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Introduction to Hypersonic Air-Breathing Propulsion

We begin a study of hypersonic air-breathing propulsion systems, engines that take the oxidizer from the surrounding atmosphere and propel vehicles to sustained speeds greatly in excess of the local speed of sound, higher than Mach 5. Hypersonic flight with air-breathing propulsion is pursued for its potential to realize cost-effective access to space and high-speed cruise. Applications include civil transports, scramjet-powered missiles to fly in the Mach 6-8 range, both tactical and strategic, single-stage space planes, and multiple-stage-to-orbit vehicle configurations. Renewed interest in hypersonic sustained flight has increased research and development activities and made substantial advances in required technologies. The remarkable performance improvements promised by high-speed air-breathing propulsion were brought within our reach by the recent development of technologies related to scramjet engines and by their demonstration in flight. Figure 1.1 depicts an artistic view of NASA's X-43A aircraft that flew at Mach 7 and 10 to demonstrate the viability of hydrogen-fueled scramjet propulsion.

Hypersonic air-breathing propulsion is based on ramjets and scramjets, the simplest jet engines to propel a vehicle to hypersonic speeds within the atmosphere. These engines have no internal moving parts, as they do not require turbomachinery (mechanical compressor/turbine) to process the ingested ambient air.

The ramjet engine has three main components: an inlet, a combustion chamber, and a nozzle. The dynamic action of the freestream air is used to produce the compression in the inlet as the vehicle flies at high speed. This action is referred to as the ram effect. The higher the velocity of the incoming air, the greater the pressure rise. The fundamental principle underlying ram compression in the ramjet inlet lies in converting the kinetic energy of the air into pressure. The compressed air then enters the combustion zone where it is mixed with the fuel and burned. The hot, high-pressure gas flow then accelerates back to a supersonic exit speed through the nozzle to develop thrust.

The most distinctive feature of the ramjet is that combustion of fuel with air takes place after the flow has been slowed internally to subsonic speeds. Moreover, the air flow is compressed in several steps, including passing through one or more oblique shock waves generated by the forebody of the vehicle or of the diffuser, deceleration of the supersonic flow in a convergent duct, transforming the supersonic flow into subsonic flow through a normal shock wave system, and further decelerating the subsonic flow in a divergent duct. Ramjets are suitable for applications where the flight Mach number is in the range 3-5 and are used mainly for supersonic flight.

Figure 1.1 Artistic rendition of scramjet-powered hypersonic cruiser.

Source: NASA.

When the flight Mach number exceeds about 5, deceleration of the ingested airflow to subsonic conditions would cause it to reach unacceptable high temperatures. To extend the flight regime above Mach 5, the scramjet was conceived. In this type of ramjet, the hypersonic inlet airflow is diffused only to supersonic speed prior to mixing it with fuel in the combustor. Hence, the combustion process takes place at locally supersonic conditions. The engine operating in this mode becomes a scramjet, an acronym standing for "supersonic combustion ramjet," a name used to emphasize that the combustion of fuel and air must occur in a supersonic flowfield relative to the engine. Scramjets in fully supersonic combustion mode begin to produce thrust flying at speeds of at least Mach 4 and would operate as long as there is sufficient air to pass and process through its inlet; the theoretical maximum operational speed for scramjets is unknown, but it could effectively reach about Mach 12.

There is a wide range of speed and altitude over which air-breathing propulsion is capable of higher specific impulse (I sp ) than is rocket propulsion. The I sp parameter indicates how much thrust the engine produces per every unit mass of propellant (fuel plus oxidizer) it uses per second. Since the air-breathing engine does not need to carry oxidizer on board, its specific impulse is much higher than that of the rocket. Scramjets are therefore the most efficient air-breathing engines, that is, with the highest (fuel)-specific impulse, at flight Mach numbers above 5. To capitalize on such advantage, much effort has been devoted to developing hypersonic air-breathing propulsion (HAP) systems to achieve hypersonic flight within the Earth's atmosphere. One such HAP concept is the dual-mode scramjet (DMSJ), an engine that operates both as ramjet (subsonic combustion) and as scramjet (supersonic combustion) in order to propel a vehicle in a flight Mach number ranging from 3.5 to 12 (the upper limit in scramjet operational Mach number is still unknown). However, due to its minimum functional speed, scramjets require acceleration by other means in order to become operational for takeoff.

For some military applications, air-breathing hypersonic vehicles can be air or ground-launched attached to a rocket motor that will accelerate the craft to the take-off speed of the scramjet. Other applications require to integrate the scramjet engine with a low-speed propulsion system (e.g. turbojet, turbofan) in order to provide the capability of propel a vehicle from the runway all the way to its maximum hypersonic speed.

Powered by scramjet engines, hypersonic vehicles scoop the oxygen required for fuel combustion from the atmosphere, and this reduces tankage requirements and airframe mass. In fact, for missile propulsion, the ramjet is very competitive with the rocket because it is simple in construction and has greater range for the same propellant weight. These characteristics are particularly attractive for military applications where simplicity and low initial cost are essential features of devices that must function on demand and never return. Moreover, hypersonic vehicles propelled by air-breathing propulsion promise affordable and rapid access to space and hypersonic cruise. Scramjet propulsion flight demonstrator programs (e.g. X-43A, X-51A, HIFiRE) have already proven that HAP vehicles are technically feasible. However, more flights and flight-test programs are required to demonstrate sustained cruise and acceleration to establish the DMSJ engine as a viable and mature hypersonic air-breathing propulsion system.

Moving at hypersonic speeds, a vehicle will naturally generate a massive amount of heat that must be properly managed. The vehicle and its integrated propulsion system must be fabricated with advanced materials designed to withstand those high temperatures, materials with high strength, and high toughness. Hypersonic vehicles travel very fast, getting hot enough to melt most traditional metals, so engineers are developing new material formulations for hypersonic craft to survive such harsh environment.

This book intends to provide the technical background to describe the fundamental characteristics of high-speed air-breathing engines, focusing on the technologies that are being developed to advance the DMSJ to power future hypersonic flight.

1.1 Hypersonic Flow and Hypersonic Flight

For air-breathing propulsion, hypersonic flight is interpreted to mean flight speeds V 0 higher than five times the speed of sound, that is,

where M 0 denotes a vehicle's flight Mach number and a 0 is the local speed of sound.

For the analysis of hypersonic air-breathing propulsion, we can define hypersonic flow as the regime where the calorically perfect gas model for air becomes invalid. For calorically perfect gas or temperatures less than 400?K, the specific heats c p and c v are constant. As the air temperature increases, in the range of temperature 400 K?<?T?<?1700 K air behaves as a thermally perfect gas, the value of the specific heats is function of temperature; and thus the specific heat ratio (? = c p /c v ) is also a function of temperature.

At temperatures above 1700?K (3000?°R), the equilibrium of specific heat (c p ) of air depends strongly upon both temperature and pressure because chemical reactions have become important. Hence, ? reaches a value of 1.286 when the formation of nitric oxide (NO) begins. When nitrogen is released during combustion, it combines with oxygen atoms to create NO, which then combines with oxygen to create nitrogen dioxide (NO2). At temperatures above 1700?K, chemical reaction and dissociation become very complicated and cannot be treated with a simple gas model.

We can also consider the value of the freestream total or stagnation temperature that would cause real gas effects to occur. Let us consider the total to static temperature ratio,

where T t0 is the freestream total temperature, M 0 is the flight Mach number, and the subscript 0 denotes the undisturbed freestream flow conditions far ahead of the vehicle as seen from the reference frame of the vehicle. When we substitute a representative value...