Starship construction has begun

Starship preparing for a rainy launch;
Source: HumanMars

Teslarati reports that construction of the first Starship has begun:

SpaceX CEO Elon Musk says that the company’s South Texas workforce has already begun to fabricate the first orbital-class Starship prototype, while Hawthorne engineers and technicians are in the midst of performing small-scale testing of the vehicle’s unprecedented stainless steel heat shield. 

To be assembled out of hexagonal tiles of (presumably) stainless steel, Starship’s metallic heat shield will be one of the most crucial aspects of the orbital spacecraft, particularly with respect to ensuring that it’s extraordinarily easy to reuse. To survive extreme interplanetary-velocity reentry conditions at Mars, Earth, and beyond and remain in a functional, flight-ready condition after landing, SpaceX will need to implement the world’s first orbital-class, large-scale metallic heat shield with an immature technology known as transpirational cooling.

By quite literally drilling tiny holes (pores) into heat shield plates at the hottest parts of Starship’s hull, transpirational cooling effectively allows a large portion of the heat of reentry to be wicked away by the flash evaporation of a liquid, typically water. For SpaceX’s Starship, it’s likely that the coolant of choice would be either liquid water or liquid methane, the former of which offers better cooling per unit of mass at the cost of added plumbing complexity and spaceship mass, while the latter would make use of the same propellant fueling the ship at the cost of worse cooling per unit of mass. Either way, SpaceX will be heading into unproven territory, demanding extensive ground and flight testing to first ensure that the concept is truly viable and then to verify that it can be made as reliable and reusable as it simply has to be. 

Tests like those shown on March 17th by Musk indicate that SpaceX is indeed deep into the process of extensive and often destructive testing, something the company has proven to be exceptionally good at. It’s unclear if the above test of Starship’s hexagonal heat shield tiles – likely made out of a variant of 300-series stainless steel – involved tiles with active or passive cooling, but Musk suggested that the hot-spots created with spin-forming torches reached temperatures as high as 1650 Kelvin (2500ºF/1400ºC. Unlike the ablative heat shields SpaceX is familiar with building and operating through its Crew and Cargo Dragon spacecraft, a stainless steel heat shield would be expected to almost entirely resist erosion (i.e. ablation) during extremely high-velocity reentries of at least 12-14 km/s (~31,000 mph, Mach 40+).

Meanwhile, Musk also confirmed that SpaceX has decided to skip constructing a replacement nosecone/fairing for Starhopper, a functional Starship prototype that will be used to conduct extremely short hop tests like Falcon 9’s development-era Grasshopper and F9R test articles. Over the last several weeks, SpaceX technicians have been rapidly assembling what was initially assumed to be the replacement fairing Musk had previously suggested would be built. Instead, the duo of stainless steel barrel sections – relying on steel much thinner [he means thicker, which is why the new cylindrical sections are unwrinkled compared with the Starhopper ] than the heavy[light]-duty stuff used to build Starhopper – are apparently the beginnings of the first orbital-class Starship prototype, said by Musk [2 months ago] to be aiming for flight-readiness as early as June 2019.

If stacked atop each other, the two new steel sections in work would likely stretch a solid 20-30 meters (65-100 ft) tall. Meanwhile, the first orbital-grade tank domes and/or conical nose section is also in the process of being welded together out of smaller segments, already fast approaching something ready to be installed inside the steel barrel sections.

On Twitter, Musk was asked whether the Starship heatshield hex tiles passed the test.  He said, "Yes.  Full duration".  He commented that "Transpiration cooling will be added wherever we see erosion of the shield. Starship needs to be ready to fly again immediately after landing.  Zero refurbishment."

A fully re-usable spaceship?  How cool is that?  And how cheap!  It means that the cost of construction and development can be spread over hundreds or thousands of launches, instead of just one.

One aside: if SpaceX is using transpiration cooling with the test, they must be using water—the risk of using liquid methane indoors in an oxygen-rich atmosphere would seem to be too great.  Water has the disadvantage that it would freeze in space, methane the disadvantage that it would coat the heatshield with carbon.  It'll be interesting to see which they will go with.

Second aside:  the current Mars/Moon timetable is 2022—first uncrewed missions to Mars; 2023—flight around the Moon; early 2025—first crewed mission to Mars.  If  tests on the real Starship can start in June, then that timetable might even be achieved.  Note that the Starship can get into orbit even without the 'Super Heavy' booster, if it carries no payload.  So orbital tests will be theoretically possible from June onwards.  That'll be a sight to see.

Third aside: compare how long it has taken SpaceX to pivot from a carbon-fibre composite model to the stainless steel one with how long NASA's SLS (Space Launch System) has taken, and how very expensive the latter has been so far.