Hyper Sonic Air

8/8/2019 Hyper Sonic Air

1/5

HYPERSONIC AIR-BREATHING PROPULSION

SYSTEMS: RAMJET AND SCRAMJETHUSEIN BHINDERWALA

K. J. SOMAIYA COLLEGE OF ENGINEERING, MUMBAI UNIVERSITY

[email protected]

ABSTRACTThis paper is intended to offer the reader an

introduction to the study of ramjet and scramjet

propulsion, including careful definitions of terms and a

unified description of the processes and characteristics of

the ramjet and scramjet engine. This paper reviews the

major knowledge base that has been accumulated through

years of theoretical and experimental research on topics

relevant to ramjet and scramjet propulsion. Later in the

paper, various innovative technological ideas or proposals

have been put forth that need great extent of research and

experimentation to follow up on. Lastly, the author has

performed a series of calculations using NASAs EngineSim

software on a predetermined ramjet model and citations of

data from various wind tunnel tests from references.

I. INTRODUCTION

Thrust is the force which moves any aircraft through theair. Thrust is generated by the propulsion system of the aircraft.

Different propulsion systems develop thrust in different ways,

but all thrust is generated through some application of

Newton's third law of motion. For every action there is an

equal and opposite reaction. In any propulsion system,

a working fluid is accelerated by the system and the reaction to

this acceleration produces a force on the system. A general

derivation of the thrust equation shows that the amount ofthrust generated depends on the mass flow through the engine

and the exit velocity of the gas.

In the early 1900's some of the original ideas

concerning ramjet propulsion were first developed in Europe.Thrust is produced by passing the hot exhaust from

the combustion of a fuel through a nozzle. The nozzle

accelerates the flow, and the reaction to this acceleration

produces thrust. To maintain the flow through the nozzle, the

combustion must occur at a pressure that is higher than the

pressure at the nozzle exit. In a ramjet, the high pressure is

produced by "ramming" external air into the combustor using

the forward speed of the vehicle. The external air that is

brought into the propulsion system becomes the working fluid,

much like a turbojet engine. In a turbojet engine, the high

pressure in the combustor is generated by a piece of machinerycalled a compressor. But there are no compressors in a ramjet.Therefore, ramjets are lighter and simpler than a turbojet.

Ramjets produce thrust only when the vehicle is already

moving; ramjets cannot produce thrust when the engine is

stationary or static. Since a ramjet cannot produce static thrust,

some other propulsion system must be used to accelerate the

vehicle to a speed where the ramjet begins to produce thrust.

The higher the speed of the vehicle, the better a ramjet works

until aerodynamic losses become a dominant factor.

The combustion that produces thrust in the ramjet occurs at

a subsonic speed in the combustor. For a vehicle travelling

supersonically, the air entering the engine must be slowed to

subsonic speeds by the aircraft inlet. Shock waves present in

the inlet cause performance losses for the propulsion system.

Above Mach 5, ramjet propulsion becomes very inefficient.

The new supersonic combustion ramjet, or scramjet, solves this

problem by performing the combustion supersonically in the

burner.

Fig1. Supersonic ramjet BrahMos missiles

Shown above are pictures of an BrahMos missile using

ramjet technology for propulsion. A rocket is used to bring theramjet up to speed before it produces thrust. Because the ramjet

uses external air for combustion, it is a more efficient

propulsion system for flight within the atmosphere than

a rocket, which must carry all of its oxygen. Ramjets are

ideally suited for very high speed flight within the atmosphere.

II. RAMJET/SCRAMJET THRUST

A ramjet engine provides a simple, light propulsion

system for high speed flight. Likewise, the supersonic

combustion ramjet, or scramjet, provides high thrust and lowweight for hypersonic flight speeds. Unlike a turbojet engine,

ramjets and scramjets have no moving parts, only an inlet, a

combustor that consists of a fuel injector and a flame holder,

and a nozzle. How do ramjets and scramjets work?


2/5

When mounted on a high speed aircraft, large amounts of

surrounding air are continuously brought into the engine inlet

because of the forward motion of the aircraft. The air is slowed

going through the inlet, and the dynamic pressure due to

velocity is converted into higher static pressure. At the exit of

the inlet, the air is at a much higher pressure than free stream.

While the free stream velocity may be either subsonic or

supersonic, the flow exiting the inlet of a ramjet is always

subsonic. The flow exiting a scramjet inlet is supersonic and

has fewer shock losses than a ramjet inlet at the same vehiclevelocity. In the burner, a small amount of fuel is combinedwith the air and ignited. In a typical engine, 100 pounds of

air/sec. is combined with only 2 pounds of fuel/sec. Most of the

hot exhaust has come from the surrounding air. Flame holders

in the burner localize the combustion process. Burning occurs

subsonically in the ramjet and supersonically in the scramjet.

Leaving the burner, the hot exhaust passes through a nozzle,

which is shaped to accelerate the flow. Because the exit

velocity is greater than the free stream velocity, thrust is

created as described by the general thrust equation (Eq. 1). For

ramjet and scramjet engines, the exit mass flow is nearly equal

to the free stream mass flow, since very little fuel is added to

the stream.

Fig.2 Schematic representation of a Ramjet engine

The thrust equation for ramjets and scramjets contain three

terms: gross thrust, ram drag, and a pressure correction. If thefree stream conditions are denoted by a "0" subscript and the

exit conditions by an "e" subscript, then:

Thrust=F= Eq. 1

1. Aerodynamicists often refer to the first term (exitmass flow rate times exit velocity) as the gross

thrust, since this term is largely associated with

conditions in the nozzle.

2. The second term (free stream mass flow rate timesfree stream velocity) is called the ram drag. This

term can be quite large for scramjet engines.3. For ramjets and scramjets, the nozzle exit velocity is

supersonic, and the exit pressure depends on the area

ratio between the throat of the nozzle and the exit of

the nozzle. Pressure correction is usually small

compared to the first term of the thrust equation. But

for completeness, this term is usually included in the

gross thrust.

III. RAMJET PARTS

For high supersonic or hypersonic flight, the

ideal propulsion system is a ramjet. A ramjet uses the forwardspeed of the aircraft to compress the incoming air and,

therefore, has fewer moving parts than a turbine engine.

Fig. 3 Computer Drawing of a typical ramjet engine

In this figure3 we show a computer drawing of a typicalramjet engine. In the computer drawing, we have cut out a

portion of the engine to have a look inside. At the front of the

engine, to the left, is the inlet, which brings outside air into the

engine. At the exit of the inlet, the air is at a much higher

pressure than free stream conditions. Fuel is injected and mixed

for combustion just downstream of the inlet. The resultingflame is stabilized in the engine by the red flame holder ring.

This part is very similar to an afterburner in a fighter

jet engine. The hot exhaust then passes through the nozzle,

which is shaped to accelerate the flow and produce thrust.

A. INLETS

y SUPERSONIC INLETS-An inlet for a supersonic aircraft has a sharp lip. Theinlet lip is sharpened to minimize the performance

losses from shock waves that occur during supersonic

flight. For a supersonic aircraft, the inlet must slow

the flow down to subsonic speeds before the air

reaches the compressor. Some supersonic inlets use a

central cone to shock the flow down to subsonic

speeds. Other inlets use flat hinged plates to generate

the compression shocks, with the resulting inlet

geometry having a rectangular cross section.This variable geometry inlet is used on the F-14 and

F-15 fighter aircraft. The inlets of the Mach 3+ SR-71

aircraft are specially designed to allow cruising flight

at high speed. The inlets of the SR-71 actually

produce thrust during flight.y HYPERSONICINLETS

Inlets for hypersonic aircraft present the ultimate

design challenge. For ramjet-powered aircraft, the

inlet must bring the high speed external flow down to

subsonic conditions in the burner. High stagnation


3/5

temperatures are present in this speed regime and

variable geometry may not be an option for the inlet

designer because of possible flow leaks through the

hinges. For scramjet-powered aircraft, the heat

environment is even worse because the flight Mach

number is higher than that for a ramjet-powered

aircraft. Scramjet inlets are highly integrated with the

fuselage of the aircraft. Thick, hot boundary layers are

usually present on the compression surfaces of

hypersonic inlets. The flow exiting a scramjet inletmust remain supersonic.

B.NOZZLES

All gas turbine engines have a nozzle to produce thrust, to

conduct the exhaust gases back to the free stream, and to set

the mass flow rate through the engine. The nozzle sits

downstream of the power turbine.

A nozzle is a relatively simple device, just a specially shaped

tube through which hot gases flow. However, the mathematics

which describes the operation of the nozzle takes some careful

thought. Nozzles come in a variety of shapes and sizes

depending on the mission of the aircraft.

Simple turbojets, and turboprops, often have a fixed geometryconvergent nozzle as shown on the left of the

figure. Turbofan engines often employ a co-annular nozzle as

shown at the top left. The core flow exits the centre nozzle

while the fan flow exits the annular nozzle. Mixing of the two

flows provides some thrust enhancement and these nozzles also

tend to be quieter than convergent nozzles. Afterburning

turbojets and turbofans require a variable

geometry convergent-divergent - CD nozzle. In this nozzle, the

flow first converges down to the minimum area or throat, then

is expanded through the divergent section to the exit at the

right. The variable geometry causes these nozzles to be heavier

than a fixed geometry nozzle, but variable geometry provides

efficient engine operation over a wider airflow range than a

simple fixed nozzle.

Rocket engines also use nozzles to accelerate hot exhaust to

produce thrust. Rocketengines usually have a fixed geometry

CD nozzle with a much larger divergent section than is

required for a gas turbine. Recently, however, engineers have been experimenting with nozzles with rectangular exits. This

allows the exhaust flow to be easily deflected, or vectored.

Changing the direction of the thrust with the nozzle makes the

aircraft much more manoeuvrable.

Because the nozzle conducts the hot exhaust back to the free

stream, there can be serious interactions between the engine

exhaust flow and the airflow around the aircraft. On fighter

aircraft, in particular, large drag penalties can occur near the

nozzle exits. A typical nozzle-after body configuration is

shown in the upper right for an F-15 with experimental

maneuvering nozzles. As with the inlet design, the external

nozzle configuration is often designed by the airframe and

subjected to wind tunnel testing to determine the performance

effects on the airframe. The internal nozzle is usually the

responsibility of the engine manufacturer.

IV. CALCULATIONS DONE ON ENGINESIM SOFTWARE

With this software you can investigate how a jet (or

turbine) engine produces thrust by interactively changing the

values of different engine parameters.

By convention, a white box with black numbers is an input box

and you can change the value of the number. A black box with

yellow or red numbers is an output box and the value is

computed by the program.

The program screen is divided into four main parts:

1. On the top left side of the screen is a graphic of theengine you are designing or testing. In the Design

Mode, the drawing is a schematic, while in Tunnel

Test Mode the drawing is an animation.

2. On the upper right side of the screen are choicebuttons which control the analysis. You can select the

type of analysis, the type of output to be displayed,and the units to be used in the calculations. You will

always see the overall engine performance displayed

as thrust, fuel flow, airflow, and computed engine

weight.

3. On the lower right side of the screen are the results ofengine performance calculations. The output can be

presented as numerical values of certain parameters,

graphs of engine performance, or as photos of the

engine parts with descriptions of their purpose. You

select the type of output displayed by using the choice

button labelled "Output:" on the upper right panel.

4. On the lower left side of the screen various input panels are displayed. You can select the input panel

by clicking on the name or the component in thegraphic at the upper left.

Flight Conditions include the Mach number, airspeed,

altitude, pressure, temperature, and throttle and afterburner

settings. There are several different combinations of these

variables available for input. The pressure and temperature are

computed as functions of the altitude by using a Standard Day

atmospheric model.

Design variables for each engine component can also be varied.

The components and variables include the Inlet (pressure

recovery), Fan (pressure ratio, efficiency, and bypass ratio),

Compressor (CPR, compressor efficiency), Burner (fuel,

maximum temperature, efficiency, and pressure ratio), Turbine(turbine efficiency) and Nozzle (maximum temperature,

efficiency, A8/A2). As you choose a different component the

part of the engine being affected is highlighted in the graphic

by changing from its default colour to yellow. Engine Size can

be specified by either the frontal area or the diameter. As the


4/5

engine size changes, the grid background changes in proportion to the size. The distance between any two grid lines is 1 foot.

Fig 4. Calculations on the EngineSim software

V. FUTURE INNOVATIONS

A. BUSSARD RAMJET

It is intended to circumvent the problems of rocket

economics by collecting fuel as it goes along. Conventionalrockets carry all their fuel with them. The vast majority of its

weight and size were taken up with fuel. An interstellar rocket,which would have to travel distances measured in light years,

would therefore be enormous, and most of the fuel would be

used accelerating other fuel. This is simply not practical.

Bussard's Solution:

The entry on Jets and Rockets explains in detail why jet

engines don't work in space. In summary, it is because a jet

engine works by accelerating a medium, such as air. In space,

of course, there is no such medium. Or is there?

Space is not, in fact, completely empty. Even between thestars there is hydrogen gas, at a density of about one or two

atoms per cubic centimetre. This is the 'medium' for the

Bussard ramjet.

As with conventional ramjets, the Bussard ramjet cannotaccelerate from a standing start. Some other drive technology

must first be used to accelerate the ship to a measureable

fraction (say 1%) of lightspeed1.

When the ship is moving fast enough, it is encountering

enough atoms of interstellar hydrogen every second to make it

worth collecting them and using them as fuel.

The Invisible Scoop

Even at these speeds, the hydrogen collector would need to be

quite large. Estimates vary, but a typical figure for the diameter

is 50,000 kilometres! Obviously, no physical collector thislarge is practical. Instead, the hydrogen collector would consist

of a vast electromagnetic field, generated by superconducting

coils on the ship. This field would ionise the hydrogen atoms

and magnetically funnel them into the engine intake. There

they undergo a fusion reaction, and the exhaust is directed outof the rear.

Journeys by Ramscoop

The pilot of a Bussard ramjet could conceivably set it to

accelerate at a constant 1g. This would be convenient, as it

would provide a shipboard environment indistinguishable from

Earth. There would be none of the inconveniences of ship-

board gravity generated by centrifugal, such as very

obvious Coriolis effects, variable gravity from circumference

to axis, and having to build the rooms with two 'down'directions, one for when the ship is accelerating and one for

when it is coasting with spin.Another advantage of constant 1g acceleration is that it would

allow the pilot to make very long journeys. To an observer on

Earth, such a ship would take hundreds of thousands of years to


5/5

reach the centre of the galaxy. Thanks to relativistic time

dilation, however, the pilot would be only 20 years older on

arrival. So, for the pilot, the centre of our galaxy is only 20

years away!

A Science Fiction Dream

Leaving aside the fact that we are not yet able to build fusion

engines or sufficiently powerful superconducting coils, the

Bussard ramjet sounds at first like an excellent prospect for

interstellar propulsion. Unfortunately, there are strong

theoretical objections to the principle of the Bussard ramjet.Fusion as generated on Earth requires deuterium, whichaccounts for only about 0.01% of interstellar hydrogen. Fusion

in the Sun uses normal hydrogen, but achieving the conditions

necessary for that would be very difficult. An optimistic

estimate would be that only 1% of the hydrogen would be

actually usable as fuel. So in fact much of the propulsive power

would be used up slogging through a soup of useless hydrogen.

Also, one of the by-products of the fusion reaction is neutrons.

Any crew compartment would need extremely heavy shielding

against this radiation, adding to the mass of the ship.

Unless these and other serious problems can be addressed, the

Bussard ramjet will remain a science fiction concept. Of

course, we literally cannot imagine the capabilities of future

technology, so the stated objections may eventually seemtrivial.

Fig. 5 Artistic rendition of Bussard ramjet

VI. CONCLUSION

This technology is still in its nascent stages and further workneeds to be done requiring hours of research and experimental

work. In the path of the author, he aims at building a model

ramjet to demonstrate the workings of the propulsion system of

which he has already made detailed drawings which would be

presented at the presentation.

Documents

Hyper Sonic Air