Thruster Swap Keeps Voyager 1 Mission Alive
Thruster Swap Keeps Voyager 1 Mission Alive
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After nearly 50 years of space travel, Voyager 1’s thrusters were clogged and compromised. To keep the mission alive, engineers 15 billion miles away on Earth activated another set of thrusters.
Voyager 1, launched 47 years ago, has reached interstellar space and continues its incredible journey deeper into the cosmos. The durable spacecraft is currently about 15.4 billion miles from Earth. Reliable communication with NASA depends on Voyager 1 being pointed at Earth; this positioning ability depends on multiple sets of thrusters. After decades in space, however, many of the thrusters are clogged to some degree with silicon dioxide, a by-product from a rubber diaphragm in the spacecraft’s fuel tank. This makes it more difficult for the spacecraft to receive commands and send scientific data.
“This is a really long time for rubber, or any material that is not completely inert, to be flown in space, but there were no known material incompatibilities at the time of launch,” said Todd Barber, Voyager’s propulsion specialist. “This problem was first seen in European spacecraft in the 1990s and was thought to be a ‘European problem’ until a few U.S. spacecraft started showing the same issues. Voyager is most definitely the trendsetter and mission duration pioneer with respect to this phenomenon.”
Voyager 1 has three sets of thrusters: two sets of attitude-propulsion thrusters and one set of trajectory-correction maneuver thrusters. During the mission’s planetary flybys, both types of thrusters were used for different purposes. But as Voyager 1 travels on an unchanging path into deep space, its thruster needs are simpler, so either thruster set can be used to point the spacecraft at Earth.
In 2002 the mission’s engineering team, based at NASA’s Jet Propulsion Laboratory (JPL), noticed some fuel tubes in the attitude propulsion thruster set that were being used to align the spacecraft were clogging, so the team switched to the second set. When that set showed signs of clogging in 2018, the team switched to the trajectory-correction maneuver thrusters, which have been used since then.
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Thruster experts convened at JPL, along with the engineers from the thruster manufacturer, to discuss the problem. No viable methods that could reverse the capillary tube plugging were identified. “The one successful case that showed some improvement with this contamination problem was on a European satellite, but that was for a contaminated thruster catalyst bed itself, not the capillary propellant inlet tube,” said Barber. “Reversal of the latter, to our knowledge, is not possible.”
Now, six years later, those trajectory correction thruster tubes are even more clogged than they were in 2018. The tubes are located inside the thrusters and direct fuel to the catalyst beds, where it is turned into gases. Where the tube opening was originally only 0.01 inches (0.25 millimeters) in diameter, the clogging has reduced it to 0.0015 inches (0.035 mm)—about half the width of a human hair. As a result, the team needed to switch back to one of the attitude propulsion thruster sets—but how?
After weeks of careful planning, the team switched the spacecraft to a different set of thrusters.
“This was a difficult decision, because it had to be made quickly and a brand-new sequence had to be developed to accomplish the switch,” said Barber. “Also, considering how quickly the TCM minus pitch thruster was clogging by the summer of 2024, the potential for losing the thruster was rather imminent.”
Given this, the team opted to accept some small risk in developing a sequence quickly to switch thruster branches to mitigate the larger risk of having fault protection swap the thruster sets. “The latter is more involved and was deemed riskier, even without considering the fact it would be swapping to very cold thrusters, another significant risk for continuing the mission,” Barber said.
Switching to different thrusters would have been easier in 1980 or even 2002. But the wear and tear of 47 years in space has introduced new challenges, primarily related to power supply and temperature.
To save power, the team had previously turned off all non-essential onboard systems, including some heaters, to conserve the gradually shrinking electrical power supply (generated by decaying plutonium).
More For You: Space Exploration and the Engineering Workforce
Although these steps helped reduce power consumption, they also made the spacecraft colder, including the attitude propulsion thruster sets. Turning them on in that state could seriously damage the thrusters, making them unusable.
The team determined that the best solution would be to warm the thrusters before the switch by turning on what had been deemed non-essential heaters. However, the spacecraft’s power supply was so low that turning on non-essential heaters would require the mission to turn off something else to provide the heaters adequate electricity.
After another brainstorming session, the engineering team determined that one of the spacecraft’s main heaters could be safely turned off for up to an hour, freeing up enough power to turn on the thruster heaters. "This was a clever solution, using a known heater [the Bay 1 heater] and its decently known cooling time constant, to buy power for the thruster branch switch,” said Barber. “This was possible because the thruster catalyst bed heater time constant is much shorter than the cooling time constant of the Bay 1 heater. However, we never turn off the Bay 1 heater glibly, since the roll propellant lines immediately start to cool, and are already quite near the freezing point of hydrazine. There was also the risk that the Bay 1 heater would fail to turn back on, but we felt relatively comfortable with this risk.
The strategy worked—on August 27 the team confirmed that the needed thruster set was back in action, keeping Voyager 1 pointed toward Earth.
“Given the risks of an autonomous swap to a different thruster set, especially with extremely cold thrusters, there is a decent chance this pre-emptive thruster set switch may have saved the Voyager 1 mission,” Barber said.
More on Space: Robotic Surgeon Makes Rounds in Orbit
“All the decisions we will have to make going forward are going to require a lot more analysis and caution than they once did,” said Suzanne Dodd, Voyager’s project manager at the JPL, which manages Voyager for NASA.
The spacecraft continues its travel through interstellar space. The mission science team works tirelessly to keep Voyager 1 going for as long as possible. How much longer can the mission last with its dwindling power supply? The team’s best guess is that it can continue with at least one science instrument into the 2030s. However, as issues become more frequent and complex with age, the mission could end at any time.
Mark Crawford is a technology writer in Corrales, N.M.
“This is a really long time for rubber, or any material that is not completely inert, to be flown in space, but there were no known material incompatibilities at the time of launch,” said Todd Barber, Voyager’s propulsion specialist. “This problem was first seen in European spacecraft in the 1990s and was thought to be a ‘European problem’ until a few U.S. spacecraft started showing the same issues. Voyager is most definitely the trendsetter and mission duration pioneer with respect to this phenomenon.”
Troubleshooting a solution
Voyager 1 has three sets of thrusters: two sets of attitude-propulsion thrusters and one set of trajectory-correction maneuver thrusters. During the mission’s planetary flybys, both types of thrusters were used for different purposes. But as Voyager 1 travels on an unchanging path into deep space, its thruster needs are simpler, so either thruster set can be used to point the spacecraft at Earth.In 2002 the mission’s engineering team, based at NASA’s Jet Propulsion Laboratory (JPL), noticed some fuel tubes in the attitude propulsion thruster set that were being used to align the spacecraft were clogging, so the team switched to the second set. When that set showed signs of clogging in 2018, the team switched to the trajectory-correction maneuver thrusters, which have been used since then.
Discover the Benefits of ASME Membership
Thruster experts convened at JPL, along with the engineers from the thruster manufacturer, to discuss the problem. No viable methods that could reverse the capillary tube plugging were identified. “The one successful case that showed some improvement with this contamination problem was on a European satellite, but that was for a contaminated thruster catalyst bed itself, not the capillary propellant inlet tube,” said Barber. “Reversal of the latter, to our knowledge, is not possible.”
Now, six years later, those trajectory correction thruster tubes are even more clogged than they were in 2018. The tubes are located inside the thrusters and direct fuel to the catalyst beds, where it is turned into gases. Where the tube opening was originally only 0.01 inches (0.25 millimeters) in diameter, the clogging has reduced it to 0.0015 inches (0.035 mm)—about half the width of a human hair. As a result, the team needed to switch back to one of the attitude propulsion thruster sets—but how?
The lesser of two risks
After weeks of careful planning, the team switched the spacecraft to a different set of thrusters. “This was a difficult decision, because it had to be made quickly and a brand-new sequence had to be developed to accomplish the switch,” said Barber. “Also, considering how quickly the TCM minus pitch thruster was clogging by the summer of 2024, the potential for losing the thruster was rather imminent.”
Given this, the team opted to accept some small risk in developing a sequence quickly to switch thruster branches to mitigate the larger risk of having fault protection swap the thruster sets. “The latter is more involved and was deemed riskier, even without considering the fact it would be swapping to very cold thrusters, another significant risk for continuing the mission,” Barber said.
Switching to different thrusters would have been easier in 1980 or even 2002. But the wear and tear of 47 years in space has introduced new challenges, primarily related to power supply and temperature.
To save power, the team had previously turned off all non-essential onboard systems, including some heaters, to conserve the gradually shrinking electrical power supply (generated by decaying plutonium).
More For You: Space Exploration and the Engineering Workforce
Although these steps helped reduce power consumption, they also made the spacecraft colder, including the attitude propulsion thruster sets. Turning them on in that state could seriously damage the thrusters, making them unusable.
The team determined that the best solution would be to warm the thrusters before the switch by turning on what had been deemed non-essential heaters. However, the spacecraft’s power supply was so low that turning on non-essential heaters would require the mission to turn off something else to provide the heaters adequate electricity.
After another brainstorming session, the engineering team determined that one of the spacecraft’s main heaters could be safely turned off for up to an hour, freeing up enough power to turn on the thruster heaters. "This was a clever solution, using a known heater [the Bay 1 heater] and its decently known cooling time constant, to buy power for the thruster branch switch,” said Barber. “This was possible because the thruster catalyst bed heater time constant is much shorter than the cooling time constant of the Bay 1 heater. However, we never turn off the Bay 1 heater glibly, since the roll propellant lines immediately start to cool, and are already quite near the freezing point of hydrazine. There was also the risk that the Bay 1 heater would fail to turn back on, but we felt relatively comfortable with this risk.
Cheers all around
The strategy worked—on August 27 the team confirmed that the needed thruster set was back in action, keeping Voyager 1 pointed toward Earth.“Given the risks of an autonomous swap to a different thruster set, especially with extremely cold thrusters, there is a decent chance this pre-emptive thruster set switch may have saved the Voyager 1 mission,” Barber said.
More on Space: Robotic Surgeon Makes Rounds in Orbit
“All the decisions we will have to make going forward are going to require a lot more analysis and caution than they once did,” said Suzanne Dodd, Voyager’s project manager at the JPL, which manages Voyager for NASA.
The spacecraft continues its travel through interstellar space. The mission science team works tirelessly to keep Voyager 1 going for as long as possible. How much longer can the mission last with its dwindling power supply? The team’s best guess is that it can continue with at least one science instrument into the 2030s. However, as issues become more frequent and complex with age, the mission could end at any time.
Mark Crawford is a technology writer in Corrales, N.M.
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