Jet engine manufacturing is a niche domain and requires cutting edge technology. The domain has been one of the most criticized domain within the Indian defense public sector companies, largely because of the partial success of GTRE Kaveri. However, GTRE Kaveri is not the only engine that India had worked on. This journey started with the Reheat version of the Orpheus 703 Engine of HF24 but it only achieved limited success.

Today, India has multiple jet engine projects running simultaneously, some of these have been successful while others are still considered to be partially successful.

  1. GTRE GTX-35-VS Kaveri
  2. HAL HTFE-25
  3. STFE (Former Manik)
  4. Shakti Engine

The working concept of all the aero engines are the same, the blades spin at high speed and compresses (squeezes) the air. The compressed air is then sprayed with fuel and an electric spark lights the mixture. The burning gases expand and blast out through the nozzle, at the back of the engine. As the jets of gas shoot backward, the engine and the aircraft are thrust forward. The difference lies in the way by which the air is compressed and pushed into the later stages.


The turbojet engine is a basic type of jet engine or first form of gas turbine power plant for the aviation industry developed by the UK with the following main components:

  1. Compressor – To compress the in-taken air to high pressure around six times of the original pressure.
  2. Combustion chamber – To burn the above compressed air with fuel mixture. The combustion chamber temperature may go up to 800 degrees.
  3. Turbine – To drive the compressor. This is achieved by passing the combustion chamber burnt gases into the turbine thus producing the shaft work output process.
  4. Nozzle – To expand the out coming gas from turbine to produce thrust

        The basic operation of the turbojet engine is based on the Brayton cycle. Accordingly, the air is the working fluid, as air flows through the compressor that brings higher pressure, energy is then added by spraying fuel in the compressed air and igniting it to combustion process which in turn generates high temperature. The high temperature as well as pressurized gas enters the turbine producing the shaft work output, which in turn drives the compressor. The unused energy coming out of the turbine can be converted into thrust by expanding the same in the nozzle. The expanding gases push the front of the engine. In general, the turbojet sucks and compresses the air which in turn shoots out of the rear nozzle pushing the plane forward.

         Turbojet engines have lower efficiency at subsonic speeds while their efficiency considerably goes up in the supersonic regime. One of the notable applications was the use of OLYMPUS 593 in Concorde which achieved the required thrust at Mach2.0 with reduced fuel consumption compared to modern turbofan engines. One of the biggest merits of Turbojet engines is their reliability which can be as high as 99%.

Indian turbo jet programme:

Aero Engines Research and Design Centre established in 1960 carries out the design and development of Gas Turbine Engines. It has successfully designed and developed small aero engines currently in operation with the defense services. Centre completed the design and development of a Hindustan Jet Engine (HJE-2500) as its first jet engine project in 1965 which was proposed for the HJT-16 aircraft. Later, a piston engine and other engine accessories including a pneumatic starter and hydraulic pump were also developed.

HJE – 2500

PTAE – 7 Engine used on the Lakshya

The engine used by the Indian remotely piloted high-speed target drone system developed by the Aeronautical Development Establishment (ADE) of DRDO is also a turbojet engine capable of producing 3.73 KN thrust.



        A turbofan engine is an air-breathing engine, a bit complex in structure, and more fuel-efficient. Hence used widely in commercial airliners. Unlike turbojet that gets all the air into the engine, only a small portion of air enters the engine, and balance air is bypassed which comes out as a cold jet which later gets added to produce hot jet. The turbofan engine consists of the following main parts,

  1. Fan – Which directs airflow towards the engine
  2. Low-pressure compressor – Compresses and increases the air pressure
  3. High-pressure compressor – Compresses and increases the air pressure
  4. Combustion chamber – Ignition of the air-fuel mixture
  5. High-pressure turbine – Power generated to drive the compressor and fan
  6. Low-pressure turbine – Power generated to drive the compressor and fan
  7. Propelling Nozzle – Expansion of gases for generating thrust

       As the air passes through the fan, it is divided and a fraction of air enters the engine which gets compressed in 12 stages under low pressure and high-pressure compressor. Then the same will get ignited in the combustion chamber and expanded in low and high-pressure turbines. The bypassed air is then sent through a low-pressure condenser which helps to cool the engine and the same is again mixed with a gas generator to produce a hot jet. The bypass can be divided into following;

  1. High bypass turbofans – With larger fan diameter and fan thrust higher than jet thrust. Example – all commercial airliners and military transport aircraft.
  2. Low bypass turbofans – With reduced fan diameter and jet thrust higher than fan thrust. In general low bypass, turbofan engines are used with afterburners where the second stage of combustion occurs by adding fuel to exhaust from the turbine and reigniting. Example – engines used in fighter jets.

Indian turbofan jet programme:

The most significant Indian program under this category is GTRE Kaveri but India has a few other projects under this category.

  1. HTFE 25 – The HAL HTFE-25 (“Hindustan Turbo Fan Engine”) is a 25 KN turbofan engine under development by Hindustan Aeronautics Limited (HAL). The engine can be used in single-engine trainer jets, business jets, and UAVs weighing up to 5 tonnes and in the twin-engine configuration for the same weighing up to 9 tonnes.
  1. The reheated variant of this engine will be called as HTFE 40.
  • Short Turbofan Engine – Former “Manik” engine is now called the ‘Small Turbofan engine’. The engine can produce 400 kgf thrust class designed and developed by GTRE to propel Nirbhay Missile.
  • AL-31 FP Engine: The AL-31 FP engine of Russian origin is being manufactured and overhauled at HAL Koraput division as a Power plant for SU-30 MKI aircraft.

Turbo Shaft Engine: –

A turboshaft engine uses the same principles as a turbojet to produce energy, that is, it incorporates a compressor, combustor, and turbine within the gas generator of the engine. The primary difference between the turboshaft and the turbojet is that an additional power section, consisting of turbines and an output shaft, has been incorporated into the design.

A turboshaft engine is very similar to a turboprop and many engines are available in both variants. The principal difference between the two is that the turboprop version must be designed to support the loads of the attached propeller whereas a turboshaft engine need not be as robust as it normally drives a transmission which is structurally supported by the vehicle and not by the engine itself.

Indigenous projects:

Shakti/Ardiden 1H1 engine is a turbo shaft engine power plant for Advanced Light Helicopter (ALH) Mk III & Mk IV and Light Combat Helicopter (LCH) operated by Indian Armed Forces. This engine designed jointly with M/s Turbomeca France and AERDC, HAL. AERDC designed Oil Pump, Heat Exchanger, Filter housing unit and Pipe lines. Initial operational clearance for operation of Shakti engines in ALH Helicopters was accorded in 2010 after conducting all the required field evaluation and certification tests.


       A turboprop engine is a form of turbine engine that drives the propeller of the aircraft and has better efficiency at lower (subsonic) speeds, unlike turbojets. The first turboprop engine was developed by Germany. While the basic components are the same as turbojets, a propeller and gearbox addition before the compressor is the main change. The compressor, combustion chamber turbine, and propelling nozzle operation remains the same as above mentioned in turbojet operations.

       The operation of compression, combustion, and expansion is the same, while part power generated by the turbine is used to power the compressor and the balance is transmitted to the reduction gearbox which in turn drives the propeller. The further expansion of gases occurs in the propelling nozzle producing the required thrust. The energy generated is mostly used to drive the propeller, leaving the exhaust gases with very negligible energy, unlike turbojets.

        At lower speeds (about 400 to 500 miles per hour) turboprop has quite a good fuel efficiency. Modern turboprops are designed to cope up with a bit higher speeds maintaining good fuel efficiency, with a change in geometry of the fans, increased number of blades, and reduced diameter of the propeller. Turboprops are quite easy to maintain with reduced fuel consumption and are also cost-effective. C130J and C295 are a couple of examples of the most fuel-efficient, suited turboprop powered planes used by major nations.

Indigenous projects:

The project is not fully indigenous but HAL does make engines for the Dornier Do 228 aircraft. The Garrett TPE 331-5 Turboprop Engine is being manufactured, overhauled, and repaired for various Customers under license from Garrett Engine Division of the Honey Well Company [earlier Allied Signal Aerospace Company, USA] since 1988 and belongs to one of the most popular series of small Turboprop Engines powering a large number of Commuter and Corporate Aircraft such as Dornier DO-228.

Balaji Jaikumar

In a world of darkness look up at the stars, because that can be a precision-guided munition

By Alpha Defense

Alpha Defense initially a solo venture but now a defense group by people from various demographics of India covering defense news and updates. We believe in unbiased analysis of every subject in hand. Our mission is to provide simplfiied defense information to the public.

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