Jeff Findley wrote on Mon, 10 Dec 2018
06:54:42 -0500:

In article ,
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.


Well, that would be cost/schedule. Given his wording about "heavy
metal" I don't expect it will be the 'conventional' metal. Back in
the dim past around here there was a guy who proposed using swaged
steels for booster construction of a 'Big Dumb Booster'. There was
some weight penalty over 'conventional' materials, but he calculated
that it was not as much as you might think and that construction costs
would be much lower.

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.


Perhaps, although I'd think a TPS would, well, 'P' from 'T' and not
rely on a conducting hull, which could leave you prone to heat damage.
Carbon fiber tends to be very heat resistant, which means that
replacing it in exposed structures like vehicle hull will probably
require some sort of refractory metal. Given the difference in
density between the two, going to such a metal hull would indeed be
heavy.

Musk was asked several times just which metal he was referring to and
avoided the question each and every time.


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw