What is the main advantage of scramjet engines?

Scramjet engines will help in achieving hypersonic speeds and it does not have any moveable parts like conventional engines.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)