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>the absolute statement that refueling will be completely unneeded. >It's fair to say that over two decades, additional capabilities will exist. Capability do

>the absolute statement that refueling will be completely unneeded. >It's fair to say that over two decades, additional capabilities will exist. Capability doesn't create need, in either a technical or economic/resources sense. Too many people are interpreting "unneeded" as: "a satisfactory amount of fuel for a long-enough lifespan, but more would be better"... When it really means: "fuel is no longer the most likely controlling/ limiting factor as to what ultimately triggers the end of webb's usable/useful lifespan". Even if fuel ends up being the actual limiting factor, no ability to extend the (likely) lifespan of ALL the other critical components makes refueling a high-risk, high-cost, low-return venture.

dredmorbius 5y ago
So, I agree that there are likely multiple paths to end-of-mission for JWST. And that fuel itself, over 20 years, could prove not to be the limiting factor. There remains the point that you've stated, and are again asserting, an absolute condition, that refueling is completely unneeded. Given that a now + 20 years JWST that is still functional except for fuel might exist, that categorical absolute remains unjustified, regardless of how probable it might be. And again, it's specifically how you're communicating this that seems to be raising objections. None of us have a perfect view of the future. JWST itself offers only an improved view of the past, after all.
- beerandt 5y ago
  If refueling was already a highly questionable proposition given 5 or 10 years of fuel, having ~20 years of fuel certainly doesn't increase the reward in risk-vs-reward analysis.[0] In what hypothetical scenario does fuel deplete at a greater than expected rate, in a way that wouldn't also call into question the reliability and expected lifespan of the remaining limiting factors? Accidental commanded release? A leak? Station-keeping being more demanding than expected? Those would all move risk-v-reward in the wrong direction. And it doesn't substantially decrease the cost or risks, other than what might be gained by an extended development period, which would need to be weighed against simultaneous advances better applied to and spent on a successor. So maybe not an absolutism, but an already highly-unlikely proposition who's chances of implementation just decreased dramatically due to the new information at hand. [0] By ending up with an older/ closer to failure but refueled satellite, and a decreased relative value of remaining potential science/observations, presuming highest value science is done first.
  - dredmorbius 5y ago
    These are good points. They are not the elements of your initial nor follow-up comments with which I've had issue. And you continue to not address those. As for what might lead to low-fuel-but-serviceable observatory: reaction-wheel degreadation shy of failure (there are multiple redundant wheels for each axis of rotation), possibly some kind of solar-storm or solar-wind interaction which increases station-keeping requirements. It's difficult to foresee what previously anticipated scenarios might emerge, for all the obvious reasons. But if I had a $10 billion low-on-fuel observatory on my hands, it was still turning in good science, and my capabilities to provide it with fuel had improved markedly, and there was nothing else obviously imminantly limiting lifetime ... I might opt for the top-up. Even at a cost of a few sheckles. Keep in mind that the opportunity cost isn't the amortised value of the observatory, but its projected future life and the cost of replacing that functionality. I don't know what the associated values here are. I could envision a reality in which those turned favourable for a refueling mission. Which, again, contradicts your original, and several-times-sustained, "completely unneeded" assertion. The horse seems quiet well flogged, and in the interests of a peaceful death, I'm begging off further beating of it.
    - beerandt 5y ago
      I think we'd be pretty much in agreement if only 5 years of fuel were left.
WalterBright 5y ago
> Capability doesn't create need, in either a technical or economic/resources sense. Of course it does! Today we have all kinds of needs that people 20, 100, 1000 years did not have. That's because the capability to do something created the need for it. For an obvious example, people need iphones. Nobody had one 20 years ago. For another, deodorant wasn't needed 100 years ago. The need for it was totally created by the ability to manufacture it.
- beerandt 5y ago
  Semantics, I suppose. Capability allows the market to exist and creates potential for demand. For certain levels of actual demand and saturation, social/cultural normalcy and expectations might lead to "need" as you describe it. But in this case, I meant specifically that Webb, given 20 yrs of remaining fuel, won't suddenly "need" refueling if the experimental technology suddenly becomes avaliable in 19 years, as the remaining system also approaches end of life. Fuel is no longer the most likely point of failure for determining lifespan. That might have been different if Webb had to use more fuel on its trajectory burn, had only a few years of fuel, and the technology was extant/eminent and cheap. The risk-v-reward calculus would completely different, but unlikely to change at this point. If my truck throws a rod on a roadtrip in Alaska and I limp upon a gas station as my 20yo dying truck burns the last of its gasoline... I don't "need" gasoline, I need a flight home and a new truck.

dredmorbius 5y ago

So, I agree that there are likely multiple paths to end-of-mission for JWST. And that fuel itself, over 20 years, could prove not to be the limiting factor. There remains the point that you've stated, and are again asserting, an absolute condition, that refueling is completely unneeded. Given that a now + 20 years JWST that is still functional except for fuel might exist, that categorical absolute remains unjustified, regardless of how probable it might be. And again, it's specifically how you're communicating this that seems to be raising objections. None of us have a perfect view of the future. JWST itself offers only an improved view of the past, after all.

beerandt 5y ago
If refueling was already a highly questionable proposition given 5 or 10 years of fuel, having ~20 years of fuel certainly doesn't increase the reward in risk-vs-reward analysis.[0] In what hypothetical scenario does fuel deplete at a greater than expected rate, in a way that wouldn't also call into question the reliability and expected lifespan of the remaining limiting factors? Accidental commanded release? A leak? Station-keeping being more demanding than expected? Those would all move risk-v-reward in the wrong direction. And it doesn't substantially decrease the cost or risks, other than what might be gained by an extended development period, which would need to be weighed against simultaneous advances better applied to and spent on a successor. So maybe not an absolutism, but an already highly-unlikely proposition who's chances of implementation just decreased dramatically due to the new information at hand. [0] By ending up with an older/ closer to failure but refueled satellite, and a decreased relative value of remaining potential science/observations, presuming highest value science is done first.
- dredmorbius 5y ago
  These are good points. They are not the elements of your initial nor follow-up comments with which I've had issue. And you continue to not address those. As for what might lead to low-fuel-but-serviceable observatory: reaction-wheel degreadation shy of failure (there are multiple redundant wheels for each axis of rotation), possibly some kind of solar-storm or solar-wind interaction which increases station-keeping requirements. It's difficult to foresee what previously anticipated scenarios might emerge, for all the obvious reasons. But if I had a $10 billion low-on-fuel observatory on my hands, it was still turning in good science, and my capabilities to provide it with fuel had improved markedly, and there was nothing else obviously imminantly limiting lifetime ... I might opt for the top-up. Even at a cost of a few sheckles. Keep in mind that the opportunity cost isn't the amortised value of the observatory, but its projected future life and the cost of replacing that functionality. I don't know what the associated values here are. I could envision a reality in which those turned favourable for a refueling mission. Which, again, contradicts your original, and several-times-sustained, "completely unneeded" assertion. The horse seems quiet well flogged, and in the interests of a peaceful death, I'm begging off further beating of it.
  - beerandt 5y ago
    I think we'd be pretty much in agreement if only 5 years of fuel were left.

WalterBright 5y ago

> Capability doesn't create need, in either a technical or economic/resources sense. Of course it does! Today we have all kinds of needs that people 20, 100, 1000 years did not have. That's because the capability to do something created the need for it. For an obvious example, people need iphones. Nobody had one 20 years ago. For another, deodorant wasn't needed 100 years ago. The need for it was totally created by the ability to manufacture it.

beerandt 5y ago
Semantics, I suppose. Capability allows the market to exist and creates potential for demand. For certain levels of actual demand and saturation, social/cultural normalcy and expectations might lead to "need" as you describe it. But in this case, I meant specifically that Webb, given 20 yrs of remaining fuel, won't suddenly "need" refueling if the experimental technology suddenly becomes avaliable in 19 years, as the remaining system also approaches end of life. Fuel is no longer the most likely point of failure for determining lifespan. That might have been different if Webb had to use more fuel on its trajectory burn, had only a few years of fuel, and the technology was extant/eminent and cheap. The risk-v-reward calculus would completely different, but unlikely to change at this point. If my truck throws a rod on a roadtrip in Alaska and I limp upon a gas station as my 20yo dying truck burns the last of its gasoline... I don't "need" gasoline, I need a flight home and a new truck.