How efficient are ramjet engines?

Ramjet engines are usually efficient when the vehicle is traveling at supersonic speeds of around Mach 3. You can read about the Hypersonic Technology Demonstrator Vehicle (HSTDV) – unmanned scramjet technology of DRDO in the given link.

Further readings:

1. Advanced Technology Vehicle – India, one of the 4 Countries To Develop Scramjet Engine
2. Reusable Launch Vehicle – Technology Demonstrator (RLV – TD)