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SpaceX has posted an update on the investigation into the destruction of its Crew Dragon test vehicle in April. In short, it was a leak and a dodgy valve wot dunnit. The incident happened at 18:13 UTC on 20 April while SpaceX was conducting static fire engine tests atop a test stand at the company's Landing Zone 1 at Cape …
Going by this description of what happened the 3d printed components of the Super Draco actually survived - it was a lump of fuel that hit a non-return valve that then blew the poor thin to bits that caused a fire (burning the Titanium piping at that point)
The thing is, this is why they test spacecraft & they used the already flown hardware because they wanted to know how it would cope when it's reused,
It's good this happened as they know what went wrong & they have a fix - better than having a "bag of mostly water" (STTNG reference there) on board when it failed.
Long term, I don't think they will keep the burst disk idea, it reduces re-usability. but short term it makes sense to get crew dragon flying, it isn't like they are using the Super Dracos for landing, only abort.
Obviously the non return valves/plumbing need re-engineering to avoid this issue in future when they do need re-usability.
I bet the moment they realised that high pressure NTO could ignite titanium that easily* was mildly hair-raising...
It's probably a fair assumption that the Superdraco's feed systems weren't primed during Demo-1 but blimey, this thing was attached to the ISS!
* For particular definitions of "easy".
Which is why ultra-large (crews much larger than a dozen people, tops) crewed space vehicles will never happen. As the size and complexity increase a combinataorial explosion will eventually lead to a literal one (or perhaps a literal decompression or loss of attitude control or similar total-loss-of-vehicle-and-crew event. There are just too many things that can go wrong which lead to everyone dying. If you only have, say, 30 thrusters attached to the ISS and they only go fatally bang once in 1000 years the MTBF is probably acceptable. What about when the vehicle's so huge it needs 30,000 thrusters? (For thrusters, read any other component that kills everyone in certain failure modes.)
That's not an argument against crewed flight, btw, it's an argument against idea of vehicles or facilities with more than a certain threshold level of complexity, There are other arguments against having any crew at all :)
Aliens because maybe they have magic fail-oroof technology.
'Which is why ultra-large (crews much larger than a dozen people, tops) crewed space vehicles will never happen. [...] There are just too many things that can go wrong which lead to everyone dying.'
And yet people still buy tickets to fly on commercial aircraft, despite so many of them having crashed with no survivors.
My point being mostly that safety is not absolute: it's a relative thing. "Safe enough" - that's what it'll take for large space crews to be fielded. I wouldn't bet on that happening for another fifty years or so, mind.
The replacement of the check valves by burst disks, which seal completely until opened by high pressure are a once only use solution but then the need to fire the SuperDraco thrusters, that are hopefully never needed, are to save the crew capsule, in the event that there is a failure of the launch vehicle.
The use of check valves was because SpaceX was conducting multiple static fire engine tests, that the abort system does not require as the SuperDracos are designed to push the spacecraft away from a failing Falcon 9 and are only to be used in the event of a launch escape scenario.
So we now have a much safer Crew Dragon capsule as the result of this fortunate unplanned disassembly of a Crew Dragon capsule and a system design, that should in retrospect of been used in the first place!!!
Titanium is a metal that given the opportunity combines readily with many elements but I wander was it the titanium, or another substance in the check valve or plumbing, that reacted with the nitrogen tetroxide with such violence!!!
The most likely reason for not using burst disks in the first place is that SpaceX have a strong company preference for directly-testable, and thus re-usable, parts.
A check valve can be physically tested to ensure it meets the spec before fitting it to the spacecraft.
Burst disks can only be batch tested, as they're one-shot. So they can't be sure that any one disk is going to rupture in the right way, only that some percentage of a batch did work correctly.
For example, one presumes that fragments of thin stainless steel are bad for the engine, so they'll want to be quite sure that the disks are going to keep hold of all the bits. Quite difficult to do when tearing metal.
Also, was there not partially hopeful "powered landing" designs? So adding on/off valves were a priority previously. As NASA have prefered parachute landing, then these can be used as emergency escape/separation rockets only. So now the design/scope has changed.
The original plan was to use the Super Draco thrusters for a powered landing on ground rather than a parachute-braked landing on water. They would also double up as escape rockets during a launch if things went wrong.
To achieve this they had to put the Super Draco motors and the highly toxic fuel and oxidiser to power them in the capsule structure beside the crew. Existing capsule escape systems use a set of rockets on a tower mounted to the top of the capsule. It's usually jettisoned in a normal flight to reduce weight and free up the docking adapter in the nose of the capsule once the point in the launch where a powered escape isn't going to be successful has been reached.
Now SpaceX has abandoned the powered landing concept they're stuck with lifting the mass of the entire Super Draco system plus fuel and oxidiser to orbit and returning to Earth with it, basically parasitic weight if things go right. It would require a major redesign to remove them from the capsule and add a disposable escape tower system like the Boeing capsule has.
They are indeed, however in most industrial applications they only have a "must not fail below" and "must fail below" spec, as they're emergency pressure relief to stop some piece of kit from actually exploding.
Debris doesn't matter a jot, as long as it's not going to react with the material to hand.
well the titanium part was in a HELIUM system, and the nitrogen-based oxidizer ended up in there. I expect that the helium system works very well for HELIUM, not explosive liquids like NTO.
apparently a small amount of NO is often mixed with the NTO to limit stress corrosion of titanium (it's in the article).
So the real problem is HOW the *FEEL* did the NTO get into the helium system in the FIRST place?
(worth pointing out, titanium is chemically and physically a LOT like aluminum but a lot of people probably do not know this)
Anyway, that much is probably obvious. Captain Obvious definitely thinks so.
You're combining a vicious oxidizer with a material that oxidizes easily. The fact that a physical fault probably *started* the whole mess is more important than a buttload of oxidizer reacting with a highly reactive metal that is freshly shattered and therefore not protected by an oxidized surface layer.
I'm guessing (SWAG) that the oxidizer hit the valve much like water hammer and simply shattered it from physical violence akin to what we imagine doing to the screaming child on an airplane. The oxidizer does its job, and promptly (in an extreme exothermic fashion) oxidizes anything it touches.
FFS, they're using a hypergolic fuel. Some mechanical part suffered a RUD and spewed shrapnel. The end result is far, far, FAR less important than the cause. Knowing that a match will burn is less important than knowing what conditions could cause it to. Especially when human lives hinge on whether or not that match burns.
As far as burst disks, they are a solidly reliable technology. Installed in a manufacturer-approved fashion, in a manufacturer-approved fitting, they will burst at the manufacturer specified pressure (within reason) every time. If your system is so touchy that the safety margin is less than the rated margin of the safety device protecting it.... you are an idiot and should let someone smarter design it.
At the very least, we can be thankful they weren't using a florine based oxidizer. Florine fires are really something to see. From a safe distance. In proper safety gear. Only once in your life. Because watching wood, steel, sand, concrete, and even a stream of halon gas burn really makes you re-evaluate the definition of "safe."
I had to insert the obligatory xkcd for this comment.
Also if you are mad enough to dare to explore fluorine / hypergloc chemistry or just want a good laugh take the time to read PDF: IGNITION! An Informal History of Liquid Rocket Propellants
Flame icon for the toasty chemistry content ;-)
Ignition has recently been reprinted. Its an excellent tome, with an intro by Isaac Asimov.
The fuel in a super draco system is one of the less hilarious fuels. Ignition details more, and some of the safety precautions and tests devised. Apparently the proper safety gear for handling chlorine triflouride is a good set of running shoes.
> At the very least, we can be thankful they weren't using a florine based oxidizer. Florine fires are really something to see. From a safe distance. In proper safety gear. Only once in your life. Because watching wood, steel, sand, concrete, and even a stream of halon gas burn really makes you re-evaluate the definition of "safe."
I was once, very long ago and very far away, trying to assay the concentration of fluorine in an experiment where I was pretty sure that the fluorine was reducing in concentration due to its reaction with the stainless-steel containment vessel. In seeking to replicate the usual test for iodine, I decided to release the gas into a vessel containing methanol and, IIRC, sodium iodide, later to be titrated with a starch solution (OK, this was 30+ years ago, some details have evaporated over time). Let's leave aside the fact that my first attempt to de-oxygenate the methanol by boiling in an Erlenmeyer flask resulted in a fire because I'd forgotten about boiling chips and a rapidly boiling solution shot up into the air like Vesuvius erupting and landed back down on the hot-plate -- it was a Sunday and no-one else was around...
Anyway, as I cracked open the valve that led to a small teflon tube immersed in the methanol solution, I noticed what seemed to be a spark at the end of the tube. I cautiously proceeded with the same result. Shut down, turn off...
I then found a book on the halogens in the University library which noted "there have been no reports of the interaction of fluorine with methanol which haven't resulted in explosions"!
I'd take a hydrogen fire any day of the week. Heck, a hydrogen fire may be darn near invisible, it may burn at an insanely hot temperature, and it may gnaw its way through darn near any material.
But cut off its oxygen, and it dies. Cut off the fuel, and it dies.
A florine fire is a special beast. It reacts with nearly anything to release oxygen. Its reactions are insanely exothermic. And it reacts with nearly anything. That means it brings its own oxygen, heat, and fuel to the area it resides in. It's basically the devil's diarrhea, turning anything it touches into its own special little version of Hell.
I don't honestly know of anything that would extinguish a florine fire. Chilled nitrogen may snuff it out, but as soon as the nitrogen is displaced it would start right back up. It seems to be one of those 'sit back and wait for it to burn itself out' deals. Not to mention the devil's fart of insanely dangerous by-products a florine fire produces.
There's just some chemicals you just 'nope' right on past and start walking faster.
So which is worse: your O2F2 or my ClF3, seeing as how both of them are exceedingly dangerous? I would think ClF3 is worse because it's more stable at STP (O2F2 decomposes rapidly in STP).
PS. I would love to know the particulars of A. G. Streng's attempt to mix the two.
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