METHOD OF PRODUCING
PROPULSION TANKS FOR
AEROSPACE APPLICATIONS, AND
TANKS SO PRODUCED
Field of the Invention
The present invention is directed to propulsion tanks useful in space-borne applications and is particularly directed to lightweight, low cost storage tanks for fuel and the like used in space-deployed structures, such as satellites.
Background of the Invention
It is well known that satellites when orbiting a planet, such as earth, require a particular orientation with respect to the planet. This orientation is especially needed so that the satellite can perform the desired tasks. More importantly, it is known to provide satellite and orbiting space vehicles with altitude position and orientation controls which permit the correction and maintaining of particular positions in all directions of freedom of motion.
Generally, one uses mini-propulsion systems and small jets for that purpose; they provide very small but very accurately metered mechanical impulses so that requisite control and corrective motion can be obtained.
The miniature or mini-propulsion systems and control jets in a space vehicle require a certain amount of fuel and, therefore, it is necessary to store such a fuel in a suitable container within the satellite. The fuels used are generally in the liquid state. Known and conveniently used single component fuels include hydrazine and derivatives thereof. Decomposing hydrazine constitutes a very useful propulsion gas that can be used directly by the propulsion system for purposes of position and altitude control as mentioned above. A pressurant gas is also generally stored in a separate tank to supply pressure necessary to convey the propulsion fuel from the tank to the propulsion jets.
Multi-component fuel is also known and is often preferred because of a somewhat larger energy control and, thus, is a preferred choice in those cases in which the space vehicle can be refueled only infrequently or not at all.
However, it is a significant drawback of these multicomponent fuels that at least some of the more desirable components are highly chemically aggressive. A typical example of such a component is nitrogen tetroxide.
One of the typical approaches known in the art is to use forged titanium or stainless steel tanks to store propellent fuel, pressurant or oxidant. The minimum weight of each tank is fixed by the wall thickness, which in turn is determined by the size of the tank and practical limits of machining such a tank shape. Such tanks are typically about 100 cm. in diameter and about 4 mm. in thickness. The weight of forged titanium or stainless steel tanks constitutes a major portion1 typically about 67%, of the propulsion system weight. Due to the substantial weight of such tanks, the useful payload of a satellite has to be reduced, as the capacity of a launching rocket is substantially fixed.
Additionally, titanium or stainless steel tanks are difficult to fabricate and expensive. They also require long lead times, typically about 18 months. In some cases, such as tanks used for storing a pressurant gas, an aluminum liner is generally required to prevent leakage of the. pressurant gas. Such a liner not only complicates the manufacturing process but also increases the cost and weight of the propulsion system.
Thus, it is desirable to produce a lightweight, inexpensive tank suitable for space-borne applications.
Lightweight, fiber reinforced, polymer composite tanks not only offer the advantage of a significant weight reduction but also offer the additional advantage of a significant reduction in cost of fabrication.
Furthermore, fabrication techniques used for producing polymer composite tanks are simple enough to significantly reduce lead times, typically by 12 or more months.
However, the aforementioned polymer composite tanks suffer from a major drawback. At present no effective method is available for providing the walls of such tanks with a puncture-resistant metal layer that can effectively protect such tanks from the corrosive action of multi-component fuels or prevent leakage of a pressurant gas.
It is known to position a non-adherent metallic film on the inner side of a polymer composite tank to protect the walls of such a tank from the corrosive action of- fuels or the components thereof. The thickness of a load-sharing metallic film in this situation is adjusted so that the combined structure achieves a desired strain when such tanks are pressurized to a targeted pressure (for example, in the case of the pressurant, about 300 k./cm.2, thereby preventing leakage of the pressurant gas). One example of such a metallic film is a titanium film with a thickness of about 350-500 microns. The strain within the aforementioned strain limit keeps the stretch of the metallic film below its ductility limit, i.e., the film deformation occurs only within the ductile range of the metallic film.As the fuel from the tank is consumed during propulsion maneuvers, the metallic film, stretched beyond its elastic limit, buckles inward. As a result, the sealing ability of the metallic film may be affected and seepage of the propellent fuel through damaged portion of the metallic film can damage the walls of the polymer composite tanks.
The present invention provides means for depositing lightweight, non-load bearing adherent metal layers on the walls of polymer composite tanks. Said layers are not damaged during pressurization and depressurization of such tanks. The invention also provides means for customizing the metal layers to substantially prevent chemical attack by the fuel components on the polymer tank walls. The invention further provides means for storing pressurant gases at high pressure without leakage.
Statement of the Invention
The invention is directed to a method of producing a tank which comprises producing a lightweight tank shell and depositing an adherent metal layer on desired surfaces of the shell.
The present invention is further directed to a tank, most often a lightweight space-deployed propulsion tank, which comprises a tank shell having a wall comprising an array of a reinforcing material impregnated with a polymer material and a substantially non-load bearing and substantially unyielding adherent metal layer disposed on the desired surfaces of the shell.
Detailed Description of the Preferred Embodiment
A propulsion system of a satellite generally comprises jet nozzles, various propulsion tanks and an operating system for providing precise impulses through the nozzles to accurately position an orbiting satellite. Since the weight of propulsion tanks forms a major portion of the overall propulsion system weight, a lightweight propulsion tank would be highly desirable for reducing the overall weight of the propulsion system. Generally, propulsion tanks are used for storing propulsion components, such as a fuel, an oxidant and/or a pressurant. The pressurant is typically an inert gas, such as helium, used to apply constant pressure to liquid oxidant or liquid fuel stored in the oxidant and fuel tanks, respectively. The pressurant gas forces the oxidant or the fuel into the propulsion operating system. Separate tanks are generally provided for storing the aforementioned components. However, a single tank having compartments to store the propulsion components could also be used.
Such a lightweight, structurally stable space deployed propulsion tank may be made from a composite shell of a reinforcing material impregnated with a polymer material. As the reinforcing material, such materials as carbon, glass or steel fibers are generally employed. Carbon fibers are preferred. However, other reinforcing materials.
such as woven cloth of carbon, glass or steel, can also be employed.
The polymer material suitable for use in the present invention may be a thermoplastic or a thermoset polymer, or a combination of both, i.e. two or more layers comprising a thermoset polymer and a thermoplastic polymer. A thermoset polymer is preferred. Suitable thermosetting polymers useful as intermediates therefor are the epoxy resins. For example, an epoxy resin or an epoxide, defined as any molecule containing more than one epoxy group (whether situated internally, terminally or in cyclic structures) capable of being converted to a useful thermoset form, may be used. Suitable thermoplastic polymers are polycarbonate and polyetherimide, including halogenated polyetherimide. Polyetherimide is preferred.
The aforementioned tank shell may be produced by a variety of methods. For example, an array of a reinforcing material, such as fibers or yarns, after being impregnated with a polymer material, such as an uncured epoxy resin, may be wrapped or wound on a disposable pattern of a desired shape, typically a spherical or torpedo shape, and the epoxy resin subsequently cured.
The aforementioned disposable pattern may be made of any suitable moldable material that is solubilizable, etchable or breakable. Suitable materials are aluminum and hydrated calcium sulfate, such as plaster of
Paris. Aluminum is preferred. The aforementioned pattern may be formed by any well known process, such as rotational molding, blow molding, injection molding, die casting, spin forming and the like.
If necessary, the disposable pattern, which is generally hollow, may be pressurized with a fluid, such as hydraulic oil, to maintain its shape during the fiber winding process. Details of the aforementioned process are provided in Morris, E., et al., High-Pressure, High
Performance Filament-Wound Carbon/Epoxy Pressurant
Tanks With Seamless Aluminum Liners For Expendable
Launch Vehicles and Spacecraft, 34th International SAMPE
Symposium (1989), incorporated herein by reference. The aforementioned paper describes various fiber materials as well. as winding devices used in the fiber winding process.
Generally, the array of reinforcing material is produced by weaving, in various directions, long strands of fiber or yarn on a disposable pattern to achieve randomization of the reinforcing fiber. Such randomization significantly boosts the structural integrity and strength of the resulting propulsion tank shell.
Upon heating and curing the array, a reinforced structure is formed. The disposable pattern may be then broken off or removed by a solubilizing agent or an etching solution to form the tank shell. Any suitable etchant, such as base, may be used, for example, when a disposable pattern of aluminum is employed.
The propulsion tank shell may also be produced by coating a disposable pattern of a desired shape with a layer of a first polymer material, preferably polyetherimide. The thickness of the layer of the first polymer material is generally about 25-50 microns. The coated surface is then wrapped with an array of a reinforcing material, such as carbon fibers, typically preimpregnated with a second polymer such as an epoxy resin, and then cured to form a reinforced structure. It should be understood that the present invention contemplates additional polymer layers, if such layers are desired. Such a tank shell has the structural strength and stiffness characteristics of the employed polymer materials.It is contemplated that by adjusting the thicknesses of various polymer layers, the overall weight of the resulting propulsion tank can be maintained at a desired level with an increase in structural strength and stiffness. The underlying disposable pattern is then removed to form the tank shell.
Once the tank shell is produced, an adherent metal layer is then deposited on the desired surfaces thereof to prevent leakage of the stored fuel component as well as to protect the tank shell walls from degradation induced by the fuel component stored therein. The thickness of the adherent metal layer is adjusted to prevent leakage of the pressurant gas when the adherent metal layer is stretched along with the shell wall by the pressure of the pressurant gas.
The surfaces on which the metal layer is deposited are preferably on the inner side of the tank shell.
However, they may be on the outer side or on both sides of the tank shell. The metal layer may be provided on the outer side of the tank shell when the resulting propulsion tank is used as a pressurant tank containing pressurant gas, such as helium. Additionally, it is contemplated that a propulsion tank shell with a metal layer on both inner and outer sides will protect its contents from damage resulting from the action of ultraviolet or gamma rays.
The adherent metal layer is preferably applied on the desired surfaces of the tank shell by electroless deposition. Prior to electroless metal deposition, adhesion between the desired surfaces of the tank shell and the metal layer deposited thereon, may be improved by methods well known in the art. For example, U. S. Patents 4,959,121 and 4,999,251, both incorporated herein by reference, disclose suitable methods for improving adhesion of a metal layer to a polyetherimide surface.
The electrolessly applied adherent metal layer comprises gold, nickel, copper or various combinations thereof. A gold layer is preferred for propulsion tanks used to store a propulsion fuel or an oxidant, and a copper layer is preferred for propulsion tanks used for storing a pressurant gas. A total metal thickness of about 20-150 microns is sufficient to prevent leakage of the pressurant gas. Alternatively, the adherent metal layer may comprise an inner copper layer followed by an outer nickel or gold layer.
The adherent metal layer may be also applied on the surfaces of the tank shell by other well known methods, such as vacuum metallization or chemical vapor deposition.
Such an adherent metal layer may comprise gold, nickel, copper, titanium, stainless steel or various combinations thereof. In case of a propulsion tank used for storing fuel components, a final layer of stainless steel or titanium may be necessary to protect the inner metal layers from oxidation.
Preferably, the metal thickness is increased from about 1-2 microns to the desired thickness of about 20-150 microns by deposition of successive metal layers.
The additional thicknesses can comprise nickel, gold, copper or various combinations thereof and are preferably electrolytically deposited. Electrolytic metal deposition is well known to those skilled in the art. However, other deposition methods, such as vacuum metallization or chemical vapor deposition, could be used.
Another aspect of the invention is a spacedeployed propulsion tank produced by any one of the aforementioned methods. The tank of the present invention comprises a propulsion tank shell having a wall made from an array of a reinforcing material, such as carbon fibers, impregnated with a polymer material, such as an epoxy resin, and an adherent metal layer disposed on the desired surfaces of the tank shell. The metal layer comprises one or more layers of nickel, gold, copper, titanium, stainless steel, or combinations thereof. Since the thickness of the adherent metal layer disposed on the desired surfaces of the tank shell is of the order of about 20-150 microns, it is essentially incapable of bearing any structural loads. Thus, as the metal layer is very thin, the overall weight of the tank is low. Additionally, since the metal layer adheres to the surfaces of the tank shell, it is substantially unyielding.
Thus, damage, such as cracking or pinholes, to the metal layer resulting from expansion and contraction of the tank shell during its working cycle is significantly minimized.
The present invention is also directed to a tank having compartments for storing propulsion components, such as an oxidant, a pressurant and a fuel.