SOURCE: Anand SG / FOR MY TAKE / IDRW.ORG
PTAE-7 Turbo jet engine developed by HAL for use on pilotless target aircraft, Lakshya likely used on recent Nirbhay Trial
In Recent successful test flight of India’s first indigenously designed and developed ‘NIRBHAY’ Long Range Sub-Sonic Cruise Missile achieved the range of 647 km for a total time duration of 50 minutes which was less than what it had covered in its second flight when it covered over 1050 km for a 1-15 minute duration due to which it was widely speculated that drop in performance was largely due to use of Turbojet engine instead of Turbofan engine for the first time .
DRDO has not confirmed nor denied use of Turbojet engine instead of a Turbofan engine nor clarified on exact reasons behind the need to use it, but this has not stopped many Defence Analysts from coming up with their own theory why a Turbojet engine was used instead of Turbofan engine used in all previous trials.
Russian supplied NPO Saturn 36MT turbofan engine
India’s worst kept secret has been that India has been using Russian supplied NPO Saturn 36MT turbofan engine for all previous tests and best two possible reasons why India decided to use older Turbojet engine could be due 1) India might be out of supplied NPO Saturn 36MT turbofan engines and Russians might have hiked prices of them . another reason could be that due to delays in certification of Indigenous Manik Turbofan engine currently under development by GTRE, India might have decided to instead use older Turbojet engine for time being rather than import fresh batch.
GTRE’s Manik Engine going though ground tests
2) Another theory is that recent flight test largely stressed on validation of the in-flight performance of the missile and its onboard systems like navigation system and software that use of older Turbojet engine didn’t matter for recent trial. Another speculation is that Manik Turbofan engine which reportedly has completed all ground-based testing successfully is going through final weight reduction measures and might be closer to production and will be ready for flight trials soon on next Nibhay trials and that recent tests were done using older Turbojet engine to validate other key parameters instead.
Use of a turbofan engine in Subsonic cruise missile is quite common and it has been used in many modern Subsonic cruise missiles like American Tomahawk long-range cruise missile and even on Pakistan developed Babur land attack cruise missiles and is widely used on other subsonic cruise missiles.
A Turbofan engine is efficient at subsonic speeds and produces less noise along with it also has better fuel efficiency. A turbofan engine is actually a further development of Turbojet engine which was developed to improve their performances in subsonic regime.
A Turbojet engine is inefficient at low-speed flight and is extremely noisy when used in subsonic regime. Turbojet engine might not be widely used engines on cruise missiles around the world this days but they are still operational around the world and are still used in many mid-range (<500km) Subsonic cruise missiles like Chinese C-802 Anti-ship Subsonic cruise missiles for example due to low cost of production and high reliability largely to cater to export market .
A Turbofan engine like Manik will always remain a preferred choice of the engine which will part of the Production version of Nirbhay Cruise missile when it enters production. Next series of Nirbhay Cruise missile will actually see the use of Manik Turbofan engine which will be a key milestone on long-delayed Nirbhay Cruise missile program.