TECHNICAL FIELD 
     Our invention relates to the field of valves. More specifically, our invention relates to valve seats in poppet valves, particularly those used in spacecraft dual-fuel thruster fuel control systems. 
     BACKGROUND OF THE INVENTION 
     Valves designed for use in spacecraft must endure great environmental extremes. The valves must be extremely reliable, yet must be lightweight. Such valves experience much shock and vibration during launch of the spacecraft, as well as extremes in temperature. The fluids the valves control are often extremely corrosive, requiring the use of special materials. Seals are particularly vulnerable since they are often made from resinous or elastomeric material that is subject to swelling or deteriorates when exposed to these fluids. 
     Dual-fuel thruster systems for spacecraft typically use corrosive fuels such as hydrazine, monomethylhydrazine, and nitrogen tetroxide, an extremely unfriendly substance that is incompatible with most materials used to make valve seals. Tetrafluoroethylene, sold under the trade name TEFLON by DuPont, is unaffected by nitrogen tetroxide, however, making TEFLON a strong candidate for use in the manufacture of valve seals. Unfortunately, TEFLON has other properties that make it difficult to use in an environment in which parts cannot be replaced. 
     TEFLON is not resilient and has no shape memory. As a result, deformations and damage are permanent and cumulative, leading to leakage when TEFLON is used to make seals and/or sealing valve seats. One source of deformation is the shock and vibration of launch. Deformations also can result from engagement between a poppet and a TEFLON valve seat. 
     Additional properties of TEFLON that cause problems are its tendency to flow under load (&#34;cold flow&#34;) and its high coefficient of thermal expansion. Any load applied to a TEFLON valve seat can cause deformations of the seat and result in leakage. The high coefficient of thermal expansion results in sensitivity to changes in temperature. Even a small change in temperature can result in expansion or shrinkage of a TEFLON valve seat that can cause leakage. 
     Thus, while valve designers use TEFLON valve seats because of its compatibility with corrosive chemicals, its other properties render such use problematic. There is a need for a TEFLON valve seat arrangement that minimizes the likelihood of deformation of the seat from shock, vibration, and cold flow. There is also a need for a TEFLON valve seat that limits exposure of the seat to changes in temperature that can cause undesirable expansion and contraction of the seat. Further, there is a need for a valve seat that reduces or eliminates leakage ordinarily resulting in the natural deformation caused by contact between the poppet and the seat. 
     Most prior art valves using TEFLON as a sealing material hold the TEFLON parts in place with threaded retainers. As they are rotated during assembly, the threaded parts slide against the TEFLON parts, causing pitting, stretching, and other deformations of the TEFLON parts, increasing the possibility of leakage through the seal. Additionally, the assembled threaded parts place a load on the TEFLON, causing cold flow of the TEFLON that can lead to improper mating of parts and further leakage. While the amount of leakage caused by such damage and cold flow may be acceptable in the applications for which these prior art valves are designed, such leakage is not acceptable in dual-fuel thruster systems for spacecraft. Consequently, there is a need for a sealing valve seat arrangement using TEFLON that is not assembled with threads and that allows extremely low leakage. 
     While an interference fit would be preferred to avoid damaging the TEFLON seat, the properties of TEFLON cause problems when such an interference fit is actually attempted. For example, when pressing the TEFLON into position, the TEFLON will tear at the interface between the metal and the TEFLON, resulting in leakage paths. The annealing temperature for TEFLON is on the order of 400° F., and TEFLON has a large coefficient of thermal expansion. These properties make it easy to expand the TEFLON so that it can be shrunk-fit onto a metal piece. However, to achieve a similar interference fit of a second piece of metal over the TEFLON seat, the metal piece must be heated to a temperature ranging from 420° F. to 600° F., depending on the exact type of metal used to make the metal piece and the desired amount of interference. Thus, the second metal piece would be heated beyond the annealing temperature of the TEFLON seat, and possibly beyond its melting point, altering the properties of the seat, if not outright destroying it. There is a need, therefore, for a method of making a valve seat arrangement using a TEFLON seat where the parts are assembled using an interference fit but without exposing the seat to damaging heat. 
     Additionally, with most prior art arrangements, a small indentation results when the poppet contacts the seat. If the poppet is not correctly guided, the poppet will create overlapping indentations, resulting in leakage. Thus, there is a need for a valve seat arrangement in which the poppet is properly guided to prevent the formation of such overlapping indentations. 
     SUMMARY OF THE INVENTION 
     Our invention provides a sealing valve seat arrangement using a TEFLON seat that allows extremely low leakage. The parts are assembled without threads using a novel interference fit method, avoiding the damage that thread use ordinarily does to TEFLON sealing members. Further, our invention greatly restricts TEFLON cold flow so that the low leakage of the valve seat arrangement can be maintained for a greater period. We also minimize deformation from shock and vibration, and we limit the ways the TEFLON can change from variations in temperature. 
     We shape the seat, its supporting piece, and its retainer so that the TEFLON of the seat can cold flow substantially only toward the poppet that engages the seat. As a result, any cold flow of the seat only moves the engagement point of the seat and poppet in the direction of motion of the poppet. We also include grooves in the surfaces of the supporting piece and the retainer to provide a more secure engagement of the seat. The grooves also create a more labyrinth-like leak path around the seat, reducing leakage. 
     The seat is shrunk-fit on its supporting piece in the usual manner, but the interference fit of the seat retainer cannot be performed the same way, as discussed above. We overcome the interference fit problem by immersing the seat and its supporting piece in liquid nitrogen to contract them. This allows us to place the retainer over the seat after removing the assembly from the liquid nitrogen. As the seat assembly warms, the seat expands into the retainer, creating an interference fit without exposing the seat to potentially damaging heat. Our resulting valve seat arrangement provides excellent poppet mating and sealing characteristics, far superior to valves assembled with threads. An additional benefit of our invention is that, because the seat has not been forced over the metal, there are no scratches in the seat that could result in leak paths. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of the valve seat arrangement of the instant invention taken along a longitudinal axis of the arrangement. 
     FIG. 2 is a flow chart schematically illustrating the method of making the instant invention. 
     FIG. 3 shows the outlet guide and seat of the invention before they are assembled. 
     FIG. 4 shows the outlet guide and seat of the invention after they have been assembled to form the seat/guide assembly and shows the retainer before it has been placed on the seat/guide assembly. 
     FIG. 5 shows a solenoid valve in which the invention can be used. 
    
    
     DESCRIPTION OF THE INVENTION 
     Our invention is particularly suited for poppet solenoid type valves such as the solenoid valve 90 shown in FIG. 5, but can be used in any suitable valve that requires extremely low leakage and high reliability. 
     In the preferred embodiment as shown in FIGS. 1 and 3-5, our invention comprises three major components: an outlet guide 10, a seat 20, and a retainer 30. The outlet guide 10 supports the seat 20 and also provides a conduit 50 for the valve 90 and a guide portion 16 for guiding the valve poppet 40. The outlet guide 10 is generally annular and preferably includes three sections 11, 12, 13 of progressively smaller outer diameters with first and second steps 14, 15 separating the sections. The second section 12 engages the retainer 30, which also abuts the face of the first step 14 of the outlet guide 10. The third section 13 and the face of the second step 15 support and constrain the seat 20 in the final assembly. The outer surface of the third section 13 and the face of the second step 15 preferably include grooves 17 for positively securing the seat 20 as will be explained in more detail below. 
     The seat 20 itself is also generally annular with a first section 21 and a step 25 to a second section 22 of reduced outer diameter. The second section 22 can instead have a reduced inner diameter as compared to the first section 21, though this would require a more complex shape for the outlet guide 10. The end 24 of the first section 21 of the seat 20 abuts the second step face 15 of the outlet guide 10. The second section 22 of the seat 20 preferably extends along the outlet guide 10 when assembled and provides a seat engagement surface 26 at its tip 23. 
     The retainer 30 is generally annular, but includes a rim 33 that engages the step 25 of the seat 20 to hold the seat 20 in place. As with the outlet guide 10, the retainer 30 has three sections, the second section 32 having a smaller outer diameter than the first section 31, the third section 33 having a smaller inner diameter than the second section 32 and comprising the rim 33. Two steps 34, 35 separate the sections: a first step 34 from the outer diameter of the first section 31 to the outer diameter of the second section 32; and a second step 35 from the inner diameter of the second section 32 to the inner diameter of the third section 33 to form the rim 33. The first and second sections 31, 32 of the retainer 30 engage the second section 12 of the outlet guide 10. The end 36 of the first section 31 abuts the first step 14 of the outlet guide 10, and the second section 32 also engages the first section 21 of the seat 20. The rim 33 of the retainer 30 engages the step 25 and second section 22 of the seat 20. Grooves 37 can be provided on the inner surface of the second section 32 and on the surface of the second step 35 of the retainer 30 to enhance engagement between the retainer 30 and the seat 20. 
     Once the valve seat arrangement 1 is assembled, the outlet guide 10 and the retainer 30 cooperate to prevent flow of the seat material in any direction but toward the poppet 40 that engages the seat 20. While flow of the seat material will change the point along the direction of motion of the poppet 40 at which the poppet 40 engages the seat 20, the flow will not alter the point on the seat engagement surface 26 at which the poppet 40 engages the seat 20. Additionally, the load applied to the seat engagement surface 26 by the poppet 40 tends to hold the seat material in its proper place, counteracting flow toward the poppet 40. Thus, the sealing properties of the valve seat 20 are maintained even where cold flow occurs. 
     In the preferred embodiment, the guide portion 16 of the outlet guide 10 is important to the sealing integrity of the seat 20 since it ensures consistent alignment of the poppet 40 and proper engagement of the poppet engagement surface 41 with the tip 23 of the seat 20. The guide portion 16 comprises an annulus attached to the third section 13 of the outlet guide 10 into the bore of a solenoid, such as the solenoid 90 shown in FIG. 5. The guide portion 16 is sized to have a very tight tolerance within the poppet 40 mounted thereon. The tight tolerance assures that the poppet 40 follows substantially the same path every time it travels along the guide 16. Preferably, the guide 16 is long enough relative to the poppet 40 that cocking of the poppet 40 within the solenoid bore is minimized, further ensuring that the poppet 40 travels substantially the same path with each engagement of the seat 20. Variations of our preferred arrangement could conceivably be used to ensure consistent poppet 40 alignment and still be within the scope of our invention. 
     As mentioned above, the assembly of our invention involves two interference fits: one between the outlet guide 10 and the seat 20, and one between the seat/guide assembly and the retainer 30. As was also mentioned above, the retainer 30 cannot be heated for its interference fit without damaging or destroying the valve seat 20. We have therefore developed a particular preferred method of assembling our invention 100, shown schematically in FIG. 2, that allows the interference fits to be achieved with no damage to the valve seat 20. Preferably, the amount of interference used in the interference fits minimizes load on the seat 20, thus preventing cold flow of the preferred TEFLON seat 20. 
     First, as indicated by box 101 and its sub-parts, we heat the seat 20 for a predetermined period and at a predetermined temperature to expand the seat 20 beyond the designed interference. For the preferred TEFLON seat 20, the heating process is also an annealing treatment (box 1011) that gives the TEFLON better sealing and durability qualities. Because TEFLON has a high coefficient of thermal expansion, the preferred annealing temperature of 375° F.-400° F. (box 1012) causes adequate expansion of the seat 20. The TEFLON seat 20 is preferably maintained at the elevated temperature for at least 30 minutes to an hour (box 1013) and placed on the outlet guide 10 to form the seat/guide assembly (box 102 and its sub-parts). FIG. 3 shows the seat 20 and outlet guide 10 as they would appear after heating of the seat 20, but before placement of the seat 20 on the outlet guide 10. The seat 20 is placed on the outlet guide 10 such that the first and second sections 21, 22 of the seat 20 are aligned with the second and third sections 12, 13 of the outlet guide 10, with the end of the first section 21 of seat 20 abutting the first step 14 of the outlet guide 10 (boxes 1021-1023). The assembly is then allowed to return to ambient temperature (box 103). The seat 20 shrinks and becomes firmly attached to the outlet guide 10 by an interference fit (boxes 1031 and 1032). If grooves 17 are included in the surfaces of the third section 13 and the second step 15 of the outlet guide 10, the preferred TEFLON seat 20 flows into the grooves 17 to provide a more secure attachment. 
     After the seat/guide assembly cools to ambient temperature, it is cooled to a second predetermined temperature for a second predetermined period to shrink the assembly (boxes 104 and 1041-1044). We prefer to force the seat assembly to contract by immersing it in liquid nitrogen (box 1043). The exact amount of time that the seat assembly is left in the liquid nitrogen depends on the ambient temperature and the temperature of the liquid nitrogen (box 1044). However, a good gauge of the proper amount of cooling and consequent shrinkage of the seat assembly is the termination of bubbling from the seat assembly. Termination of bubbling indicates that the seat assembly has cooled below the vaporization temperature of nitrogen (box 1042), which should cause adequate shrinkage of the seat assembly. The assembly is then removed from the liquid nitrogen and the retainer 30 is placed on the seat/guide assembly (box 105). FIG. 4 shows the seat/guide assembly and retainer 30 as they would appear just before the retainer 30 is placed on the seat/guide assembly. Preferably, as indicated in boxes 1051-1053 and as shown in FIGS. 1 and 5, the retainer 30 abuts the second step 15 of the outlet guide 10; the first and second sections 31, 32 of the retainer 30 are aligned with and engage the second section 12 of the outlet guide 10 and the first section 21 of the seat 20; and the third section or rim 33 of the retainer 30 abuts the step 25 of the seat 20 with retainer step 35. The third section 33 of the retainer also engages the second section 22 of the seat 20. The parts are then allowed to return to ambient temperature (box 106), causing the seat assembly to expand into the retainer 30 (box 1061). As with the seat 20 and outlet guide 10, the retainer 30 is manufactured with enough interference to ensure its secure attachment by an interference fit on the seat/guide assembly when the assembly returns to ambient temperature (box 1062). The completed valve seat 20 arrangement is washed with alcohol once it reaches ambient temperature to remove condensation and then vacuum dried for 30 minutes to an hour. If grooves 37 are included on the inner surface of the second section 32 of the retainer 30, the preferred TEFLON seat 20 flows into the grooves 27 to provide a more secure connection between the retainer 30 and the seat 20. 
     The method of manufacture 100 is important since it allows assembly of the valve seat arrangement without threads, preserving the integrity of the seat 20 even when the seat 20 is made from TEFLON. The seat/guide assembly is cooled in liquid nitrogen because the annealing temperature of TEFLON is close to or below the temperature to which the retainer 30 would need to be heated for an ordinary shrink fit. Though the temperature to which the retainer 30 must be heated is dependent on the exact amount of expansion required and the particular material used, the temperature can be as low as 420° F. or as high as 2100° F. Thus, damage to and leakage through the seat 20 could result if the retainer 30 were heated for a shrink fit. 
     PARTS LIST 
     1 Valve seat arrangement 
     10 Outlet guide 
     11 First section of outlet guide 
     12 Second section of outlet guide 
     13 Third section of outlet guide 
     14 First step of outlet guide between second and third sections 
     15 Second step of outlet guide between first and second sections 
     16 Guide for poppet 
     17 Grooves of outlet guide 
     20 Sealing valve seat 
     21 First section of valve seat 
     22 Second section of valve seat 
     23 Tip or end of valve seat 
     24 End of first section of valve seat 
     25 Step between first and second sections of valve seat 
     26 Seat engagement surface 
     30 Retainer 
     31 First section of retainer 
     32 Second section of retainer 
     33 Third section or rim of retainer 
     34 First step between first and second sections of retainer 
     35 Second step between second and third sections of retainer 
     36 End of first section of retainer 
     37 Grooves of retainer 
     40 Poppet 
     41 Engagement surface of poppet 
     50 conduit 
     90 solenoid valve 
     100 Method of assembly 
     101 Step of heating seat 
     1011 Indication that step of heating seat anneals seat 
     1012 Sub-step of controlling temperature within specific range 
     1013 Sub-step of maintaining elevated temperature for at least a minimum predetermined period 
     102 Step of forming seat assembly 
     1021 Sub-step of placing seat over outlet guide 
     1022 Sub-step of aligning first and second sections of seat with second and third sections of outlet guide 
     1023 Sub-step of placing seat on outlet guide so that first section of seat abuts second step of outlet guide 
     103 Step of allowing seat to achieve ambient temperature 
     1031 Indication that step of allowing set to achieve ambient temperature causes seat to shrink onto outlet guide 
     1032 Indication that first interference fit results from allowing seat to reach ambient temperature 
     104 Step of cooling seat assembly 
     1041 Indication that cooling shrinks seat assembly 
     1042 Sub-step of controlling temperature below particular temperature 
     1043 Sub-step of immersing in liquid nitrogen 
     1044 Sub-step of maintaining reduced temperature 
     105 Step of placing retainer over seat assembly 
     1051 Sub-step of aligning first section of retainer with first sections of outlet guide and seat 
     1052 Sub-step of aligning second section of retainer with second section of seat 
     1053 Sub-step of placing retainer on seat assembly so that retainer step abuts seat step 
     106 Step of allowing seat assembly and retainer to return to ambient temperature 
     1061 Indication that seat assembly expands into retainer 
     1062 Indication that second interference fit results from expansion of seat assembly into retainer
A valve seat arrangement that uses a TEFLON valve seat. The seat is held in place and kept from cold flowing in undesired ways by an outlet guide and a retainer. The valve seat arrangement is assembled using interference fits to avoid damage to the TEFLON seat. To overcome technical difficulties associated with the interference fit, a particular method of assembly is used. Specifically, the seat is heated and mounted on the outlet guide. Once the guide/seat assembly returns to ambient temperature, it is immersed in liquid nitrogen to force it to shrink. After removing the guide/seat assembly from the liquid nitrogen, the retainer is mounted thereon and the assembly is allowed to return to ambient temperature, thereby achieving another interference fit. If the retainer were heated to force it to expand, its temperature upon mounting on the seat would damage or destroy the seat, causing leakage through the seat.