Power plants – Reaction motor – Liquid oxidizer
Reexamination Certificate
1998-08-31
2001-08-07
Freay, Charles G. (Department: 3746)
Power plants
Reaction motor
Liquid oxidizer
C060S039010, C060S039440
Reexamination Certificate
active
06269630
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the structure of a liquid-fueled rocket engine, and, more particularly, to the structure at the injector end of the combustion chamber.
A typical liquid-fueled rocket engine includes a generally cylindrical combustion chamber, with an injector attached to its injector end and a flared nozzle attached to its nozzle end. A liquid propellant including fuel and an oxidizer flows through injector ports in the injector and into the combustion chamber. The propellant mixes and ignites in the combustion chamber. The hot gas resulting from the combustion expands through the nozzle and drives the rocket engine and the attached rocket structure in the direction opposite to that in which the nozzle is pointed.
In order to achieve maximum efficiency of the rocket engine in terms of thrust per weight of propellant and maximum specific impulse, the mixing of the propellant components must be complete to encourage complete combustion. An internal step structure at the injector end of the combustion chamber had been previously developed to promote the complete mixing and combustion. However, early versions of the rocket engine having the internal step structure within the combustion chamber suffered from two problems. First, their efficiency, while better than that of a rocket engine of comparable weight but having no internal step structure, was not as good as desired. Second, in some cases there was premature failure of the wall of the combustion chamber during fabrication or during test firings.
The second problem is of particular concern, because a preferred embodiment of the rocket engine is to be used in commercial communications satellites. The rocket engine boosts the satellite from low-earth orbit to geosynchronous orbit or initiates interplanetary missions. If the rocket engine combustion chamber fails prior to completion of its mission, the entire satellite may be lost to service because it does not achieve the required geosynchronous orbit.
Thus, while the rocket engine with internal step structure in the combustion chamber offers important potential benefits in terms of improved efficiency, those benefits have not been fully realized in initial forms of the rocket engine. There is a need for a design which achieves maximum efficiency with high reliability and low likelihood of premature failure. The present invention fulfills, this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a rocket engine having an internal step structure at the injector end. The configuration of the step structure is optimized for high combustion efficiency and high specific impulse while protecting the wall of the combustion chamber and maintaining a relatively cool injector end. The attachment of the step structure to the injector and to the wall of the combustion chamber is selected to achieve sufficient structural strength and seating against leakage of hot gas, and also to prevent damage to the combustion chamber wall. The resulting rocket engine has high efficiency and also good reliability.
In accordance with the invention, a rocket engine comprises a generally cylindrical annular combustion chamber having a wall with a chamber outer diameter and a chamber inner diameter. The chamber has an injector end, a throat, and a nozzle end. The injector is attached to the injector end of the combustion chamber. The chamber length is measured from the injector face to the throat. A generally cylindrical annular step collar fits within the combustion chamber adjacent to the injector end thereof. The step collar has a step collar outer diameter, a step collar inner diameter, and a step collar length measured from the injector face to the end of the step. There is an attachment of the combustion chamber, the injector, and the step collar.
In one form of the invention, the step collar outer diameter is less than the chamber inner diameter by a clearance amount or gap. The clearance allows the fabrication of the rocket engine to be performed without damaging the wall of the combustion chamber. The clearance also allows the step collar to expand during the heating experienced when the rocket engine is operated, without cracking the wall of the combustion chamber. That is, the gap should be sufficiently great that it is not closed and a clearance remains when the rocket engine is operated and the step collar expands radially outwardly toward the wall of the combustion chamber. On the other hand, the gap should not be larger than necessary to provide for this clearance during operation, so that hot gas does not flow into the gap by a backdraft effect to damage the inner surface of the wall of the combustion chamber. In a preferred embodiment of the rocket engine wherein the inner diameter of the wall of the combustion chamber is about 1.78 inches, the difference in the step collar outer diameter and the chamber wall inner diameter is from about 0.020 inches to about 0.024 inches, measured at room temperature. That is, the gap between the step collar outer diameter and the chamber wall inner diameter is from about 0.010 inches to about 0.012 inches.
In another embodiment, the step collar length is from about 21 percent to about 31 percent of the chamber length. This step collar length achieves an optimized performance of the engine while avoiding damage to the inner surface of the wall of the combustion chamber by hot combustion gasses.
In yet another embodiment, the combustion chamber is made of a first material, and the step collar is made of a second material different from the first material and having greater corrosion and erosion resistance than that of the first material in the combustion chamber environment, in order to withstand the combustion environment that it experiences from being in direct contact with the combustion gases. Examples of preferred materials of construction of the step collar include an alloy of platinum and rhodium, an alloy of columbium, or a ceramic.
The injector, step collar, and combustion chamber must be joined together at the injector end of the combustion chamber by the attachment which provides sufficient strength, seals against the leakage of hot combustion gas from the interior of the combustion chamber, and prevents damage due to the large dimensional changes experienced during the firing of the rocket engine. In the preferred approach, the attachment includes a step collar/injector joint structure joining the step collar and the injector, and a clip structure joining the combustion chamber and the step collar. The clip structure comprises a C-shaped annular clip having a first leg extending parallel to the chamber length and with an inner diameter of about that of the outer diameter of the combustion chamber and affixed thereto, a second leg extending parallel to the chamber length and with an outer diameter of no greater than that of the outer diameter of the step collar and affixed thereto, and a web extending between the first leg and the second leg. This C-clip provides the necessary strength by acting in the manner of a circumferential rib to withstand hoop stresses produced during the manufacturing and the firing of the rocket engine and also the thermal expansion stresses. It seals the injector end, and also allows the step collar to differentially expand without loading excessive additional stress into the wall of the combustion chamber. The clip is preferably made from a molybdenum alloy, most preferably an alloy of 50 weight percent molybdenum, 50 weight percent rhenium whose surface is gold plated.
The present invention thus provides a rocket engine which has improved efficiency of operation and improved specific impulse due to the presence of the internal step within the combustion chamber. It also has good reliability because its structure withstands the combustion temperatures to which it is exposed, and absorbs the additional stresses caused by the presence of the step collar. Other features and advantages of the present invention will be apparent from th
Bronson David
Kreiner Kurt B.
Neiderman Joel M.
Stechman Carl R.
Woll Peter E.
Freay Charles G.
Gudmestad T.
Hughes Electronics Corporation
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