Aeronautics and astronautics – Aircraft structure – Fuel supply
Reexamination Certificate
1999-10-29
2001-09-04
Barefoot, Galen L. (Department: 3644)
Aeronautics and astronautics
Aircraft structure
Fuel supply
C244S158700, C220S563000
Reexamination Certificate
active
06283412
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to propellant tanks for launch vehicles and, in particular, to a liquid propellant tank for providing a desired level of slosh attenuation with reduced mass and complexity of construction.
BACKGROUND OF THE INVENTION
Launch vehicles are generally used to launch payloads, such as satellites or scientific equipment, from the Earth's surface into space. Once in space, other on-board spacecraft propulsion systems may be utilized for orbital adjustments or transfers and attitude control. It will thus be appreciated that unlike such other on-board spacecraft propulsion systems and associated propellant tanks, launch vehicle propulsion systems and propellant tanks operate within the atmosphere and are not limited to operation in a low or zero g environment. Indeed, launch vehicles often experience a high g environment as a result of the substantial thrusts generated by the launch vehicle. As a result, launch vehicles are subject to substantial jostling due to wind and movement of thrust deflectors or the like during maneuvering.
Liquid propellant engines are used for many launch vehicle applications. Such liquid propellant engines have a number of desirable qualities including the ability to turn the engines off, as may be desired, after ignition. Accordingly, many launch vehicles include liquid propellant tanks for containing one or more liquid propellants. Among the important liquid rocket engine propellants are RP
1
and LO
2
.
Although liquid propellant engines are advantageous for many applications, they also entail certain potential complications for launch vehicles. One of these complications relates to propellant sloshing. As noted above, launch vehicles may be subject to substantial jostling while traversing the atmosphere. As a result of this jostling, lateral forces are exerted on the liquid propellant tending to cause sloshing, i.e., non-uniform fluid flow along a longitudinal extent of the propellant tank wall and associated non-uniform distribution of the propellant across width of the tank.
Such sloshing is not a trivial concern. For example, for one proposed launch vehicle design, the portion of the total weight of the liquid propellant which sloshes will be about 24,725 pounds in the case of LO
2
and 17,425 pounds for RP
1
. The sloshing motion of this propellant can induce significant structural loads and rigid body disturbances in the vehicle as well as potentially affecting control system operation. The potential for excitations of the sloshing resonances and their interaction with the vehicle control system are primarily driven by the location of the sloshing masses relative to the vehicle center of gravity and center of percussion, the modal frequencies and lateral modal deflections in the vicinity of the sloshing mass and the damping of the sloshing resonances. Most of these parameters cannot easily be changed should propellant sloshing dynamics become a critical issue. One exception is the propellant slosh damping.
Conventional approaches to slosh damping have generally involved installing ring baffles within the propellant tank to attenuate sloshing. Specifically, the propellant tank generally includes a cylindrical propellant containment area defined by a side wall. In some cases, the inner side wall is formed from an isogrid structure including a number of intersecting, raised ribs defining a network of cells. These isogrid structures provide a desirable combination of lightweight construction, load bearing strength and rigidity. A number of baffle rings are installed within these side walls to attenuate sloshing. Generally, these baffle rings are annular in shape and extend inwardly from the side wall, for example, six to twelve inches or more in the case of tanks having a diameter of about 12 feet. These baffles may be spaced at a longitudinal distance of, for example, two to four feet, throughout a tank slosh zone, e.g., throughout the entire propellant containment area or a portion thereof where slosh is of concern.
It will be appreciated that such baffling may involve substantial mass/weight. In one case, the dry weight of such baffles was projected to be about 350-400 pounds for a launch vehicle booster. This weight, of course, affects the total thrust requirements and/or available payload. Moreover, the fabrication and installation of these baffles increases construction complexity and overall costs.
SUMMARY OF THE INVENTION
The present invention is directed to a method and apparatus for attenuating slosh in liquid fuel tanks of a launch vehicle without undue mass and additional structure dedicated to slosh control. It has been recognized that, for a variety of launch vehicle applications, the desired level of slosh attenuation or damping can be achieved through either a smaller number of larger ring baffles spaced at greater longitudinal distances across a slosh zone or a larger number of smaller ring baffles spaced at smaller longitudinal distances across the slosh zone. The present invention is based in part on the recognition that, in certain cases, a desired level of slosh control can be obtained using a slosh attenuation system that is substantially contained within a space adjacent to the tank wall that is significantly narrower than that of ring baffles that have previously been employed. In one particularly advantageous application, the slosh attenuation system can be integrated into an appropriate structural grid such that structure dedicated exclusively to slosh attenuation can be substantially reduced or eliminated. The present invention thus allows for achievement of a level of slosh attenuation as desired for certain launch vehicle liquid propellent tank applications while reducing mass, simplifying construction and reducing costs.
In accordance with one aspect of the present invention, a liquid propellant tank for a launch vehicle includes a low profile slosh attenuation system. The propellant tank has a containment area for containing a liquid propellant such as, for example, LO
2
or RP
1
. The containment area has a longitudinal axis and a width and, in operation, the propellant tank is generally oriented such that the longitudinal axis is aligned with the flight path of the launch vehicle. It will thus be appreciated that lateral forces experienced by the launch vehicle tend to induce slosh defined by a nonuniform distribution of the liquid propellant relative to the width of the containment area; that is, side-to-side displacement of the propellant with attendant sloshing along the inside wall of the containment area
The containment area further includes a slosh zone defined relative to the range of locations of the surface of the liquid propellant as the propellant is expended during a launch operation. In this regard, the propellant in the tank can be modeled as being divided into two distinct segments. The first is designated the “rigid” propellant and, for purposes of analyzing the effects of slosh, can be treated as part of the vehicle dry structure which moves with the vehicle rigid body translational/protational modes. The rest of the propellant located at the top of the tank is designated the “sloshing” propellant and is assumed to move laterally with respect to the tank at a specified frequency when lateral excitations are provided to the tank due to rigid body and/or structural accelerations. It will be appreciated that the location of this sloshing propellant varies as fuel is expended. Accordingly, the slosh zone may encompass the entire containment area or any portion of the containment area where it is desired to attenuate sloshing.
The low profile slosh attenuation system allows for slosh attenuation to a desired level in the slosh zone. For example, in one implementation as will be described in detail below, the low profile slosh attenuation system provides a slosh damping of at least 0.5% for slosh angles of 5 degrees, as specified by launch vehicle design requirements. The low profile slosh attenuation system includes a number of slosh barriers
Barefoot Galen L.
Lockheed Martin Corporation
Marsh & Fischmann & Breyfogle LLP
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