Why do planes not have rocket engines?


Lexicon> letter T> engine

Definition: a drive unit, usually for an aircraft or a rocket

More specific terms: jet engine, rocket

English: engine

Categories: Basic Concepts, Power Machines and Power Plants

Author: Dr. Rüdiger Paschotta

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Original creation: January 19, 2020; last change: 08/20/2020

URL: https://www.energie-lexikon.info/triebwerk.html

Not everything that is sometimes referred to colloquially as an engine technically falls into this category.

An engine in the broader sense is a machine that is used to drive a vehicle or other object. In a narrower sense, however, the term is restricted to drive units that generate propulsion without direct interaction with solid objects - for example, through interaction with a fluid such as air or water. In this sense, for example, is a car engine no Engine, as it drives wheels that interact with the solid ground. The same applies to the drive of a cable car or a freight elevator. On the other hand, engines in the narrower sense include the propulsion units of aircraft and spacecraft, for example jet engines, rocket engines and ion engines, all of which function on the principle of recoil by the expulsion of gases and therefore do not require a firm contact with the earth's surface. The same applies to ship propulsion systems if the devices for power transmission to the water (such as a ship's propeller) are included.

A fuel is what is used as an energy carrier by an engine.

Engines are usually operated with chemical energy, mostly from an entrained liquid substance that is referred to as fuel (and sometimes also as fuel). On the other hand, the operating materials of engines that do not directly produce thrust, but deliver mechanical power (e.g. via a shaft), are better referred to as fuels.

Most engines generate thrust (propulsion, propulsive power) by ejecting hot exhaust gases (and possibly also cold air) against the direction of flight at high speed. The exhaust gases are created either by the combustion of the fuel with oxygen in the air (air-breathing engines) or (in the case of a rocket engine) through the chemical reaction of the fuel with an oxidizing agent carried along.

Types of engines

Engines in the narrower sense can initially be divided into air breathing engines, which use the oxygen in the air as an oxidizing agent, and non-air breathing enginesthat carry the oxidizing agent (e.g. pure oxygen or a solid compound). The latter is particularly the case with rocket engines. Their main advantage is that they can work independently of the atmospheric air, i.e. also in a vacuum.

In the case of air-breathing engines, a further distinction is made between machines with an aircraft engine and jet engines:

  • An aircraft engine is usually an internal combustion engine, but in some cases it is also an electric motor, which usually drives a propeller. The combination of engine and propeller can be viewed as one engine. For the energy source used, however, the term fuel is more appropriate than fuel.
  • Jet engines are engines that generate thrust directly with a gas jet - but some of them also have an externally visible propeller or a disguised fan (a fan) contain. In the case of commercial aircraft in particular, most of the thrust is usually generated by a fan.

Jet engines are mostly used today for aircraft with higher cruising speeds, e.g. B. at about 800 to 900 km / h, while propeller planes are limited to lower speeds, often below 400 km / h.

Power, thrust and efficiency of an engine

The power of an engine is seldom given in units of watts as is the case with an engine. H. as the amount of mechanical energy released per unit of time. Instead, the thrust (the propulsive force) is usually given in units of Newtons (or with the old unit kilopond = kp).

The effective drive power of an aircraft engine is then the product of the thrust and the speed of the aircraft in relation to the air. So it is zero as long as the engine is operated at a standstill; It then only gives off energy to the gases emitted, not to the aircraft. The best drive efficiency is obtained at a certain speed, e.g. B. close to the typical cruising speed of an airplane. At higher speeds, the efficiency decreases again because the thrust decreases, e.g. B. because strong eddies reduce the efficiency of the force generation. The specified thrust of an engine therefore applies to a certain speed in relation to the air. Often it does not depend dramatically on this speed in wide areas, but at some point it decreases drastically if the speeds are too high.

In connection with engines, a Propulsion efficiency specified. This matches with Not the effective drive efficiency discussed above and is also not a directly measurable variable, but only takes into account the efficiency with which the mechanical energy generated is converted into effective drive energy. For example, the efficiency of the engine of a propeller aircraft is not taken into account. A high propulsion efficiency does not automatically mean a high efficiency of the entire drive.

The propulsion efficiency is something completely different from the effective drive efficiency!

The propulsion efficiency of an aircraft engine can be calculated as ηv = 2 vf / (vG + vf), in which vf means the speed of the aircraft in relation to the air and vG the speed of the emitted gases in relation to the aircraft (i.e. the ejection speed). So it becomes 1 (= 100%) for vG = vf, but then the thrust disappears. In practice it must vG be greater than vfto get a boost that is proportional to vG − vf is. The propulsion efficiency then drops. Values ​​in the region of 80% are realistic for aircraft with a turbofan engine (turbofan engine) when the cruising speed is reached; At low speeds (take-off and landing), however, the propulsion efficiency decreases accordingly. Since large losses occur in the engine's gas turbine, the effective drive efficiency is much lower than the propulsion efficiency - in the order of 30%. Propulsion efficiencies a little above 90% could perhaps be achieved in the future with so-called open rotor engines with two propellers rotating in opposite directions, which, unfortunately, have so far been quite loud.

In spacecraft and space probes, the propulsion power of an engine loses its importance, so to speak, because it depends on the speed, but this speed is relative. H. depending on the selected reference system. In space, the ambient air is no longer a natural reference system. The energy efficiency of an engine can then no longer simply be quantified by means of an efficiency.

Specific fuel consumption

The specific fuel consumption specified. This is the fuel consumption (e.g. in kilograms per hour) divided by the thrust generated for a certain entry speed of the air (e.g. at the cruising speed of the aircraft driven by it). The specific fuel consumption is thus expressed in units of kg / (N s) (SI base unit) or z. B. kg / (kN h) (= kilograms per hour and kilonewtons). From this, together with the airspeed and the fuel's heating system, the effective propulsion efficiency can then be calculated.

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See also: jet engine, fuel
and other articles in the Basic Concepts, Engines, and Power Plants categories