Elon Musk could use metal hot structures for rocket re-entry. This is a decades old concept. I do not think it has been used in flight since the early X-plane program days. There has been more lab and ground experiments on the concepts.
This could be another bold technology choice for SpaceX to once again raid the NASA warehouses of promising but underdeveloped technology. The X-planes did not reach orbit and did not re-enter from orbit. Elon Musk and SpaceX plan to have the Super Heavy Starship re-enter Earth and Mars at far higher speeds than have been tested before. These demands will put more stress and heat on the rocket.
Elon has tweeted that the redesigned Super Heavy Starship (aka BFR) would use a lot of metal in the design. He said it would be a radical counter-intuitive design. He mentioned that the tanks, airframe and heatshield were redesigned but the shape or counters would be roughly the same. In September when Elon was announcing the Japanese billionaire, he showed simulations showing the BFS re-entering unpowered like a human parachutists before the parachute opens. SpaceX Starship needs to come in fast, unpowered and survive for the planned thousands of re-uses with ideally no maintenance.
I am going to make the guesses on the material. I thnk Titanium alloy or nickel alloy for the hot metal structure will be used in the Super Heavy Starship. We will probably get confirmation in January 2019 when pictures and information could be released.
The new design is metal
— Elon Musk (@elonmusk) December 9, 2018
Fairly heavy metal, but extremely strong
— Elon Musk (@elonmusk) December 9, 2018
Previously Elon talked about changing the airframe, tanks and heatshield.
Contour remains approx same, but fundamental materials change to airframe, tanks & heatshield
— Elon Musk (@elonmusk) November 25, 2018
Metal Hot Structure for Re-entry
During re-entry a blunt body about 90% of friction heat is carried away by the bow shock wave and only 10% of the energy would reach the spacecraft.
All the reusable heat shields have had insulation behind the heat shield. The airframe of the rockets have been light-weight aluminum. The thinking was to protect the aluminum from heat but carry the weight of the insulation.
What was considered in the when the Space Shuttle was being designed was to make the airframe of the rocket with high-temperature alloys. There would be far less insulation to protect the crew. The main structure would reach about 1,000 degrees Celsius. There would be more mass to re-radiate away the heat.
X-15 was the fastest manned plane ever built
Paul Werbos has been a proponent of the Boeing hot structures system designs. Ray Chase of ANSER did a lot of materials testing for hot structures.
It has been discussed on the Power Satellite Forums and on the Lifeboat Foundation mailing lists.
SpaceX Used Pica Materials for Their Heat Shield Already
Elon Musk and SpaceX did rapidly develop their heat shields using Pica materials. The Pica materials was virtually unused in flight before SpaceX began relying heavily upon them.
Elon Musk and SpaceX could again take concepts that have been floating around NASA for decades and then use them heavily and perfect them.
Elon Musk Will Leverage His Thermal Protection SpaceX Skunk Works and the 51% Rule of Testing
Skunk Works at SpaceX for Thermal Protection
SpaceX built an entire lab for thermal protection system research for re-entry vehicles. It became the best lab in the world in nine months starting from an empty room. They use rapid development cycles based on processes used for rapid fighter pilot reaction.
The OODA loop uses
observe
orient
decide
act
The faster a fighter pilot could through this loop of iterations, then the more likely they were to win a dogfight.
SpaceX engineers working 80-hour weeks don’t have to iterate between themselves and another engineer to get effective use of their time. Very fast engineering reaction also means avoiding meetings with managers or for review unless absolutely needed.
The Skunk Works was the small team at Lockheed that famously made the huge advances to make the SR-71 mach 3+ spyplane.
51% Experimentation Rule
SpaceX will test once they have a 51% chance of success. They will take the least amount of material that will get them valid results. NASA would wait until they had 80-90% chance of success and there would be lot more meetings about the work or process.
This more rapid testing leads to more rapid development.
Hot Metal Re-Entry Background Research
This has been considered with ceramics and other materials over the years.