A surface transformed by fire
At Baikonur Cosmodrome, launch pads endure some of the most extreme conditions on Earth. When a Soyuz rocket ignites, its engines release a torrent of superheated gas. The exhaust plume from its kerosene and liquid oxygen (LOX) propellant can reach temperatures of 3,500 K (approximately 3,200°C). This thermal shock is inflicted upon the launch structure, particularly the massive concrete flame trench designed to deflect the exhaust. Over decades of repeated launches, this extreme environment has physically and chemically altered the launch pads, creating a unique, self-protective armor.
The most famous of these structures, Gagarin's Start (Pad 1/5), was first used in 1957 and hosted over 500 launches before its final mission in 2019. The pad's flame trench—a concrete channel more than 43 meters deep—is subjected to intense heat and the force of corrosive gases moving at supersonic speeds. The surface of the heat-resistant concrete does not erode. The top layer melts under the thermal assault and then rapidly cools after the rocket ascends. This process, vitrification, creates a glassy, amorphous silicate layer. This dark, obsidian-like coating seals the porous concrete beneath it, protecting it from moisture intrusion and the damaging effects of future launches. It is an unintended and effective form of large-scale material hardening.
The science of refractory concrete
Concrete used in launch facilities is not a standard construction mix. It is a specialized formula known as refractory concrete, designed to withstand extreme temperatures. These mixtures often use cements with oxides that absorb heat less readily and exhibit reduced thermal expansion, minimizing the formation of microfractures. Even so, repeated exposure to temperatures that can melt steel causes cumulative damage. When moisture trapped in the concrete's pores turns to steam, it can expand violently, causing the surface to break apart in a process called spalling.
The vitrified layer at Baikonur mitigates this spalling. By forming a non-porous barrier, it prevents moisture from seeping into the concrete substrate between launches. Material analysis shows this glassy shield is composed primarily of melted silicates from the concrete, but it also incorporates residual carbon from unburnt kerosene fuel and other trace elements from the rocket exhaust. This gives the surface its characteristic dark coloration. While engineers at other spaceports often repair launch pad damage with specialized ceramic epoxy coatings, the pads at Baikonur have, in a sense, learned to repair themselves. The very force that seeks to destroy the structure is what forges its protective shield.