5 ms·

I'm curious to know how this compares to other new aircraft testing, since there are already plenty of commercial companies in that space. Admittedly, spacecraf

by netfire 14y ago

I'm curious to know how this compares to other new aircraft testing, since there are already plenty of commercial companies in that space. Admittedly, spacecraft is a a bit different from normal aircraft, but something going horribly wrong in either situation still means almost certain death.

ballooney 14y ago
It's a whole nuther ballgame for commercial airliners, for example. They undergo hundreds of test flights and all manner of subsystem tests before getting certification. Of course rockets are single use. Most rocket designs will never fly even half the number of test program flights a commercial airliner has to undergo. The cost would be (hohoho) astronomical. A lot of the reliability estimates of spacecraft are really high speculative, and that's a very polite euphemism. I know personally a few people at Nasa involved in human spaceflight, and they say, in unguarded moments at the end of a long day, things like 'well... what the hell does 'man rated' actually mean anyway? does anyone know?'. No one does. Obviously to say 'it will be 99.9% reliable' is very odd - who is going to pay for the several thousand tests required to make a statement like that with any confidence? So yes, take all talk of safety, especially quantitatively, with a pinch of salt. Interestingly the Skylon (reusable) Spaceplane is down to be qualified to the same standards as a commercial airliner. That might yield some data.
- btilly 14y ago
  Of course rockets are single use. Actually one of the design goals for SpaceX is to develop reusable rockets. According to Elon in an ideal world, this would reduce the cost of a launch by something like a factor of 100. Elon claims that a realistic target is to reduce it by a factor of 10. Of course as soon as you design a rocket to be reusable, the task of making sure it is still safe after a dozen or a hundred launches becomes much harder.
- natep 14y ago
  Actually, the largest cost is building a whole new structure. With a fully reusable rocket (as SpaceX is going to use), the only costs are fuel and maintenance, which is more expensive than an airplane (~$50k per flight, IIRC), but negligible compared to the initial cost of designing and building the structure. Could you imagine if airplanes were single use? There would no way Southwest could achieve a 45-minute turnaround time And reliability numbers for man rated parts don't come out of nowhere. They come from extensive simulations and tests, which are then extrapolated. It's not the same guarantee as running thousands of end-to-end missions, but it's better than you imply.
  - ballooney 14y ago
    > With a fully reusable rocket (as SpaceX is going to use) This is the sort of thing where you need to be careful with what you infer, the the sort of thing that often causes engineers to be overconfident in performance and reliability estimates.In reality, they're doing some very initial experiments in vertical landing with a view towards exploring reusability. That is different to your implication that reusability is a done deal. > They come from extensive simulations and tests, which are then extrapolated. It's not the same guarantee as running thousands of end-to-end missions, but it's better than you imply. And that's the problem. Notice you're talking about man rated 'parts' and I'm very deliberately not. Many of the mission failures or anomalies in launch vehicles so far have come from parts that work fine on the bench as individual subsystems. It's the lack of full-scale, realistic tests of complete systems that cause problems. There's just not the money for it nowadays. For example, Orion's crew vehicle had budgeted 2 aeroplane parachute drop tests. Apollo's landing module had over 230. Interestingly, they recorded anomalies on over 210 of those. As for simulations, well one of the catch-phrases in the rocket engine business is 'plumbing never leaks in simulations'. As for extrapolation, as a datapoint related to a field I have worked in (parachutes for space systems), quite a few of the high profile parachute failures were colloquially summarised as 'they extrapolated without a license'. All the Mars landers the USA have landed so far have used disc-gap-band parachutes of the same design and size that were explored in a set of very expensive and extensive tests performed at high altitudes for the 70s Viking Lander. It's called the 'viking box' and people at JPL know you do not just 'extrapolate' out of it because they've seen what happens when smart, well intentioned engineers do. That's why they called it a box :) Going back to simulation for a moment, I am familiar with the state of the art of parachute simulation (and fluid-structure interaction simulation in general), and so are they people in charge of the space missions, and that's why they stick to the Viking box. We can barely match that viking data in sims, let alone start wondering out of it into unexplored territory. Finally a little anecdote from Charles 'Chuck' Lowry, the guy who designed the apollo landing systems, about testing. On Apollo 15 reentry, one of the 3 parachutes failed, the first and only recorded failure of an apollo chute during operations. It was traced back to being because the landing module thrusters had vented their fuel out before landing, but this had ignited on the still hot nozzles on the way out, causing a load of burning fuel to go fly up into the chute and destroy it. Thank god, he said, that it only caught the one and not a second one, else it could have ended very badly. The parachute system tested perfectly, and the thrusters performed admirably during their entire qualification program and all previous flights. But the combination of these two systems, under real conditions, interacted in such that the consequences were a significant risk to life. 'You ain't tested it till you've tested it', he said. HN is full of similar examples of outages of things like AWS due to an interaction of failures of parts, systems, and bob the technician not putting the circuit breaks back in exactly the right place after routine maintenance. The space of possible failures rises exponentially with the number of parts, when you consider all the ways they can interact. It's a hard problem to solve and the people at the top are under no illusions about the reliability numbers, they're made for congress and journalists.

by netfire 14y ago

ballooney 14y ago

It's a whole nuther ballgame for commercial airliners, for example. They undergo hundreds of test flights and all manner of subsystem tests before getting certification. Of course rockets are single use. Most rocket designs will never fly even half the number of test program flights a commercial airliner has to undergo. The cost would be (hohoho) astronomical. A lot of the reliability estimates of spacecraft are really high speculative, and that's a very polite euphemism. I know personally a few people at Nasa involved in human spaceflight, and they say, in unguarded moments at the end of a long day, things like 'well... what the hell does 'man rated' actually mean anyway? does anyone know?'. No one does. Obviously to say 'it will be 99.9% reliable' is very odd - who is going to pay for the several thousand tests required to make a statement like that with any confidence? So yes, take all talk of safety, especially quantitatively, with a pinch of salt. Interestingly the Skylon (reusable) Spaceplane is down to be qualified to the same standards as a commercial airliner. That might yield some data.

btilly 14y ago
Of course rockets are single use. Actually one of the design goals for SpaceX is to develop reusable rockets. According to Elon in an ideal world, this would reduce the cost of a launch by something like a factor of 100. Elon claims that a realistic target is to reduce it by a factor of 10. Of course as soon as you design a rocket to be reusable, the task of making sure it is still safe after a dozen or a hundred launches becomes much harder.
natep 14y ago
Actually, the largest cost is building a whole new structure. With a fully reusable rocket (as SpaceX is going to use), the only costs are fuel and maintenance, which is more expensive than an airplane (~$50k per flight, IIRC), but negligible compared to the initial cost of designing and building the structure. Could you imagine if airplanes were single use? There would no way Southwest could achieve a 45-minute turnaround time And reliability numbers for man rated parts don't come out of nowhere. They come from extensive simulations and tests, which are then extrapolated. It's not the same guarantee as running thousands of end-to-end missions, but it's better than you imply.
- ballooney 14y ago
  > With a fully reusable rocket (as SpaceX is going to use) This is the sort of thing where you need to be careful with what you infer, the the sort of thing that often causes engineers to be overconfident in performance and reliability estimates.In reality, they're doing some very initial experiments in vertical landing with a view towards exploring reusability. That is different to your implication that reusability is a done deal. > They come from extensive simulations and tests, which are then extrapolated. It's not the same guarantee as running thousands of end-to-end missions, but it's better than you imply. And that's the problem. Notice you're talking about man rated 'parts' and I'm very deliberately not. Many of the mission failures or anomalies in launch vehicles so far have come from parts that work fine on the bench as individual subsystems. It's the lack of full-scale, realistic tests of complete systems that cause problems. There's just not the money for it nowadays. For example, Orion's crew vehicle had budgeted 2 aeroplane parachute drop tests. Apollo's landing module had over 230. Interestingly, they recorded anomalies on over 210 of those. As for simulations, well one of the catch-phrases in the rocket engine business is 'plumbing never leaks in simulations'. As for extrapolation, as a datapoint related to a field I have worked in (parachutes for space systems), quite a few of the high profile parachute failures were colloquially summarised as 'they extrapolated without a license'. All the Mars landers the USA have landed so far have used disc-gap-band parachutes of the same design and size that were explored in a set of very expensive and extensive tests performed at high altitudes for the 70s Viking Lander. It's called the 'viking box' and people at JPL know you do not just 'extrapolate' out of it because they've seen what happens when smart, well intentioned engineers do. That's why they called it a box :) Going back to simulation for a moment, I am familiar with the state of the art of parachute simulation (and fluid-structure interaction simulation in general), and so are they people in charge of the space missions, and that's why they stick to the Viking box. We can barely match that viking data in sims, let alone start wondering out of it into unexplored territory. Finally a little anecdote from Charles 'Chuck' Lowry, the guy who designed the apollo landing systems, about testing. On Apollo 15 reentry, one of the 3 parachutes failed, the first and only recorded failure of an apollo chute during operations. It was traced back to being because the landing module thrusters had vented their fuel out before landing, but this had ignited on the still hot nozzles on the way out, causing a load of burning fuel to go fly up into the chute and destroy it. Thank god, he said, that it only caught the one and not a second one, else it could have ended very badly. The parachute system tested perfectly, and the thrusters performed admirably during their entire qualification program and all previous flights. But the combination of these two systems, under real conditions, interacted in such that the consequences were a significant risk to life. 'You ain't tested it till you've tested it', he said. HN is full of similar examples of outages of things like AWS due to an interaction of failures of parts, systems, and bob the technician not putting the circuit breaks back in exactly the right place after routine maintenance. The space of possible failures rises exponentially with the number of parts, when you consider all the ways they can interact. It's a hard problem to solve and the people at the top are under no illusions about the reliability numbers, they're made for congress and journalists.