METHOD OF PRODUCING
PROPULSION TANKS FOR
The present invention is directed to the production of propulsion tanks useful in space--borne applications and is particularly directed to lightweight, low cost storage tanks for fuel and the like used in spacedeployed structures, such as satellites.
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 maintenance 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 portion, 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: no effective method has been 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 multicomponent 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 loadsharing 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 kg./cm.2, whereby leakage of the pressurant gas is prevented. One example of such a metallic film is a titanium film with a thickness of about 350-500 microns. The strain produced 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 portions of the metallic film can damage the walls of the polymer composite tanks.
Copending, commonly owned application Serial
No. [ RD-21925) discloses and claims a method for making lightweight polymer composite tanks having a satisfactory metal film for these purposes. Said method comprises producing a lightweight tank shell and depositing an adherent metal layer on desired surfaces of the shell. The present invention is an alternative method, which may in some instances be more convenient.
The invention is a method for producing a tank which comprises depositing a metal layer on the outer surface of a disposable pattern having the desired shape, forming a lightweight tank shell on the exposed side of said metal layer, and removing said pattern.
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 andlor 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 by the present invention.
In the first step of the method of the invention, a metal layer is formed on the outer surface of a disposable pattern. Said pattern may be made of any suitable formable material that is solubilizable, etchable or breakable.
Illustrative materials are hydrated calcium sulfate, metals, and polymers. The preferred pattern materials are metals which melt at relatively low temperatures and thus can be easily removed by melting, and solvent-soluble thermoplastics such as polycarbonate or polyimide (including polyetherimide) which may also be easily removed.
The 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, a hollow pattern may be pressurized with a fluid, such as hydraulic oil, to maintain its shape during subsequent operations.
It is within the scope of the invention to deposit an inner polymer layer on the outer surface of the pattern, prior to metallization. This step is particularly useful when an inner polymer layer on the tank is desired, which may be the case, for example, when it is to be used to store pressurant gas or any other inert substance. The polymer for the inner polymer layer is most often a thermoplastic polymer such as polycarbonate or polyetherimide, including halogenated polyetherimide. Polyetherimide is often preferred.
The inner polymer layer may be applied by any conventional coating method, such as powder, electrostatic, dip or solvent coating or rotational or spin molding.
Rotational molding is often preferred. The inner polymer layer typically has a thickness of about 20-30 microns.
A metal layer is then deposited on the pattern.
Its purposes are to prevent leakage of the stored material and to protect the subsequently fabricated tank shell walls from degradation induced by any fuel component stored therein. The thickness of the metal layer may be adjusted to prevent leakage when the metal layer is stretched along with the shell wall by the pressure of the contents.
The metal layer is preferably applied by electroless deposition. Prior to such deposition, adhesion between the surface of any inner polymer layer and the metal layer deposited thereon may be improved by methods 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.
A metal layer applied by electroless deposition may comprise 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 metal layer may comprise an inner copper layer followed by an outer nickel or gold layer.
The metal layer may be also applied by other methods, such as vacuum metallization or chemical vapor deposition. Such a layer may comprise gold, nickel, copper, titanium, stainless steel or various combinations thereof.
Preferably, the metal thickness is increased from about 1-2 microns to the desired thickness of about 20-150 microns by deposition of a second metal layer or successive layers. The additional layers can comprise nickel, gold, copper or various combinations thereof; copper is often preferred since it can be easily treated to increase its adhesion to the tank shell. Electrolytic metal deposition, often employed to increase metal thickness, is well known to those skilled in the art and is preferred.
However, other deposition methods, such as vacuum metallization or chemical vapor deposition, could be used.
It is usually further preferred to treat the exterior metal surface to promote its adhesion to the tank shell to be formed thereon. Suitable treatment methods are known. In the case of copper, for example, a conventional black oxide treatment can be employed.
In the second step of the method of the invention, a lightweight tank shell is formed on the exposed side of the metal layer. It is typically fabricated as 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 fabrication of the tank shell 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 the aforementioned polycarbonate and polyetherimide, with polyetherimide often being preferred.
The 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 the outer surface of the metal layer and the epoxy resin subsequently cured.
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 tank shell. Upon heating and curing the array, a reinforced structure is formed.
The tank shell may also be produced by coating the metal layer with an intermediate polymer layer, preferably of polyetherimide. The thickness of the intermediate polymer layer is generally about 25-50 microns. The coated surface is then wrapped with an array of a reinforcing material, such as carbon fibers, typically pre-impregnated with further 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 layers, the overall weight of the resulting tank can be maintained at a desired level with an increase in structural strength and stiffness.
Following formation of the tank shell, the pattern is removed. Its removal may be by any suitable method such as dissolution, melting or breakage and removal of the pieces. Particularly advantageous are melting in the case of a low melting metal pattern, and dissolution in the case of a resinous pattern.
Since the thickness of the metal layer deposited by the method of this invention is of the order of about 20150 microns, it is essentially incapable of bearing any structural load. 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.