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  #251  
Old 09-01-2016, 10:15 AM
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Shit.
  #252  
Old 09-01-2016, 10:44 AM
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Being a fan of SpaceX is sure exciting, though not always in a positive direction.

Fortunately, no one was hurt in the accident. I hope this turns out to be a problem with the pad and not the rocket, but that's just blind hope. Not enough details yet (and already a huge amount of misinformation) to even remotely narrow down the cause. It happened 3 minutes before the scheduled static fire, but that's all we know. Could be propellant loading or a million other things.
  #253  
Old 09-01-2016, 12:53 PM
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Originally Posted by Dr. Strangelove View Post
Being a fan of SpaceX is sure exciting, though not always in a positive direction.

Fortunately, no one was hurt in the accident. I hope this turns out to be a problem with the pad and not the rocket, but that's just blind hope. Not enough details yet (and already a huge amount of misinformation) to even remotely narrow down the cause. It happened 3 minutes before the scheduled static fire, but that's all we know. Could be propellant loading or a million other things.
I assume by "a problem with the pad and not the rocket", you mean "the cause of the incident was something about the pad or other support facilities, and not the vehicle itself". Because there was most definitely a "problem" with the vehicle: it blew up. Took its payload with it.

Reports are confused, unsurprisingly. A few things I've gleaned from sources:
  • This was not a reused 1st stage. This was a new 1st stage that was going to be the first to be reused in its next mission.
  • SpaceX has minimal commentary, other than the event was an "anomaly". (Duh.)
  • Apparently, a full-up test firing? They do that? With payload and everything?
  • The payload was a Facebook satellite. So, nothing of value was lost?
  #254  
Old 09-01-2016, 01:38 PM
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The early reports are that the explosion occurred during or just following propellant loading, which is a semi-autonomous operation (no personnel on pad during the loading operation). Those familiar with the vehicle will note that supercooled liquid oxygen (SLOX) is used to increase propellant density to achieve better total impulse performance for high inclination/orbit or heavy payload missions, and so SLOX loading has to be done just prior to test or launch in order to maintain temperature. It is unknown if the failure is related to the SLOX loading system but it has been a challenge for SpaceX and the additional complexity and hazard with supercooled cryogenic fluids is why the rest of the rocket propulsion industry has generally avoided propellant densification.

SpaceX does perform a pre-launch "static fire" to verify the Stage 1 propulsion system workmanship and function at a system level. While they do perform hotfire operations on the individual engines and on the Octaweb assembly, the only time to perform a full up functional test including the tankage and propellant feed system is once the Octaweb is assembled to the stage. This is typically done for a few seconds to allow the engines to get up to full liftoff thrust. This is actually an old practice that harkens back to the early days of rocket launch vehicles. Most launch vehicle operators today do not perform this because of cost, risk, and feasibility, and SpaceX has discussed eliminating this test as well.

SpaceX is correct in not commenting on the cause of the explosion because they need to perform a root cause investigation and determine cause and corrective action, especially since this clearly represents a public hazard and impacts the use of the Falcon launch vehicle for planned crewed operation. Releasing information before completing an investigation or making unsubstantiated comments encourages unqualified speculation which may impact both the budget and the industry confidence in SpaceX.

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  #255  
Old 09-01-2016, 02:57 PM
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Space.com has some incredible footage of the explosion here.

Quote:
The rocket's upper stage exploded at 9:07 a.m. EDT (1307 GMT), setting off a dramatic, fiery cascade...

The initial fireball quickly fills the entire left half of the video's field of view; a few seconds later, Amos-6 falls to the ground, setting off another series of explosions. More detonations follow over the course of the video's 5.5 minutes; indeed, about 20 separate explosions can be heard.
  #256  
Old 09-01-2016, 03:24 PM
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It's really sad to see the payload fall down from the rocket and break up into bits.

I'm not sure about it being a 2nd stage explosion. It looks like the explosion originated near the umbilical. Though I'm not sure if a hydrogen leak would create such a bright initial explosion?

Last edited by scr4; 09-01-2016 at 03:25 PM.
  #257  
Old 09-01-2016, 03:52 PM
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It's really sad to see the payload fall down from the rocket and break up into bits.

I'm not sure about it being a 2nd stage explosion. It looks like the explosion originated near the umbilical. Though I'm not sure if a hydrogen leak would create such a bright initial explosion?
It isn't hydrogen. The upper stage is a single Merlin 1D Vacuum+ engine, an RP-1/LOX engine.

Guessing from the video, the explosion started in or just outside of the Stage 2 tankage area. See Figure 2-1 on Page 10 of the SpaceX Falcon 9 Launch Vehicle Payload User's Guide (warning: PDF). Hard to say what happened, but that could have been oxygenated kerosene detonating (in open air).
  #258  
Old 09-01-2016, 04:37 PM
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Tool late to ETA, but Elon Musk's twitter has the following tweet
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Originally Posted by Musk, via Twitter
Loss of Falcon vehicle today during propellant fill operation. Originated around upper stage oxygen tank. Cause still unknown. More soon.
Which is where the first Falcon 9 failure (Flight 19) occurred, although the circumstances were different.

That's just tough. I wonder how much flak SpaceX will take for their reasoned risk acceptance of doing static test firing full-up? It was a gamble, after all.
  #259  
Old 09-01-2016, 05:04 PM
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That video was pretty nuts. I can't believe how fast the initial explosion was. You can step the 60 fps video frame-by-frame, and it goes from (apparently) nominal to a 25 m long fireball over a single frame. Unless I'm misjudging things by a lot, that's >700 m/s, which is >mach 2. So it seems like a detonation as compared to deflagration (the later fireballs are clearly deflagration).

Not sure what could cause this. If there was a leak and kerosene and LOX pooled together or otherwise mixed, could they detonate?

It's hard to be sure, but the source certainly seems to be outside the vehicle; maybe at the fueling connection point.
  #260  
Old 09-01-2016, 06:32 PM
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I'm not sure about it being a 2nd stage explosion. It looks like the explosion originated near the umbilical. Though I'm not sure if a hydrogen leak would create such a bright initial explosion?
Neither the vehicle nor the payload use liquid hydrogen as a fuel. However, while a hydrogen flame is not as bright as hydrocarbons combustion, it would still make a very bright flash, albeit not with the red and black smoke seen in this incident.

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That's just tough. I wonder how much flak SpaceX will take for their reasoned risk acceptance of doing static test firing full-up? It was a gamble, after all.
SpaceX was not performing a static fire test at the time of the incident. If they were, the plume from the first stage would be very apparent. This is more likely a result of their vehicle fueling system and the need for densified propellants for the F9v1.1 Upgrade vehicle.

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That video was pretty nuts. I can't believe how fast the initial explosion was. You can step the 60 fps video frame-by-frame, and it goes from (apparently) nominal to a 25 m long fireball over a single frame. Unless I'm misjudging things by a lot, that's >700 m/s, which is >mach 2. So it seems like a detonation as compared to deflagration (the later fireballs are clearly deflagration).

Not sure what could cause this. If there was a leak and kerosene and LOX pooled together or otherwise mixed, could they detonate?
One of the problems with LOX, and especially SLOX, is that by virtue of being so cold they actually have a lot of latent thermal energy; that is, by being at such a temperature differential to the ambient environment they can undergo radical state change which can result in instabilities. In the case of LOX, if is heated it will rapidly evaporate and disperse when heated, creating an oxidizer-rich environment in which anything combustable will burn with the slightest impetus. If you've ever 'played' with liquid oxygen you'll understand what I mean.

Although the explosion was visually impressive, the first event does not appear to be a detonation (e.g. there is no characteristic detonation shock wave, and the event does not appear to have immediately destroyed the downstage or ejected the payload fairing upward). It looks more like a thermobaric event (you can see LOX vapor being pushed outward by the combustion wave), followed by rupture of the upper stage RP-1 tank which created the downward red streamers, and then failure of the first stage LOX and RP-1 tanks which splash and then mix (the big flash). A few seconds after the initial explosion (starting at 1:20) you can see the payload fairing pitch over and fall, which does appear to have partially detonated the hypergolic fuel in the payload.

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  #261  
Old 09-01-2016, 08:40 PM
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I've heard chatter that aside from the investigation into the RUD SpaceX is grounded until they can repair/rebuild LC 40. However SpaceX also has LC 39A. Work on the pad was completed earlier this year and it's ready for Falcon 9 and Falcon Heavy launches. Is there a reason why they would not be able to use 39A once they are ready to resume launch operations?
  #262  
Old 09-02-2016, 07:21 AM
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So, Stranger - you got anything for us other than a hunch?

Thanks as always for your informative and well-supported posts.
  #263  
Old 09-02-2016, 09:25 AM
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SpaceX was not performing a static fire test at the time of the incident. If they were, the plume from the first stage would be very apparent. This is more likely a result of their vehicle fueling system and the need for densified propellants for the F9v1.1 Upgrade vehicle.
A lot of early reports conflated "fueling in preparation for static fire test" with "performing a static fire test", so that's where that impression came from. In practice, they would have had to fuel the vehicle full up even if they didn't do any engine test. So there really won't be any rational criticism of the test firing regime -- launch prep never got that far, and the anomaly occurred during a necessary and inevitable process.

As long as the press gets it right that the static fire test wasn't a factor at all. It never got a chance to become a factor.
  #264  
Old 09-02-2016, 09:53 AM
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Originally Posted by Stranger On A Train View Post
Neither the vehicle nor the payload use liquid hydrogen as a fuel.
Thanks for the correction.


Quote:
A few seconds after the initial explosion (starting at 1:20) you can see the payload fairing pitch over and fall, which does appear to have partially detonated the hypergolic fuel in the payload.
At least all the nasty hydrazine got burnt up good, right??
  #265  
Old 09-02-2016, 10:09 AM
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Space.com has some incredible footage of the explosion here.
I wonder what caused the large explosion around 4:00, I wouldn't expect there would be much left of the rocket by then.
  #266  
Old 09-02-2016, 11:21 AM
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At least all the nasty hydrazine got burnt up good, right??
I certainly hope so. Hydrazine and its associated compounds, and nitrogen tetraoxide are very toxic. Even if they have fully combusted, the site it going to require substantial remediation as it is adjacent to Merritt Island National Wildlife Refuge.

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I wonder what caused the large explosion around 4:00, I wouldn't expect there would be much left of the rocket by then.
My guess would be some kind of ground support equipment fuel reservoir (generator or chiller for the LOX).

I'm not going to speculate as to the cause (and I don't have anything more than the same video to go from) but this is certainly a setback for SpaceX and the commercial space industry overall. However, I'll also say that when you are handling this volume of combustable fluids (and especially cryogenic oxidizers, which pose particular hazards) and large structures, this kind of failure is always a risk that requires constant vigilance. Every single operator of large liquid propellant launch vehicles has experienced these kinds of failures.

The key to recovering from them and regaining confidence is a thorough root cause investigation and corrective action to reduce or eliminate design flaw or process errors which caused it. In the case of the Space Transportation System ('Shuttle') the process of making design and process improvements on such a flawed vehicle eventually ground the program to a near halt. SpaceX has shown more flexibility in terms of making rapid design changes, and more importantly, they instrument the hell out of everything, pasting on GoPro cameras like they are BeDazzlingTM the vehicle and support equipment.

So, hopefully SpaceX is able to quickly get to root cause and then move on to correct the issue. My concern is that this may impact their intention to use densified propellants, which is key to their "F9 Upgrade" configuration and achieving higher payload masses and orbital energies, which itself was key to getting to a critical portion of the large satellite market that F9v1.1 wasn't able to service. I suspect that SpaceX is cutting their cost margins a lot closer than Musk and Shotwell would like to admit (and I'm still skeptical about the extent of cost savings on reuse of Stage 1), and while SpaceX has accomplished a number of impressive technical milestones at the end of the day they still need to turn a profit and demonstrate high reliability to their high payload value customers, especially for the EELV contract.

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  #267  
Old 09-05-2016, 09:08 PM
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Although the explosion was visually impressive, the first event does not appear to be a detonation (e.g. there is no characteristic detonation shock wave, and the event does not appear to have immediately destroyed the downstage or ejected the payload fairing upward).
To be clear, I wasn't referring to anything past the first frame--that is, the first ~17 ms. Shortly after that, the tank has clearly ruptured and you get the "standard" deflagration as the LOX and RP1 in the upper stage mix. You can see the initial flare in the second image here.

However, upon reflection I do think you're right. I had thought there was fire the entire length of the illuminated area, but there's no reason to believe this--instead, I think a large portion of the "fireball" is really just clouds of water vapor illuminated by the, uh, energetic event. The actual amount of fire involved at that point might not be much.

Still no real news. Probably for the best that they aren't relaying every stage of their internal investigation, even if it is frustrating to interested bystanders.
  #268  
Old 09-05-2016, 10:04 PM
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One interesting item of note is that there is a large panel (on the order of 1 m2 area) of some kind that is blown free of the initial explosion and only later starts to combust (presumably being saturated with liquid oxygen but ejected in advance of the initiation of combustion). This would seem to indicate some kind of rupture of a pressurized vessel preceding he conflagration. I have some private speculation as to what could have contributed to this but we'll see what the investigation concludes.

One immediate impact from this has been a 41% decline in market valuation Spacecom, the operator of the AMOS-6 satellite. This will most likely result in a cancellation of the planned acquisition of the company by Xiunwei Technology Group, and a series of lawsuits. It is a good week to be a lawyer, and not anyone else involved in the launch.

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  #269  
Old 09-05-2016, 10:20 PM
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Its a lousy few weeks to be Elon Musk thats for sure.

I just cannot make up my mind about the guy. I love his visions and crazy ideas. But, some nagging part of me feels that he is a future perp walk candidate.
  #270  
Old 09-09-2016, 03:58 AM
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It's not much, but Musk has tweeted a few more scraps:
Still working on the Falcon fireball investigation. Turning out to be the most difficult and complex failure we have ever had in 14 years.

Important to note that this happened during a routine filling operation. Engines were not on and there was no apparent heat source.

Particularly trying to understand the quieter bang sound a few seconds before the fireball goes off. May come from rocket or something else.

There is in fact a (relatively) quiet pop sound a few seconds before the fireball, but doesn't seem to be associated with any visible change.

As far as I know, SpaceX considers static fires to be essentially a launch rehearsal, and so would have all of their telemetry and other stuff going as they would with a real launch. So they should have plenty of data.
  #271  
Old 09-26-2016, 04:34 PM
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A bit of an update, and some good stuff coming up.

SpaceX released an update on their anomaly investigation, which narrows the problem to their cryogenic helium system. Although they weren't more specific, it's almost certainly a breach in the COPV, or "carbon overwrapped pressure vessel", which is the tank that stores the pressurized helium, and is located inside the liquid oxygen tanks. COPVs are quite strong but store an immense amount of energy and a failure could easily cause the explosion.

In other news, SpaceX has released some sweet pictures of their new Raptor engine in action. Not clear yet how much of the engine is complete yet--it seems likely that it's pressure-fed at this point, with the turbopumps to come later, but in any case a test firing is good to see. They claim 300 bar chamber pressure--that's huge! 50% more than the Shuttle main engines, and 20% more than anything the Russians have in production.

And in other other news, SpaceX will livecast their Mars Architecture talk from the International Astronautical Congress at 1:30 CST tomorrow. Should be interesting! Hopefully will get to learn a lot more about Raptor, MCT, and the BFR.
  #272  
Old 09-27-2016, 12:28 AM
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This can't be right!

SpaceX couldn't have developed a methane engine!
According to ULA (Remember all those old rockets? ULA still makes 'em!), Blue Origin was to create the first methane engine - it even has a name! It's called the "BE-4" and is the engine which will allow ULA to stop using those dirty Russian engines!

It says so right here: https://www.blueorigin.com/be4
SpaceX's firing is just a bad dream - the Country will soon awaken!
  #273  
Old 09-27-2016, 08:12 AM
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My pre-coffee sarcasm detectors aren't fully functioning yet. So I'm going to take usedtobe's post at face value and blather a bit about the BE-4 and the Raptor.

First of all, that Raptor test firing was not a full-scale production engine. It's a smaller scale prototype, likely developed with Air Force funding to develop a higher-performance upper stage engine. Conceivably, if SpaceX designed a new upper stage that used it, they could get a significant performance and capability boost for payloads going to high orbit. In any case, it'll take a lot more work to develop the full-scale engine.

Second, while there haven't been as much publicity, it seems that BE-4 development is nearly complete. Last I heard, ULA was waiting for a final full-scale engine test in the next few months, before committing to the BE-4 for the Vulcan rocket. Blue Origin also plans to use the BE-4 to power New Glenn, a new super-heavy lift vehicle with estimated payload capacity comparable to the Block 1 SLS. They've even started building the factory that will build this rocket.

Without getting into fanboy pissing contests about BO vs SpaceX, I'm just going to say that it's amazing that there are two privately funded, ambitious, and credible new approaches for space exploration.

Last edited by lazybratsche; 09-27-2016 at 08:12 AM.
  #274  
Old 09-27-2016, 05:16 PM
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Some good stuff in the IAC preso. I'll write up some stuff tonight unless someone beats me to it.

Some really impressive pics of a tank prototype. In fact this is more impressive to me than the Raptor shots; it's hard to tell from those if the engine is 5% complete or 50% complete, but this is a seriously huge tank, and has apparently already survived their deep cryo testing.

300 tons to LEO fully reusable. Smart architecture, too. They recognize that the Mars component has a limited number of reuses just by virtue of the trip taking so long. The booster stage and refueling ships can be reused far more frequently and so the costs amortize to almost nothing.

They plan on landing the booster stage right back on the pad. Based on their proven accuracy for the barge landing, this seems... almost reasonable.
  #275  
Old 09-27-2016, 05:41 PM
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If you're writing a substantive post, it's probably worth starting a new thread. Someone else will at any rate, and it'd be kinda lame to refer them to the end of a thread where they're not interested in the first couple hundred posts...
  #276  
Old 09-27-2016, 05:46 PM
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If you're writing a substantive post, it's probably worth starting a new thread. Someone else will at any rate, and it'd be kinda lame to refer them to the end of a thread where they're not interested in the first couple hundred posts...
Yeah, this probably warrants a new thread. Ton of cool tidbits in the preso. Picked up a cold so it depends on how I'm feeling tonight, though...
  #277  
Old 09-28-2016, 12:04 PM
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SpaceX released an update on their anomaly investigation, which narrows the problem to their cryogenic helium system. Although they weren't more specific, it's almost certainly a breach in the COPV, or "carbon overwrapped pressure vessel", which is the tank that stores the pressurized helium, and is located inside the liquid oxygen tanks. COPVs are quite strong but store an immense amount of energy and a failure could easily cause the explosion.
I would caution against drawing the conclusion that the COPVs, which are just one type of component in a complex pressurized system for tank ullage. If the COPV was designed and qualified per the indusry standard (ANSI/AIAA-S-081A), the tank should fail in a non-catastrophic leak before burst (LBB) mode in the overwrapped section of the liner, and the neck portion should have sufficient margins that the overwrapped section fails first. Although an LBB could result in LOX tank overpressure and rupture if the leak were large enough to increase tank pressure beyond the capability of the relief valve, it should be gradual enough to be seen in telemetry (and in fact SpaceX has had a problem with COPVs leaking previously on the pad, although never catastrophically) and the kinetic energy imparted shouldn't be enough of a mechanical shock by itself to rupture the LOX tank. Rupture of the pressurized lines (the plumbing that connects the tanks to a regulator or servovalve), or catastrophic failure of the regulator or valve could potentially create a tank rupture with sufficient mechanical shock to initiate combustion or detonation, and may not be readily observable in telemetry. Having to operate in a deep cryogenic environment complicates analysis of the system because it not only has to withstand flight acceleration and dynamic loads but also both thermal fatigue and coefficient of thermal expansion stresses. The COPV itself, it properly designed and autofrettaged, should be robust against thermal stresses as the overwrap keeps the liner in compression at all times, but evaluating thermal stresses on the overall plumbing and mounting structure can be quite challenging.

If the root cause or most likely cause is determined to be a COPV, that is potentially a significant problem, indicating either a problem with the design margins, manufacturing methods, and inspection processes. Since the Falcon 9 uses the same COPV in dozens of locations on the Stage 1 and Stage 2 vehicles, they would potentially have to change the design on hundreds of tanks in inventory or installed on in-process stages. COPVs are process intensive to build, requiring 100% inspection of welds on the liner/neck, carefully controlled winding of the overwrap, controlled autofrettage cycling to deform the liner to ensure compressive prestress even in the worst case conditions, and the post-autofrettage inspection to assure that uniform liner deformation and no signs of incipient cracking.

Stranger

Last edited by Stranger On A Train; 09-28-2016 at 12:06 PM.
  #278  
Old 09-28-2016, 04:59 PM
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Thanks for the correction, Stranger. My impression from elsewhere is that COPVs tend to only fail catastrophically, but I see that's not true. Question: is there a significant difference here between winding techniques? Specifically, is a "braided" winding (using interleaved fibers as compared to large sections of linearly oriented ones) more resistant to catastrophic failure?

Autofrettage is a fun word. I went down the Wikipedia rabbit hole for some time when running across that word a few days ago.

Slight self-correction: the C in COPV is for "composite", not "carbon", though in this case we are talking about carbon fiber.
  #279  
Old 10-01-2016, 12:07 PM
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Wovern fabric--termed 'broadcloth'--is used for large structural layups and conformal structures, or as the surface layers on a honeycomb panel, where it is laid down in mats, trimmed to size, and typically vacuum bagged or pressed in a mold to consolidate the the fibers and minimize voids or porosity in the matrix that can result in disbonds. Because the mats have non-joined edges (i.e. you can't readily 'weld' the fibers together), they aren't especially good at containing internal pressure without significant reinforcement.

Pressure vessels and overwraps are made by continuous winding of a roving (set of parallel tows) at a controlled helical angle. The structure may have local broadcloth patches (often termed "doilies") for reinforcement around attachment points or apertures, and may have circumferential reinforcements in a cylinder section, but by controlling the spacing and wind angle can get a very optimized design. The art of continuous fiber winding is pretty mature (there are actually 6 and 7 axis CNC machines that can now wind significantly more complex shapes than cylinders and ellipsoids); the challenge of building COPVs is actually in the impermeable metallic liner, which has to be designed and pre-stressed to ensure that it shares the bulk of the pressure load evenly with the overwrap so that it doesn't develop weak spots that can result in through cracks, or experience tensile stresses that can create low cycle fatigue damage.

COPVs are supposed to fail in leak before burst mode, which doesn't mean that they always do, especially if difficult to inspect flaws or post-proof damage degrades the overwrap or the liner separates from the overwrap. Despite their price tag (a COPV the size of a basketball runs in the tens of thousands of dollars, and the ones used on the Shuttle program were hundreds of thousands of dollars apiece, plus the cost of regular inspections) there are only a handful of companies that make aerospace-grade COPVs because of the difficultly in producing COPVs reliably. On the aforementioned Shuttle, COPVs were a reliability driver insofar as a failure could both lease a critical system without fluid or fuel, and could pose a contamination hazard.

But there are other things that could have caused an overpressure failure in this case, so I would again caution against drawing conclusions without further evidence.

Stranger

Last edited by Stranger On A Train; 10-01-2016 at 12:07 PM.
  #280  
Old 10-01-2016, 12:42 PM
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Going from science to gossip, the Washington Post has a story today stoking the idea that ULA sabotaged the test:

https://www.washingtonpost.com/busin...mepage%2Fstory

And by "stoking," I mean throwing the idea out there and then walking back from it. Like Trump talking about how nice it is that he didn't say something mean.
  #281  
Old 10-01-2016, 03:03 PM
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Seems pretty irresponsible of them to write it up like that. Some kind of intentional damage--whether by a competitor or just an idiot with a high-powered rifle--has to be on their fault tree, so it's not surprising that they would be chasing down any leads in that direction. But yeah, putting "sabotage" right in the title is just stoking the rumor-mill. I guess we live in the age of clickbait titles.
  #282  
Old 10-01-2016, 04:49 PM
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I guess we live in the age of clickbait titles.
Yes, we do.
  #283  
Old 10-04-2016, 12:56 AM
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Yet another sabotage article. Please don't click on the link; you'll only encourage them. Instead, enjoy this music video.

Even if it was their primary theory, which I doubt, no good can come of this rampant media speculation. On the other hand, I don't really see any means of them shutting it down. Bleh.
  #284  
Old 10-28-2016, 10:36 PM
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Relatively small update here. This bit seems most significant:
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Through extensive testing in Texas, SpaceX has shown that it can re-create a COPV failure entirely through helium loading conditions. These conditions are mainly affected by the temperature and pressure of the helium being loaded.
No true root cause yet, though it's not clear if that's because they haven't quite confirmed that their recreation had exactly the same conditions as the failure scenario, or whether they haven't yet ruled out all other possibilities.

I wonder if this will prove to be something that escaped their qualification process because of too many dimensions. They obviously can't test every combination of LOX pressure, LOX temperature, He pressure and He temperature (where one might wish to try 10 values of each)--not to mention variations in duration. Perhaps they used sparse sampling across the range, or picked a handful of what they thought were the worst-case conditions. That might leave certain bad combinations untested.
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Old 10-28-2016, 10:53 PM
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I wonder if this will prove to be something that escaped their qualification process because of too many dimensions. They obviously can't test every combination of LOX pressure, LOX temperature, He pressure and He temperature (where one might wish to try 10 values of each)--not to mention variations in duration. Perhaps they used sparse sampling across the range, or picked a handful of what they thought were the worst-case conditions. That might leave certain bad combinations untested.
There's been some speculation that SpaceX was testing new LOX and He filling procedures during this static fire. That aligns with some of the comments by Shotwell about the fix being "business process" related, e.g. "stop testing new procedures when your customer's payload is sitting on a GIANT BOMB." If true, that's pretty damning... and yet, SpaceX is advancing so rapidly in part because they are squeezing a lot of testing and development around paying missions. In the short to medium term, they might get away with continued low reliability if they can keep learning valuable things (besides "someone figured out a new way to hammer in a sensor... backwards!") while providing cost-competitive launch services.
  #286  
Old 10-28-2016, 11:10 PM
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If true, that's pretty damning... and yet, SpaceX is advancing so rapidly in part because they are squeezing a lot of testing and development around paying missions.
Yeah--we know they've done that for their booster recovery efforts. One could argue that these efforts had zero impact on mission success, and naively that seems to be true, but there are always going to be small risks. The landing legs could have broken and caused instabilities; the extra helium lines could have burst; etc. So in some sense we already know that SpaceX is risking customer payloads to make their R&D cheaper.

It does seem a little irresponsible to do the same thing with propellant/helium loading, but maybe they thought the risk was infinitesimal.

At any rate, I don't think they'll be doing their pre-launch testing with payloads attached from now on.
  #287  
Old 11-05-2016, 10:20 PM
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Just a thought -

AIUI, the use of super-cooled fuel (and helium?) means that the window from fueling to launch is on the order of 30 minutes.

If they are using this extreme fueling procedure to get an extra X% thrust on the Falcon 9, could they use stable fuel and get the extra boost required by strapping on solid rockets?
Yeah, they'd have to go outside to buy the boosters (or buy somebody who already builds motors), and they need to do something to jettison the boosters if they want to land the first stage. That should not be difficult.

How common are such push-the-envelop technologies used?

For that matter: could they get this "Extra X%" by simply using all fuel aboard the booster and sacrifice the booster? How much fuel is used for the recovery? If you are willing to burn that fuel to deliver the payload, could you use ambient-temperature fuel?

Alternately, they could come up with payload "fairings" with Dragon-style escape boosters and parachutes. j/k
  #288  
Old 11-06-2016, 07:57 AM
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Just a thought -

AIUI, the use of super-cooled fuel (and helium?) means that the window from fueling to launch is on the order of 30 minutes.

If they are using this extreme fueling procedure to get an extra X% thrust on the Falcon 9, could they use stable fuel and get the extra boost required by strapping on solid rockets?
Yeah, they'd have to go outside to buy the boosters (or buy somebody who already builds motors), and they need to do something to jettison the boosters if they want to land the first stage. That should not be difficult.

How common are such push-the-envelop technologies used?

For that matter: could they get this "Extra X%" by simply using all fuel aboard the booster and sacrifice the booster? How much fuel is used for the recovery? If you are willing to burn that fuel to deliver the payload, could you use ambient-temperature fuel?

Alternately, they could come up with payload "fairings" with Dragon-style escape boosters and parachutes. j/k
My information comes from some of Stranger's posts on earlier SpaceX flights using densified O2. As I remember it, the concept was considered as far back as the 60's and rejected as being impractical/not worth the effort. So I believe no one else besides SpaceX has tried this technique. A major problem with it is the very short time window inherent in the technique. One has be launch very soon after fueling, if something goes wrong (say an intrusion into the flight safety area), the launch has to be scrubbed.
It is worth noting that a recent review of launch procedures by a NASA panel of astronauts and flight experts was unanimous in rejecting the idea of having crew present during fueling. This last accident isn't going to give the astronauts much confidence in the idea.
  #289  
Old 11-06-2016, 02:50 PM
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SpaceX isn't the only one using densified propellants--the Antares rocket used sub-chilled oxygen, as did the Russian N1 rocket (which was where the Antares engines came from).

However, no one else has sub-chilled quite to the same degree, nor has anyone else used submerged carbon-fiber COPVs. It appears that this combination was what proved disastrous and led to the formation of solid oxygen. This reacted with the carbon, and... boom.

SpaceX seems to have a somewhat unusual approach to design. They take neither the "big dumb booster" path nor do they pursue delta-V at any expense. So they have a basic two stage rocket (when three stages would be more efficient for GTO missions) that uses kerosene (when a hydrogen upper stage would be a win). But on the other hand they really push the limits of what they have with densification and composites.

The payload increase from densification is certainly less than the payload hit from reusability. So they could make that tradeoff if they wanted. Densification gives more thrust as well, but that's also not hugely important for them.

It seems to me that the "astronauts aboard during propellant loading" question does not have an immediately obvious answer. The risks during loading were certainly demonstrated, but simply being in the vicinity of a fully-loaded rocket is dangerous. So there are risks involved in just taking the elevator up to the capsule, among other things.

On the other hand, the capsule is arguably the safest place within a 1-mile radius. The escape system is operational during the whole process, so if the astronauts are already inside, there is no point where humans are nearby a skyscraper full of fuel and oxidizer while not also being protected by a safety system. The same can't be said for late boarding.
  #290  
Old 11-06-2016, 05:25 PM
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I thought the helium tank's placement within the tank the helium is to purge was a cute and efficient arrangement - no tubing, fewer/no fittings to fail.

But I see the liquid temp of helium (−269 C) is lower than the Freezing point of oxygen (−218.79 C) - if you want liquefied helium, you are risking solidified oxygen.

I really do hope they weren't trying to inject liquid helium (does it even exist in commercial quantities?) into a tank buried in LOX.

Would the NASA manned shuttle to ISS require the super-densified fuels they were using for the failed test?
  #291  
Old 11-06-2016, 09:00 PM
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Yep, they were injecting liquid helium into a LOX tank.

It is now official: A chunk of solidified oxygen (!!!!) bumped into the carbon-fiber overwrap of the helium tank, and ignited it. I don't see anything resembling an ignition source.
On a wild guess, I'd think they'd know to ground all metal bits anywhere near LOX.

My mind is now (once again) officially blown.

Just found Ad Astra Rocket company. Warp drive coming up...

Would you like some oxygen ice cubes in that Hemlock?
  #292  
Old 11-07-2016, 08:10 AM
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The He in the second stages isn't supposed to be liquid, the temperatures they are using are never supposed to be that cold. If there was indeed liquid He in the tank, then something very strange happened.

The working theory I've head is not that there was solid oxygen floating around, but that liquid oxygen soaked into the carbon overwrap on the helium tanks, then froze. As the tanks were pressurized, the solid oxygen was compressed as the carbon overwrapped stretched, and it was that compression that provided the ignition source.
  #293  
Old 11-07-2016, 11:56 AM
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Although the NK-33 engines used on the N-1 rocket and the later Antares do require LOx that is cooled below the liquid temperature of oxygen at standard pressure, it is only done to about -196 ℃ and is done to prevent LOx vaporization and combustion instability in the engine during operation. SpaceX is cooling their oxidizer down to -207 ℃ for the Falcon 9 specifically to densify the propellant, and in fact their performance margins for the uprated vehicle are dependent upon using densified propellants. The difference of -11 K may not sound like much but at those temperatures it is extremely difficult to keep the liquid insulated against conduction from the ambient environment, hence why SpaceX has experienced repeated aborts due to exceeding propellant temperature limits. This presents the sort of mission critical sequencing that stands in contradiction to making launches cheaper and less prone to error. As I've noted previously, densified propellants have been studied since almost the beginning of the space launch era (since at least the Titan I ICBM development) because they present the only practical way of getting higher performance from a size limited package due to the fundamental limitations of combustion chemistry, but they carry such severe constraints and risks that no one has implemented it in operation. As recently as the late 'Nineties, McDonnell Douglas (now Boeing) and Lockheed Martin both looked at using densified propellants to achieve better performance from existing designs and concluded it was not cost effective.

I'm not certain where the notion that there is liquid helium in the COPV tanks but this is not the case. SpaceX loads chilled gaseous helium into the COPVs which is heated and released for ullage (filling up the volume left by consumed propellants to maintain sufficient pressure) during stage operation. It is true that COPVs are not typically submerged into cryogenic fluid in operation, but one of the ways of testing COPVs that are used for low temperature operation is to thermal cycle them using a supercooled liquid nitrogen bath to ensure that they can endure sufficient temperature cycles without seeing liner separation or thermal stress cracking. The explanation for the failure appears to be an energetic reaction between the LOx and the carbon overwrap, presumably ignited by friction within the overwrap. SpaceX has not, as far as I am aware, presented evidence of testing or analysis to substantiate this but it seems like a plausible mechanism, albeit one that has potential design implications.

The claim that "The payload increase from densification is certainly less than the payload hit from reusability," is very probably not true. The amount of propellant necessary to return the first stage back to a powered landing depends upon the trajectory but can be as low as 5% for a downrange (barge) landing, and even a return-to-launch-site landing is probably only ~15%, because the post-separation booster is so light and much of the energy to slow the stage is provided by atmospheric drag. Densification is giving SpaceX up to a 30% improvement in total impulse delivered to the payload and is absolutely crucial to achieving the upper range of the advertised capability for payload mass or GTO/GSO parameters. SpaceX would realize some amount additional delivered capability by removing the landing legs, attitude control system, and other hardware associated with the first stage return capability but that is actually a relatively small portion of the dry (unloaded) mass of the stage and is only represents a small fractional increase per unit mass of final payload.

The notion that it is 'safe' for the crew to be in the capsule during propellant loading because it has a launch escape system (LES) is equivalent to placing your faith in automotive airbags to allow you to drive recklessly down the highway. A LES is a last resort system to try to remove astronauts from a hazardous situation by subjecting them to accelerations and motion that is near human tolerance. There has only been one actual use of a LES in practice (Soyuz T-10-1) and in that case had sufficient information to actuate a couple of seconds before the rocket it was sitting on exploded. In the case of the F-29 failure, there was no warning of the imminent failure from instrumentation, and thus, the LES would have minimum time to respond. Fueling rocket launch vehicles is not like fueling your car, and it is always considered a hazardous operation to be done without people present on pad since the early days of rocketry after a few very near misses. A system that obligates the crew to be in capsule during the fueling operation is inherently risky to a degree that is beyond the normal tolerance for risk for crewed missions even by 'Sixties standards.

Although SpaceX has demonstrated the technical capability to recover the first stage, has successfully static fired the returned engines, and plans to refly a stage in the near future, the fiscal case for doing so remains unclear. Elon Musk and Gwynne Shotwell were originally promising order-of-magnitude reductions in cost (>90%), then 75% (the purported portion of cost of a Falcon 9 vehicle), more recently a modest 30%, and within the last month that discount dropped to 10%. They also continue to present the rationale that once a stage has been flown it is inherently more reliable by virtue of flight demonstration, which ignores the fact that modern rocket launch vehicles, including the Falcon 9, are not as mature or have sufficient ability to safely recover from a failure automobiles or airliners and just by virtue of operating near material capability limits under extremes of vibration and thermal stress have a very limited life under conditions in which the failure of a single subsystem can result in loss of vehicle.

The use of solid propellant boosters to augment liftoff thrust and get the vehicle moving (thus reducing gravity drag losses even though solid boosters have relatively poor propellant efficacy) as merit in general but would require structure to mount to the Falcon 9, and given their horizontal processing flow (vehicle is assembled laying on rails and then erected at the launch pad) is not really practical, nor does it really fit with SpaceX plans of high launch volumes. There are only two companies still producing large solid propellant rocket motors, and neither does so economically at high volume. Solid propellant motors also have significant logistical limitations that are not consonant with the way SpaceX operates so I doubt they would consider using solids for thrust augmentation even if it would work technically.

Stranger
  #294  
Old 11-07-2016, 08:45 PM
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The claim that "The payload increase from densification is certainly less than the payload hit from reusability," is very probably not true. The amount of propellant necessary to return the first stage back to a powered landing depends upon the trajectory but can be as low as 5% for a downrange (barge) landing, and even a return-to-launch-site landing is probably only ~15%, because the post-separation booster is so light and much of the energy to slow the stage is provided by atmospheric drag. Densification is giving SpaceX up to a 30% improvement in total impulse delivered to the payload and is absolutely crucial to achieving the upper range of the advertised capability for payload mass or GTO/GSO parameters.
SpaceX did achieve ~30% extra performance from their "Full Thrust" upgrade package, but not all of that is from densification. They stretched the second stage, reduced structural mass, and increased overall thrust. Some/most of the thrust was due to subcooling but it was partly just uprating. Saying that the full 30% is attributable to densification seems dubious, though I couldn't say what the exact breakdown is.

SpaceX has said that the payload hit for barge landing is about 15% and 30% for return-to-launch. They could probably optimize that further but it increases their risk. They already failed one landing because the remaining fuel required a three-engine "hoverslam" that their control system couldn't quite handle. They'd rather use a more comfortable but less-efficient single-engine landing.

So perhaps "certainly less" was stronger than is warranted; I'll say "in the same ballpark of" instead.

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The notion that it is 'safe' for the crew to be in the capsule during propellant loading because it has a launch escape system (LES) is equivalent to placing your faith in automotive airbags to allow you to drive recklessly down the highway.
That's not quite my argument. There's no doubt that fueling a rocket is a dangerous operation. But a fully-fueled rocket is also a danger. Not as much, certainly, but it is still a highly dynamic system and under the wrong conditions could easily fail.

Boarding a fueled vehicle is therefore a risk. This isn't exactly controversial--the Shuttle famously had a tower escape system that transported the astronauts to a protected bunker in case of an accident. Obviously, this system had some limits and would probably only be effective when there was some kind of failure that hadn't quite progressed to the giant fireball stage.

I'm just suggesting that, depending on how the numbers work out, it isn't obviously wrong that the gains from protecting astronauts from "static" pad accidents could exceed the potential losses during fueling accidents.

To be more concrete (using some made-up numbers), suppose that rocket X fails 2% of the time during fueling and 0.5% of the time while fueled during the boarding process. The LES has a 95% success rate while the tower escape is 50%.

Late boarding thus has a 0.5% * 50% = 0.25% chance of crew loss. Half exposure to boarding failures but no exposure to fueling failures.

Early boarding would have a (2% + 0.5%) * 5% = 0.125% chance of crew loss. It has exposure to the more dangerous fueling operation but somewhat mitigated by the LES.

Like I said, made-up numbers, and I'd expect that both NASA and SpaceX have a more sophisticated performance model, but in any case I can see them coming to different conclusions even if both optimized for overall crew safety.
  #295  
Old 11-07-2016, 08:54 PM
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As the tanks were pressurized, the solid oxygen was compressed as the carbon overwrapped stretched, and it was that compression that provided the ignition source.
Yeah. Here's a video of some LOX and oil being shocked by a falling weight. It detonates quite nicely. Trapped liquid oxygen would presumably be squeezed out from the COPV layers, but solid oxygen would be trapped and get squeezed at high pressure. I wonder if anyone's characterized the reaction between oxygen and carbon at COPV pressures.
  #296  
Old 11-08-2016, 03:51 AM
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...

Although SpaceX has demonstrated the technical capability to recover the first stage, has successfully static fired the returned engines, and plans to refly a stage in the near future, the fiscal case for doing so remains unclear. Elon Musk and Gwynne Shotwell were originally promising order-of-magnitude reductions in cost (>90%), then 75% (the purported portion of cost of a Falcon 9 vehicle), more recently a modest 30%, and within the last month that discount dropped to 10%. They also continue to present the rationale that once a stage has been flown it is inherently more reliable by virtue of flight demonstration, which ignores the fact that modern rocket launch vehicles, including the Falcon 9, are not as mature or have sufficient ability to safely recover from a failure automobiles or airliners and just by virtue of operating near material capability limits under extremes of vibration and thermal stress have a very limited life under conditions in which the failure of a single subsystem can result in loss of vehicle.

The use of solid propellant boosters to augment liftoff thrust and get the vehicle moving (thus reducing gravity drag losses even though solid boosters have relatively poor propellant efficacy) as merit in general but would require structure to mount to the Falcon 9, and given their horizontal processing flow (vehicle is assembled laying on rails and then erected at the launch pad) is not really practical, nor does it really fit with SpaceX plans of high launch volumes. There are only two companies still producing large solid propellant rocket motors, and neither does so economically at high volume. Solid propellant motors also have significant logistical limitations that are not consonant with the way SpaceX operates so I doubt they would consider using solids for thrust augmentation even if it would work technically.

Stranger
I wonder if SpaceX Martian ambitions are not, in part, behind their recoverable first stage business. As mentioned before they are not shy of piggybacking their R&D to commercial launches; as you say the economic case for reusability is sketchy but if they'd have a further incentive for developing technology that could be used on a rocket that can land and take off again in, oh let's say Mars, it may explain why they'd pursue that path.

A rocket that can land and take off again, on Earth, is cool but arguably doesn't make much sense (economically at least), but a rocket that can land and take off again from Mars is the only thing that would make sense.
  #297  
Old 11-08-2016, 11:37 AM
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I'm not certain where the notion that there is liquid helium in the COPV tanks but this is not the case. SpaceX loads chilled gaseous helium into the COPVs which is heated and released for ullage (filling up the volume left by consumed propellants to maintain sufficient pressure) during stage operation. It is true that COPVs are not typically submerged into cryogenic fluid in operation, but one of the ways of testing COPVs that are used for low temperature operation is to thermal cycle them using a supercooled liquid nitrogen bath to ensure that they can endure sufficient temperature cycles without seeing liner separation or thermal stress cracking. The explanation for the failure appears to be an energetic reaction between the LOx and the carbon overwrap, presumably ignited by friction within the overwrap. SpaceX has not, as far as I am aware, presented evidence of testing or analysis to substantiate this but it seems like a plausible mechanism, albeit one that has potential design implications.
In a little more detail, Musk stated that their leading hypothesis was that frozen oxygen formed between the fibers of the COPV. With expansion of the COPV, that could produce sufficient pressure for ignition.

However I'm not exactly clear on the temperatures and pressures involved. For that explanation to be true, clearly SpaceX believes that something is below the freezing point of oxygen. Liquid helium would be cold enough. But I've also seen COPV tank pressure figures in the several thousand PSI range. At the temperature of the surrounding oxygen, I believe that means the helium is a supercritical fluid.

I've also heard that helium in some phases will actually get colder as it is compressed, unlike most fluids. Though I haven't seen any solid figures showing that this phenomenon is relevant at the temperature and pressures involved.

Can anyone shed a little more light on this?
  #298  
Old 11-08-2016, 11:58 AM
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I can see the grad papers now "Effects of interstitial compression on various crystallographic phases of oxygen"
  #299  
Old 11-08-2016, 04:35 PM
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I wonder if SpaceX Martian ambitions are not, in part, behind their recoverable first stage business. As mentioned before they are not shy of piggybacking their R&D to commercial launches; as you say the economic case for reusability is sketchy but if they'd have a further incentive for developing technology that could be used on a rocket that can land and take off again in, oh let's say Mars, it may explain why they'd pursue that path.

A rocket that can land and take off again, on Earth, is cool but arguably doesn't make much sense (economically at least), but a rocket that can land and take off again from Mars is the only thing that would make sense.
IMO (and I doubt that is some great insight nobody else has noticed) is yes THAT is a major component of Musk's plans.

And it probably makes more sense than a rocket with wings that lands like an airplane...
  #300  
Old 01-02-2017, 06:07 PM
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Probably the final anomaly update.

The helium pressure vessels (COPVs) have an aluminum liner which prevents the helium from escaping (the carbon-fiber overwrap, although it has a high tensile strength, is porous to helium). The update claims that it had "buckled", which I suspect means something like the small creases you get when smoothing aluminum foil onto a curved surface.

The buckles alone aren't a problem as the liner is not a structural component of the COPV; all that matters is that there are no breaks in the surface. However, they did allow a place for liquid oxygen to pool (since LOX can infiltrate the carbon overwrap as well).

The trapped oxygen was in contact with carbon fiber and the loading was a potential cause of friction, which can cause it to ignite. Worse, the temperatures were such that solid oxygen could form, which is more subject to ignition and would not get squeezed out.

A change to their loading procedure should eliminate the problem, although a COPV design change to eliminate the buckles is the longer term solution and will allow them to resume their fast loading method.

I'm a little surprised that SpaceX allowed these buckled liners to pass their QA, but maybe this is a known defect with COPVs and not traditionally considered a problem.
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