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by hilbert42
1031 days ago
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In recent years I've occasionally wondered what the type/part number of that TWT is. Given its history, its heater and cathode emission seems to have held up remarkably well. Another question I'd like answered, do the Voyagers cycle down or turn off the heater during TX downtime? Turning it off risks metal/cycling fatigue, leaving it on risks reducing the cathode life—or even poisoning it from stray ions. A final point, presumably the TWT's output has dropped over the decades, how is this monitored and do we know the percentage drop in power output (from the TWT not the Pu power source)? The fact the TWT is still working seems quite remarkable—but then perhaps I shouldn't be overly surprised, one of my TV sets, a 23" Sharp, is 43 years old and the CRT still works well (and it's switched on for several hours every day). |
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I suspect that now the heaters are turned off during Tx downtime, but I think this is probably because they Pu power is much lower and so thy want to conserve as much power as possible. Even if they did have more power, Voyager only makes contact once per day so I think the increased thermal cycling is not much compared to reduction in cathode life so they probably go this route anyway. I do know that other missions I have worked on that have much more frequent contacts, we do keep the Tx heaters on so there is probably some number of cycles where one becomes more dominant.
> A final point, presumably the TWT's output has dropped over the decades, how is this monitored and do we know the percentage drop in power output (from the TWT not the Pu power source)?
This is telemetry from the TWTAs on the helix current and the anode voltage. Along with the DC power draw of the TWTAs I think you can interpret some of what the power output is and how it changed over life. Some of this might be curve fitting to ground-based life tests so there could be some estimation. The ground is also certainly measuring the received power so they can estimate power output from the spacecraft from that as well.
> The fact the TWT is still working seems quite remarkable
You're not the only one. From this article [0] a JPL engineer says "Nobody can explain why the Voyager TWT is still working".
[0] https://spinoff.nasa.gov/Traveling-Wave-Tubes-Travel-Far