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A conversation with Elon Musk



 
 
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  #11  
Old October 15th 19, 12:26 AM posted to sci.space.policy
David Spain
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Default A conversation with Elon Musk

On 2019-10-14 3:08 PM, JF Mezei wrote:
On 2019-10-14 12:30, David Spain wrote:

That is really a remarkable comment. Not as bad? How do you know? You
building Starships?


I was noting that Musk compared the cost of raw material per pound of
material. It is a nice PR stunt but doesn't provide correct comparison
since one does not use the same amount of carbon fibre (by weight) as
you would steel to build a structiure of same strength.

Note: I didn't state that carbon would be cheaper, just that his
cpomparison exagerated the cost difference.


"With carbon composite, you need to cut the fabric, impregnate it with
high-strength resin, which can be difficult and then make 60 to 120
layers!


Modern structures lay not carbon fibre "fabric" but rather
pre-impregnated carbon fibre strands. A robot arm strings the strand
along in precise location/direction. This allows very procise strength
is specific location of the "object" being built where the machine will
striong more strands over a specic location and in various directions
(instead of the fabric that ahs just 90° strand crossing each other)

This sounds like a hand wave to me. Any idea how expensive that is for a
large object? Is that even feasible for a 9m diameter rocket? How does
that compare in cost to laying fabric? Numbers?

After the laying is done, the object is then cured in an oven. The resin
used for the pre-impregnation requires heat instead of a catalyst
chemical to trigger the curing process.

Some structures such as large wing surfaces still make use of fabric.
For instance, the Bombardeir X-Series (Now Airbus A220) uses fabric for
wing siurface, but teh IArbus A350 uses a pen "drawing" system to
populate the surface with fibre strands. The later takes much mroe
programming than hand laying fabric, but results is more spophisticated
and mroe precise construction.

Okay so for large surfaces what Tim Dodd says holds I presume. You did
note the 60X figure (cost of composite vs stainless to mfg.) in the quote?

That's probably why SpaceX's composite casting rig went from this:

http://compositesmanufacturingmagazi...rocket-bfr.jpg


to this:


https://www.teslarati.com/wp-content...c-1024x346.jpg




"There is a further reason for using stainless steel rather than carbon
composite – fracture toughness. Carbon composites are typically very
brittle, and small flaw can initiate cracks which rapidly spread and
destroy the structure.



Which is why the PROCESS is extremely important when making any
composite object since you want a process that will not have a risk of
imperfections, airbubbles etc. Hence my comment about this being great
when you need to produce a whole bunch (since work to program the robots
to do the perfect job is spread amongts many produced units whereas if
you are producing only a handful, then it is a lot of work to setup the
robots for only a few units produced.


Not just mfg. process but handling too. *Again* I re-quote from previous
post:

" ...A subsequent lengthy investigation determined that the case of the booster had likely been slightly damaged during ground handling, providing a flaw which initiated a crack which rapidly spread."


Noted: "...*slightly* damaged during ground handling" & during launch
not while on the ground. A structural failure in a composite due to a
defect caused by a mishandle while on the launch pad that lead to a
quick demise. If structural integrity of Starship depended upon hull
integrity of a composite rocket it would still fail, liquid fueled or
not. Possibly in a fashion and for reasons quite hard to detect while on
the ground. In this case it's not performance (weight) uber alles. I'll
take that trade of some resilience for weight. Again I'm not second
guessing Elon here.

I don't really think you are either. But I don't consider your point
that Elon is being too dismissive as valid. He had already spent big
money on tooling for composite mfg. before a big change in direction and
the switch to stainless steel. I suspect he had given this much, much
thought.

Dave



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  #13  
Old October 15th 19, 11:51 AM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,001
Default A conversation with Elon Musk

In article ,
says...
In the case of SpaceX, if they have unlimited power from unlimityed
number of engines, then they are less concerned with weight. But at the
end of the day, I doubt that they have unlimited ower available and
weight will become a big concern when this goes beyond the neat demo
flights stage.

But for now, tyhat steel allows Musk to make prototypes fast and cheap
and that is a big advantage.


It also allows for a much hotter structure during/following earth
reentry and landing. None of your aircraft examples have to endure the
extremely high temperatures of reentry and landing.

Musk has said several times that the higher temperature stainless steel
structure allows for a thinner and lighter heat shield. And aluminum or
carbon composite structure would require far more insulation on the heat
shield to protect it from reentry heat. So in the end, your Starship
might be heavier if you make it out of carbon fiber.

Again, we're trying to optimize the lifetime cost/dry mass of the entire
Starship, not just the structure. Carbon fiber just does not make sense
for this application, despite the "conventional wisdom" of aerospace
engineers in decades past (i.e. the X-33 which canceled after its carbon
fiber cryogenic propellant tanks failed during ground testing).

Jeff
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These posts do not reflect the opinions of my family, friends,
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  #14  
Old October 21st 19, 12:46 PM posted to sci.space.policy
Jeff Findley[_6_]
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Default A conversation with Elon Musk

In article ,
says...

On 2019-10-15 06:51, Jeff Findley wrote:

Musk has said several times that the higher temperature stainless steel
structure allows for a thinner and lighter heat shield. And aluminum or
carbon composite structure would require far more insulation on the heat
shield to protect it from reentry heat. So in the end, your Starship
might be heavier if you make it out of carbon fiber.


The way I see it, we are still very much at prototype stage, and steel
is cheaper and faster to get built, it makes much sense for the
portotype stage.


At the time Musk announced he was switching to steel, the argument was
that the weight gain from steel would be offset by weight loss due to
lighter/no heat shield.

The problem is that since heat shield had not been decided/final yet,
that math couldn't have happened.


Bull****. The amount of insulation and/or cooling could be calculated
for the different materials. Regardless of what heat shield would be
used, it would still be *lighter* for stainless steel than the other
materials due to the much *higher* temperature tolerance of stainless
steel.

They've moved to transirational cooling and now to tiles. They've
changed the shape of the rocket. The design is advancing, evolving.


Yes, but that does not negate the fact that choosing a different
structural material with a *higher* temperature tolerance allows for a
*lighter* cooling/insulating solution.

And since the design is changing, it makes much sense to test things in
easy steel. No point building design specific tooling/mandrels for
ccarbon rocket when the design is still changing.


True.

So I am still not convinced that the long term "productized" sharship
will be steel.


You keep being you, but you're wrong on this. Stainless steel allows
for far cheaper and *faster* production of Starships and Super Boosters.
And given SpaceX's application for even *more* Starlink satellites,
they're going to need a lot of launches of Starship to put them all into
orbit. Cite:

https://www.businessinsider.com/spac...atellites-itc-
filing-30000-additional-42000-total-2019-10

So, 42,000 satellites need to be launched. Someone else online
estimated that you could stuff 325 Starlinks inside a Starship. So
that's about 130 launches of Starship. So if you launch four times per
week, that's 2.5 years to launch the entire constellation. So, SpaceX
is going to need *several* Starship/Super Booster combinations to meet
this high flight rate.

Depending on how much shielding they actually need with steel, the

math
may return to favouring carbon fibre or aluminium.


Bull****.

Going to LEO will work with heavy steel. But once they set the target to
Mars, they may need all the mass savings they can get.


Again, Musk has said stainless steel allows for a *lighter* Starship
design because the thermal protection system can be much lighter. It's
an excellent design trade. You keep hand waving without providing the
evidence that your assertions are true. I trust that SpaceX ran the
numbers and Musk is reporting the results of an actual engineering
analysis when he makes this point.

Also, the goal is to lower the cost of access to space. Cheap stainless
steel allows for that far more than something "exotic" and more
expensive like carbon fiber composites with heavier thermal protection.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #15  
Old October 21st 19, 01:14 PM posted to sci.space.policy
David Spain
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Posts: 2,595
Default A conversation with Elon Musk

On 2019-10-21 7:46 AM, Jeff Findley wrote:

Also, the goal is to lower the cost of access to space. Cheap stainless
steel allows for that far more than something "exotic" and more
expensive like carbon fiber composites with heavier thermal protection.

Jeff


Jeff I have to agree with all the excellent points you made in the
previous post. Stainless steel looks to be a winner here. A key point
that is absolutely CENTRAL to the SpaceX design philosophy is the
ability to *rapidly* turn around design changes. There is plenty of
experience and skill set working stainless. The ability to manufacture
and *re-manufacture*, and *tweak* using stainless cannot be
underestimated. The techniques for working stainless and other forms of
steel have been worked through over decades. Esp. in the area of
Brownsville and the East Coast of Texas' gas and oil fields. I doubt
you'll find many construction folks out there with vast experience
working ccarbon. This is really really key.

I love the idea of polishing the outer hull of SH/SS with WD-40. LoL.

Also now would be a good time to remind everyone of another prototype
that used stainless steel. The XB-70 Valkyrie. It actually flew and had
the capability of incredible speeds for a bomber (Mach 3+). But too
costly to develop and deploy. It's function was eventually evolved into
and superseded by the slower, but more capable B1 Lancer.

https://en.wikipedia.org/wiki/North_...XB-70_Valkyrie


Dave
 




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