Scramjet Technology to fulfil Avatar (reusable launch vehicle) program of ISRO
ISRO made a successful test of its Scramjet (or Supersonic Combusting ramjet) engine. The space agency's Advanced Technology Vehicle rocket was able to fly at Mach 6 (six times the speed of sound) speeds, propelling India into the league of only four nations to be capable of doing so. ISRO plans to use the scramjet technology for its Avatar program.
Scramjet requires air to be sucked in from the front, burn fuel in the middle and make exhaust gas come out of the back generating thrust. Factors like air inlet design, air compressibility, complex flows, high temperatures, fuel injection, etc - make it really difficult to design a suitable scramjet power plant. Scramjets are highly inefficient at low speeds, with their efficiency increasing at supersonic speeds.
Why are scramjets important for ISRO? ISRO currently uses rocket launch vehicles like the PSLV to deliver satellites into orbit. PSLVs are expendable, meaning that can only be used once, and are designed to carry both fuel and oxidizer with for launch. Scramjets use ambient air to burn fuel, thus saving the need to carry an oxidizer - thus increasing the payload of a craft.
ISRO claims that using Avatar for satellite launches will cut down launch costs by half. Since there are no rotating parts in a scramjet, the chances of failure are also measurably reduced.
Thus, the spacecraft will use different engines for different stages of the flight - with ramjet used at lower speeds, scramjet at hypersonic speeds and cryogenic engines when the craft reaches the edge of the atmosphere.