Abstract:
The jet ( 40 ) has an air-fuel combustion chamber ( 42 ) and a plurality of rocket engines ( 11 ) arranged upstream from the combustion chamber ( 42 ), each rocket engine having its own combustion chamber with the wall thereof being cooled by lateral injection of fuel through said wall.

Description:
TECHNICAL FIELD 
     The invention relates in general to hypersonic propulsion, i.e. making use both of rocket type propulsion and of air-breathing propulsion of the ramjet or super-ramjet type. Propulsion in rocket mode serves to accelerate the vehicle to a supersonic speed that is sufficient to allow operation in ramjet mode. The invention relates more particularly to arranging a plurality of rocket engines of a particular type in a ramjet in order to obtain a ramjet that can be used for a very long duration, possibly being reusable. 
     BACKGROUND 
     It is known, e.g. from an article by W. B. Scott published in the journal “Aviation Week and Space Technology” of Jul. 5, 1999, to arrange a plurality of small rocket engines in a ramjet. However that article remains vague about the particular design of the small rocket engines. 
     A major problem is cooling the walls of the combustion chambers and the nozzles of those small rocket engines, given that the temperature reached by the combustion gas is greater than 3000 K. This makes the length of life of such systems quite problematic, particularly if is desired to produce a machine that is reusable. 
     BRIEF SUMMARY 
     The invention enables this problem to be solved by a particular choice in the type of rocket engine used. 
     More particularly, the invention provides a supersonic jet, characterized in that it comprises a ramjet comprising an air-fuel combustion chamber and a plurality of rocket engines arranged in the airstream upstream from said combustion chamber, and in that such a rocket engine fed with a mixture of propellant components comprises a tubular body provided internally with a coaxial wall forming a screen that defines an annular fuel injection zone extending over the major fraction of the length of said body, and an oxidizer injector opening out axially into the inside of said screen in the vicinity of the front end thereof. 
     The combustion chamber of the rocket engine is essentially constituted by the inside volume of said screen, and this type of fuel injection ensures that the screen is cooled. 
     This structure enables all of the fuel or fuel-rich gas to be injected laterally through the wall of said screen. This concept implements a cooling principle referred to as “sweating”. This type of fuel injection enables the wall of the combustion chamber to be cooled simply by forming a protective film. It is thus possible to provide rocket engines of small diameter and to group them together to constitute narrow injector pylons. A “pylon” is constituted by a plurality of rocket engines grouped together side by side and extending parallel to one another in a single row. Such pylons may be arranged so as to constitute a grid installed in the airstream upstream from the combustion chamber of the ramjet. The grid presents relatively little obstruction to the airstream. 
     Thus, by way of example, the wall of the combustion chamber is cooled by creating, on demand, a film of hydrogen-rich gas that is injected laterally and that protects the wall of the combustion chamber. Until now, this type of cooling has been underestimated, since it has in principle the effect of reducing unit performance of a rocket engine of this type. However, in the context of installation in a ramjet, this loss is compensated downstream by the excess fuel (i.e. the hydrogen that was used for cooling) mixing with the air passing through the ramjet and engaging in post-combustion. Consequently, the invention provides overall performance that is at least equal to, or even better than, that of conventional rocket engines fed with a mixture of oxygen and hydrogen. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be better understood and other advantages thereof appear more clearly in the light of the following description of a rocket engine and a supersonic jet fitted with a plurality of such rocket engines, given purely by way of example and made with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic longitudinal section view of a rocket engine in accordance with the invention; 
         FIG. 2  is a detail view in perspective partially cut away and in section showing the front portion of a rocket-engine pylon; and 
         FIG. 3  is a diagrammatic view of a supersonic jet in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference more particularly to  FIGS. 1 and 2 , there can be seen a rocket engine  11  suitable for being installed in a ramjet as described below. 
     The rocket engine  11  comprises a generally cylindrical body  12  having a nose  15  that tapers forwards and an exhaust nozzle  17  including a throat  19  forming a flow restriction in order to increase the speed with which gas is ejected. An oxidizer injector  21  is located axially at the front and opens out into a combustion chamber  23 , here generally cylindrical in shape, that extends between the injector orifice  21  (at the front) and the nozzle  17  (at the rear). The wall  27  of the combustion chamber  23  is coaxial with the wall of the body  12 . Between them these two walls define a cylindrical annular space  29 . 
     In the example described, the rocket engine  11  forms part of a pylon  39  associating a plurality of similar rocket engines that are disposed side by side longitudinally. Such a pylon comprises a front block  14  that forms the tapering nose  15  for all of the rocket engines, and a rear block  13  having all of the exhaust nozzles  17  incorporated therein. Two side plates  16  interconnect the front block  14  and the rear block  13 . They ensure that the pylon  39  is completely faired and they protect the bodies  12  of all the rocket engines. 
     The structure of the pylon is described in greater detail below. 
     According to an important characteristic of the invention, the wall  27  of the combustion chamber is arranged as a screen and the space  29  between the two walls constitutes an annular fuel-injection zone that thus extends over the major fraction of length of the body. A fuel injector  33  is situated at the front. It communicates with the space  29 . The term “screen” is used herein to mean any wall that allows fuel or a fuel-rich gas to pass through a multitude of perforations that are distributed over its entire area. Such a screen may be constituted by a porous material or by a cylinder perforated with small-diameter holes  34 , as shown. This arrangement cools the wall by “sweating” with a protective film of fuel being formed all along the inside face of the screen, enabling the wall  27  to remain intact in spite of the high temperature that exists inside the combustion chamber  23 . 
     The distribution of the holes may vary along the combustion chamber so as to obtain a film that is sufficient, including for protecting the throat  19 . 
     Advantageously, additional injector means  35  are provided for injecting fuel or fuel-rich gas. These means open out into the combustion chamber immediately ahead of the throat  19  so as to reinforce the effectiveness of the protective film flowing along the wall of the throat. The fuel-rich gas may come from the same source as feeds the combustion chamber or it may be colder, by adding an additional quantity of fuel. 
     Furthermore, channels  37  for passing and injecting fuel are embedded in the space available around the throat  19 . The purpose of these channels and the manner in which they are provided are described below. 
     Such a rocket engine may operate for example by reacting to propellant components such as for example a gas rich in hydrogen for the fuel passing through the screen and liquid oxygen for the oxidizer. 
     As shown in  FIGS. 2 and 3 , because of its small diameter, the structure of such a rocket engine is advantageous for making narrow pylons  39  and thus obtaining a tight grid of pylons that provide little obstruction for the airstream inside a ramjet  40 , upstream from the mixing and combustion chamber  42  thereof. 
     As shown in  FIG. 3 , a plurality of rocket engines as described above are installed inside the ramjet  40 . The ramjet comprises an air inlet  41 , the combustion chamber  42  that receives the fuel of the ramjet, and an outlet nozzle  43 . The injector means for injecting fuel into the ramjet are not shown. The air inlet leads to the combustion chamber. The rocket engines  11  are installed in the airstream upstream from the combustion chamber  42 . They are disposed parallel to the gas flow direction. 
     As shown, they are arranged side by side in pylons, each pylon  39  extending transversely through the airstream. The tapering nose  15  is common to all of the rocket engines in a given pylon. It is generally in the form of a forwardly-streamlined blade and it contains a fuel feed channel  45  and an oxidizer feed channel  47 . The fuel feed channel  45  communicates with all of the annular spaces  29 , while the oxidizer feed channel communicates with all of the combustion chambers  23  via parallel ducts  49  opening out axially into the front ends of respective combustion chambers  23 . 
     A plurality of pylons  39  may thus be placed parallel to one another in the airstream (or in a ring for an annular airstream), being spaced apart by a distance that is sufficient to form a grid that presents relatively little obstruction to the airstream. 
     In operation, the film of fuel or fuel-rich gas that cools by sweating is burnt in post-combustion in the ramjet. 
     Similarly, the channels  37  deliver fuel to the combustion chamber  42  of the ramjet. They are not used while the rocket engines are in operation, but during the stage of supersonic operation. The efficiency of combustion is improved because of the small spacing between the pylons.