4 ms·
> there is nothing inherent about that rocket that means you can't put a crew capsule on it But there is; gold standard for human rated missions is 98% reliab
by ddalex 4y ago
> there is nothing inherent about that rocket that means you can't put a crew capsule on it
But there is; gold standard for human rated missions is 98% reliability rate. This means testing every single component, and making sure that Product(Px) > 0.98 where Px is the probability failure of component x. This drives the cost up exponentially. E.g. if the rocket has Px = 0.99 and the crew capsule has Pcapsule = 0.99, then Psystem = 0.98, barely clearing the threshold.
Falcon 9 had 3.5 failures out of 185 launches, giving it a Pfalcon9 = 0.98; PfalconHeavy then is,at most, 0.98^3 = 0.84, way too low to make it human-rateabale.
The difference between Procket 0.84 and P0.99 is exponential cost.
- pilsetnieks 4y ago0.98^3 is 0.94, not 0.84
- ddalex 4y agoOf course, you are right and I am wrong - fat fingered the calculation. The point however still stands - every extra point of reliability costs 10x.
- philipwhiuk 4y agoPrior to this launch SLS was 0/0. It's now 1/1. Falcon Heavy has never had a launch failure. It's 4/4 If you're arguing it's had a failure because it shares heritage then SLS was 133/135 before the launch which is worse and so shouldn't fly crew until it does way more than just one flight.
- ddalex 4y agoFirst, one should not apply statistics retroactively until you hit large numbers. Second, I am talking about design goals, not track records. You don't evaluate the safety of a large system by running it a lot of times and see what happens - you design the thing towards a goal of safety. So this involves designing each component to the specific failure rate, and testing individually, then then testing together, documenting every step, to provide traceability of every change on every component. The comparison to historical data on Falcon is not entirely valid, because we don't know the design goals of Falcon, etc. Just because Falcon 9 is human rated, doesn't mean Falcon Heavy is human rated, etc.
- bumby 4y agoSmall nit: this assumes all components are series. You can also increase reliability through parallel (i.e., redundancy). Obviously, this may not work for all systems.
- ddalex 4y agothere are lots of ways to increase reliability - redundancy, graceful degradation (e.g. Abort to Orbit mode of STS) etc... but they cost, a lot :)
- deleted 4y ago[deleted]
- bumby 4y ago“Abort to Orbit” constitutes a mission failure and therefore doesn’t increase reliability. It decreases severity (and therefore lowers risk) by limiting loss of crew/spacecraft, but it’s not quite the same as increasing reliability