Tsiolkovsky conceived a number of ideas that have been later used in rockets. They include: gas rudders (graphite) for controlling a rocket's flight and changing the trajectory of its center of mass, the use of components of the fuel to cool the outer shell of the spacecraft (during re-entry to Earth) and the walls of the combustion chamber and nozzle, a pump system for feeding the fuel components, the optimal descent trajectory of the spacecraft while returning from space, etc. In the field of rocket propellants, Tsiolkovsky studied a large number of different oxidizers and combustible fuels and recommended specific pairings: liquid oxygen and hydrogen, and oxygen with hydrocarbons. Tsiolkovsky did much fruitful work on the creation of the theory of jet aircraft, and invented his chart Gas Turbine Engine. In 1927, he published the theory and design of a train on an air cushion. He first proposed a "bottom of the retractable body" chassis. However, space flight and the airship were the main problems to which he devoted his life. Tsiolkovsky had been developing the idea of the hovercraft since 1921, publishing a fundamental paper on it in 1927, entitled "Air Resistance and the Express Train" (). In 1929, Tsiolkovsky proposed the construction of multistage rockets in his book "Space Rocket Trains" ().