Comment by Mikhail_K

Photon drive is not "thrust without reaction mass", because photons do have mass (they have zero rest mass, but are never at rest). Mass of a photon is, unsurprisingly, E/c^2, where E=h\omega. If photons had zero mass, they would not have been influenced by gravity, and bending of light by the solar mass has been measured in the beginning of 20th century.