Method of fabricating a rocket engine combustion chamber
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 86-568(72 Stat. 435; 42 U.S.C. 2457) 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods of making liquid rocket engine combustion chambers. 
     2. Prior art 
     It is known to make a rocket engine combustion chamber by casting a liner and then forging it into the desired hourglass shape. Cooling channels are then cut in the outer surface of the liner, followed by filling the cooling channels with wax. A thin layer of copper is then electrodeposited over the filled channels and a thicker layer of nickel is electrodeposited over the copper layer. The wax is then removed from the channels to leave them clear and a multi-part clamshell jacket is fitted around the liner and the entire assembly is welded together to form a combustion chamber. 
     A major disadvantage of this process is that there are numerous welds which cannot be inspected. Failure of one of the welds can result in a catastrophic failure of a space shuttle being lifted into orbit by the rocket motor. 
     Another disadvantage of this conventional process is that it is very expensive and time consuming, the electrodeposition of the copper and nickel layers alone requiring thousands of man hours and great expense. 
     SUMMARY OF THE INVENTION 
     A process for making a combustion chamber for a rocket engine wherein a copper alloy in particle form is injected into a stream of a heated carrier gas in plasma form which is projected onto the inner surface of a cast, hollow metal jacket having the configuration of a rocket engine combustion chamber. The particles are in the plasma stream for a sufficient length of time to heat the particles to a temperature such that the particles will flatten and adhere to previously deposited particles but will not spatter or vaporize. After a layer of the alloy has been built up, cooling channels are cut in the layer. The cooling channels are filled with a filler material and a second layer of alloy is then applied to cover the cooling channels. The filler material is then removed from the cooling channels to leave open cooling channels through which a cooling liquid can be circulated. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a cast metal jacket which serves as a form for making the combustion chamber. 
     FIG. 2 is a cross sectional view of the jacket of FIG. 1 after a first layer of an alloy liner has been applied to the inner surface of the jacket. 
     FIG. 3 is an enlarged fragmentary view taken on line 3--3 of FIG. 2 showing cooling channels which have been cut in the first layer of the alloy liner and then filled with a filler material. 
     FIG. 4 is a cross sectional view of the apparatus of FIG. 2 after a second layer of alloy liner has been applied. 
     FIG. 5 is a greatly enlarged fragmentary view taken on line 5--5 of FIG. 4 showing the completed cooling channels after the filler has been removed from them. 
     FIG. 6 is an enlarged fragmentary view showing the vacuum plasma spray nozzle positioned to spray the heated alloy particles onto the inner surface of the jacket. 
     FIG. 7 is a schematic view showing the structure used with the vacuum plasma spray nozzle to form the combustion chamber. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now in detail to the drawings, there is shown in FIG. 1 a one-piece, cast metal jacket 11 inside of which a liner is to be formed to make the combustion chamber for a liquid rocket engine. The jacket 11 is cast from a suitable metal, with the preferred metal being one of several alloys, Inconel 718, stainless steel 347 or JBK-75. These alloys have the following compositions: 
     
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          Inconel 718                                                     
                  Stainless 347                                           
                               JBK-75                                     
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nickel      50-55%     9-13%       29-31%                                 
chromium    17-21%    17-19%       14-16%                                 
niobium +   4.75-5.5% 0.8%         0%                                     
tantalum                                                                  
molybdenum  2.8-3.3%  0%            1-1.5%                                
titanium    0.85-1.15%                                                    
                      0%           2.1-2.5%                               
aluminum    0.2-0.8   0%            0.1-0.35%                             
iron        balance   balance      balance                                
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     While the above alloys are preferred, other metals and alloys can be used provided that the metal or alloy can withstand the high temperatures encountered in vacuum plasma spraying (described below) and has properties suitable for use in a liquid rocket combustion chamber. 
     FIG. 2 shows the jacket of FIG. 1 after a layer of a copper alloy forming a liner 12 has been applied to the inner surface of the jacket. This liner is applied by a vacuum plasma spray system wherein an alloy in particle form is heated and projected at a high velocity into impact with the inner surface of the jacket 11 under vacuum or low pressure conditions, i.e., 40 to 200 torr. 
     FIG. 6 shows a cross sectional view of the spray head 15 used to form the liner 12. The head 15 includes a tungsten cathode 18 and a copper anode 19, the anode 19 being surrounded by a water jacket 22 for cooling the anode. A DC voltage of 40 to 70 volts from a supply 23 is applied across the cathode and anode to cause a current flow of 800 to 1,200 amps between the cathode and the anode. 
     A carrier gas of about 80%-100% argon and up to 20% helium or hydrogen from a supply 24 is passed through the head 15 and toward the jacket 11 at a velocity of Mach 2 to Mach 3. A metal alloy in powder form from a supply 27 is injected through ports 28 into the gas stream. The DC current flowing in the form of an arc from the cathode 18 to the anode 19 heats the gas to a high temperature, i.e., 5,000 to 20,000 Kelvin and ionizes it to form a plasma. 
     The metal alloy is preferably an alloy of 3.2 to 3.7 weight percent silver with the balance being copper or an alloy of 2.75 to 3.25 weight percent silver, 0.3 to 0.7 weight percent zirconium with the balance being copper. The alloy is in powder form with a particle diameter within the range of 5 to 45 microns. 
     The head 15 is positioned at a distance from the surface being sprayed such that the powder is in the heated gas stream for a time sufficient to heat the powder particles to a high enough temperature that the particles will flatten and adhere to previously deposited particles. However, the temperature of the plasma and time period that the particles are in the plasma should not be great enough that the particles are melted to the point where they spatter when they strike the surface of the jacket. 
     FIG. 7 schematically shows apparatus used with the spray head 15. The jacket 11 is supported inside a vacuum chamber 28 by a spindle 31 secured to an arm 32 mounted on a shaft 33 which extends through the wall of the chamber 28. The spindle 31 may be rotated to rotate the jacket 11 and the shaft 33 can be turned to pivot the jacket 11. 
     The spray head 15 is mounted on the end of a control arm 35 operated by a motion control unit 37-. attached to the top of the vacuum chamber 28. The arm 35 can be manipulated by the control unit 37 to traverse the spray head 15 around the inner surface of the jacket 11. The arm can also be moved toward or away from the control unit 37 to move the spray head 15 in a direction parallel to the surface of the jacket 15. Also, the arm 32 can be traversed about an axis perpendicular to the axis of the arm to vary the distance of the spray head 15 from the inner surface of the jacket 11. The control unit 37, which is a known piece of equipment, is not a part of this invention. 
     After the liner has been formed to a depth of 0.6 to 1.5 cm, the jacket 11 with its liner 12 are removed from the vacuum chamber 28 and allowed to cool. Cooling slots 38 (FIG. 3) are then cut with a slotting saw into the inner surface of the liner 12 in a known manner. In the completed combustion chamber liquid hydrogen will flow through these slots to cool the combustion chamber and prevent it from reaching temperatures which would damage or destroy it. These slots are best shown in FIG. 3, where the slots are shown filled with a filling material 39. 
     The purpose of the filling material is to allow the slots 38 to be opened by removal of the material after the chamber is completed. The material may be any material which can withstand a spraying operation such as described above and which can be dissolved out of the slots 38 by the use of water, acids, etc. Some examples of filler material are aluminum, mild steel, titanium, lithium flouride, calcium chloride, ceramics and other salts. If a metal such as mild steel is to be used as the filler material, a mild steel rod having a cross sectional configuration and a contour to fit into the slot 38 will be used. The slot will be cut with a slotting saw and the mild steel rod will be placed in the slot. 
     The inner surface of the liner 12 is then machined to remove any excess of the filler material. The jacket and liner are then returned to the vacuum chamber 28 and the spraying continued as described above until an additional liner thickness of 0.05 to 0.15 cm has been achieved. This additional layer covers the filled slots 38. The mild steel 39 is then removed from the slots 38 to leave open cooling channels in the liner 12, as best shown in FIG. 5. With this, the combustion chamber is completed and ready for testing. 
     It can readily be seen that no welding is involved n making the combustion chamber itself. Thus, the risk of catastrophic failure is substantially reduced. Also, substantial savings are made in the time and expense required to make the combustion chamber.