How a Jet Engine Operates

A turbojet engine is essentially a machine designed for the only purpose of producing high-velocity gases, which are discharged through the jet nozzle at the rear of the engine. The engine is started by rotating the compressor with a starter, then igniting the mixture of fuel and air in the combustion chamber³ with one or more igniters. When the engine has started and its compressor is rotating properly, the starter and the igniters are turned off. The engine will then run without further assisstance as long as fuel and air in the proper propor-tions continue to enter the combustion chamber.

The gases created by a fuel and air mixture burning under normal atmospheric pressure do not expand enough to do useful work. Air under pressure must be mixed with the fuel before the gases produced by combustion can be successfully employed to make a turbojet ope-rate. The more air an engine can compress and use, the greater is the power or thrust it can produce.

In a jet engine the fuel and air mixture is compressed by means of a centrifugal compressor. The power necessary to drive the compressor in a turbojet engine is very high. To indicate how much power is absor­bed by the compressor of a moderately large turbojet, let us assume that we have an engine that produces 10,000 pounds of thrust for take-off. In this engine, the turbine has to produce approximately 35,000 shaft horsepower⁴ to drive the compressor when the engine is operating at full thrust. About three-quarters of the power generated inside a jet engine is used to drive the compressor. Only what is left over is available to produce the thrust needed to propel the airplane.

Single stage centrifugal compressors are practical for pressure ratios up to about 4:1. Higher pressures can be achieved, but at a decrease in efficiency. It is possible to obtain higher pressures by using more than one stage of compression.

Liquid Propellants

The energy developed in the rocket engine for propulsion purposes is derived from the thermochemical energy of the propellants. Their chemical reaction, called "combustion" is accompanied by the genera­tion of large quantities of gases at high temperature. Since the fuels employed are hydrocarbons, the products of combustion usually con­tain carbon dioxide (CO₂), carbon monoxide (CO) and water (H₂O) in the form of steam, as the principal constituents. The temperature attained by the reaction and the composition of the reaction products are influenced to a large extent by the mixture ratio¹ of the propel­lants. The pressure in the combustion chamber² where the reaction takes place influences the completeness of the reaction. The higher the chamber pressure, the more complete the reaction. High chamber pressure gives improved performance; however, the improvement dec­reases for valves of chamber pressure above approximately 300 psia.

The ideal liquid rocket propellant is one that meets the following requirements:

The heat value⁴ per pound of propellant shou

1. ld be as high a possible.

2. The density should be high to keep space requirements low.

3. The propellant should be easily stored and present no special handling problems.

4. The corrosiveness of the propellant should be low.

5. The performance of the propellant combination should not be greatly affected by temperature changes.

6. The ignition should be reliable and smooth.

7. The propellant should be stable for reasonable lengths of time.

8. The viscosity change with temperature change should be low so that the pumping work at low temperatures will not be excessive.

None of the known liquid rocket propellants satisty all these requi­rements.

The Temperature Problem

The problem which has become of increasing importance as the speeds of aircraft have become higher is that of temperature. The tem­peratures associated with very high energies dissipated during re­entry² of a missile are frequently above the melting point of most materials. Even the temperatures associated with the leading edge³ of airplanes in supersonic flight are high enough to reduce severely⁴ the strengtn characteristics of the structural materials. Three methods have been used to overcome the temperature prob- lem. To certam missile re-entry application, it is possible to construct the body with a shielding of material that is able to absorb the heat generated during the re-entry manoeuvre by merely melting or burning away the smelding , leaving the main structure undamaged. In cases where such an approach would be unsatisfactory, efforts have been made to combat the temperature by utilizing cooling systems, such as feeding water under pressure through the leading edge and absorbing the excess heat by converting it to steam. At lower speeds, tem­ perature-resistant materials, such as stainless steel or titanium or even certain aluminum alloys, have proved a very satisfactory ap­ proach. ■

StoLs and vtoLs

STOL stands for short take-off and landing. STOL looks like conventional aircraft, but depends on powerful engines and stabili­zation devices for landing and take-off. These might include large retractable flaps¹ to increase wing area at low speeds and to deflect the airstream downward for increased lift.

Being faster than helicopters but requiring more space to land STOLs might be used in intercity operations between suburban airports.

VTOL stands for vertical take-off and landing. It should be noted that VTOL craft can also operate in the STOL mode where landing space is available. All VTOLs pose difficult technical problems. While an ordinary aircraft can develop lift slowly by increasing speed along a runway³, the VTOL must take off without this kind of help. It seeks all its initial lift without any forward speed. This requires a great amount of lifting power, which is likely to be needed only for take-off and landing. The result is lower payload, higher costs, and shorter range.

Operating costs are improving, but are still higher than those of conventional aircraft. Nevertheless⁴, there is no question that there is a place for VTOLs — assuming a satisfactory design can be found.

A number of different kinds of VTOL have been built or are under study.

A model of the strange-looking ADAM II has already been built and is being tested. ADAM stands for Air Deflection and Modulation. Turbofan engines⁵ will be located right in the wings and nose. To obtain upward thrust , the fixed-wing design diverts⁷ the airflow downw­ard through a series of louvers⁸ or slats. ADAM is planned as a high-sonic craft, which may bring it into the 600-mph class. Finally, work is proceeding⁹ on several supersonic, jet-driven VTOLs. These, as well as ADAM, are the kind of high-performance craft, that must sacrifice¹⁰ payload and economy of operation to obtain this high performance. Therefore now they are of more interest to the military than to commercial operators. The future, however, may see even more novel designs.

Rocket Propulsion Fundamentals

The chemical rocket engine is not dependent upon air as its oxidizer source and therefore can operate outside the earth's atmosphere to propel space vehicles. This is an advantage over other types of jet propulsion engines¹. A rocket engine functions perfectly in vacuum or near-vacuum conditions since it does not have to overcome the drag that is created in atmospheric conditions.

The rocket engine differs also from other types of jet propulsion in that its thrust depends entirely upon the effective velocity of the exhaust and does not depend upon a momentum difference². Since its thrust depends only upon the effective jet velocity, it is not affected by the speed at which the vehicle travels if the propellant consump­tion rate is constant. Thrust equations:

The thrust of a rocket engine is composed of the sum of two terms: momentum thrust³ and pressure thrust⁴. The momentum thrust is simply the change of momentum which results from the acceleration of the propellant particles. The equation for momentum thrust is often called the simplified thrust equation because it assumes "comp­lete expansion" of the exhaust gases in the nozzle⁵. In other words it assumes that the gases have expanded to the point where the nozzle pressure is the same as the pressure surrounding the rocket nozzle. The equation for the momentum thrust is:

Th = ^G~V_e, where Th — momentum thrust in lbs;

G— weight rate of flow of propellant in lbs. per second; g— acceleration due to gravity (32.2 ft/sec²); V_e — velocity of gases at nozzle exit in ft. per second.

Combustion-Driven MHD¹ Generator

The combustion-driven MHD generator is remarkably simple — nothing more than a relatively low-pressure rocket, a combustion cham­ber attached² to a rather long nozzle, with the whole assembly inserted inside a magnet. And because of the ability of the generator to handle very-high-temperature gases, a MHD powerplant will run at effi­ciencies which may exceed 60%. Its high efficiency could drastically reduce — even eliminate — thermal pollution³ of lakes and rivers. In wide use, it could also significantly reduce sulfur dioxide pollution of the atmosphere, and turn out sulfuric and nitric acids⁴ as bypro­ducts. The performance of MHD, moreover, improves with increased generator size.

But the conceptual simplicity of MHD does not, in itself, cinch⁵ its application. In many ways, the situation is closely analogous to that of the rocket engine, which the generator so closely resembles⁶.

Ability to utilize a very-high-temperature, high-energy heat source distinguishes⁷ the gaseous MHD generator as a power source. In MHD, the power-production process takes place throughout the gas volume. The gas container — the MHD channel — can be cooled, and so can operate at much lower temperature than the generating gas. Consequently, in principle, an energy source at any temperature may be employed. Ability to operate at high temperature means high thermodynamic efficiency and large power density. As a practical matter, however, the gaseous working fluids of primary interest come from the energy of chemical combustion and the solid-core nuclear reactor. For combus­tion-driven MHD, this means a maximum gas temperature below about 5200 F, except in special instances involving very high energy fuels. For the nuclear heat source, maximum attainable temperatures are much lower, well below 3500 F for advanced systems, and below 2000 F for more conventional systems.

Finally, engineers confront a multitude of problems related to the other equipment for a complete powerplant. These include the develop­ment of the regenerative high-temperature heat exchangers to preheat the combustion air to 2000—3000 F.

Information Theory, Codes and Messages

The general problem of transmitting and interpreting (decoding) messages is considered by information theory, a close relative¹ of thermodynamics, which, a little by design and a little by chance, uses the statistical concept of entropy as a starting point.

In the general communication problem considered by Claude Shannon, the inventor of information theory, the following basic elements are introduced: A message

A transmitter: the thing that is sending the message A receiver: the instrument that reads and decodes the message A channel: medium through which the message is transmitted A code: set of symbols used to write the message Noise: an undesirable signal that interferes with the whole process and cannot be eliminated

A simple example is provided by the telegraph. There is a code given by a sequence of lines, dots², and periods of silence; a transmitter, which serves to send the message in the form of an electromagnetic signal; a channel — the air; a receiver, which includes the operator who decodes the message. Noise is distributed throughout: there may be electrical discharges interfering with the real signal, errors caused by the operator, etc. In devising³ his dot-and-dash code⁴ Morse fol­lowed the principle of using the shortest symbols — the fastest to transmit — for the most common letters. This method is still used in more sophisticated codes.