Future rocket engines will be combined in some manner with turbomachinery so that they can utilize the atmosphere during ascent to orbit. The great strength of the turbojet is the large static thrust that it produces. Therefore spaceplanes so-equipped would easily be able to taxi around a spaceport and take off under their own power. Turbojets are giant vacuum cleaners, sucking in enormous quantities of ambient air, compressing it and combining it with a relatively small amount of fuel. The exhaust is then jetted out the rear of the engine, usually through a subsonic nozzle, to provide the thrust. Turbojets can achieve a specific impulse of 10,000 s, a value which gradually declines with speed.
Turbofans have a larger frontal area, and use this to transfer bypass air around the compressor and mix it with the exhaust to give added thrust.
In a turbo-ramjet, a turbojet is mounted inside the structure of a ramjet, so that the integrated engine can operate from zero forward speed to ramjet speeds. This tends to reduce the efficiency because of the added weight of the turbojet, which ceases to function on its own at about Mach 3.
Turborockets, or air-turborockets, can reach Mach 5 or 6, and are much lighter than turbojets, with a better thrust-to-weight ratio, but have a low specific thrust at low speeds, and a lower specific impulse than the turbojet. By using elements of well-understood turbojet technology, the safety factors of spaceplanes will inevitably rise to the standards common in commercial aviation.
One special type of turborocket is the SABRE precooled hybrid air-breathing rocket engine, shown in Fig. 8.5, and designed by Reaction Engines Limited to power the Skylon SSTO spaceplane. The SABRE engine includes several components, each vital for the efficient operation of the turborocket: an air inlet cone, turbo-compressor, helium loop, small ramjets, and two rocket engines. An outgrowth of the earlier LACE engine designs, which actually liquefied air and separated out the oxygen component, the SABRE engine cools incoming air to just above the point of liquefaction - the vapor boundary. The reason the air is not liquefied is because in LACE engines, too much hydrogen fuel is required to power the system, and the engines become inefficient as a result. To accomplish this, a continuous liquid helium loop is used, which is itself cooled by onboard liquid hydrogen. The helium is an inert element, and therefore ideally suited for double duty as air precooler and energy supply for the onboard turbo-machinery. By cooling down the incoming air, helium is in turn heated, which gives it thermodynamic energy and the potential to do useful work. This heated helium can then turn the compressors, which further squeeze the frigid air prior to being delivered to the combustion chamber. Since the incoming airflow has already been chilled significantly by the closed helium loop, it poses little danger in terms of melting the compressors and other turbo-machinery inside the SABRE. Therefore the entire design can be made much lighter in weight. This is of vital concern in the design of any rocket engine. The two-mode SABRE turborocket can operate
as an air-breather from a standstill on the runway up to speeds of Mach 5.5, at which point the rocket engine mode takes over.11
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