After a flawless reentry and splashdown on August 27th, SpaceX’s first thrice-flown Cargo Dragon spacecraft completed its latest mission, arriving in Port of Los Angeles aboard SpaceX vessel NRC Quest.
The successful completion of NASA Commercial Resupply Mission 18 (CRS-18) means that SpaceX is officially the first and only company to launch the same orbital spacecraft three times. Meanwhile, Cargo Dragon capsule C108 also happened to mark the first known orbital flight test of hardware that may be destined for use on SpaceX’s next-generation Starship launch vehicle, taking the shape of four ceramic tiles installed as part of its ablative PICA-X heat shield.
Cargo Dragon’s CRS-18 mission successfully lifted off on its way to the International Space Station (ISS) on July 25th and was berthed to the ISS roughly two days later, completed its delivery of several tons worth of cargo. During the launch webcast, one of the SpaceX hosts noted that black tiles visible on Cargo Dragon’s heat shield – distinct beside its silvery water-sealed PICA-X tiles – were prototypes of a ceramic heat shield material being analyzed for possible use on Starship.
CEO Elon Musk confirmed this after the first launch attempt was scrubbed by weather, stating that SpaceX was looking into the use of “thin [ceramic] tiles” to protect Starship’s windward (atmosphere-facing) half during orbital reentries. Prior to this development, Musk had proposed and posted videos of real-world tests of a steel Starship heat shield concept, in which extra energy could be wicked away by ‘transpiring’ liquid oxygen or methane through microscopic holes on each tile’s leading edge.
Although particular species of stainless steel do feature exceptionally high melting points and structural characteristics at ultra-high temperatures (> 1400C/2500F), some unofficial analyses of the numbers involved indicated that the density and weight of steel could rapidly hinder any benefits derived from its use as a heat shield. Musk appeared to confirm this in his July 24th comments, indicating that thin ceramic tiles on the windward side and nothing on the leeward side of Starship looked like the “lightest option”.
Indeed, ceramics were so prevalent on the Space Shuttle – the only semi-routinely reusable space plane ever developed – in large part because they can be made spectacularly light. The Shuttle’s main ceramic tiles had a density of 155 kg/m³ (9 lb/ft³), about five times denser than styrofoam or roughly the same density as freshly-fallen snow and balsa wood. Stainless steel is about 50 times denser, on average. To use Musk’s own 2017 turn-of-phrase, adding thick steel tiles to Starship’s already-steel skin was probably a bit too much like “building a box in a box”, whereas prioritizing ceramic tiles presumably cuts the shield’s mass by a factor of something like 20-100+.
Although the Shuttle did make extensive use of ceramic shielding, that shielding – specifically, reinforced carbon-carbon (RCC) tiles about as fragile as the material people are familiar with – and a mixture of organizational ineptitude infamously lead to the death 7 NASA astronauts and was generally a nightmare to deal with. SpaceX certainly won’t have to deal with the foam and solid rocket boosters that a lot of Shuttle’s ceramic problems can be traced to, but the company will likely be laser-focused on producing a form of ceramic shielding that isn’t nearly as fragile as Shuttle-derived materials.
The fact that Cargo Dragon’s ceramic Starship tile prototypes appear to be almost completely unscathed after their first orbital reentry is an excellent sign that SpaceX is making progress in the materials design and certification department, or is at least taking flight-testing extremely seriously.
SpaceX CEO Elon Musk is expected to provide an official update on Starship no earlier than late September, a presentation that will likely include details about the route the company is taking with the massive spaceship’s heat shielding.
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