Comment by gruturo | Hacker News Reader

Comment by gruturo | Hacker News Reader

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Yes, but mostly no, also no, and, frankly, why would we.

Yes: we could lift off a much heavier spacecraft, give it plenty of fuel, and many of its parts would be lighter than their 1970s equivalent, giving us lots of room for modern sensors

No: the "battery" in the old ones is nuclear and it's going to be difficult to beat that (but not impossible: a nuclear stirling engine, either powered by fission or by decay, e.g. https://www.nasa.gov/technology/rps/stirling-convertor-sets-...)

Also no: the old Voyagers benefited from a lot of gravity assists from half the solar system, thanks to an alignment which won't happen again until 2151 (https://space.stackexchange.com/questions/5075/when-is-the-n...), so unless you're not in a hurry, we won't have that.

Why would we: the _point_ of the Voyager crafts was to do close flybys and collect plenty of data from the outer planets, not to go as far away and as fast as possible. You want to be as slow as possible near them, so you have science time. You're rushing this part in order to get right away to the centuries of nothing which follow?

Just a small aside, are Stirling engines used on other spacecraft? The Wikipedia article suggests development was largely abandoned a decade ago. My first glance concern would be that because it's reliant on moving parts it is liable to fail sooner than a purely thermoelectric RTG (due to part wear, lubricant leaks, fatigue, etc). This would seem to be quite important on the timeline of a probe that is going to take a long time to reach the subject of its investigation.

Which, to your point, only works against the idea of hypothetical Voyager 3.

We probably do not need to wait for ideal planets position to outrun Voyagers. We have ion thruster now with much higher specific impulse!

Voyager-2 gained about 10 km/s at Jupiter, about 5 km/s at Saturn, about 2 km/s at Uranus, and lost about 2 km/s at Neptune. [1]

Dawn spacecraft gained 11.5 km/s from ion thrusters.

So just gravity assist maneuver just around Jupiter alone + massive tank for ion thrusters gas might give 20+ km/s

Though I agree that value of such a mission would be low.

[1] https://www.planetary.org/articles/20130926-gravity-assist

Unfortunately a gravity assist maneuver is almost "incompatible" with an ion thruster.

If you approach e.g. Jupiter you gain speed (it's pulling you in), which you then lose as you get away from it (as it's still pulling you in, meh). Gravity assists work because you use your chemical rocket right when you are closest and speed away, "robbing" Jupiter of the chance to claim the energy it lent you on approach.

Ion thrusters have very low thrust, so you would accelerate veeeeery slowly away from Jupiter's gravity well - and in this time it will keep affecting you and slowing you down, and the whole thing would be barely worth doing.

You could bolt on a very simple solid rocket booster just for the gravity assist, of course, but its ISP will be lower and you'll have to carry its mass until you can expend it.

I think you are mixing up gravity assist with the oberth effect. I'm pretty sure Voyagers did no massive burns close to the giant planets for example. A (so far theoretical) Solar Oberth maneuver on the other hand...

Whoops, you're completely right. I stand corrected. Thanks, I just learned the difference between the 2.

> the "battery" in the old ones is nuclear and it's going to be difficult to beat that

Newer atomic batteries can theoretically last centuries instead of decades.

V'ger 1 atomic battery is based on Plutonium-238 which has a halflife of ~88 years. It's down to ~210W output from initial 470W at launch time.

Americium-241 has a half-life of over 400 years.

The longer the half life, the lower the power output, no free lunches. But by putting the same suitable radioactive lump in a stirling engine you can extract 4 times as much electricity: https://en.wikipedia.org/wiki/Stirling_radioisotope_generato...

So yes maybe with Americium-241 we could have something which lasts 4 times a much _and_ gives us the same amount of power thanks to a SRG.

But it seems to involve moving parts, so I am not sure the reliability trade-off is worth it for decades/centuries long missions.

Yes, it's being tested (NASA ran one for ~15 years continuously, which seems promising, I don't know if the test is still running), but I don't know if any were actually launched and are used in space.

That seems actually quite impressive.

Using a sterling motor would give you mechanical wear.

Finding “V’ger” hilarious.

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