Seal for a joint or juncture – Seal between fixed parts or static contact against... – Contact seal for other than internal combustion engine – or...
Reexamination Certificate
2000-06-27
2002-09-10
Knight, Anthony (Department: 3676)
Seal for a joint or juncture
Seal between fixed parts or static contact against...
Contact seal for other than internal combustion engine, or...
C277S641000, C277S652000, C277S654000, C277S537000, C277S938000, C239S265110
Reexamination Certificate
active
06446979
ABSTRACT:
TECHNICAL FIELD
This invention relates to sealing devices and more particularly to a rocket motor joint construction which includes a thermal barrier structure in conjunction with elastomeric primary and secondary O-ring seals between casings of a solid rocket motor.
BACKGROUND ART
Assembly joints of current solid rocket motor cases are generally sealed using high performance, elastomeric O-ring seals. The 5500° F. propulsion gases commonly produced during the relatively short firing interval of the rocket motors are kept a safe distance away from compounds are used to fill insulation gaps leading to the seals to prevent a flowpath of propulsion gases to the seals.
Normally, these two stages of protection are enough to prevent a direct flowpath of the 900-psi hot gases from reaching the seals. Occasionally, seals have experienced charring due to parasitic leakage paths that open up in the joint-fill compounds during rocket operation. Inspection during disassembly of Space Shuttle solid rocket motor nozzle joints from RSRM-44 and RSRM-45 revealed O-ring erosion of Joint
3
primary O-ring seals. Subsequent improvements in joint-fill compound application techniques have apparently overcome the Joint
3
charring problem. However, a number of nozzle joints including the nozzle-to-case joint and Joint
2
continue to show hot gas penetration through the joint-fill compound. The current nozzle-to-case joint design incorporates primary, secondary, and wiper (innermost) O-rings and polysulfide joint-fill compound. In the current design, one out of seven motors experiences hot gas to the wiper O-ring. Though the condition does not threaten motor safety, evidence of hot gas to the wiper O-ring results in extensive reviews before resuming flight. Because of the severe conditions which exist in a rocket motor firing environment, further assurance is desired that the joints and the primary and secondary O-ring seals therein are not jeopardized.
It is essential to design rocket motors and their casings to maximize the available thrust from the motor. Solid rocket motors are typically manufactured in sections for assembly in an elongated structure. Not only does the sectional design facilitate manufacture, but the casing sections can be retrieved after firing, refurbished, filled with fuel and reassembled for further firings.
Reusable rocket sections present additional design constraints in that after each firing the casings have experienced considerable stresses and have been exposed to environmental conditions which may change the properties and/or dimensions of the casing structure. The primary and secondary O-ring seals and any additional sealing components must be designed to accommodate such changes. This is in addition to the initial design constraints for the joint structure and the seals therein which may experience relative movement between casing sections due to vibration or the thrust forces produced by the rocket motor.
An additional design constraint imposed upon joint structures including the primary and secondary O-ring seal arrangement is that once the casing sections are assembled, the seals are buried within the structure at a location which is highly tolerant to the physical stresses imposed during motor firing, but not readily accessible to verify the integrity of the seal arrangement. The usual technique for verifying seal integrity is to provide a seal test port which extends between the primary and secondary seals. This test port when pressurized, provides an indication of sealing integrity of the seals under static conditions. However, the condition of materials and structures which make up the mechanical joint between rocket motor casing sections positioned radially inward of the O-ring seals, may have an effect upon pressurization of the seals. It is therefore important that any materials or structures placed radially inward of the O-ring seals must allow gas flow through them so as not to provide misinformation about the integrity of the seals (e.g., false positive) during pressure tests.
Much design and development activity has been directed to finding suitable materials and/or arrangements for the joint configuration between rocket motor casings. Such an arrangement must assure the primary sealing function of the O-ring seals is properly performed and yet accommodate those conditions which are imposed during firing conditions. It has been proposed to include a further thermal barrier structure in the joint between casing sections, radially inward of the primary and secondary O-ring seals. However existing materials have proven unsuitable for this purpose. Thus, there exists a need for a joint and seal structure for casing sections of a rocket motor which provides enhanced protection for the O-ring seals and greater assurance of joint integrity under firing conditions.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide an improved thermal barrier, primarily for a rocket motor application.
It is a further object of the present invention to provide a thermal barrier which is used in conjunction with primary and secondary O-ring seals, including wiper seals, in rocket motor casing joints.
It is a further object of the present invention to provide an improved thermal barrier for rocket motors which can withstand rocket motor firing temperatures for a longer period of time.
It is a further object of the present invention to provide an improved thermal barrier for rocket motors that can drop the temperature of incoming jets of hot gas and spread these narrow jets to reduce their damaging effects on downstream O-rings.
It is a further object of the present invention to provide an improved thermal barrier for rocket motors which is of braided configuration and which allows pressurization of the main O-ring seals during rocket motor firing and which does not materially affect main seal integrity pressure tests before rocket firing.
It is a further object of the present invention to provide a casing joint for a rocket motor casing which has enhanced integrity and reliability.
These and other objects of the invention will become apparent in the following Best Modes for Carrying Out Invention and the appended claims.
Rocket motor propulsion gas temperatures reach a level on the order of 5500° F. The burn time in typical applications is of the order of only a few minutes. As flows of hot gases occur it is important to prevent such gases from impinging on the O-rings which seal adjacent rocket motor casing sections. Exposure to such hot gases can cause O-ring char and erosion limiting O-ring sealing ability and possibly leading to joint failure, as experienced in the loss of the Space Shuttle Challenger.
Rope seals have been developed previously and braided designs are used in gas turbine engine applications. In these applications the seals are commonly made of ceramic fibers and superalloy wires. These seals provide advantages not only as seals but also as compliant mounts under aggressive temperature and pressure requirements. However, these seals are generally being used in environments at peak temperatures in the 1500-2000° F. range. Such seals could not stand up for more than a few seconds to the 5500° F. temperatures commonly encountered in rocket motors. This is because ceramics have a melt temperature of about 3500° F. and common superalloy metals melt at about 2500° F.
It is a teaching of this invention that a braided rope type thermal barrier may be used to achieve advantages in rocket motor type applications. Such a barrier when comprised of carbon fibers provides superior protection to the primary and secondary O-ring seals (including wiper seals) of the motor and additional advantages to the joint structure between motor casing sections. Carbon fibers are used in an exemplary embodiment because of their relatively high heat conduction, low linear expansion coefficient, high corrosion resistance and thermal stability as well as their high strength and low density. It is known that carbon fibers oxidize and lose mass over long intervals wh
Dunlap, Jr. Patrick H.
Steinetz Bruce M.
Knight Anthony
Pickard Alison K.
Stone Kent N.
The United States of America as represented by the United States
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