Introduction¶
This class is focused on propulsion engines for aerospace applications. In any propulsion engine, the primary goal is to provide a net force on the craft to accelerate it or to balance an opposing force to maintain a constant speed. This force is commonly called the thrust and is generated by changing the momentum of a fluid as it passes through the propulsion engine.
The change of fluid momentum is due to the forces acting on the fluid, according to Newton’s second law. Newton’s third law says that the fluid provides an equal and opposite force on the boundaries of the flow, thus producing the desired thrust force. These kind of propulsion engines are therefore also called reaction motors and flow machines.
There are a number of common ways of characterizing propulsion engines. First, we can categorize the engine based on whether it requires fluid from its environment, or whether the propulsive fluids are carried entirely onboard the craft. The former type of engine is typically called air-breathing and the latter is a rocket.
Another way to characterize propulsion engines is by the types of thermodynamic processes they undergo. In this sense, we can categorize based on whether the fluid acquires a net work input, a net heat transfer input, or both. The types of engines we will discuss in this class are shown in Table 1.
Type |
Engines |
|---|---|
Net Work Input |
Propeller |
Net Heat Transfer Input |
Turbojet, Ramjet, Scramjet, Pulsejet, Rocket |
Both |
Turbofan |
All of the devices that use a net heat transfer input commonly on chemical energy, released in the form of combustion, to operate. For space-based applications, other options are available to energize the propellant. Nuclear thermal propulsion uses the energy from a nuclear reactor to heat the propellant, while resistojets use electrical resistance to heat the propellant, and arc jets use a plasma arc to do the same. Finally, devices that accelerate ions or molecules tend to be very high efficiency but very low thrust, and are only suited for long-term use in space.
Overview of the Material¶
In the next few sections, we will review the fundamental equations relevant to propulsion engines. Following that, we will discuss each of the propulsion devices listed above at a high level to determine their basic performance characteristics and compare them. Then, we will dive in to detail of each of the devices.