On Monday, 17 December 2018 at 07:56:31 UTC+5:30, Robert Clark wrote:

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"Jeff Findley" wrote in message news:MPG.36781e113...@news.eternal-september.org...

In article <d9vr0edt8do0rbil4...@4ax.com>,
fjmc...@gmail.com says...

I saw a report today that SpaceX was dropping composites for tanks and
main structure on BFS in favor of using "heavy metal" (Musk's phrase).

He Tweeted about it in replies to Everyday Astronaut. https://twitter.com/elonmusk/status/1071578086418788352

From above:

Everyday Astronaut: their new Starship and Super Heavy will be all
carbon composite (mostly)

Elon Musk: The new design is metal

Elon Musk: Fairly heavy metal, but extremely strong

Everyday Astronaut: It seems like you?ve made some really big changes recently. Is this why we didn?t get a big technical rundown at #dearmoon
or IAC this year? So what?s with the big mandrels at the port? Or those carbon tanks shown off for the past couple years?

Elon Musk: Yes

Malcom Head: Is Super Heavy in development at all, or just starship
right now?

Elon Musk: Both, but demo Starship is being built now, whereas Super
Heavy hardware will start getting built in spring

All articles I've seen afterwards don't contain any new information,
just speculation beyond the original Tweets which I quoted above.

It's unclear why the change, although it's probably a cost/schedule
move. It's also unclear if this change is permanent or if it's only
for early ships. Also unclear what impact this will have on payload,
since BFR will now be lifting a much heavier BFS than was originally planned.

Could be they decided composites posed too much technical risk, so
they've retired that risk by switching to a more conventional metal
design.

All these changes are both bad and good. They're bad because of
impacts to weight and such. They're probably good in that they
indicate that we're close to actual development and that is driving changes.

I'm also wondering if the changes had to do with the thermal protection scheme for BFS/Starship. The nice thing about metal is that it conducts
heat better than a carbon fiber composite would. That might be an
advantage during reentry.

Jeff
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Actually some new metal alloys are even better on strength-to-weight than carbon composites. Elon had referred to the change to metals as counter-intuitive. I think this is because the prevailing view was carbon composites had to be used for rockets to save weight. See discussion here:

https://exoscientist.blogspot.com/2018/06/darpas-spaceplane-x-33-version-page-2.html

You are right though that the metals have the additional advantage of having better heat resistance. This would mean reduced weight for thermal protection.

Bob Clark

----------------------------------------------------------------------------------------------------------------------------------
Carbon nanotubes can revolutionize 21st-century technology IF they can be made arbitrarily long while maintaining their strength.
Some proposals to accomplish that here:
From Nanoscale to Macroscale: Applications of Nanotechnology to Production
of Bulk Ultra-Strong Materials.
American Journal of Nanomaterials.
Vol. 4, No. 2, 2016, pp 39-43. doi: 10.12691/ajn-4-2-2 | Research Article. http://pubs.sciepub.com/ajn/4/2/2/ ----------------------------------------------------------------------------------------------------------------------------------

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