Cryogenic Mirror Fixing Device Using Compressive Stress
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Solution Overview
Problem
Large spacecraft mirrors, particularly those made of glass-ceramic materials like Zérodur, face significant stress issues during launch and operation due to vibration loads and temperature variations, as existing fixing devices primarily exert tensile and bending stresses, which glass-ceramic materials are not well-suited to withstand effectively.
Innovation Solution
A fixing device that applies pure compressive stresses using a T-shaped glass protrusion with a clamping part that secures the heel of the protrusion, employing pads and screws to distribute and control forces, and materials with low thermal expansion coefficients like Invar and aluminum to minimize deformation and stress on the mirror's active face.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional fixing devices are used to secure the mirror to the support structure, then the mirror can be fixed and withstand launch vibrations, but tensile and bending stresses are exerted on the glass-ceramic mirror which it cannot withstand effectively
Solution Approach 1:
The invention inverts the stress application approach by using a T-shaped glass protrusion that converts tensile and bending stresses into pure compressive stresses. The heel of the T-shaped protrusion contacts the clamping part such that tightening screws apply force along the axis of the intermediate part, creating compression rather than tension on the mirror substrate.
Solution Approach 2:
The invention changes the stress state parameter from tensile/bending to compressive by modifying the geometry of the fixing device. The T-shaped protrusion with its specific orientation and the clamping part geometry are designed to transform the force application mode, exploiting the fact that glass-ceramic has higher compressive strength than tensile strength.
2Reliability
If the fixing device applies strong clamping forces to secure the mirror, then the mirror remains stable during launch, but thermal expansion differences cause deformations at cryogenic operating temperatures
Solution Approach 1:
The invention explicitly addresses thermal expansion by selecting materials with matched coefficients of thermal expansion: Invar for the clamping part (very low expansion) and aluminum for the pads (high expansion). This differential expansion compensation allows the assembly to maintain proper clearance and stress distribution across the temperature range from launch to cryogenic operation.
Solution Approach 2:
The design incorporates preliminary compensation for thermal effects by creating adjustable clearances and using materials that counteract each other's thermal expansion. The screw mechanism allows pre-adjustment of the clamping force to account for anticipated thermal contraction at cryogenic temperatures, preventing excessive deformation during operation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively withstands vibration loads during launch and reduces thermal-induced deformations at cryogenic temperatures, ensuring the mirror's stability and operational integrity by converting stresses into compressive forces that glass-ceramic materials can better resist, thereby maintaining the mirror's precision and functionality.
Implementation Method 1
The invention proposes a fixing device implementing constraints of the pure compression type, that is to say in a direction perpendicular to the direction Dx, the resistance of the glass to this type of constraint being much better than the resistance of the glass to tensile or shear stresses
Implementation Method 2
said device comprises a clamping part clamping the end part of the protrusion (the heel) in a direction parallel to the intermediate part and means for fixing said clamping part to the on-board structure
Implementation Method 3
the pads comprise a first part in contact with the terminal part of the protrusion having a flat face and a convex face and a second part having a concave face complementary to that of the first part (thus producing a pivoting head)
Implementation Method 4
the clamping part is made of a material with a very low coefficient of thermal expansion of the iron-nickel alloy type of the Invar type
Implementation Method 5
the pads are made of a material with a high thermal expansion coefficient of the aluminum type
Implementation Method 6
the surface of the pads in contact with the terminal part of the protrusion further comprises a layer of soft material of the gold type making it possible to improve the contact with respect to the surface states and therefore to homogenize constraints
Implementation Method 7
the clamping part can further comprise force cells between the screws and the pads
Data Source
Figure 1~2
Figure 3~4
Figure 5a~5d
AI summary
The device has a locking part (33) compressing a terminal part of a glass excrescence (11) according to direction parallel to an intermediary part of the excrescence. A fixation unit fixes the part (33) on an embarked structure. The unit (33) is made of invar type thermal dilatation material with low coefficient and has openings with a locking screw (77) to block the part (33) against the excrescence. Skids assure contact between the terminal and locking parts and are made of aluminum type high thermal dilatation coefficient material. Surfaces of the skids have a layer of gold type material.