FIELD OF THE INVENTION

The present invention relates to an electrical heater assembly for liquid propellant chemical rocket engines, and more particularly to such heater assembly comprising a heating cable and a heater support, wherein the heater support exhibits a groove in which the cable is accommodat- ed, and wherein the heating cable is attached to the support by means of brazing and/or by means of an enclosing member, as well as a liquid propellant chemical rocket engine provided with the heater assembly.

BACKGROUND ART

In order to pre-heat the reactor in liquid propellant chemical thrusters, also referred to as liquid propellant chemical rocket engines, electrical pre-heating of the thruster is typically used. The thrusters, or small rocket engines, typically have a thrust of from 0.1 N to about 5 kN. Such thrusters may advantageously be operated on reduced risk liquid storable propellants, such as Ammonium DiNitramide (ADN) based liquid monopropellants, and bipropellants, e.g. High Performance Green Propulsion (HPGP) monopropellants, and bipropellants, or on Hydroxyl Ammonium Nitrate (HAN) based liquid monopropellants, and bipropellants. Thrusters for the above reduced risk liquid storable propellants require preheating of the ther- mo/catalytic reactor of the thruster to a temperature of about 300-400°C in order to achieve nominal ignition. In the prior art, preheating has been accomplished by means of an electrical heater assembly comprising a metallic heater support, to which support a coil of a resistive heating cable is attached by brazing, which heater support is located outside the thrust chamber of the en- gine with a heat conductive connection in between. In practice this has been accomplished using e.g. a heater support comprising a hollow cylindrical metal housing of Molybdenum (Mo) or a Mo alloy, e.g. TZM, around which housing a resistive heating cable, having an outer tube of In- conel or a similar metal which contains Nickel (Ni), is wound into a coil and fixed to the heater support by means of a braze filler. The heater support has a circular cross-section. Three flanges may typically extend radially from the cylindrical housing of the heater support on the inside thereof. The flanges are attached to the thrust chamber assembly and serve to conduct heat to the thruster reactor. However, the present inventor has found that the brazing becomes brittle over time, and that cracks appear in the brazing, which may lead to detachment of the heating cable, and can ultimately lead to failure of the heater assembly.

Accordingly, it would be desirable to be able to eliminate crack formation, or at least significant- ly reduce cracking growth to improve the life time of an electrical heater assembly for a liquid propellant chemical rocket engine.

SUMMARY OF THE INVENTION For a heater assembly of the prior art, comprising a metallic heater support 10, to which support a coil of a resistive heating cable 30 is attached, the above problem has been solved by means of a heater assembly 1, wherein the support exhibits a groove 17, in which groove the heating cable coil is accommodated. The heating cable is attached to the support by means of brazing 40 and/or by means of an enclosing member 45. Thereby, detachment of the heating cable, and as- sociated impaired contact between cable and heater support will be alleviated.

Heater support 10 is threaded with a groove 17, the dimensions of which groove match the heating cable, and the heating cable is accommodated in the groove. The groove allows for improved thermal contact of the heating cable with the support, and, in embodiments wherein brazing is used for attaching the cable to the support, for a reduced amount of brazing to be used for attaching the cable as compared to the prior art. The use of a brazing will further improve the thermal contact. The reduced amount of braze also corresponds to a reduced thickness of the brazing as compared to the prior art. With a reduced thickness the brazing becomes less prone to crack, and crack formation is thereby reduced. Also, by means of the groove any detachment tendency of the coil from the heater support due to expansion of the coil during brazing will be avoided, especially in embodiments wherein the groove, and the heating cable coil, is provided on the inside of the support. In preferred embodiments the brazing 40 is Nickel free. Thereby, cracking formation and growth will be at least reduced or delayed, thus increasing the life time of the heater assembly.

In embodiments wherein the heating cable is brazed to the support the surface of the heater sup- port, i.e. the surface of the groove, to which surface the heating cable 30 is attached by brazing, preferably exhibits a thin metal layer 50 of a Ni free metal alloy having a melting point above the melting point of the braze filler 40 for the heating cable. Thereby, crack formation will be markedly even further reduced, and thus the thermal cycling resistance of the heater assembly will be substantially improved.

In preferred embodiments the heating cable is embedded in the heater assembly. In such embodiments the heating cable is covered by a tubular member 45 enclosing the coil between the support and member. Thereby, since the cable is held in place by the enclosing member, brazing of the cable to the support can be omitted. Omission of brazing will allow for movement of the coil in the axial direction thereof during thermal cycling thereof, thereby reducing the risk of fracture, crack formation or the like in the cable sheath and/or rupture or breaking of the leads inside the cable due to different coefficients of thermal expansion of the cable sheath, the support and the cable leads, respectively. In order to improve thermal contact of the heating cable with the enclosing member 45 said member preferably exhibits a corresponding groove 17 (not shown) to accommodate therein the heating cable.

The inventive heater assembly is primarily intended for liquid propellant chemical rocket en- gines with a thrust within the range of 0.1 N to about 5 kN, but could possibly also be used with larger engines, especially in bipropellant applications. The inventive heater assembly can also be used with liquid propellant gas generators.

In one aspect the invention consequently relates to a liquid propellant chemical rocket engine provided with the inventive heater assembly.

The inventive heater assembly is configured to be located upstream of the catalyst bed of the Further advantages and embodiments will be apparent from the following detailed description and appended claims.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

FIGURE 1 shows a 1 N liquid propellant chemical rocket engine, i.e. thruster, including a heater assembly 1 of the invention, wherein the groove 17 and cable 30 are located on the inside of support 10. FIGURE 2 shows heating cable 30 of the heater assembly in the form of a coil, configured to fit heater assembly 1 shown in FIG. 1.

FIGURE 3 is a cross-sectional detailed view of heater support 10 of FIG. 1, which support is provided with a groove 17 on the inside thereof to accommodate heating cable 30 shown in FIG. 2.

FIGURE 4 shows a cross-sectional detailed view of heater support 10 (shown in FIG. 3) with cable 30 (shown in FIG. 2) accommodated in groove 17 of the support. FIGURE 5 is an enlarged view of the encircled area in FIG. 4, denoted Det. A. In FIG. 5, for reasons of simplicity, the cross-section of the heating cable has been indicated as solid. Accordingly, e.g. the leads on the inside of the cable are thus not shown. The outer surface of the heating cable 30 is typically comprised by an Inconel tube. FIGURE 6 shows a partial view of an embodiment of the inventive heater assembly 1, wherein the groove 17 is located on the outside of the support 10. The heater assembly shown is attached to the wall of a rocket engine at a location outside the heat bed of the engine.

FIGURE 7 shows a partial view of an embodiment of the inventive heater assembly 1 having embedded heating cable 30, wherein the groove 17 is located on the outside of the support 10, and which embodiment also includes an outer, enclosing member 45. The heater assembly is attached to the wall of the rocket engine at similar location as in FIG. 6. FIGURE 8 shows a partial view of an embodiment of the inventive heater assembly 1, wherein the groove 17 is located on the inside of the support 10. The heater assembly is attached to the wall of a rocket engine at similar location as in FIGS. 6 and 7. FIGURE 9 shows a partial view of an embodiment of the inventive heater assembly 1 having embedded heating cable 30, wherein the groove 17 is located on the inside of the support 10, and which embodiment also includes an inner, enclosing member 45. The heater assembly is attached to the wall of a rocket engine at similar location as in FIGS. 6-8. DETAILED DESCRIPTION

The present inventor has found the cracking observed in the brazing of prior art heater assemblies to be associated with thermal cycling of the heater assembly. The brazing in the prior art heater assembly has been found to become brittle over the thruster's thermal cycling life due to liquid metal embrittlement due to the presence of molybdenum and nickel. More particularly, nickel (Ni), which is used in the brazing alloy for its excellent wetting properties, has been found to form a brittle alloy with molybdenum (Mo) in the interface between braze and heater support. Mo alloys, e.g. TZM, have good properties and are frequently used in space applications, e.g. in the heater support, and are not easily substituted. Due to the brittleness of the thus formed Ni/Mo alloy, said alloy is prone to crack, especially during thermal cycling.

The outer tube of the heating cable is formed from a high-temperature resistant metal alloy. Such alloys are typically Ni alloys, such as e.g. Inconel alloys. In embodiments wherein a Ni free brazing alloy 40 was used, migration of Ni from the outer Ni containing Inconel tube of heating cable 30 into the braze was observed. The migration of Ni depletes the Inconel tube of Ni and weakens the tube. Also, formation of the brittle Ni/Mo alloy was eventually observed in the interface between braze and heater support when a Ni free braze was used, although the formation was delayed. Accordingly, a Ni free braze will suppress or at least delay formation of cracks in the brazing.

As shown in FIG. 3, the heater support 10 exhibits a threaded surface comprising a groove 17, in which groove the heating cable 30 is accommodated, as shown in FIG 4. The groove allows for improved thermal contact between cable and support, and, when a brazing is used, improved attachment of the heating cable to the heater support, and, also less braze to be used for attachment of the cable to the support. The dimensions of the groove should be configured to accommodate therein the heating cable as shown in FIG. 5. The dimensions of the groove should thus match the dimensions of the heating cable. Without a groove to accommodate the heating cable in the support, more brazing and a larger thickness of brazing would be required, and, due to different thermal expansions of the brazing and the heater support, the brazing would then be more prone to cracking. In embodiments wherein the cable is attached to the support by means of brazing, the location of the brazing is in the groove of the support. According to the invention, any conventionally used brazing alloy 40 may be used. Preferably, the braze alloy 40 should however be Ni free.

In one embodiment of the invention migration of Ni from the heating cable tube to the Mo-alloy of the heater support is suppressed by means of a barrier layer 50 on the heater support, as shown in FIG. 5. In the figure, barrier layer 50 is indicated by a solid thick line. Accordingly, in a preferred embodiment the inner surface of the heater support, to which surface the heating cable is attached by brazing, exhibits a migration barrier 50 of a thin metal layer. Such barrier can be accomplished by a e.g. 10 to 50 μπι thick layer of a Ni free metal alloy having a melting point above the melting point of the braze filler 40 for the heating cable. Preferably, the metal of the barrier layer is selected from the group consisting of noble metals, e.g. gold, platinum, palladium. An especially preferred metal alloy for the barrier layer is an Au/Pd-alloy. This allows for a heating cable tube containing Ni to be used, such as Inconel. For a heater assembly using a heating cable with an Inconel tube, such migration barrier in combination with the threaded heater support has been found to improve the thermal cycling resistance by a factor of about 25 as compared to prior art heater assemblies.

In alternative and preferred embodiments the outer tube of the heating cable is Ni free. Preferably the heater tube is formed from a Pt/Rh alloy, such as e.g. 90% Pt/10% Rh, or from a Pt/Ir alloy. Thereby, migration of Ni into the braze, and associated formation of brittle Ni/Mo alloy with any Mo contained in the heater support is eliminated. In such embodiments no barrier layer is required. Even during prolonged testing of such embodiment of the inventive heater assembly, wherein a Ni free braze was used, failure could not be induced during the testing period. Accordingly, a Ni free heater tube can further substantially improve the thermal cycling resistance, and thereby also life time, of the inventive heater assembly. An especially preferred braze 40 for this embodiment is gold (Au), or a gold-based braze. For the latter and preferred embodiment of the invention the improved thermal cycling resistance has been assessed to be significantly greater than a factor of 25, as compared to a prior art heater assemblies. In embodiments wherein the heating cable is not embedded, it is preferred that the groove is provided on the inside of the support.

In embodiments with embedded heating cable, such as shown in FIGS. 7 and 9, the enclosing member 45 is preferably threaded so as to exhibit as groove (not shown) corresponding to the groove the support. The enclosing member can then be threaded on to the support with cable coil. The enclosing member is preferably has a annular cross-section.

Attachment of the heater assembly 1 to the rocket engine can be accomplished in various ways as long as heat from the heater can be conducted, e.g. via the attachment, to the engine, so that the heater will be able to heat the engine. The assembly is attached to the upstream end of the engine, such as e.g. to the engine wall outside the heat bed of reactor 20, as shown in FIG. 1. The location of attachment of the support to the engine should be upstream of the catalyst bed of the engine, as the temperature of the outer wall of the engine during operation thereof will be extremely high outside the catalyst bed and downstream, such as e.g. about 1,600°C. In a pre- ferred embodiment heater support 10 is attached to the engine outside the heat bed of the engine, such to the reactor wall. The heater may also be attached to the injector of the engine. During pre-heating of the engine such location of attachment will allow for efficient transfer of heat from the support to the heat bed via the reactor wall. During operation of the engine such location of attachment will allow for the heater support 10 to serve as a radiator dissipating heat from the reactor wall, which heat being generated from the downstream combustion could otherwise be conveyed via the reactor wall upstream to the injector and e.g. possibly cause the propellant in the feed tube to boil. Heat transfer upstream inside the engine during operation thereof will effectively be prevented by the heat bed, which bed is being cooled by the propellant being injected into the engine, and will thus act as a thermal isolator during operation. The heater support will thereby serve to prevent injector and upstream parts to be overly heated. For optimum radiator performance, the heater should be attached as far downstream as possible outside the heat bed. Attachment can e.g. be accomplished by means of one or more flanges 15 extending from the heater support to the point of attachment on the engine. In the embodiments shown in FIGS. 3 and 4 flanges 15 are provided in the upstream end of the heater support 10. During operation of the engine, the heater assembly will typically be heated to about 800°C.

The temperature range of the heater assembly during pre-heating is typically within the range of about 300°C to 800°C, such as about 400 to 500°C.

In order to allow for thermal expansion of the heater support and heating cable, and thereby reducing thermal stress in the support, cable, and brazing, the heater support may be provided with one or more slots (not shown) running alongside the heater support 10, along the portion thereof where the coil is located, preferably extending all the way to one end of the heater support, so that the circumference of such end of the heater support is disrupted by the slot(s). With reference to FIG. 1, the downstream end of the heater support 10 could exhibit one or more slots, e.g. 1-6 slots, running along the support up to the location of the support where the flanges are located.

In preferred embodiments the heater support is cylindrical and has a circular cross-section, such as a hollow cylindrical metal support.

The heating cable coil is preferably concentric with the longitudinal axis of the heater support.