Cause of SpaceX Crew Dragon Explosion Revealed (cbsnews.com) 81
On April 20, a SpaceX Crew Dragon capsule, designed to take humans to the ISS, exploded during a routine test fire at Cape Canaveral Air Force Station. The cause has now been identified as a leaky valve in a propellant pressurization system. Thelasko shares a report from CBS News: Hans Koenigsmann, SpaceX vice president of build and reliability said: "We believe that we had a liquid slug of the (NTO) in the pressurization system. When we opened the valves and pressurized the propellant system, we think that this slug was driven back into the check valve. That basically destroyed the check valve and caused an explosion."
He said no one expected that "NTO driven into a titanium component would cause such a violent reaction. We then performed tests ... with the help of NASA, and we found out when the pressure is high, the temperature is high and you drive a slug with a lot of energy into a titanium component that you can have these rather violent reactions." Additional work is needed to rule out other less likely culprits but SpaceX is pressing ahead with plans to replace the valves in question with pressure-activated "burst discs" that have no moving parts and cannot leak.
He said no one expected that "NTO driven into a titanium component would cause such a violent reaction. We then performed tests ... with the help of NASA, and we found out when the pressure is high, the temperature is high and you drive a slug with a lot of energy into a titanium component that you can have these rather violent reactions." Additional work is needed to rule out other less likely culprits but SpaceX is pressing ahead with plans to replace the valves in question with pressure-activated "burst discs" that have no moving parts and cannot leak.
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Space X seems to have a solid testing plan (Score:5, Interesting)
It's not like SpaceX has not seen some explosions and failures. But happily none of them have injured anyone, and Space X seems to have a pretty good record of finding all of the strange edge cases like this explosive slug issue, before it affects people or many major missions.
Of all of the major space ship providers SpaceX is probably the one I'd feel most comfortable flying in, except maybe NASA... but eventually SpaceX may overtake NASA's safety record (they would need to start flying any humans first to even start to compare).
Keep up the good work SpaceX...
Re:Space X seems to have a solid testing plan (Score:5, Interesting)
They absolutely do, but titanium.. man.
Titanium has a long record of failures in the rocket business. It's a particularly reactive metal, and also sucks to try and machine or weld. You have an extremely small temperature and time window to work in. Go read Ignition! and check out some of the stories of disasters with Ti.
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> It's not like SpaceX has not seen some explosions and failures. But happily none of them have injured anyone
A SpaceX employee was killed at their McGregor test site in 2014, but I'm having trouble finding the details.
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Thanks for the link, I had not heard about that. :-(
So I guess there is at least one so far...
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But even that article says the death was not related to a rocket test, so it's not even relevant to the context you were discussing
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It's not like SpaceX has not seen some explosions and failures. But happily none of them have injured anyone...
Not exactly. [spaceflightinsider.com] If you only consider explosions and failures involving rocket hardware then yes, they've never lost anyone. But in all their operations, they've lost at least one. That was a machine shop explosion and fire.
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It's not like SpaceX has not seen some explosions and failures. But happily none of them have injured anyone
Well considering it's all unmanned and the only interaction they've had with people is the cargo Dragon docking with the ISS, that's not very surprising. Sure, they are handling some hazardous chemicals on the ground but all the rocket science bits have huge safety ranges. It's when you start sending humans to space you have human casualties.
Of all of the major space ship providers SpaceX is probably the one I'd feel most comfortable flying in, except maybe NASA
It's Lockheed Martin building the Orion, even though the contract is a bit different there's extremely little SpaceX does with the Crew Dragon on their own. Right now I
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It's not like SpaceX has not seen some explosions and failures. But happily none of them have injured anyone
Yet.
Even as a child, I could see the hubris that existed until Apollo 1 had it's fire, killing three Astronauts. Or the Challenger disaster. Or Columbia. Hubris.
The problem that made this Dragon Capsule explode took them by surprise. A lot of the initial blame was on testing an already flown capsule. And I am curious about the idea of replacing a check valve with a burst disc. I've been on projects that use both, and they are kinda different things. But that's a different matter.
No, my concerns are
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No, my concerns are that Spacex is following a path that is pretty familiar, and a lot of that attitude is what causes bad accidents.
It's a complex business with small margins. Is there anybody else who can do a manned space project without accidents ?
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No, my concerns are that Spacex is following a path that is pretty familiar, and a lot of that attitude is what causes bad accidents.
It's a complex business with small margins. Is there anybody else who can do a manned space project without accidents ?
That's my point in all of this. No one can do this without accidents. If you watch a Spacex launch on Youtube, the audio mix is of the crowd cheering in orgasmic glee with each event. Launch, stage separations The mix has their ecstasy drowning out the launch noise, the mix has them drowning out the Flight Communicator's announcements. It seems like Spacex is trying to turn it into the Oscars or a football game.
I don't have any problem with them otherwise I find what Spacex is doing pretty exciting - j
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There's logic to both check valves and burst discs.
SpaceX has always preferred the idea of fully reusable craft with minimal turnaround that you can test repeatedly (including, for all systems that aren't only testable in-flight, pre-flight testing). E.g., with valves you can test them out before you launch. With burst discs, if you test it, that's it - it's used, you can't reset it and then fly. So you're always flying with an untested burst disc.
But, burst discs are a lot simpler. So there's that.
Regardl
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Also, there's been a progressive scaling back of ambition for Dragon 2 as SpaceX has been accelerating Starship. Initially it was supposed to even be able to land on Mars. Then Red Dragon got cancelled and it was just propulsive landings on Earth, and flyby moon trips. Then the flyby moon trips got moved to Starship, and NASA didn't want propulsive Earth landings, so development of them stopped too. And the closer we get to Starship flying, the less SpaceX cares about Dragon 2 except for what's needed t
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There's logic to both check valves and burst discs.
I'm a little outside of my area of expertise here - so I could write something dumb.
The check valve is of course, designed to keep fluid from returning to the direction from which it came. A Burst disc is designed to fail after pressure reaches some point. But the burst disc is not generally placed in a circuit - it routes the fluid away from the circuit. While the check valve is more akin to an electronic diode - the fluid is constrained to one direction.
So I would be thinking that rather replace the
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strange edge cases like this explosive slug
I've been eating slugs for years and all I ever got was rat lungworm. Fucking rich bastards get all the good toys.
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SpaceX doesn't have any space ships. They have rockets that deploy satellites.
The SpaceX press release (Score:5, Informative)
In case you were wondering... (Score:2)
NTO is Nitrogen Tetroxide, which is oxidizer component of the rocket fuel for SpaceX.
interim solution (Score:4, Interesting)
Apparently, the burst discs that will replace the faulty valves aren't reusable. There's time pressure to maintain their schedule, but hopefully a more elegant fix will be forthcoming.
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If you have access to the safety device, swapping a burst disc is a fast and simple procedure.
1. Unscrew cap.
2. Remove used burst disc
3. Put new burst disc in.
4. Torque cap.
This assumes the burst disc vents to wherever the disc is located. If it's got pipes on both sides it gets a bit more complicated.
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If you have access to the safety device, swapping a burst disc is a fast and simple procedure.
1. Unscrew cap.
2. Remove used burst disc
3. Put new burst disc in.
4. Torque cap.
This assumes the burst disc vents to wherever the disc is located. If it's got pipes on both sides it gets a bit more complicated.
It has pipes on both sides.
Re:interim solution (Score:5, Informative)
Apparently, the burst discs that will replace the faulty valves aren't reusable. There's time pressure to maintain their schedule, but hopefully a more elegant fix will be forthcoming.
After NASA nixed the idea of propulsive landing the SuperDracos are there just for the abort system so on missions that go according to plan they wouldn't be used at all. So they'll be looking into whether just going to space means they need to be replaced between missions or not, obviously if the abort system fires they're not reusable but then nobody's going to reuse anything anyway. So it might very well become the permanent solution.
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Currently, NASA doesn't want to fly astronauts on reused capsules anyway, even if the flight was nominal.
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Currently, NASA doesn't want to fly astronauts on reused capsules anyway, even if the flight was nominal.
The capsule itself no, but I would think they can scavenge parts on a case-by-case basis. I don't know that for a fact, but I had that impression from other parts like the grid fins the boosters use. As long as they're good they can be used even if the booster is retired, assuming it lands and is not expended of course.
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The Hooman rate Dragon Capsule will be reused for un-hoomaned cargo flights.
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Apparently, the burst discs that will replace the faulty valves aren't reusable. There's time pressure to maintain their schedule, but hopefully a more elegant fix will be forthcoming.
Right, and the reason they originally went with valves is because they allow testing before use. The burst discs must be replaced after every test.
lack of foresight? (Score:5, Informative)
This one line says it all: no one expected that "NTO driven into a titanium component would cause such a violent reaction".
I used to do a lot of work with various metals at high temperatures in oxidizing environments. The self-ignition of many alloys (such as titanium) is well known and should be respected under conditions of high temperature, pressure, and exposure to oxidants. Since NTO is an excellent oxidizer, they should have investigated this. Alas.
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Check out "Ignition!" from your favorite book source for a reasonably approachable history of the search for usable rocket fuels. I read it on the recommendation of someone on Slashdot and had a good time. EBook warning: the version I read had crappy OCR on organic chemistry diagrams.
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Pretty sure this is a very low temperature environment, as the fuels are cryogenically stored - but apparently at those sort of pressures, with the NTO "slug" impacting hard enough to destroy the titanium valve, then that was enough to create a spark and away we go.
Low on their fault tree maybe, but they did find it. My money was on a salt-immersion issue with the SuperDracos, but apparently titanium isn't safe enough even at those temperatures. They may have to re-evaluate a number of their components.
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Dragon relies on hypergolic fuel (oxidizer NTO and MMH) - they are not cryogenically stored - they are liquid in normal temperature. Additionally it wasn't a mechanical impact of NTO which destroyed the titanium valve, it was an explosive (exothermic) oxidizing reaction of high pressure, hence high temperature NTO reacting with titanium.
Falcon boosters use cryogenic oxygen, but that's a different story.
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Thanks for the correction about the cryogenics, I should have read the report [spacex.com] more closely. But I note that it says
A slug of this NTO was driven through a helium check valve at high speed during rapid initialization of the launch escape system, resulting in structural failure within the check valve. The failure of the titanium component in a high-pressure NTO environment was sufficient to cause ignition of the check valve and led to an explosion.
To me that sounds like the structural failure occurred before ignition, as a direct consequence of the NTO slug's high speed, and the ignition was caused by the failure rather than the reverse.
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To me that sounds like the structural failure occurred before ignition, as a direct consequence of the NTO slug's high speed, and the ignition was caused by the failure rather than the reverse.
Yes and no. Yes - the explosion has nothing to do with the engine, however the point was that with the NTO pressure and temperature as it was in this case during pressurization the titanium itself becomes a very good fuel, as in this environment the titanium valve did not break but exploded - as far as I understood the description, but maybe it's just my not clear explanation.
Re:lack of foresight? (Score:5, Insightful)
I'm sure these properties of titanium were also well known to the designers, but that they thought it would be operating in the safe zone.
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I bet that diamond plus NTO equals burning diamond...
Fix allows post-firing failure? (Score:2)
I can imagine the following scenario:
Propellant tanks are pressurized, bursting the burst disks. Very shortly after, the valves to the combustion chambers are opened, and Super Dracos fire. Later, valves close, Super Dracos shut off, leaving fuel and oxidizer in the tanks. Now the helium pressurization lines and the tanks are at equal pressure. Sloshing causes fuel and oxidizer to travel up these lines (with no pressure differential to prevent it) and eventually meet, causing a small explosion which breaks
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This is almost certainly the case. The SuperDracos are used for mission abort, so it makes sense for them to be fired once as much as possible - there's not much reason to leave any fuel behind. For comparison, the launch-abort systems used in Apollo, Soyuz, (and, if it ever flies, on Orion) etc. are solid rocket motors, and those have no choice but to burn until they are fully exhausted.
Video Describing Failure (Score:3)