This invention relates generally to supporting structures for use in spacecraft, and more particularly, to structures suitable for supporting space-based telescopes or optical benches. Space telescopes and other spacecraft payloads containing an optical bench are extremely sensitive to structural distortions. Distortions may be due to inertial forces caused by acceleration of the spacecraft, or by thermal effects when the spacecraft is exposed to radiation from the sun or other sources.
A space telescope must normally be supported as a payload on a spacecraft structure. The spacecraft and telescope combination must also include commmunications systems, power systems, possibly a propulsion system, and various other systems needed to support the spacecraft and telescope in a desired operational orbit. The spacecraft is typically placed in its operational orbit by first being launched from earth in a space shuttle vehicle or an expendable launch vehicle, which lifts its cargo to a relatively low-altitude parking orbit. If the desired operational orbit is of greater altitude than the parking orbit, the spacecraft is propelled to its operational orbit by an on-board propulsion system or by means of a separate propulsion stage.
There are two seemingly incompatible requirements for a mounting structure linking the telescope to the spacecraft. First, structural deformations of the spacecraft should ideally not be transmitted to the telescope. these deformations may be caused by thermal gradients, elastic or plastic deformation of the spacecraft during launch or assembly, or by other sources. The second requirement is that the mounting structure must provide sufficient strength and rigidity to transmit launch loads from the telescope to the launch vehicle, and also provide sufficient rigidity to permit precision pointing of the telescope by the spacecraft. An associated requirement is that the supporting structure be compatible with suitable launch vehicles, such as the shuttle orbiter and expendable launch vehicles, as well as with integral propulsion systems or other propulsive stages.
Although it is a relatively simple matter to provide a mounting structure that rigidly connects a telescope to a spacecraft, such a rigid interface would also transmit spacecraft deformations to the telescope. In a kinematic mounting structure, a mounted body is constrained from movement in only six degrees of freedom, namely movement along each of the three spatial axes, and rotation about the three axes. If a supported body is constrained in more than six degrees of freedom with respect to a mounting structure, it is said to be over-constrained. In an over-constrained mounting, some distortions of the mounting structure are transmitted to the supported body.
Prior space telescope support structures have not successfully addressed these difficulties. The National Aeronautics and Space Administration (NASA) Space Telescope project employs a design in which a telescope is supported by a collar-like spacecraft structure, contacting the telescope in substantially a single plane. The telescope is basically cantilevered out from this support structure, and is therefore free of any transmitted distortions that might have been present if both ends of the telescope had been secured to the spacecraft.
However, there are a number of disadvantages to the NASA Space Telescope approach. First, it is effective only if the greater part of the mass of the telescope is concentrated near the plane of support. Any other mass distribution will give rise to significant support problems both during launch and in maneuvering the telescope in space. Even if the mass is concentrated near the support plane, re-orienting the telescope, or performing other orbital maneuvers, can stress and distort the cantilevered telescope. Furthermore, the collar-like spacecraft structure is inconvenient, and therefore costly, to assemble about the telescope. The resulting telescope-spacecraft structure is, in a sense, an integrated or unitary one, although there may be a kinematic mounting interface between them. In fact, some of the support subsystems are disposed on the telescope rather than on the spacecraft itself.
A more recent NASA project, the proposed Gamma Ray Observatory (GRO), does not use the cantilevered approach of the Space Telescope, but instead employs an over-constrained mounting to secure an instrument payload to a spacecraft. However, the GRO has a much less stringent need for pointing accuracy than the Space Telescope, and distortions of the instruments are therefore of less significance.
It will be appreciated from the foregoing that there is a need for a spacecraft mounting structure that addresses these problems and provides an ideal support for a space telescope or optical bench. The mounting structure must not be over-constrained, should constrain movement in six degrees of freedom, and should isolate the telescope from distortions of the spacecraft. The present invention is directed to this end.