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I'll note that the mars exploration rovers Spirit and Opportunity were meant to last 90 sols (Martian days). Spirit lasted 2,210, and (assuming it doesn't reply

I'll note that the mars exploration rovers Spirit and Opportunity were meant to last 90 sols (Martian days). Spirit lasted 2,210, and (assuming it doesn't reply) Opportunity lasted 5,111. When you have everyone working on the mission trying to make sure their part isn't the one that causes the mission to end before the 90 days are up, you get a long-lived rover.

dingaling 8y ago
The flip side to over-engineering is ever-ballooning budgets, fewer missions and more chance of random events curtailing the few missions that are flown. If the requirements state 90 days, design for 90 days - even if 180 days is achievable 'for free'. Otherwise there is no feedback loop to inform future requirements and 90 becomes an implicit 2000.b If JWST was designed for two years and a hard shutdown it would have flown years ago and we'd be on JWST Gen 3 by now, doing actual science.
- avar 8y ago
  You're making the false assumption that designing something that last at least 90 days, but ends up lasting 2000 days is more expensive than something intended to last at least 90, and at most 180. Planned obsolescence of hardware operating on another planet can be hard, much harder than over-engineering it. Consider the wheels on the rover as a simple example (Curiosity has had structural issues with those). Next time they can either just say spend an extra 10 kg of payload to make the metal thicker, and be assured that it's "definitely 90, or way more" and move on to the next task. Or, they can consider them lasting beyond 180 a failure. Then they'll need tens of thousands or hundreds of thousands of hours of design and testing to ensure that various types of Martian terrain wear them out at just the right rate, and no more or no less. Easier to just say "this isn't worth micro-optimizing", eat the fixed cost of the extra payload, and move on. There's going to be tens of thousands of components on the rover that are like that, e.g. try finding a camera sturdy enough to operate on Mars but just fragile enough to reliably fall apart in 180 days. In aggregate that's why these rovers are exceeding their life span. If it's truly important that missions are extended for whatever reason that can just be decided as a matter of policy. Stop talking to the rover after 90 days, or hand it over to hobbyists.
  - nkozyra 8y ago
    > You're making the false assumption that designing something that last at least 90 days, but ends up lasting 2000 days is more expensive than something intended to last at least 90, and at most 180. I think this demonstrates the risk op was getting at. Designed for 90 and lasting more is not an implicit assurance it will last up to 180. It changes expectations and that always Cascades.
- Ankaios 8y ago
  If JWST was designed for two years and a hard shutdown it would have flown years ago and we'd be on JWST Gen 3 by now, doing actual science. JWST's design lifespan is not significantly driving its cost. Its cost derives from its complexity, and (crudely) its complexity follows from the aperture diameter and temperature it requires to achieve the scientific objectives chosen for it.
- Rebelgecko 8y ago
  I wouldn't say it's overengineering. If you design something to have a 99.9% chance of lasting 90 days, the odds are really good that it's still going to be working at 180 days. I don't know what the bathtub curve for a Mars rover looks like, but I imagine a disproportionate amount of failures occur in the first week. If you can make it through that, you're going to make it until major systems fail or your solar panels die. You could get rid of hardware redundancies like 2 of the wheels, but that'll just increase the odds of you getting stuck on a rock while the rover is still functional.

dingaling 8y ago

The flip side to over-engineering is ever-ballooning budgets, fewer missions and more chance of random events curtailing the few missions that are flown. If the requirements state 90 days, design for 90 days - even if 180 days is achievable 'for free'. Otherwise there is no feedback loop to inform future requirements and 90 becomes an implicit 2000.b If JWST was designed for two years and a hard shutdown it would have flown years ago and we'd be on JWST Gen 3 by now, doing actual science.

avar 8y ago
You're making the false assumption that designing something that last at least 90 days, but ends up lasting 2000 days is more expensive than something intended to last at least 90, and at most 180. Planned obsolescence of hardware operating on another planet can be hard, much harder than over-engineering it. Consider the wheels on the rover as a simple example (Curiosity has had structural issues with those). Next time they can either just say spend an extra 10 kg of payload to make the metal thicker, and be assured that it's "definitely 90, or way more" and move on to the next task. Or, they can consider them lasting beyond 180 a failure. Then they'll need tens of thousands or hundreds of thousands of hours of design and testing to ensure that various types of Martian terrain wear them out at just the right rate, and no more or no less. Easier to just say "this isn't worth micro-optimizing", eat the fixed cost of the extra payload, and move on. There's going to be tens of thousands of components on the rover that are like that, e.g. try finding a camera sturdy enough to operate on Mars but just fragile enough to reliably fall apart in 180 days. In aggregate that's why these rovers are exceeding their life span. If it's truly important that missions are extended for whatever reason that can just be decided as a matter of policy. Stop talking to the rover after 90 days, or hand it over to hobbyists.
- nkozyra 8y ago
  > You're making the false assumption that designing something that last at least 90 days, but ends up lasting 2000 days is more expensive than something intended to last at least 90, and at most 180. I think this demonstrates the risk op was getting at. Designed for 90 and lasting more is not an implicit assurance it will last up to 180. It changes expectations and that always Cascades.
Ankaios 8y ago
If JWST was designed for two years and a hard shutdown it would have flown years ago and we'd be on JWST Gen 3 by now, doing actual science. JWST's design lifespan is not significantly driving its cost. Its cost derives from its complexity, and (crudely) its complexity follows from the aperture diameter and temperature it requires to achieve the scientific objectives chosen for it.
Rebelgecko 8y ago
I wouldn't say it's overengineering. If you design something to have a 99.9% chance of lasting 90 days, the odds are really good that it's still going to be working at 180 days. I don't know what the bathtub curve for a Mars rover looks like, but I imagine a disproportionate amount of failures occur in the first week. If you can make it through that, you're going to make it until major systems fail or your solar panels die. You could get rid of hardware redundancies like 2 of the wheels, but that'll just increase the odds of you getting stuck on a rock while the rover is still functional.