The turbojet, turboramjet, and turborocket are air-breathing engines. The minimum mass is 10 tons. Turbofan and turboprop fly at subsonic velocities only, so they are not suitable for space transportation.
M. Minoda, K. Sakata, T. Tamaki, T. Saitoh, and A. Yasuda, "Feasibility Study of Air-Breathing Turboengines for Horizontal Takeoff and Landing Space Planes," Journal of Propulsion and Power, Vol. 7, No. 5, September-October 1991, pp. 821-827.
Giuseppe Bussi, Guido Colasurdo, and Dario Pastrone, "Analysis of Air-Turborocket Performance," Journal of Propulsion and Power, Vol. 11, No. 5, September-October 1995, pp. 950-954.
A continuous stream of air is compressed by a turbocompressor, mixed with fuel, ignited, and expanded through a turbine. The turbocompressor and the turbine are on the same shaft. The best engines have the thrust-to-weight ratio of about 10. Turbojet was invented by Frank Whittle of Great Britain in 1928.
The ramjet is one of the best air-breathing engines, but it cannot generate thrust at takeoff. A turbojet is placed inside the ramjet to accelerate the ramjet to operating speed (about 1 km/s). The contraption is called turboramjet.
The turborocket is heavier than the turbojet but performs better at high velocity. The turborocket turbine is powered by a gas generator exhaust. A heat exchanger in the combustion chamber heats up hydrogen fuel before it reaches the gas generator. The turborocket is sometimes called "air-turborocket" or "gas generator cycle air-turboramjet."