CRS-7 Launch Update • Hacker News Board

Re: CRS-7 Launch Update

#61

Post by jballanc » Mon, Jun 29, 2015, 3:58 PM UTC

drzaiusapelord wrote: post #41

nkoren wrote: post #34

Earlier quoted context omitted.

Rocket physics are actually relatively trivial and, yes, can be fully simulated (and are). Rocket plumbing is always the hard part. Perhaps the threads on some connector got stripped as it was screwed together. Perhaps there was a manufacturing flaw or unexpected bit of corrosion in a particularly vital bolt. Perhaps a bit of contamination in a fuel line caused something catalytic to happen. Perhaps a bit of excess H…

I guess my point is, can we do a full simulation of every screw, material, plumbing, liquid dynamics, weather dynamics, etc and augment those with known fail scenarios and other fuzzy data to build out a real world KSP that predicts fails reliably? We should understand how things like corrosion and condensation work on a rocket engine. Considering the low cost of incredible amounts of CPU power, granular level simula…

The issue is that rocket physics (essentially Newton's laws of motion) are for the most part first- and second- order, linear, ordinary differential equations with exact solutions. The plumbing, on the other hand, is governed by fluid dynamics, heat transfer, and the like. These are systems of complicated partial differential equations without exact solutions, requiring numeric methods.

Re: CRS-7 Launch Update

#62

Post by Osmium » Mon, Jun 29, 2015, 3:59 PM UTC

drzaiusapelord wrote: post #32

colomon wrote: post #16

Earlier quoted context omitted.

Ugh, now imagining trying to debug a really sneaky bug that costs $100 million every time it triggers...

Stupid question perhaps, but don't we have the ability to 100% simulate the rocket physics and the code it runs on in a simulation environment? Or are they just punching out code, shoving it onto rocket controllers, and testing in the real world? I'm curious as to the QA system used here. I imagine with proper simulation this should have been catchable. I wonder if SpaceX's low cost approach means cutting certain cor…

> I imagine with proper simulation this should have been catchable.

We don't know in this specific instance, but generally this is just fundamentally not true. You're way over-estimating the abilities of computers and way under-estimating the complexity of these systems.

I haven't been down voting your comments, but if I had to guess as to why other people are, it's because comments like this are typical of a certain kind of arrogance. I think we've all had that moment where we think "what, that sounds easy!" about someone else's problems, only to realise when we try to solve it ourselves we're suddenly confronted with that problem's true complexity. These are very smart people working on these problems, and your comments come across as if you're suggesting they've been negligent–while that could be true, it wouldn't be my first guess. Rocket science is hard, and things go wrong, and no amount of computing can change that.

Re: CRS-7 Launch Update

#63

Post by rohit89 » Mon, Jun 29, 2015, 3:59 PM UTC

drzaiusapelord wrote: post #41

nkoren wrote: post #34

Earlier quoted context omitted.

Rocket physics are actually relatively trivial and, yes, can be fully simulated (and are). Rocket plumbing is always the hard part. Perhaps the threads on some connector got stripped as it was screwed together. Perhaps there was a manufacturing flaw or unexpected bit of corrosion in a particularly vital bolt. Perhaps a bit of contamination in a fuel line caused something catalytic to happen. Perhaps a bit of excess H…

I guess my point is, can we do a full simulation of every screw, material, plumbing, liquid dynamics, weather dynamics, etc and augment those with known fail scenarios and other fuzzy data to build out a real world KSP that predicts fails reliably? We should understand how things like corrosion and condensation work on a rocket engine. Considering the low cost of incredible amounts of CPU power, granular level simula…

A lot of simulation already happens but simulating every imaginable scenario is impossible.

Re: CRS-7 Launch Update

#64

Post by mikeash » Mon, Jun 29, 2015, 4:02 PM UTC

grecy wrote: post #59

mikeash wrote: post #48

Earlier quoted context omitted.

It's not something you merely "enable." These cargo missions are using Dragon 1 capsules, whereas the manned system is Dragon 2. The abort system in Dragon 2 is integral to the capsule (it doubles as the landing system used after a normal flight) and isn't something you can just retrofit to a different vehicle.

Can Dragon 1 have parachutes?

Yes, and it does on every flight since it's used to return things from the space station back to Earth. However an abort system also needs something to move the capsule away from the (probably exploding) rocket when something goes wrong, which is typically a strong and short-firing rocket on the capsule itself.

In this particular case, it sounds like Dragon survived and probably could have been recovered using the parachutes, but they weren't armed since survival without an abort system isn't something they really try for. It would be interesting to make the attempt, but not informative for manned flights since those will have a proper abort system.

Re: CRS-7 Launch Update

#65

Post by HCIdivision17 » Mon, Jun 29, 2015, 4:05 PM UTC

drzaiusapelord wrote: post #41

nkoren wrote: post #34

Earlier quoted context omitted.

Rocket physics are actually relatively trivial and, yes, can be fully simulated (and are). Rocket plumbing is always the hard part. Perhaps the threads on some connector got stripped as it was screwed together. Perhaps there was a manufacturing flaw or unexpected bit of corrosion in a particularly vital bolt. Perhaps a bit of contamination in a fuel line caused something catalytic to happen. Perhaps a bit of excess H…

I guess my point is, can we do a full simulation of every screw, material, plumbing, liquid dynamics, weather dynamics, etc and augment those with known fail scenarios and other fuzzy data to build out a real world KSP that predicts fails reliably? We should understand how things like corrosion and condensation work on a rocket engine. Considering the low cost of incredible amounts of CPU power, granular level simula…

I think a critical part of his point is that what you build and what you model are neccessarily two different things. You can totally simulate it, and then when the rocket inevitably fails, you can go "aha! This bit deviated from the simulation!" But you can't feed that forward beforehand to prevent the failure, since you can't expect random acts of poor workmanship or crafting. You can only prepare and hope (and you can do that with shockingly high confidence - but rockets are more than equally shockingly complex.)

Part of what makes the idea of bringing the first stage back to the pad so important is that we so rarely get to use the same engines multiple times in the field (where all the really nasty reality checks are done). Being able to reuse stages allows us to far better model how they will perform in the future. Otherwise, we're using test beds to feed parameters into sims to inform our launches; it's good practice, but more physical evidence is always better.

Re: CRS-7 Launch Update

#66

Post by mmedellin » Mon, Jun 29, 2015, 4:05 PM UTC

JshWright wrote: post #29

phkahler wrote: post #21

My thoughts on this. Overpressure in the LOX tank could be due to 1) failure to vent properly or 2) something heating it up. Option 1 is based on the fact that it's constantly evaporating and needs to vent. This seems really unlikely given that it passed all test on the pad 2 minutes before. It also sounds like this was likely ruled out based on Elon's tweet about a "counter intuitive cause". Option 2 sounds like fir…

There are plenty of other scenarios... The IDA is the heaviest thing they have ever carried in the trunk, AFAIK. Maybe its mounting bracket failed and it impacted the top of the second stage, buckling the LOX tank. A suddenly induced crack allowed some LOX to escape (the initial 'puff'), and the suddenly reduced pressure allowed the rest of the LOX to boil off and the tank to BLEVE, causing the catastrophic failure o…

Falcon typically has a trunk-facing camera on top of the second stage, sometimes broadcasted during separation with Dragon. I wonder if CRS-7 had the camera? If the mount did fail, the footage could show that (assuming SpaceX successfully received the broadcast).

Example footage here: https://youtu.be/p7x-SumbynI?t=25m45s

Re: CRS-7 Launch Update

#67

Post by nitrogen » Mon, Jun 29, 2015, 4:11 PM UTC

drzaiusapelord wrote: post #45

makmanalp wrote: post #37

Earlier quoted context omitted.

I don't think we have the ability to 100% simulate anything - I don't doubt for a second that a lot of this stuff is being simulated already, and I think you may be severely overestimating just how useful any doable simulation is in terms of catching a bug like this one. You have literally hundreds of systems working in concert and tied to more hundreds of physical components coming under extreme temperature and pres…

>You have literally hundreds of systems working in concert and tied to more hundreds of physical components coming under extreme temperature and pressure conditions This is exactly what computers are for: doing hard stuff we can't do on paper or just by real world prototype testing. I imagine this is a hard problem, but it may be so because from a time/budget perspective it may just make financial sense to let stuff…

The problem with simulating failures is that there are an infinite number of them. Should you simulate the effects of omitting each individual molecule in the whole assembly? Or adding one of every possible contaminant molecule at every possible location? What about more than one? It's a combinatorial impossibility.

Re: CRS-7 Launch Update

#68

Post by Osmium » Mon, Jun 29, 2015, 4:11 PM UTC

grecy wrote: post #60

Osmium wrote: post #13

Earlier quoted context omitted.

> we could've hypothetically saved the payload. As I understand it, the primary payload (the new international docking adaptor for the ISS) wasn't in the Dragon capsule itself, so it wouldn't have been saved even if the Dragon was recovered (sadly).

If it wasn't in dragon, how was it going to get to the ISS?

Sorry, I wasn't very clear–it is in the Dragon, but it's not in the pressurised 'nose' part (which I believe is the part that's jettisoned in an emergency, but I'm not actually sure about that? perhaps I'm wrong). Instead, it was in the unpressurised 'trunk'.

See this image: http://www.nasaspaceflight.com/wp-content/uploads/2014/04/Z8...

"Dragon also will use its unpressurized trunk to deliver the first International Docking Adapter to enable future commercial crew vehicles to dock to the station."

https://www.nasa.gov/sites/default/files/atoms/files/spacex_...

They've previously used the trunk for a payload on CRS-2, CRS-3 and CRS-5. You can see a video of how it works in CRS-5 here:

https://www.youtube.com/watch?v=yXfOOIYWGF8

(Really cool video by the way!)

Re: CRS-7 Launch Update

#69

Post by HCIdivision17 » Mon, Jun 29, 2015, 4:15 PM UTC

drzaiusapelord wrote: post #45

makmanalp wrote: post #37

Earlier quoted context omitted.

I don't think we have the ability to 100% simulate anything - I don't doubt for a second that a lot of this stuff is being simulated already, and I think you may be severely overestimating just how useful any doable simulation is in terms of catching a bug like this one. You have literally hundreds of systems working in concert and tied to more hundreds of physical components coming under extreme temperature and pres…

>You have literally hundreds of systems working in concert and tied to more hundreds of physical components coming under extreme temperature and pressure conditions This is exactly what computers are for: doing hard stuff we can't do on paper or just by real world prototype testing. I imagine this is a hard problem, but it may be so because from a time/budget perspective it may just make financial sense to let stuff…

Oh gosh, having worked on a fairly large vacuum system, I can tell you that o-rings are monsters. Very minor errors in dimensions can mess up the seal, and temperature/humidity/wear/elasticity and all that can subtly mess with the dimensions in crazy nonlinear ways. You can simulate the ever loving garbage out of it and an imperceptible change in composition due to an undetectible mixing error when extruding the ring can cause a seal to slightly leak. Mayhem ensues. (And most likely any attempt to directly detect it will destroy the integrity of the o-ring or take so long to render the test useless, since there are usually hundreds of o-rings (or in the thousands - o-rings are all over the place).)

I'm not even talking about jackquesm's note about the failure mode, either. Just real insidious errors in manufacturing that can't be detected in any sort of reliable, sane way. Even the Challenger's o-ring wasn't guaranteed to fail, and indeed most didn't. In fact, most of that entire o-ring didn't fail.

I've seen some really freaky things amplify what are essentially chaotic edge cases. You can certainly figure them out, but you'd never get anything done for any level of affordability in time for any ship date if you didn't just calculate risk and go ahead.

TL;DR: risk is always there because the world's imperfect. At best you just tighten the statistical confidence, but that's super hard.

Re: CRS-7 Launch Update

#70

Post by nitrogen » Mon, Jun 29, 2015, 4:16 PM UTC

rezistik wrote: post #22

DanielBMarkham wrote: post #15

Earlier quoted context omitted.

Not a linguist but a writer here who loves language. I'm seeing a lot of folks dropping articles. It's also becoming quite common to eliminate pronouns at the beginning of sentences, i.e., instead of "I went down to the store" you write "Went to the store" I use this purposefully to jar the reader. I have no idea what the underlying linguistic reasons are.

Implied pronouns, perhaps a trend from Facebook usage? On Facebook and Twitter you have an implied "I" that makes saying you did something redundant. This could transfer over to other formats of conversation.

Other languages are a bit ahead of English in that regard, but their conjugations make it clear who the subject is.