Author Topic: BFR/ITS risk due to composites  (Read 6286 times)

Offline ZachF

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Re: BFR/ITS risk due to composites
« Reply #20 on: 05/02/2017 04:49 PM »
Composites have been around since what? the 1980's.  Glock made the first composite framed pistol back in the 1980's. ... I used Glock as an example because ITS will have gravitational stresses taking off and landing. ...

It's not a great example, since those frames are made out of fiberglass reinforced nylon, not carbon fiber reinforced epoxy. The mechanical properties are very different, nearly as different as AlLi alloy and CFRP.

I'm pretty sure modern polymer framed pistols aren't fiberglass reinforced... just plastic, with stamped metal inserts for the rails the slide sits on. The first polymer framed pistol was an HK too, IIRC.

Fiberglass reinforced as in glass-filled nylon. It looks exactly the same as nylon unless you cut it open. It doesn't look like a familiar fiberglass layup (e.g. boat hull) in any way.

But composites are used in a lot of high-shock applications, including automotive, aerospace, high-performance bikes, etc.

Maybe for some rifle stocks, but pistol frames are just plastic with metal inserts, here a few pics of Glocks that have kaboomed:

http://www.reno4x4.com/attachment.php?attachmentid=48049&d=1383017506
http://www.frontsight.com/Images/Glock21ReloadDamage/glock02.jpg

Offline envy887

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Re: BFR/ITS risk due to composites
« Reply #21 on: 05/02/2017 05:47 PM »
Maybe for some rifle stocks, but pistol frames are just plastic with metal inserts...

Glass-filled nylon IS "just plastic". ;) In the sense that plastic is a general term that includes glass-filled polymers. Thermoplastics like glass-filled nylon tend to be reasonably tough (at least relative to their tensile strengths) compared to the epoxies typically used in carbon fiber composites. But higher glass content tends to increase strength and reduce toughness.

Offline DAZ

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Re: BFR/ITS risk due to composites
« Reply #22 on: 05/05/2017 01:07 AM »
From what I remember, the problems with the Boeing 787 composites and the long delays associated weren’t really with the composites themselves.  There were 2 primary problems.  One was a chronic shortage of aerospace certified fasteners to bolt the composite parts together.  The 2nd problem was that these parts were manufactured in greatly dispersed factories.  These factories were located across the world in other countries.  Some of these companies were very good at holding their manufactured part tolerances to the required levels.  But there were a few of the companies that could not.  So when these parts were brought together for the 1st time they could not be bolted together.  Boeing literally had to send people to these factories and teach them how to build parts that Boeing had already paid them to build.

Once these parts were built and tested there was absolutely no doubt from the results that Boeing was able to meet all of their design performance goals.  An example of this is when they did the iron Eagle tests.  This is where they build a mockup of all the structural elements of the aircraft and stress them on the ground hydraulically to simulate all of the loads they will undergo in the aircraft’s lifetime.  Because of the newness of this design along with everything that had been learned in the past the FAA required this to be the most stringent test performed to date.  After they had done all the fatigue tests and stressed it to its design ultimate limit (150% of maximum design ultimate load) this then passed all of the FAA’s required tests. 

Boeing then did one additional test not required by the FAA.  This test was only for the Boeing engineers.  It was a test to determine how much performance (how much stronger than necessary and thus extra weight than needed was added) was left on the table.  There is always a certain amount of uncertainty when designing the parts.  You obviously never want the failure to be less than the design goal.  Decades ago being only 10% over was considered good.  With modern designs, 5% is more likely the design goal.  When Boeing tested to find the true ultimate load the failure was at just over 151% and failing at the predicted point in the structure.  So the Boeing engineers left just a little over 1% on the table.  This is actually much better than they could do with aluminum parts.  It illustrates just how well the engineers understand these composite materials.

Offline Nomadd

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Re: BFR/ITS risk due to composites
« Reply #23 on: 05/05/2017 01:50 AM »
 The 787 had a six month delay because of premature composite delamination during the wing load test. It wasn't really the composite's fault. A wing design change in Japan wasn't properly considered and caused greater loads at one point. Part of the fix was actually removing some material to increase flexibility at one point and move the load where it should be. Most of the other composite related problems were from Alenia doing substandard work that caused a lot of their barrels to be scrapped.
 It's the kind of problem that could be even worse when your margins are smaller, like in a spaceship. The effects of every little change has to be considered a million different ways since everything is so tightly designed. Making one part a little stronger can mess up a lot of calculations.

Online meekGee

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Re: BFR/ITS risk due to composites
« Reply #24 on: 05/07/2017 06:22 AM »
I did not include specific program names in this initial post.  If I did the 787 would have been an example of the very concern that I have.  The 787 cost twice as much to develop as originally planned (40 billion) and the initial delivery was 40 months late.  Not all because of composite issues but a good part.  The break even point is now the 1100th aircraft, wow!  Boeing thought it understood the design issues and bet its future on that aircraft. It had the financial resources to recover.  I not sure that SpaceX would fare as well since it internally funded without a large back order ledger for BFR and ITS.

The question I am asking is the risk worth the reward for the interplanetary  transportation system?

Worth noting that the experience of building the 787 is likely to make building the ITS easier. As with any pathfinder project, everything afterwards using similar technology is easier.
Also worth noting that a very big part of 787 schedule issues had to do with loss of control over outsourcing.

The  wrapped fuselage barrel sections were actually a good idea.

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Offline docmordrid

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Re: BFR/ITS risk due to composites
« Reply #25 on: 05/07/2017 09:46 AM »
Just a reminder: the ITS tank was built by Janicki, and they'll set up at your site.

>
One secret is where this is?  I'll assume it was assembled (and possibly components manufactured) at the location.  If they are going to test it on a barge, it has to be somewhere close to water and without lots of low clearance road structures.
Not that much of a secret.
They said it was built by Janicki Industries.
So it is either in Sedro-Woolley  or Hamilton Washington.
« Last Edit: 05/07/2017 09:46 AM by docmordrid »
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Offline Oersted

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Re: BFR/ITS risk due to composites
« Reply #26 on: 05/07/2017 10:06 PM »
The ITS couldn't be conceived without composites. They are absolutely essential for the numbers to add up.

Offline spacenut

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Re: BFR/ITS risk due to composites
« Reply #27 on: 05/07/2017 10:16 PM »
I disagree, ITS could be done with aluminum, but would loose payload ability.  How much?  Don't know.  Remember Sea Dragon was all steel pressure fed single engine per stage two stagebut could get 500 tons to LEO.  It was around 70-75' in diameter.  Aluminium could be a fall back, but would affect payload. 

Offline Robotbeat

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Re: BFR/ITS risk due to composites
« Reply #28 on: 05/09/2017 12:49 PM »
Aluminum could be a fall back but won't because SpaceX doesn't get scared off by new technologies like NASA might.

The main issue for composites is oxygen compatibility with cracks, and the fallback there is a metal liner in the oxygen tank. Musk said this himself.

It's not like aluminum doesn't have issues. Aluminum doesn't have an endurance limit under fatigue like steel does, in other words, aluminum keeps getting fatigue even with low loadings. But that's just something you take into account and move on. You don't build airplanes out of steel.

I actually think half the reason for composites is the manufacturability improves for some shapes. It's not just performance.
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Offline rsdavis9

Re: BFR/ITS risk due to composites
« Reply #29 on: 05/09/2017 12:54 PM »

It's not like aluminum doesn't have issues. Aluminum doesn't have an endurance limit under fatigue like steel does, in other words, aluminum keeps getting fatigue even with low loadings. But that's just something you take into account and move on. You don't build airplanes out of steel.

I actually think half the reason for composites is the manufacturability improves for some shapes. It's not just performance.

what does composites look like on this very nice graph?
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Offline Star One

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Re: BFR/ITS risk due to composites
« Reply #30 on: 05/09/2017 02:42 PM »
Aluminum could be a fall back but won't because SpaceX doesn't get scared off by new technologies like NASA might.

The main issue for composites is oxygen compatibility with cracks, and the fallback there is a metal liner in the oxygen tank. Musk said this himself.

It's not like aluminum doesn't have issues. Aluminum doesn't have an endurance limit under fatigue like steel does, in other words, aluminum keeps getting fatigue even with low loadings. But that's just something you take into account and move on. You don't build airplanes out of steel.

I actually think half the reason for composites is the manufacturability improves for some shapes. It's not just performance.

That's a rather blanket statement about NASA being scared off. Why people feel the need to cast aspersions on a body like NASA just to argue a point escapes me.

Offline Oersted

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Re: BFR/ITS risk due to composites
« Reply #31 on: 05/09/2017 07:37 PM »
I disagree, ITS could be done with aluminum, but would loose payload ability.  How much?  Don't know.  Remember Sea Dragon was all steel pressure fed single engine per stage two stagebut could get 500 tons to LEO.  It was around 70-75' in diameter.  Aluminium could be a fall back, but would affect payload. 

Remember, the ITS is not about lifting stuff into Earth orbit but about going to Mars and back. The margins delivered by composites are absolutely needed.

Offline john smith 19

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Re: BFR/ITS risk due to composites
« Reply #32 on: 05/09/2017 10:16 PM »
The impressive performance numbers realized for the BFR and ITS are in part due to extensive use of composites in the vehicles structure, airframe and tanks. 
Since ITS has not flown yet that would be the anticipated impressive performance numbers.
Quote from: testguy
Composites have been proven problematic when used in other aerospace projects in the past.  Problems have been revealed in parts processing, inspection, repair and durability amongst others that I'm sure this forum can identify.  SpaceX, no doubt appreciates the composite issue as demonstrated by their early demonstration of the ITS oxidizer tank and their copy experience. The very size of the BFR and ITS  make it difficult to test the structures other than in flight.  How else can they subject the stages to the extreme thermal, structural and dynamic environmental conditions that must be survived on multiple cycles.  After all, if you think about it, each stage is the size of a small sky scraper.
 the Mars

This is not the first time very large LV's have been built, or large aerospace structures for that matter. Concorde (for example) was tested using an airframe covered in electric heaters and hydraulic actuators feeding a "wiffle tree" to simulate both the thermal and structural load cycling of multiple flights in order to avoid surprises

You're right however that the multiple loads together IE mass, thermal, vibration and acoustic, is going to be very tough short of an all up build and launch.
 
Quote from: testguy
My concern is that extensive composite use may once again be a rabbit hole that could sink the Mars aspirations.  Could a composite issue identified during flight testing be too late to recover from? 
TBH there are always the "unknown unknowns." as the pad explosion on SLC40 proved.  :(

However SX is in quite a strong position for several reasons.

ITS will be its 3rd (or 4th, depending on how different you see FH as being from F9) major LV. So they have a clear sense of a development cycle and areas which have caused particular issues in the past for them.

They now have substantial knowledge about both the launch and reentry environments around Earth (and provided all goes will will have some of the entry corridor to the Martian surface). That gives them a lot of data for their various environmental models for CFD and FAE analysis.

Lastly they have a number of CFRP structural elements on all of their LVs which (AFAIK) have gone through several iterations. This is valuable as historically there has been a tendency in the industry to treat CFRP as "Black Aluminum" and design a part the same way as it would be in metal alloy, much as early 3d printed parts copied existing parts while later iterations made better use of 3d printing's strengths. In CFRP this has resulted in parts that are don't realize the weight savings that the materials specific strength predict should occur.

Worse, a part shaped to take advantage of Al alloys properties may be exceptionally prone to particular CFRP failure modes, so it has to be strengthened (and hence heavier) than the part it replaces, a double failure in design.

I suspect this is what you mean by the "rabbit hole" of CFRP development. SX seems quite aware of CFRP's differences and of leveraging it's benefits while minimizing it's issues.

 
Quote from: testguy
I am not an expert, just witnessed many development problems over the years.  The intent of opening this discussion is to solicit thoughts pertaining to composites for BFR and ITS.  Why will SpaceX be successful this time?  Should all the design eggs be in one basket?  It is even feasible to have a viable less risky design.  With billions needed for development, with source of funding being internal, it appears that SpaceX must get it right the first time.
To an extent. But SX has the advantage that it's setting the timescale. They can always regroup if they hit a serious problem.
« Last Edit: 05/09/2017 10:26 PM by john smith 19 »
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