METHOD OF FABRICATING A COMBUSTION CHAMBER
The invention relates to a method of fabricating a combustion chamber by assembling together preformed shells; more particularly the invention relates to the way in which assembly is performed, eliminating any need for seam welding. The invention applies advantageously to fabricating so-called "reverse-flow" combustion chambers.
The invention also relates to a forward-flow combustion chamber obtained by implementing the method, and to a turbojet fitted with a combustion chamber of the invention.
A so-called reverse-flow combustion chamber is generally made up of metal sheet stamped to constitute shells. The shells are assembled together. For assembly purposes, the shells often have annular tongues that are assembled flat thereto by seam welding.
These welded-on tongues project outside the combustion chamber, thereby leading to head losses in the stream of air flowing around the combustion chamber. In addition, mechanical weakness remains in these tongues, particularly in a reverse-flow combustion chamber, while the outer bend of the chamber is being subjected to bending.
In addition, that assembly technique leads to thermomechanical stresses and raises problems of accessibility if it is desired to use a laser to perforate the combustion chamber.
Attempts have recently been made to reduce the number of annular tongues by making use of butt welded assembly techniques. Nevertheless, the solutions that have been envisaged until now have not made it possible completely to eliminate seam welding.
The invention makes it possible to achieve that objective.
More particularly, the invention provides a method of fabricating a combustion chamber essentially made up

of welded-together shells, the method being characterized in that it comprises making separately two subassemblies of such shells by butt-welding the shells together, with an intermediate connection ring being welded to one end of a first subassembly, the intermediate ring including an assembly surface; engaging one end of a second subassembly on said surface; and welding it to said intermediate ring.
In order to fabricate a so-called "reverse-flow" combustion chamber, the first subassembly is mainly constituted by outer shells and the second subassembly is mainly constituted by inner shells. The shells of each subassembly are assembled together by butt-welding. A flat-bottomed shell constitutes a chamber end wall for carrying the injectors, and this chamber end wall constitutes a portion of one of the subassemblies prior to final welding.
By way of example, one of the subassemblies includes such a chamber end wall and one of the ends of said chamber end wall constitutes the end of said second subassembly that is to be welded to said intermediate ring.
In a manner that is itself known, butt-welding is always performed by adjusting the docking of the two annular parts concerned by means of radial expander tools that enable the parts to be abutted edge to edge for welding purposes.
The intermediate connection ring is a part that is machined at least in part, having accurate dimensions. It can therefore perform a centering function at the moment when the two subassemblies are finally assembled together by orbital welding. Such assembly can be performed by laser welding or by tungsten inert gas (TIG) welding.
In addition, the intermediate connection ring itself includes or constitutes the filler metal needed for welding to the second subassembly.

A combustion chamber of the invention is thus characterized in that it is made up of a plurality of preformed shells including a chamber end wall, which shells are assembled together by butt-welding with the exception of a junction between two subassemblies of such shells, said junction being made by interposing an above-mentioned intermediate connection ring.
The invention can be better understood and other advantages thereof appear more clearly in the light of the following description of a method of fabricating a "reverse-flow" combustion chamber in compliance therewith, given purely by way of example and made with reference to the accompanying drawings, in which:
• Figures 1A to 1C show various welding operations
for making up a first subassembly;
• Figures 2A to 2C show various welding operations
for making up a second subassembly; and
• Figure 3 shows an orbital welding operation for
uniting the two subassemblies to constitute a reverse-
flow combustion chamber.
The drawings briefly described above are diagrammatic half-sections showing annular shells or other annular parts united in succession to make up a reverse-flow combustion chamber. With the exception of the welding shown in Figure 3, which is orbital welding specific to the invention enabling two already welded-together shell subassemblies to be themselves connected together, the other operations described with reference to Figures 1A to 1C and with reference to Figures 2A to 2C can be effected in another order. In contrast, the operation shown diagrammatically in Figure 3 is the last welding operation.
The butt-welding mentioned with reference to Figures 1A to 1C and 2A to 2C is indicated by arrows.
In Figure 1A, butt-welding is performed under an inert gas; a metal sheet stamped to the shape of a shell 11 forming the outer bend of the combustion chamber is

welded to an optionally machined connection ring 12. The ring is for use subsequently to make a connection between the chamber outlet and a high pressure turbine.
As shown in Figure IB, butt-welding is then performed under an inert gas between the other end of the outer bend and one end of a cylindrical shell 13 forming the outer wall of the combustion chamber.
As shown in Figure 1C, the other circular end of the cylindrical shell 13 is butt-welded to an intermediate connection ring 14 for use during final welding. The annular welding is performed under an inert gas. As mentioned above, the intermediate connection ring 14 has a cylindrical mounting surface 15 of reduced diameter. Its dimensions are determined by machining. For example, it may have a chamfer in the vicinity of the shoulder defining the surface of reduced diameter. The chamfer may serve to provide filler metal during the final welding operation.
At the end of the operation shown in Figure 1C, a first subassembly 20 forming the entire outer portion of a "reverse-flow" combustion chamber has been made.
In accordance with the operation shown in Figure 2A, a stamped sheet metal shell 21 for forming the inner bend of the combustion chamber is butt-welded with a connection ring 22 having the same function as that of Figure 1A and likewise intended for subsequent connection to the turbine.
In accordance with Figure 2B, the other end of the inner bend of the combustion chamber is butt-welded under an inert gas with a cylindrical shell 23 that is to form the inner wall of the combustion chamber.
Thereafter, in accordance with Figure 2C, the other end of the cylindrical shell 23 is butt-welded under an inert gas, to the inside edge of a shell 24 that is to constitute the end wall of the combustion chamber on which the injectors will be mounted.

At the end of the operation shown in Figure 2C, a second subassembly 30 constituting the entire inner wall of the future combustion chamber has been made, together with the end wall of the chamber.
In accordance with Figure 3, the two subassemblies 20 and 30 are docked one with the other, the intermediate connection ring 14 being fitted against the end wall of the chamber 24 and welded thereto by orbital welding, e.g. using a laser, and under an inert gas. As mentioned above, the filler metal required is provided by said intermediate connection ring.
It should be observed that, given the structure of the intermediate connection ring, the final welding operation does not disturb the flow of air around the combustion chamber.
In addition, the fact that the combustion chamber obtained in this way has a "smooth" outer wall makes it easier to position the laser equipment used for making the multiple orifices perforated in the wall of the combustion chamber.
The type of welding used ensures best possible thermomechanical behavior for the combustion chamber. Fabrication cost is reduced.

CLAIMS
1. A method of fabricating a combustion chamber
essentially made up of welded-together shells, the method
being characterized in that it comprises making
separately two subassemblies (20, 30) of such shells by
butt-welding the shells together, with an intermediate
connection ring (14) being welded to one end of a first
subassembly (20), the intermediate ring including an
assembly surface; engaging one end of a second
subassembly (30) on said surface; and welding it to said
intermediate ring.
2. A method according to claim 1, characterized in that a
said butt-welding is performed by adjusting the docking
of the two shells concerned by using radial expander
tools.
3. A method according to claim 2, characterized in that
said intermediate connection ring (14) is made at least
in part by machining.
4. A method according to any preceding claim,
characterized in that said intermediate connection ring
(14) includes or constitutes the filler metal needed for
welding with said second subassembly.
5. A method according to any preceding claim,
characterized in that other connection rings (12, 22) are
butt-welded with respective other ends of the two
subassemblies.
6. A method according to any preceding claim,
characterized in that, in order to fabricate a "reverse-
flow" combustion chamber, an outer first subassembly (20)
and an inner second subassembly (30) are built up by
butt-welding preformed shells together, one of the
subassemblies including an end wall of the chamber (24)

with one end thereof constituting the end of said second subassembly that is to be welded to said intermediate ring (14).
7. A combustion chamber, characterized in that it is
constituted by a plurality of preformed shells (11, 13,
23) including a chamber end wall (24) that are assembled
together by butt-welding, with the exception of a
junction between two subassemblies (20, 30) of said
shells, said junction being made with an interposed
intermediate connection ring (14).
8. A combustion chamber according to claim 7,
characterized in that said intermediate connection ring
(14) includes an assembly surface (15), in that it is
butt-welded to the first shell subassembly (20), and in
that it is welded to a second shell subassembly (30) by
welding on said assembly surface.
9. A turbojet including a combustion chamber obtained by
implementing the method according to any one of claims 1
to 6 .
10. A turbojet including a combustion chamber according
to claim 7 or claim 8.