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SpaceX is ready to fly rockets again. An expert talks about the reason a Falcon 9 blew up last year

Posted by Jason Davis

10-01-2017 6:02 CST

Topics: commercial spaceflight, mission status

On September 1, 2016, a Falcon 9 rocket inexplicably exploded on the launch pad during preparations for an engine test at Cape Canaveral, Florida. It was the second loss of a Falcon 9 in 15 months; the first occurred during a cargo flight to the International Space Station in June 2015. 

The 2015 accident put SpaceX out of commission for six months. This time, the delay was a little shorter, at four months. SpaceX says they figured out why their rocket blew up last year, and that they know how to keep it from happening again. The company is now ready to launch a fleet of 10 communications satellites from Vandenberg Air Force Base; liftoff was supposed to have happened yesterday, before lousy weather forecasts and range conflicts pushed the mission back to Saturday, Jan. 14. 

All of the information available about last year's accident comes from a series of updates posted to SpaceX's website, and the occasional tweet from either the company or its CEO, Elon Musk.

To get a deeper understanding of what happened, I spoke with an independent expert about the problem. The technical nitty-gritty is fascinating, and provides a glimpse into why SpaceX has been so successful. The company continually pushes the technological envelope in ways other firms do not, even if that means creating an occasional jaw-dropping fireball in the process. But how will that risk-reward equation change when astronauts start boarding Falcon 9 rockets?

DSCOVR contrail


DSCOVR contrail
An exhaust contrail forms behind a SpaceX Falcon 9 rocket as it pushes the DSCOVR satellite toward space.

Last year's accident

Like the 2015 accident, last year's pad explosion was traced to high-pressure helium bottles inside the rocket's upper stage liquid oxygen tank. Liquid oxygen tanks are pressurized with an inert gas like helium. In SpaceX's case, the helium bottles are stored inside the oxygen tank. This isn't uncommon; the mighty Saturn V moon rocket, for instance, used this configuration. 

In 2015, SpaceX said a strut securing one of the helium bottles broke, rupturing the bottle and the oxygen tank. During last year's accident, the company believes a pocket of liquid, or possibly solid, oxygen trapped against one of the helium bottles ignited, blowing up the bottle and liquid oxygen tank. (Technically, liquid oxygen isn't flammable, but it makes everything else around it ultra-flammable.)

Pushing boundaries: Composite tanks and chilly oxygen

SpaceX's helium bottles aren't made of a single, solid material like aluminum. Instead, a thin layer of aluminum—which helium can't pass through—is used as an inner lining. To keep the thin aluminum from bursting under pressure, it is wrapped with tiny strands of carbon fiber. This design is called a Composite Overwrapped Pressure Vessel, or COPV.

COPVs are much lighter than their solid metal counterparts. That makes them attractive for space applications, where every bit of mass matters. On rockets, COPVs commonly hold gases used to open and close propellant valves, among other things. A space shuttle orbiter, according to one NASA document, had up to 24 COPVs staggered throughout the vehicle

It took quite a bit of searching to find a third-party COPV expert, and when I did find her, she agreed to speak only on the condition of anonymity. I'll call her Lindy—she currently works with pressurized systems for a major launch vehicle manufacturer, and was formerly employed by an aerospace COPV supplier.

Lindy is a big fan of COPVs. But she said most rocket manufacturers stick with all-metal designs when they dunk their helium tanks in liquid oxygen. I asked her if she was aware of any company besides SpaceX that uses COPVs in this configuration.

"Not that I'm aware of," she said.

Oxygen liquefies between -183 and -219 degrees Celsius. These cold temperatures cause the tank and everything inside it to shrink. This can be especially troublesome near what Lindy called "interfaces"—places where the helium tank is physically secured to the oxygen tank. In order to make sure everything shrinks at about the same rate, most rocket manufacturers use the same material for both the helium and liquid oxygen tanks.

The environment inside a rocket's liquid oxygen is pretty harsh. In a Falcon 9 tank, it's even harsher—ever since the company began using superchilled liquid oxygen. The colder your liquid oxygen, the denser it gets, meaning you can squeeze more of it into your propellant tanks and increase the performance of your engines. But supercool liquid oxygen can be finicky—for starters, it has to be loaded a lot closer to liftoff time so it won't have a chance to warm up. George Sowers, the vice president for advanced technologies at United Launch Alliance, said his company doesn't bother: "not worth the trouble, small gain for lots of headaches," he tweeted last year. 



SpaceX's Crew Dragon rockets away from the launch pad during a pad abort test on May 6, 2015.

The cause and the fix

After last year's pad explosion, SpaceX recovered some helium COPVs that weren't part of the accident. The tanks' inner aluminum liner was buckled in places, leaving gaps between the liner and carbon fiber overwrap. SpaceX did not speculate on why the liner buckled.

"Thermal issues seem to be a part of it," Lindy said, noting she could only draw on publicly available information. "The aluminum may have shrunk more than expected relative to the carbon overwrap. Its CTE (coefficient of thermal expansion) is much lower than aluminum. I mean, it's not even in the same ballpark."

According to SpaceX, supercool liquid oxygen may have seeped into the gaps that formed between the two liners. Helium doesn't liquify until -269 degrees Celsius—just four degrees warmer than absolute zero. That means the gaseous helium inside the COPV could have chilled the liquid oxygen trapped in the liner gaps even further—enough to solidify it, which occurs at -219 degrees Celsius.

At that point, SpaceX believes either friction or the snapping of a carbon fiber strand near the gap started the explosion that destroyed the rocket. Lindy told me she thought the explanation was credible—it doesn't take much to start a catastrophic conflagration in a liquid oxygen environment. And SpaceX said tests showed the colder the oxygen, the more likely this scenario was to occur.

"I think the general concept is feasible," she said. "The explanation, for me, is a bit simplistic, but an engineer is always going to say that."

SpaceX says they'll be using warmer helium from now on, in a "prior flight proven configuration based on operations used in over 700 successful COPV loads." (The company did not say when—or why—the configuration was changed out of the prior flight proven configuration.)

In the long run, SpaceX plans to fix the buckling issue altogether, though Lindy said this could be challenging.

"COPVs are a bit on the delicate flower side, is how I put it," she said. "Their care and feeding is not simple."

Risk versus reward

SpaceX is a private company backed by a billionaire and other investors. It succeeds precisely because it pushes the boundaries of rocket science. In the process, it has placed the entire launch industry on notice: Innovate and lower costs, or risk getting left behind.

When SpaceX sacrifices a rocket in the process, it has to answer to a much smaller group of stakeholders than organizations like NASA (which answers to American taxpayers) or United Launch Alliance (which answers to its parent companies, publicly traded Boeing and Lockheed Martin, both of which have wider aerospace interests).

By the time SpaceX starts shipping astronauts to the International Space Station, the company says it will have finished making major design changes to the Falcon 9. That's good news for astronauts that will entrust their lives to the launch vehicle. And even if an accident does occur, it would hardly be unprecedented; NASA lost 14 people in two space shuttle accidents because of bad bureaucratic choices.

In NASA's case, reminders of the two accidents are prominently posted at the agency's human spaceflight facilities. Placards at the Michoud Assembly Facility and Kennedy Space Center Vehicle Assembly Building remind workers that lives are literally on the line when rockets are being assembled and stacked.

Right now, only profit is at risk when SpaceX bolts a vehicle together. But that will soon change. When it does, will SpaceX's risk-reward equation look any different?

See other posts from January 2017


Or read more blog entries about: commercial spaceflight, mission status


Karen: 01/10/2017 08:39 CST

I don't trust LOX + composites rockets further than I can throw the guy who thought that would be a good idea. They just don't play nicely. Random example: Maybe some day some of the various research projects trying to figure out how to get them to do so will find a way. But a commercial delivery vehicle is not a research project. Keep your LOX and your organics apart. It's just a general rule: organics pop/flash/char in LOX upon impact/shock/heat/etc and develop microcracks, which make them even more vulnerable to the former with time. The more the pressure, the less the ignition energy needed, up to the point of being essentially hypergolic. Also as a general rule for LOX there comes a pressure where any said pop/flash/char is self sustaining, aka, detonation. For some it's less than atmospheric pressure. For epoxy, it's more. But not infinite. According to the LOX handling guidelines I've read, the only carbon-based compounds considered to be truly (or at least largely) LOX-compatible are fluoropolymers.

Karen: 01/10/2017 08:43 CST

All of the guidelines I've read for design of LOX storage tanks on the ground say the same thing. Aluminum or steel tanks. No organics in the system, anywhere. Generally no silicone either. If you have to use a polymer, use a fluoropolymer. And now this company that's had two COPV failures, one directly related to a LOX/composite reaction, plans to make the largest rocket in history - by far - entirely out of composites. Don't get me wrong, I *really* want to be proven wrong. I *really* want them to succeed. But....

VirgilSamms: 01/10/2017 03:03 CST

@Karen Marcia at Spacepolicyonline has a different take: "-SpaceX determined that although a single definitive cause could not be identified, the most likely cause was-" Cause could not be identified. Fly it anyway. And NASA is going to risk their people on this thing? It is like the space shuttle all over again except.....this time they should know better.

VirgilSamms: 01/10/2017 03:12 CST

I would add (since Jason is showing pictures of it) the escape system on the toxic dragon is a very poor design probably intended to keep space stations positioned and not very useful for escape or landing. Landing with it is a really bad idea. Because of so many questionable design features I refer to the falcon as "the hobby rocket." Only a bored billionaire with political connections could get away with such a project.

Scott B: 01/10/2017 04:55 CST

I thought this particular rocket was the first one to be refueled after a prior launch. Is that right? I'm not seeing any analysis in that regard.

VirgilSamms: 01/10/2017 05:24 CST

There are quite a collection of falcon first stages that have been landed back either to a barge or the cape- but none of them have flown again. I doubt they will. It worked to keep the public interested but if those stages are now un-reusable junk (they probably are) it has to eventually backfire as a P.R. device.

Tomasso: 01/10/2017 06:56 CST

VirgilSamms, why do you say the Dragon escape system is "not very useful for escape"? It looks very useful from the video of the pad abort test.

VirgilSamms: 01/10/2017 07:48 CST

"-why do you say the Dragon escape system is "not very useful for escape"?" The standard tractor escape tower used on most human-rated systems is very hard to improve upon for several reasons you can find out for yourself. Sitting in a capsule with a ton and a half of nitrogen tetroxide and monomethylhydrazine wrapped around you is not my idea of "useful." Anything goes wrong and those toxic hypergolic propellants will make it very hard to clean up anything that is left. Compared to a solid fuel escape tower it is a crummy design.

im.thatoneguy: 01/10/2017 08:43 CST

@Karen "And now this company plans to make the largest rocket in history - by far - entirely out of composites." It's not going to be 100% composites. They've already stated the LOX tank will be lined. They could line the LOX tank interior with Aluminum and the composites would never come in contact with LOX anywhere on the rocket while still benefiting from the weight savings. Also they've really only had one COPV failure. The other wasn't a COPV failure so much as a plumbing failure coming out of the COPV.

im.thatoneguy: 01/10/2017 08:47 CST

@VirgilSamms That's a lot of tough talk about the dangers of hypergolic rockets considering the last crew killed by an engine failure was due to a solid rocket booster failing.

harukichou: 01/10/2017 08:48 CST

"In NASA's case, reminders of the two accidents are prominently posted at the agency's human spaceflight facilities." -- And yet, the 1st reminder didn't prevent the 2nd accident.

VirgilSamms: 01/10/2017 09:22 CST

"-they've really only had one COPV failure." They are not sure what failed. "@VirgilSamms That's a lot of tough talk-" Oh, it's nasty stuff to pack into a capsule. What you are saying is a straw man. They fixed the SRB's. "-the 1st reminder didn't prevent the 2nd accident." And that has absolutely nothing to do with SpaceX, does it?

VirgilSamms: 01/10/2017 09:42 CST

The shuttle was a pay-for-itself-do-everything-cargo-bay-of-dreams that wasted most of the lift of a Saturn V class launch system sending a 75 ton glider into LEO so it come right back down. The falcon is a mediocre launcher not much different than the Saturn I of 1962. It essentially is the same scam- promising much and delivering little. There is no cheap.

Stargazed: 01/11/2017 03:00 CST

VirgilSamms, you seem to be very critical of all things SpaceX. May I ask whom you work for?

ProfBarba: 01/11/2017 07:20 CST

@VirgilSamms I'm taking a print of your brilliant (irony to avoid cursing) comment, for the day SpaceX re-flies a booster. You have no idea about how much you're been laughed about right now on Reddit lol

VirgilSamms: 01/11/2017 07:41 CST

"May I ask who you work for?" "You have no idea about how much you're been laughed about right now-" The NewSpace mob does this on any forum where the flagship company or it's products are criticized. They mock and humiliate any who dare blaspheme their cult. NewSpace is the classic bait and switch con. The first part is the appeal to basic human greed by offering something for next to nothing- in this case by way of the miracle of "entrepreneurship." The second part is to substitute a hopelessly inferior product/service while advertising it as the only possible solution. In this case the "reusable" hobby rocket. The worst thing that has ever happened to space exploration,

RocketMan: 01/11/2017 08:43 CST

@VirgilSamms Why don't you make like LOX...and chill.

VirgilSamms: 01/11/2017 09:12 CST

@RocketMan Why don't you make like...a comment about space and not about me? I am expressing my views- no different than the thousands of comments that have been made damning the Space Launch System. If you want to comment go ahead but telling me to not comment? I have zero respect for any of the cyberthug hero-worshipers who have patrolled the internet for years silencing SpaceX critics. Like I said, the worst thing that has ever happened to space exploration. Worse than both shuttle disasters. The flexible path has crippled and set back Human Space Flight a decade at least.

paul_wi11iams: 01/11/2017 05:35 CST

Can anyone (Jason/Lindy...?) please confirm or refute that before LOX meets a buckle fault in the aluminum liner, it must first percolate through the thickness of the carbon fiber overwrap which would then be saturated in LOX. Is the fact of LOX percolating through the carbon fiber planned or not planned ? A short reply would be welcome, but a good place to develop the subject is a forum discussion about this here: The subject of interest here is more about physics of course, rather than new/old space issues ;).

Jason Davis: 01/11/2017 05:43 CST

Folks, a reminder again: Please don't post information about users' identities. Paul, this is a good question (and on topic, thanks!). I'll see if I can find an answer.

VirgilSamms: 01/11/2017 06:30 CST

Well, let me try and drift back on topic. "-a glimpse into why SpaceX has been so successful." Not that successful really, considering recent explosions and being years behind schedule. SpaceX has delivered much hype and some satellites into orbit, mostly on the taxpayers dime, but little else. The best "glimpse" would be a look at the check Musk wrote for the Obama campaign. Considering the billions in tax dollars, subsidies, and free NASA support this bought, it was one of the slickest political influence sales in recent history.

RocketMan: 01/11/2017 06:44 CST

"a glimpse into why SpaceX has been so successful." Agreed! SpaceX has a truly impressive launch schedule. They don't drag their feet unlike some, er, other rocket companies.

VirgilSamms: 01/11/2017 07:27 CST

"-a truly impressive launch schedule. They don't drag their feet-" I think you have your companies mixed up: ULA launched 12 in 2016 for 115 successful launches in a row. That is impressive. Blowing up is not.

ClioMarsden : 01/11/2017 07:45 CST

Never mind that SpaceX is ahead of Arianespace on flight reliability 30 missions into Ariane 5 program. Never mind that one of the parents of ULA had 3 failures in a row on Delta III, which went a long way in ironing out issue of upper stage in Delta IV (Just a stretch of the same upper stage)... Never mind even with tons of cash sunk into Titan IV they still managed to blow up 800+ million dollar payloads. And before, the one whom shall not be named, speaks an electrical short in the guidance system was preventable even back in the late 1990s. Never mind SES has signed up to fly on a used booster @E.E. Smith character of the week says he doubts it will it doubted they would be able to land a stage in the first place. Never mind @E.E. Smith character of the week said SpaceX would be out of business after CRS-7 failure, now he is saying they will never fly a USAF/DoD payload, except they are still on the manifest. You always need to keep track where the goal line is when dealing with these types. Starting to get the point here? Someone starts out at position x finds a bunch of junk to throw up that seems to claim position x, rather than an honest assessment of the data.

VirgilSamms: 01/11/2017 07:57 CST

Everybody should understand that you have been cyberstalking me for over a year now. Jason should have banned you for your previous post but it is his blog. If you could post just about Space instead of personally attacking me then you might be somewhat believable. You mixed in some ancient history but your purpose is plain, your mission is to try and silence me. You are one of the legion of cyberthug Musk groupies that make sure any critics are bullied into silence. So transparent.

Opcode: 01/12/2017 12:48 CST

I suspected when the accident happened that it might have resulted from delamination of the composite material, as has happened several times to other manufacturers. I also suspect that the whistling sound heard shortly before the explosion is a pressure relief valve opening or closing. I believe that several lines of evidence suggest that the explosion itself took place external to the rocket, very close to a pipe flange. So, my proposed scenario is that a helium tank leaked pressurized helium into the LOX tank, tripping the pressure relief valve. The oxygen fill line backed up until the flange failed, which lead to ignition.

paul_wi11iams: 01/12/2017 04:30 CST

@Opcode CF. 01/12/2017 12:48 CST That looks like a slow scenario that would leave a long and clear history in the telemetry so would be known about despite ITAR and whatever. However, if you floated that theory and/or others on a forum anywhere, can you post a link ? This would provide a more efficient "pressure release valve" for this blog comments section.

paul_wi11iams: 01/12/2017 04:54 CST

@ Jason Davis, Planetary CF 01/11/2017 05:43 CST Thankyou for this (and thanks belatedly for a great blog-page) If you can find an answer, it will be appreciated by several, on both sides of the Atlantic :)

Thorbo: 01/12/2017 07:22 CST

SpaceX says it was a strut that caused the rocket in 2015 to go boom, but NASA wasn't convinced and I'm not either. These carbon wrapped tanks seem to be more trouble than their worth. In my non expert opinion, it seems like they should at the very least wrap the whole works in another layer of aluminum so the cf isn't directly exposed. Also: @VirgilSamms makes valid points - for the most part. Calling people "the legion of cyberthug Musk groupies" is tacky and just makes you look like a child. That said I look forward to 2017. Even if Musk's plans for cheap reusable space flight fissile out, he at least tried. There is nothing to lose and everything to gain from rooting for him.

Arbitrary: 01/12/2017 09:33 CST

SpaxeX has been moving forward surprisingly fast and successfully. Taking advantage of the best technologies and management practices. They say that this year (although their calendar has often had a lazy Moon) the Falcon 9 will reach its final top performance. Because it is planned to be mature enough to be certified for human space flight. The worst that could happen is that they give up some aspect of their modernization and settle for a slightly less capable launcher version. It is not all or nothing any more. They've pushed it to the limit and now focus on their Mars project. (And I hope the Falcon Heavy, finally gets enough priority).

ClioMarsden: 01/12/2017 11:12 CST

@Thorbo SpaceX and NASA agreed the strut/strut attachment failed. They did not agree that it was proven that a fault in the strut mfg was the reason. NASA cited also installation and material selection as possible reasons for strut to fail at the attachment point. Hence the conflict.

VirgilSamms: 01/12/2017 05:59 CST

"Calling people "the legion of cyberthug Musk groupies" is tacky and just makes you look like a child." You are one of them obviously. Thanks for the insult and for self-identifying. "SpaxeX has been moving forward surprisingly fast and successfully." This NewSpace Orwellian newspeak has been repeated so many thousands of times that it is now taken for granted. Anyone who throws the B.S. flag is immediately shouted down by the legion of cyberthug Musk groupies. The only surprising thing is how gullible and easy to fool the media and the public is. When was that Falcon heavy supposed to fly originally? "SpaceX first announced the Falcon Heavy in April 2011, and at that time said the vehicle’s first launch would take place from Vandenberg Air Force Base in California in 2013."

ClioMarsden: 01/13/2017 09:32 CST

@VirgilSamms Please keep on digging the hole you're in.

Seb: 01/13/2017 10:47 CST

Those are some pretty harsh comments. Realistically they did take risks in their rocket design, which have been avoided by other rocket manufactures. Example, putting the pressurization tanks, their cabling, etc, into the LOX tank. Since Apollo 11 we know that doing this is a very bad idea. Looking at the hyperloop and poor design of their mars rocket, It is pretty obvious that this company is looking for big PR dollars. I'm sure that most of their employees are good engineers, but the company leadership is very poor.

ClioMarsden: 01/13/2017 11:29 CST

@Seb Lots of rockets submerge pressure bottles in LOX tank including Soyuz and Saturn S-IC. Submerging COPVs is novel, but that is how you move forward. It's called progress.

Jason Davis: 01/13/2017 11:50 CST

Paul, According to the expert from my story, the short answer is yes. Any voids between the inner liner and overwrap will tend to get filled by LOX. Beyond that, the answer gets a little complex. The carbon fibers are coated with epoxy. There are two methods: wet winding, in which the fiber is coated just before wrapping, and pre-preg, where the fiber is coated ahead of time. We don't know which method SpaceX uses. Pre-preg leaves more potential for voids, whereas during wet winding, the fibers can shift and fill in gaps. Wet winding, however, can create small bubbles. All in all, the overwrap is never completely filled with epoxy and fiber, so there will always be the potential for percolation through cracks, voids and bubbles in the overwrap. Apparently this is most prevalent near flanges (such as, in my layperson's terms -- where the helium line leaves the bottle) where there is a transition between the overwrap back to all metal.

Dennis: 01/16/2017 09:05 CST

Why don't they use vaporised oxygen gas to pressurise the LOX? As done in normal LOX tanks on the ground. Why use helium?

rocketengineer: 01/16/2017 12:03 CST

Dennis - one reason for helium is for weight savings. I estimated that the Falcon 9 LOX first stage would need about 8-10 lbs of helium vs about 700-800 lbs of oxygen. My numbers are rough since I simplified a lot :)

ClioMarsden: 01/17/2017 10:12 CST

@Dennis The He COPVs serve as pressurant for both the LOX and the RP-1 tanks (which can't be pressurized any other way). At that point you mine as well do both via He bottles vs running plumbing down the stage to heat exchanger on the engine for LOX and still need bottles for the RP-1 tank.

Paul_Wi11iams: 01/18/2017 07:02 CST

Thankyou Jason for your 01/13/2017 11:50 reply "Any voids between the inner liner and overwrap will tend to get filled by LOX...". It would be good to be able to fill those voids by concretion according to the possible principle of the martian spherules discovered by Oppy ! Reading around, COPV is one of hundreds of specialized subjects each of which occupies one small place on the launch vehicle. The one that gets publicity is of course the one that went wrong. We're *not* talking about all the other components that worked okay but in silence. Maybe in a year from now we'll have forgotten COPV as we've forgotten subcontracted tank support structures and everything esse that failed at some point. Lindy's contributions remind us of all those industry-wide technologies that makes planes and cars work. Here's a glimpse of all the things that must work correctly on *any one part* a launcher or any other system, just looking at the syllabus for a course on COPV: PS Although linking to here, I did copy your commented information onto two other forums because its a bit submerged in this blog section. Hoping that's okay. Could it be worth your appending that info to the main text above ? Thanking you and the rest of the Planetary blogging team. I *must* join to help cover costs, and will do this time !

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