Traditional heat shield design isolates the functions of the thermal protection system (TPS) from the underlying load-carrying structure. This is due to the use of brittle TPS material that cannot carry the structural loads and that ablate during planetary entry. To isolate the TPS, the traditional approach has been to either bond a strain isolation pad between the TPS and structure or to design the structure significantly thick to eliminate bending loads transferred to the TPS. Adhesive is used to bond the TPS to the pad and/or structure. These bond lines can be a source of stress concentrations and manufacturing flaws.
The Multifunctional Hot Structure (HOST) Heat Shield integrates the function of the TPS with the primary load-carrying structural component. An advanced carbon-carbon (ACC) material system is being evaluated for the load-carrying structure, which will be used as the outer mold line (OML) component, and will operate as a hot structure exposed to the severe aerodynamic heating associated with a planetary entry. The use of ACC on the OML also has the benefit of less recession than ablative TPS materials experience when exposed to aerodynamic heating. Flexible blanket insulation will then be used underneath the ACC to achieve the desired inner mold line temperature. By integrating functions, there is the potential to both save weight and increase available volume in the spacecraft.
The HOST concept eliminates the need for a strain isolation pad or the need for thick structural designs by using a structural material on the OML that is capable of carrying significant loads at elevated temperatures. The use of flexible blanket insulation in the HOST concept, as opposed to ceramic material bonded to the underlying structure, also eliminates the need for adhesive bonding while offering a lighter-weight alternative.