Lightweight composite structures are required to provide space vehicles with increased thrust-to-weight ratio and durability. New methods for toughening composites that also add functionality, such as making high temperature structural composites more thermally and electrically conductive, can increase spacecraft performance. Infoscitex proposes to enhance toughness of a thermal-oxidatively stable structural composite by more than 50 percent (measured by G1C) by depositing dispersed carbon nanofibers within insulating, resin-rich, interlaminar regions. Our approach also increases z-direction thermal and electrical conductivity, enabling dissipation of heat and electrical charge from hot spacecraft power supply and propulsion enclosures. In Phase I, we will process commercial carbon nanofibers and high temperature composite prepreg into nanofiber-toughened and control composite test panels using an autoclave. Cured composite test panels will be tested for G1C, open hole tensile and short beam shear strength, thermal and electrical conductivity and failure analysis to demonstrate feasibility. In Phase II, we will refine the composite toughening process and materials, conduct compression-after-impact tests, scale-up and produce a prototype multifunctional composite spacecraft structure for testing. Our team includes commercial prepreg and carbon nanofiber suppliers, a space vehicle structure manufacturer and experts in high temperature composite processing.