Course Objectives

Thrust and Engine Design

Momentum of incoming air is per unit time.

Momentum of outgoing air is per unit time.

By Newton's 2nd Law of Motion, force is equal to the rate of change of momentum. The thrust is the reaction to this force, acting in the direction opposite to the increase in momentum.

As shown above, the thrust of a jet engine is given approximately by

, the increase in momentum of the propellant,

where is the mass of propellant flowing out of the engine per unit time, and is the increase in velocity of the propellant through the engine. Thus if a jet engine takes in (and expels) 100 kg/s of air, and increases its velocity from 200 m/s to 300 m/s, its thrust is approximately 10,000 Newtons (1020 Kgf, or 2248 lbf). This equation is surprisingly useful for jet engines, where the contribution from "pressure thrust" is small or zero, as we will see later, and the fuel mass flow rate is only about 1 to 2% of the air mass flow rate.

Thus, we can design engines to either

a) accelerate a small amount of air through a large velocity increment(examples: rockets, ramjets, turbojets), or

b) accelerate a large through a small(examples: turbofan, turboprop, propfan )

 Approach Advantages Disadvantages Typical Applications (a) small , large 1. small engine area & weight. 2. better high-speed performance 3. better high-altitude performance 1. less efficient conversion of thermal to kinetic energy 2. noise 3. jet blast rockets, ramjets, high-altitude flight; turbojets, low-bypass turbofans. (b) large , small 1.High efficiency 2. Better low-altitude performance 3. Better performance below Mach 1. 4. Low noise 1. Large engine diameter 2. More complex engine 3. Slower acceleration Commercial aircraft: high-bypass turbofans, propfans, turbojets.

In the following sections, we see the factors which determine the maximum or "ideal" efficiency of a given design: this can be calculated from basic thermodynamics if we know a very few crucial parameters of the design, without worrying about the precise shape or complexity of a particular engine.

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