US 3899815 A
A thermal bed screen pack is fabricated from stacked wire mesh discs, the compressed pack having a brazed outer periphery that is threaded so that the pack may be screwed into position adjacent an injector of, for example, a gas generator device. The thermal screen pack is then mechanically interlocked with the threaded grooves in the gas generator housing, thus inhibiting compaction and settling.
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Description (OCR text may contain errors)
United States Patent [191 Maddox THERMAL BED SCREEN PACK  Inventor: James P. Maddox, Sherman Oaks,
 Assignee: Rockwell International Corporation, El Segundo, Calif.
 Filed: Sept. 28, 1973  Appl. No.: 401,922
 U.S. Cl 29/163.5 R; 228/160; 228/182;
228/190  Int. Cl B23k 31/02  Field of Search 29/47l.1, 471.3, 472.3,
 References Cited UNITED STATES PATENTS 3,061,912 11/1962 Kalil 29/4723 X Aug. 19, 1975 3,127,668 4/1964 Troy 29/502 3,193,858 7/1965 Kahn 29/456 X 3,475,815 11/1969 Girdner 29/463 X 3,793,700 2/1974 Maikish 29/481 X Primary ExaminerDonald G. Kelly Assistant ExaminerMargaret M. Joyce Attorney, Agent, or Firm-L. Lee Humphries; R. G. Upton [5 7 ABSTRACT A thermal bed screen pack is fabricated from stacked wire mesh discs, the compressed pack having a brazed outer periphery that is threaded so that the pack may be screwed into position adjacent an injector of, for example, a gas generator device. The thermal screen pack is then mechanically interlocked with the threaded grooves in the gas generator housing, thus inhibiting compaction and settling.
8 Claims, 5 Drawing Figures PATENTEBAUG I 91975 FIG. 3
BED LENGTH F I G. 5
L BED LENGTH L EXIT LAST SCREEN O INLET FIRST SCREEN PRIOR ART SCREEN BED PACK FIG. 4
THERMAL BED SCREEN PACK BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to thermal bed screen packs. A screen pack is initially heated to chemically decompose a propellant such as hydrazine when the liquid is injected into the heated pack. The resultant gas generated in the pack further traverses through multiple interstices in the pack, the high surface-to-volume ratio of the pack serving to cool down the gas by means of endothermic ammonia dissocciation prior to manifolding the accelerated gas onto turbine blades commonly associated with rocket engine hardware.
2. Description of the Prior Art There are two primary methods utilized to decompose propellant such as hydrazine in the prior art, one of which is a catalytic pack that is comprised basically of compacted particles or fines that need no secondary heat to react with the hydrazine. The second type is a thermal bed screen pack which utilizes a series of wire mesh discs stacked and packed one upon the other against, for example, an injector face of a gas generator. The compacted screens are retained against the injector face by a perforated disc fastened within the gas generator tube or housing.
The first of the prior art methods, the catalytic pack, initiates a chemical decomposition of, for example, hydrazine, upon contact with the pack. Normally hydrazine is injected through an injector directly into the catalytic pack, whereupon the chemical reaction occurs. The resultant gas continues to pass through the pack, reducing the gas outlet temperature by NH (ammonia) dissociation to a level that will not adversely affect, for example, turbine blades that are driven by the gas generator. While the catalytic pack does not require preheating, as does the thermal bed screen pack, it has a number of limitations. Generally, catalytic packs are subject to degratation since they are very sensitive to vibration. The particle fines" within the pack are free (not bound together), hence they become easily rejected from the pack. For example, if a small break were to develop in the case surrounding the pack, the entire contents of the pack might easily by lost. Additionally, catalytic packs tend to be water-sensitive and require frequent purging to prevent pack degradation. Another problem associated with catalytic packs is known as channeling. When channels are created within the pack, liquid tends to collect, the trapped liquid then is quite subject to explosion, particularly when the liquid is hydrazine.
Conventional state-of-the-art thermal bed screen packs also suffer from a number of limitations. The thermal beds, like the catalytic packs, are initially packed against an injector in a gas generator application. A perforated disc securely entraps the pack against the injector face, as heretofore described. The thermal beds are preheated prior to injection of liquid hydrazine therein. After initial operation of the gas generator, the thermal bed compacts, resulting in a gap between the injector face and the thermal bed. This condition creates a large local liquid trap volume within the gas generator which, of course, is undesirable because of increased probability of detonations occurring as the result of ignition delay. Additionally, the heated thermal beds yield circumferentially, leaving an annular channel (liquid trap) between the interior wall of the gas generator housing and the peripheral edge of the compacted screens, thus reducing combustion efficiency and increasing the detonation hazard. Yet another disadvantage of the conventional thermal bed packs is the wire mesh orientations, one against the other. When the individual wire mesh discs are stacked one upon the other, the resulting disc-disc orientation is of a random nature. The screen pack active surface/volume ratio and flow are not predictable, resistance is less repeatable from pack to pack, resulting in varying performance levels from one thermal bed pack to another. Yet another disadvantage in the prior art thermal beds is the structural requirement that the perforated retaining disc that restrains the compacted thermal pack against an injector face must take the full AP load of the decomposed gases escaping through the pack. The instant invention eliminates the disc requirement.
There are a number of patents that disclose, for example, solid propellant grains that are retained within a rocket engine motor case by providing screw threads on the exterior surface of the solid propellant grains so that solid grains are retained within the case by the threaded interconnection. However, none of the prior art devices discloses gas generators having a series of compacted wire mesh screens that make up a thermal bed screen which is subsequently screwed into a gas generator housing directly adajcent an injector face.
SUMMARY OF THE INVENTION This invention describes a method to fabricate a precompacted, brazed, thermal screen pack. The thermal pack, after it has been peripherally brazed, is threaded on its brazed outer diameter and mated with an internally threaded gas generator tube. The precompacted thermal screen pack is normally positioned adjacent the face of a liquid injector commonly associated with a gas generator device. The interior of the gas generator housing or tube is threaded to accept the threaded, precompacted thermal screen pack. The thermal pack bed thread is preferably a small angle, square type thread that is one or two screen thicknesses wide at the maximum pitch diameter, thereby forming a mechanical interlock with the threaded groove in the gas generator tube wall. Thus, a relative screen-to-screen motion is prevented, i.e., wherein one individual wire mesh screen disc moves relative to an adjacent disc. With this type of mechanical interlock, even if the brazethreaded portion of the thermal pack mechanically fails, the impacted screens cannot escape or be ejected out of the exhaust of the gas generator. Thus, it can be seen that the thermal pack bed screen settling associated with the prior art thermal packs is substantially eliminated due to the mechanical interlock of the compacted layers of screens with the threads of the gas generator tube. Additionally, the axial drag force due to bed AP is evenly supported by the tube walls, thereby eliminating the high stress concentrations at the small wire'to-wire contact points and the resulting deformation that occurs when this stress exceeds the yield strength of the material. In the present invention, there is no reaction force at the bed outside diameter and the bed AP must be accommodated by the small contact asperities at wire-to-wire interfacing. As heretofore described, in conventional thermal bed packs that are retained within a gas generator tube by a perforated retainer disc, the first Screen receives a very low force but the last screen, located just upstream of the bed retainer disc, is exposed to the full bed AP force. Since each of the individual screens, compacted one upon the other in the present invention, are interlocked into in dividual threads located along the inner wall of the gas generator, there is an even distribution of loads relative to the gas flow through the screen. In addition to eliminating compaction, the threaded design also prevents formation of a bed chamber wall gap which has been observed to occur in conventional designs and can lead to liquid channeling down the wall liquid trap, and detonation. In the threaded design, the thread on a typical thermal screen pack provides a clearance which accommodates for a thermal expansion and eliminates yielding and dishing of the instant screen pack.
Each wire mesh disc or layer making up the precompacted thermal screen pack is carefully oriented so that the mesh in an individual disc is aligned by a fixed phase angle of approximately 45 to the mesh of an adjacent disc, thus providing a repeatable rate of flow of decomposed gases through the thermal bed pack. After alignment of each disc in the precompacted screen pack, the outer peripheral edges of the assembly are brazed or electro-deposited, the braze or electrodeposited layer penetration being approximately twice the thread depth required so that the thermal bed pack will retain its mechanical integrity after the thread machining operation is completed. Ideally, the thread pitch is calculated to encompass two screens per tooth of the threaded assembly. The brazed thermal screen packs must retain their integrity long enough to be assembled within the gas generator, i.e., placing the screen pack adjacent a liquid injector. Once the screen packs are installed, the bed is constrained and independent of the mechanical integrity of the braze because the screens are mechanically interlocked in the threaded grooves of the gas generator housing.
Therefore, it is an object of this invention to provide a precompacted thermal screen pack which will retain its integrity in as much as it is threaded within the interior tube of a gas generator, thereby eliminating circumferential and axial shrinking and loosening which is prevalent in conventional thermal bed screen packs.
More specificilly, it is an object of the invention to provide a thermal screen pack which is interlocked within a gas generator tube by means of threading the thermal pack within the gas generator tube, the individual screens within the precompacted screen pack being interlocked within the threaded grooves within the gas generator housing.
Thus, an advantage over prior art thermal bed packs is the method in which the instant thermal pack is interlocked within the threads of the gas generator tube, thereby preventing loose thermal packs within the gas generators, and the like.
Still another advantage over the prior art is the retention of the present thermal pack even though the diameter of the pack may be smaller after gas generator operation because it is retained within threads of the gas generator housing, thereby eliminating liquid traps and leak paths commonly associated with conventional packs.
Yet another advantage over the prior art is that there is no screen settling of the instant thermal bed pack. The pack will not settle since each of the stacked wire mesh screens of the assembly is individually interlocked within the threads of the gas generator housing.
The potentially dangerous liquid trap volume adjacent to the injector face is thereby eliminated.
Still another advantage over the prior art is the elimination of the high AP loads across convention thermal bed packs.
The above noted objects and advantages of the present invention will be more fully understood upon a study of the following description in conjunction with the detailed drawing.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view of a typical gas generator illustrating the gas generator housing, injector, thermal bed screen pack, and adjacent turbine;
FIG. 2 is an exploded, perspective view of a thermal bed screen pack;
FIG. 3 is an enlarged cross-sectional view of the assembled thermal bed screen pack wherein the peripheral edges of each of the wire mesh discs are embedded in the brazed threaded portion of the pack;
FIG. 4 is a graph illustrating the parametric properties of the prior art thermal bed screen packs; and
FIG. 5 is a graph of the parametric properties of the instant thermal bed screen pack.
DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to FIG. 1, the gas generator generally designated as 10 is comprised of housing 12, having an inlet conduit 16 which directs liquid, such as liquid hydrazine, into an injector 14 which, in turn, injects the hydrazine into a thermal screen pack generally designated as 22. The liquid hydrazine is decomposed into ammonia, hydrogen, and nitrogen by the heated thermal screen pack 22, the pack being heated by heater device 44. A typical pack heater consists of a length of heatable Nichrome wire placed adjacent 'or within the thermal pack, or the heat may be provided by hot gas from a smaller gas generator. As the gas traverses through the screen pack from surface 28 to surface 30 of the pack, the ammonia dissociates endothermically and cools sufficiently to prevent damage to the blades 19 of turbine 20. Typically, the cooled gas mixture is accelerated through throat region 18 of gas generator 10 before impacting turbine 20.
The thermal pack 22 is comprised of a multiplicity of compressed and stacked wire mesh screen discs 24. The discs 24 are brazed around their peripheral edges, thus forming an outer rim or layer of brazed material. The composite structure is then machined to form threads 27 in the brazed portion 26 of the screen pack 22. As is illustrated in FIG. 1, the thermal bed pack is threaded into housing 12, engaging threads 23 within the inner wall of the housing. The pack is threaded into the housing so that it is positioned adjacent to the face of the injector 14. It is clearly evident then that the pack cannot be ejected out of the gas generator since it is mechanically interlocked with threads 23 of the housing. A heater 44 is utilized to preheat the thermal pack to a temperature range of between 800F to 1,000F to effectively decompose a liquid propellant, such as hydrazine. If bipropellants are utilized, such as nitrogen tetroxide/ammonia or liquid oxygen/ammonia, the NH,, decomposing function of the thermal bed screen pack 22 is also applicable to provide turbine inlet temperature control.
Turning now to FIG. 2, the series of spaced apart discs 24 making up the screen pack 22 are carefully stacked so that the weave 32 of the individual wire mesh discs is oriented so that the interstices formed by the weave of screens are alternately aligned 45 out of phase relative to one another. Proper relative orientation of the weave of the multiple disc packs will assure that each pack satisfactorily performs within predicted ranges from pack to pack.
FIG. 3 illustrates the wire mesh thermal bed screen pack in its assembled state. It should be noted here that the peripheral edges of each of the wire mesh discs 24 extend well into the brazed portion 26 so that each disc is firmly interlocked within the solid brazed portion. The depth of the braze into the peripheral edges 25 of disc 24 is indicated by numeral 34. It should also be noted that the depth of the threaded portion (minor axis) 38 extends no more than half-way into the brazed portion 26. A typical thread depth 38 would be preferably an ACME thread on, for example, a two-inch diameter pack, the depth being 0.051 inch so that a five mil clearance would be provided between the maximum pitch diameter and wall thread groove (threads 23 of housing 12) to accommodate for thermal expansion and eliminate yielding and dishing of the screens, as previously described. The preferred thread type (36) allows for expansion and contraction of the thermal pack 22 while providing maximum protection against liquid channeling down the wall between threads 27 of the thermal pack and threads 23 of the housing 12. The threads provide a tortuous path for the fluid between the thermal pack 22 and the housing 12.
It should be additionally pointed out that other methods to form the solid portion 26 encompassing the discs 24 may be incorporated. For example, the solid layer entrapping the peripheral edges 25 of the wire mesh disc 24 may be electro-deposited to a sufficient depth 34 to provide a solid base for the thread machining operation. A sintered powder, metallurgical process using nickel or gold powder could also be utilized to form the solid rim portion 26. Since the peripheral edges 25 of the wire mesh disc 24 extend into the threaded portion 27 of pack 22, it can readily be seen, in FIG. 1, that the individual discs 24 are interlocked with the threads 23 of housing 12. The edges 25 of at least two out of every four individual discs 24 extend into the major diameter 38 of the threads 27, thereby physically locking these affected discs within threads 23 of housing 12.
The wire mesh screen discs 24 are formed from metal wire (e.g., lnconel 600, manufactured by Huntington Alloys Products, Division of International Nickel, Inc., Huntington, W. Va.), having openings or interstices ranging from 0.030 to 0.050 inch in a typical thermal bed screen pack.
Turning now to FIG. 4, the graph illustrates the AP load that is subjected to the downstream face of a conventional prior art screen pack. The line 50 illustrates the axial force on the screens, the line 52 indicates the compressive stress on the screens, and the dotted line 54 illustrates the material yield strength of the screens. The shaded area 56 depicts plastic deformation of the conventional screen packs, clearly indicating the AP loads on the downstream portion of the pack.
FIG. 5 is a similar graph illustrating the parametric properties of the thermal bed screen pack of the instant invention. The parallel lines 60 and 62 illustrate substantially no axial or compressive stress forces on the screen. The AP loads on the screen pack 22 are evenly distributed, primarily due to the individual support of each of the wire mesh screens 24 interlocked within the threads 27 engaged with threads 23 in housing 12. Since there is no particular stress on the screens, there is very little plastic deformation of the screens during thermal pack operation, as is indicated by the dotted yield strength line 64.
It will, of course, be realized that various modifications can be made in the design and operation of the present invention without departing from the spirit thereof. Thus, while the principal, preferred construction and mode of operation of the invention have been explained, and what is now considered to represent its best embodiment has been illustrated and described, it should be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.
1. A method to fabricate a thermal bed screen pack comprising the steps of:
stacking a multiplicity of individual wire mesh discs;
compressing said stack of wire mesh discs to form said pack;
forming. metallurgically, a solid rim portion around the peripheral edges of said compressed stacked discs to trap each individual disc within said metallurgically formed rim; and
machining said metallurgically formed rim to form threads therein to enable said threaded compressed screen pack to be threadably engaged with threads machined in the interior wall of a gas generator device.
2. The invention as set forth in claim 1 wherein said individual wire mesh discs are fabricated from lnconel 600 wire having interstices formed by said discs ranging from 0.030 inch to 0.050 inch.
3. The invention as set forth in claim 1 wherein said metallurgically formed rim portion is a layer of braze material.
4. The invention as set forth in claim 1 wherein said metallurgically formed rim portion is a layer of electrodeposited nickel material.
5. The invention as set forth in claim 1 wherein said metallurgically formed rim portion is a layer of sintered powdered nickel metal.
6. The invention of claim 1 wherein said edges of at least half of the multiplicity of said individual wire mesh discs forming said screen pack extend into the extended major diameter of said threads machined into said rim portion, thereby locking at least half of said discs within said threads formed in said interior wall of said gas generator device.
7. The invention of claim 1 wherein said solid rim portion is twice the depth of the minor axis of said screen pack.