Performance Factors

In discussing the different objectives of propulsion, several key words or concepts arise on a regular basis, namely, acceleration, speed, endurance, and range. Because these concepts can be quantified, they provide “performance factors” that can be used to evaluate how well a propulsion system meets its objectives. To clarify this point we will spend a moment reviewing how these performance factors relate to propulsion objectives and will briefly introduce the key parameters that control each performance factor.

Speed is how fast something is moving (how quickly its position changes in time). High speed is important for getting from point A to point B quickly (transportation) and outrunning predators during a long chase (elusion). High speed is obtained when the resistance to motion (drag) is low and the force causing the animal or vehicle to move (thrust) is large.

Acceleration is the rate of change of velocity, and it determines how quickly a high rate of speed can be obtained. Rapid acceleration is important for pursuing and intercepting prey or an enemy threat, and for eluding a predator. To accelerate quickly, it is important to generate a lot of thrust and have a low mass. This is also known as having a high thrust-to-mass ratio.

Endurance indicates how long motion can be sustained. Usually (but not always) endurance refers to how long an animal or vehicle can keep moving at a constant velocity (known as cruise operation). Endurance is important for migration, hunting, and eluding predators. Endurance depends on how much energy is available (as fuel or food) to generate thrust and how efficiently the energy is used by the propulsion system.

Range is how far an animal or vehicle can go without eating or refueling. Having a large range is important for shipping, migration, and some hunting and intercept activities. A large range requires high endurance coupled with high speed.

Example Problem: Fuel Economy

Maneuverability is another factor mentioned frequently (especially in regard to predators and prey), but maneuverability is determined primarily by special devices (such as fins fish or control surfaces on airplanes) used for turning the vehicle or animal and not by the propulsion system itself. Exceptions to this rule are squid and the F-22, both of which vector thrust from the propulsion system to enhance maneuverability. Because maneuverability is not directly related to the propulsion system in most cases, we will not discuss it in detail.

The key parameters (thrust, drag, efficiency, etc.) that control the performance factors are all governed by fundamental laws of physics as described in the Principles section. The laws are the same regardless of what is being propelled or the propulsion system used. Different methods are available, however, for utilizing these laws to generate propulsion. The result is a wide variety of propulsion system designs used in both nature and engineering. Different designs that utilize the laws of physics in different ways to achieve different objectives are discussed in the Design section.