Low-density ablative heat shield fabrication

Abstract

Spacecraft heat shields are fabricated as one-piece assemblies using low-density ablative thermal protection materials. The heat shield assembly is built from modular pieces formed by ablative impregnation processing. Once the full-size heat shield is assembled from the modular blocks, heat treatment is used to bond the individual blocks together by facilitating polymeric cross-linking of impregnant material within and / or between each block. This provides a structurally integral one-piece heat shield assembly that can be further machined to final dimensions and attached directly to a spacecraft structure or a carrier panel separately attached to the spacecraft

Description

[0001]This application claims priority to U.S. Provisional Patent Application Ser. No. 60 / 785,930, titled “Low-Density Ablative Heat Shield Fabrication,” filed Mar. 24, 2006 and incorporated herein by reference.[0002]The invention described herein was made by nongovernment employees, whose contributions were done in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202). This invention was made with Government support under contract NNA04BC25C awarded by NASA. The Government has certain rights in this invention.BACKGROUND OF THE INVENTION[0003]1. Field of the Invention[0004]The present invention relates to the space program, and more specifically, it relates to processes for making ablative heat shields that provide protection to spacecraft during the severe heating conditions of atmospheric entry.[0005]2. Description of Related Art[0006]The making of large ablative heat shields that provide protection to spacecraft durin...

AI Technical Summary

Benefits of technology

[0015]The fabrication of large (larger than 1 meter diameter) spacecraft heat shields as one-piece assemblies using low-density ablative thermal protection materials (TPS) formed from rigid substrates has been constrained by limits on available component matrix material sizes and processing requirements. Methods are provided that allow large uni-piece heat shields to be fabricated for use on future space vehicles that require protection from atmospheric entry heating at severe conditions. These fabrication methods provide such large assemblies by building a heat shield assemb

Problems solved by technology

The making of large ablative heat shields that provide protection to spacecraft during the severe heating conditions of atmospheric entry is a difficult problem.

Limitations due to inherent physical properties and the inability to fabricate large assemblies due to processing and / or machining requirements have prevented the use of the most promising materials in spacecraft design in several instances.

In the specific cases of low density and mid-density ablative materials, difficulties in controlling the impregnation of fiber matrix materials with polymeric resins, and difficulty in processing larger fiber matrix substrates within density specifications to achieve uniform material properties have limited the size that billets or blocks of these materials can be made.

These limitations have required the use of less capable materials or the use of fabrication and assembly methods that lead to complex and costly final products.

Carbon phenolic is a very effective ablative material but also has high density and resulting high conductivity which is undesirable.

If the heat flux experienced by an entry vehicle is insufficient to cause pyrolysis then the TPS material's conductivity could allow heat flux conduction into the TPS attachment and spacecraft structures, thus leading to TPS failure.

This requires that each tile be machined to a finished size prior to being individually attached to spacecraft structure in a complex and costly operation and, in the case of the Space Shuttle, fabric filler materials were required to fill gaps between tiles that exist before entry heating.

For the Apollo capsule heat shield, the required one-piece ablative heat shield was fabricated by injecting an epoxy resin into a phenolic honeycomb in a costly, complex, and hard to control process.

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