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takyon writes:

SpaceX's Starship and Super Heavy (formerly Big Falcon Spaceship and Big Falcon Booster, or Big Falcon Rocket) have undergone further changes following a "final" iteration of the design in September. Elon Musk also said that a downscaled Starship hopper (for vertical takeoffs and landings) will "hopefully" be tested starting in March or April 2019, which is months sooner than a "late 2019" estimate made by SpaceX CEO Gwynne Shotwell in September.

Recent photos taken of SpaceX's operation in Boca Chica, Texas have shown a stainless steel nose cone being built. The new stainless steel design was confirmed by Elon Musk, along with numerous other details. Musk said that stainless steel can beat carbon fiber composites due to its superior strength-to-mass ratio and "mirror-like" thermal reflectivity. SpaceX is using an on-site foundry to create its own steel "superalloy", although some steel parts will be made by a supplier. Finally, the test hopper will feature three "radically redesigned" Raptor engines while being slightly shorter than the full-scale Starship, although it will share the same 9-meter diameter:

While the suggestion that Raptor's turbopumps (basically fuel pumps) would need at least 100,000 HP per engine seems to indicate that the flight design's thrust has been appreciably uprated, a past figure of ~2000 kN (450,000 lbf) per engine suggests that Starship V0.1 could weigh as much as an entire Falcon 9 Block 5 rocket (~1.2 million pounds, 550,000 kg) and still having a solid 80-100% of Falcon 9's liftoff thrust. Put simply, the rocket that appears to be coming together in the boonies of South Texas could rival almost any other liquid fuel rocket booster in service, while still being the testbed for BFR's upper stage alone.

While it's ambiguous if several additional comments applied to the Starship prototype, the final product, or both, Musk also indicated that some of the biggest benefits of a shift away from carbon composites to stainless steel would be relative ease with which the material handles extreme heating. Thanks to the fact that stainless steel can ultimately be polished to mirror-like levels of reflectivity and that mirrors are some of the most efficient reflectors of thermal energy (heat), shiny and unpainted steel would ultimately perform far better than carbon composites and could end up requiring "much less" heat shielding for the same performance.

Perhaps most unintuitive is the fact that steel can apparently beat carbon composites when it comes to usable strength-to-weight ratios at supercool temperatures. According to Musk, steel also performs "vastly better" at high temperatures and appreciably better at room temperatures. A comment made on Saturday may lend additional credence to what seems at face value to contradict basic material intuition – at least some of the stainless steel SpaceX is examing would be a special (presumably SpaceX-engineered) alloy that has undergone what is known as cryogenic treatment, in which metals are subjected to extremely cold conditions to create some seriously unintuitive properties. Ultimately, cold-formed/worked or cryo-treated steel can be dramatically lighter and more wear-resistant than traditional hot-rolled steel.

Elon Musk hinted at a "delightfully counter-intuitive" redesign in November, which was almost certainly a reference to the use of stainless steel instead of carbon fiber composites. Here's a video (10m14s) which offers some speculation about how a steel Starship could effectively conduct and radiate away heat.

Also at Business Insider.

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