Joints and connections – Utilizing thermal characteristic – e.g. – expansion or... – Members having different coefficients of expansion
Reexamination Certificate
2002-03-11
2003-09-30
Hong, John C. (Department: 3726)
Joints and connections
Utilizing thermal characteristic, e.g., expansion or...
Members having different coefficients of expansion
C403S404000, C403S408100, C029S458000
Reexamination Certificate
active
06626603
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the fabrication of jointed structures for operation under cryogenic conditions, and more particularly to extending a threaded stud above a laminated composite material so that another structure can be fastened to the stud.
BACKGROUND OF THE INVENTION
The problem to which the invention is directed arose in the context of fabrication of propellant tanks for space vehicles. In spacecraft, the launch weight is of primary importance, as every ounce of excess weight which can be removed from fixed structure can be used for storage of propellant, to thereby provide a longer operational lifetime. Thus, all parts of a space vehicle and its launcher are subject to intense efforts to reduce weight. The propellant storage tanks are prime subjects for weight reduction, as they tend to be among the largest structures on the spacecraft and/or its launcher. Past efforts at weight reduction of propellant tanks have led to innovations such as the use of laminated composite materials, made from multiple layers of strong, light fibers, such as graphite or carbon fiber, impregnated with a polymer. Many techniques are known for fabricating such tanks, as described, for example, in U.S. Pat. Nos. 5,427,334, issued Jun. 27, 1995 in the name of Rauscher, Jr., and 5,441,219, issued Aug. 15, 1995 in the name of Rauscher, Jr., which deal with fabrication of composite tanks having integral structures such as pipes and flow control devices.
A major problem with the application of composite materials to the storage of propellants in a spacecraft context is that the propellants are often cryogenic fluids, and some of the fluids, such as hydrogen, have very small molecules. The composite materials tend to have relatively large coefficients of thermal expansion in directions perpendicular to the reinforcing plies, so that relatively large changes in dimensions of the composite structure tend to occur when propellant is introduced into the tank, and then when the propellant is withdrawn. In the case of relatively small tanks, the plumbing required to carry the propellant to the engine may be made integral with the tank, as described by Rauscher, Jr., but large tanks require removable structures, such as hatches for ingress and egress, and flange attachments for attaching large-diameter pipes associated with large propellant flux.
FIG. 1
a
is a simplified plan view of the upper portion
10
of a prior-art propellant tank having an outer surface
10
os
, showing a central aperture
12
surrounded by a ring
14
of threaded apertures, some of which are marked
14
a
,
14
b
, and
14
c
. Both aperture
12
and the ring
14
are concentric with an axis
8
. The central aperture
12
may be for any purpose, such as ingress or egress of personnel, or for flow of fuel.
FIG. 1
b
is a perspective or isometric view of a portion of a propellant pipe
16
with a flange
18
which defines a ring
20
of apertures
20
a
,
20
b
adapted to fit over, and register with, the threaded apertures of ring
14
, for accommodating bolts for connecting pipe
16
to the aperture
12
of upper tank portion
10
of
FIG. 1
a
. For completeness of understanding, bolts
22
a
and
22
b
of a set of 22 of bolts is illustrated adjacent flange
18
. Each bolt
22
a
,
22
b
, . . . of set
22
of bolts is threaded to match the threads of one of the threaded apertures
14
a
,
14
b
, . . . of ring
14
of threaded apertures.
FIG. 2
a
is a cross-section of upper tank portion
10
in a region near threaded aperture
14
a
of
FIG. 1
a
. In
FIG. 2
a
, the outer surface
10
os
is at the top of the FIG., and the inside surface
10
is
is at the bottom. The thickness of the laminated composite material is selected to be as thin as possible for weight reduction, but thick enough to withstand the forces associated with the mass of the propellant being stored, the anticipated acceleration, the safety factor, and possibly other factors or considerations. Among these other considerations is that of preventing leakage of propellant directly through the composite material. Considering that a propellant may have molecules as small as hydrogen, which is notorious for its ability to leak through the most minute apertures, the thickness of the laminated composite must be adequate to reduce leakage to an acceptable level. As illustrated in
FIG. 2
a
, the threaded aperture
14
a
includes threads
214
t
defined in a metallic insert
214
. Metallic insert
214
is fastened into a portion of a cylindrical aperture
215
centered on a local axis
208
, either adhesively, by a force-fit, or both. The threaded end of bolt
22
a
extends into the threads
214
t
. An additional aperture portion
215
a
extends below the cylindrical aperture
215
, for accommodating a bolt length which, when torqued, extends below threaded insert
214
. It has been discovered that the tension applied to the insert
214
during torquing of the bolt
22
a
when fastening flange
18
of
FIG. 1
b
to tank outer surface
10
os
has the potential to cause delamination, as illustrated by delamination cavity
220
in
FIG. 2
b
. Such delamination can also result from the use of a bolt which contacts the bottom portion
215
b
of aperture
215
during torquing. In addition, delamination can result from lateral forces applied to the head end of a bolt threaded into the insert
214
. Thus, there are many possible causes of delamination. Delamination, by its nature, is not well controlled, since it literally involves disintegration, or the breaking up of an integral or monolithic material, at least in a local region. The damage associated with delamination may extend toward the inner surface, and compromise the ability of the tank to contain propellant.
The large tanks which are fabricated to carry propellant for a launch vehicle or space vehicle are expensive items. Delamination damage to the apertures
215
of
FIG. 2
a
, as illustrated in
FIG. 2
b
, must be repaired in a suitable manner, or the entire tank discarded. These repairs are rendered difficult by the need to seal against egress of the cryogenic propellant regardless of the changes in dimension of the laminated composite due to its coefficient of thermal expansion as it makes the transition between room temperature (or above) and cryogenic temperatures.
Improved composite tank fabrication and repair are desired.
SUMMARY OF THE INVENTION
A method according to an aspect of the invention is for fastening a threaded stud to project above a first surface of a composite laminate in a substantially leakproof manner as to cryogenic liquid gases. The laminated composite structure is undesirably subject to crushing above a predetermined pressure and delamination under excessive tension perpendicular to the plies. The method comprises the step of forming a through aperture through the composite material at the location at which the stud is to be installed. The through aperture should have a first diameter adjacent the first surface of the composite laminate, a frustoconical surface in the form of the frustum of a cone lying adjacent a second surface of the composite laminate, and a second diameter, smaller than the first diameter, in a region lying between the frustoconical surface and the portion of the aperture having the first diameter. An internally threaded insert is installed from the first side into the aperture so as to fasten the insert within the portion of the aperture having the first diameter. A bolt is obtained which includes a head, a nonthreaded shank portion adjacent the head, and a threaded portion remote from the head. The threaded portion of the shank should mate with the internal threads of the insert. The bolt is made from a material having a known coefficient of thermal expansion which is less than or lower than the coefficient of thermal expansion of the laminated structure. A generally cylindrical collar is obtained. The collar should be made from a material having a particular coefficient of thermal expansion and a thickness or length in an axial direction. In one
Gudaitis Charles Newell
Wright Richard Joseph
Hong John C.
Lockheed Martin Corporation
Morris LLP Duane
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