Thanks to Curbina for posting the following comment and link in the Always On Thread
Leif Holmlid and Sindre Zeiner-Gundersen published a paper in Acta Astronautica that is about an experimental relativistic drive that has an efficiency of 1300 times the energy consumed by the laser used to estimulate the anhihilation-like behavior, and the kinetic energy produced. In other words, COP of 1300x!!!
Title of Paper: “Future interstellar rockets may use laser-induced annihilation reactions for
From the Abstract:
Here, a useful method for relativistic interstellar propulsion is described for the first time. This method gives exhaust at relativistic speeds and is a factor of at least one hundred better than normal fusion due to its increased energy output from the annihilation-like meson formation processes. It uses ordinary hydrogen as fuel so a return travel is possible after refuelling almost anywhere in space. The central nuclear processes have been studied in around 20 publications, which is considered to be sufficient evidence for the general properties. The nuclear processes give relativistic particles (kaons, pions and muons) by laser-induced annihilation-like processes in ultra-dense hydrogen H(0). The kinetic energy of the mesons is 1300 times larger than the energy of the laser pulse. This method is superior to the laser-sail method by several orders of magnitude and is suitable for large spaceships.