Starship

Stainless steel Starship at Boca Chica, photo by Maria Pointer

SpaceX's new names for the BFR (Big Frigging Rocket) and BFS (Big Frigging Spaceship) is Starship for the spaceship and "Super Heavy" for the booster.  Along with the new name, SpaceX has also pivoted away from carbon fibre composites to .... steel.  But wait a minute, steel might be strong as, but isn't it too heavy?  Well, it has some big advantages over carbon fibre composites.

From Teslarati:

Taking to Twitter once more to reveal additional information about SpaceX’s radically changing BFR/Starship prototype program, CEO Elon Musk has made an unexpected visit to the company’s South Texas BFR testing facilities to oversee rapid progress with a low-fidelity hop test prototype of the orbital Starship spacecraft.

Included in a handful of replies that followed the tweeted image, Musk described some of the key advantages of a stainless steel Starship – including mirror-like thermal reflectivity for hot reentries and a “usable” strength-to-mass ratio superior to carbon fiber.

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 [in fact, he said, slightly worse at room temps]. 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.

[Read more here]

Scott Manley has an interesting video on the topic here.  He points out that the switch is most likely due to the need to deal with the heat of atmospheric reentry.  With a steel body, external heat-shielding isn't needed.  Instead there is internal heat-shielding, or components to cool the steel skin, such as using the fuel at cryogenic temperatures.  Also, the shiny, mirror-like steel will be very effective in reflecting the heat generated by re-entry.  Essentially, the heat is generated by compressed air in front of the spaceship and this is the radiated onto the skin of the spaceship.  Reflecting the heat outwards may be as effective as a heat shield which absorbs the heat.

The test vehicle will be as wide as the real Starship (10 metres), but much shorter.  It will start hopper tests from Boca Chica (SpaceX's new facility in southern Texas) before the end of April.

Musk's style is so different to a bureaucracy's.  One can't imagine that NASA or any of the other space agencies and private sector would pivot so quickly to a new technology, and not be wedded to the way things have always been done.  SpaceX will know within a few months whether this change will work.  The money it's spent on it will be a fraction of what a large bureaucracy would spend.  If successful, it will usher in a new era of very cheap space flight and an expedition to Mars.  If it fails, no one can doubt that Musk and SpaceX will go back to the drawing board and come up with something better.  Truly remarkable.  And in the meantime we can look forward to the first videos of the Starship's test flights.