If you change your fundamental approach to thermal design, you're going to have to prove to the customers that it still works,” a process Bernstein says can take several years.
Less drastic changes may be possible, however, such as harmonizing bolt positioning for base plates on competing satellite platforms.