SpaceX's futuristic spaceship Starship can epitomize the golden age of science fiction in a number of ways: in addition to (theoretically) taking passengers from planet to planet, there can be a shiny stainless steel skin that makes it look like the pulp covers of yore.
The founder and CEO Elon Musk has shown the possibility in a picture posted on Twitter under the motto "Stainless Steel Starship". This is a full scale spaceship. This is just part of a test vehicle that the company intends to use to evaluate various systems during the short "hopper" flights in 201
As with most Musk tweets, this triggered a storm of speculation and contention on Twitterverse.
19659002] The choice surprised many, because modern spaceflights have relied for years on advanced composite materials such as carbon fiber, which combine desirable physical properties with low weight. When metal is needed, aluminum or titanium are much more common. While some takeoff components, such as the upper stage of the Atlas 5 rocket, have used plenty of steel, this is definitely not an obvious choice for a ship like the Starship, which has to deal with both space and repeated reentries.
As Musk pointed out in later comments, however, stainless steel has some advantages over other materials in extremely hot or cold temperatures.
This is a special full-hardness steel alloy, referred to as the 300 series of high-strength, heat-resistant alloys – not the rich, supple stuff that we all have in our kitchens and buildings. Musk also mentioned another "superalloy" called the SX500, developed by SpaceX's metallurgists for use in Raptor engines that will propel the vehicle.
Why stainless steel? It is most likely all about reentry.
Many vehicle and reusable stages that are exposed to the heat of entry into the atmosphere at high speed use an "ablative" heat shield that dissolves in a controlled manner and dissipates heat from the vehicle.
This is unlikely to be an option for Starship, as replacing and repairing this material will require downtime and crews wherever and whenever it lands, and the ship should (possibly) be a fast reversing ship with maximum reusability. Heat shielding that reflects and survives is a better choice – but a huge design problem.
Scott Manley has put together a beautiful video that illustrates some of these ideas and speculations in detail:
Musk said before the Starship (then still as BFR ) "Almost always when it comes back, it only tries to brake while the force is distributed over the largest possible area." Reentry will likely look more like a Space Shuttle-like glider than the ballistic descent and the engine braking of a first-stage Falcon 9.
The conversion to stainless steel has the pleasant side effect that the vehicle looks really cool – more in line with it Sci-fi books and comics when their readers might ever hope. Painting would burn off immediately, Musk said:
One can not expect it to remain shiny for a long time; It may be stainless, but like a pan you left on the stove, stainless steel can still burn, and the base of the starship will probably look rather rough after some time. It's all right – spacecraft developing patina are a charming development.
There are few details, and from what we know, SpaceX could redesign the vehicle based on the test results. Next year, the first hopper flights will be for Starship hardware, and possibly the Super Heavy sub-level, which will lift its great luster from the lower atmosphere.
Musk's promised technical documentation should arrive in March or April, but whether it will solely refer to the test vehicle or take a look at the spacecraft SpaceX intends to send to the moon, one can only guess. In any case, you should assume that more information will spontaneously be revealed at the discretion of Musk – or the absence thereof.