The Soviets (later Russia) have been the masters of developing efficient and reliable semi-cryogenic engine for the past 7 decades. The earliest known example is the RD-107-8D74 used to launch R-7 Semyorka ICBMs, Sputnik, Vostok and Voskhod launch vehicles. The RD-107 was designed under the direction of Valentin Glushko at the Experimental Design Bureau (OKB-456) between 1954 and 1957. Sadly, the development of RD-107's predecessor the RD-105 represented German-designs developed by the Nazis. The V-2 motor was the world's first large, liquid-fuel rocket engine and powered the first ballistic missile, the German-Nazi V-2 rockets of World War II.

It is worthy to note here that during the 1990s, the Chinese acquired two or three models of the RD-120 (and advanced version derived from the RD-107), and possibly some unauthorized documentation. This enabled them to bootstrap their indigenous kerosene staged combustion engine program, the YF-100 and YF-115 engines.

Now India is at the cusp of developing its own semi-cryogenic engine to power its future Heavy and Super Heavy launch vehicles as per a reported The Organiser.

The new semi-cryogenic engine will be tasked to do much of the heavy-weight lifting for the Agency. It is an advancement in rocket technology that is superior to the current cryogenic engine which is currently under use by ISRO for GSLV (Geo-Synchronous Launch Vehicle) The GSLV is a three-stage heavy-lift launch vehicle used to launch heavy satellites into space. It is also expected to power ISRO's upcoming NGLV rockets (previously planned as ULV) as well as ISRO's future reusable rockets based on Reusable Launch Vehicle-Technology Demonstrator (RLV-TD). On April 02, 2023 ISRO successfully conducted an autonomous landing test of the RLV-TD spaceplane.

Cryogenics is the study of the production and behaviour of materials at very low or extremely low temperatures (minus 150 degrees). A cryogenic engine provides a greater force with each kilogram of cryogenic propellant which is far more effective than the solid/liquid fuel propellant. The Cryogenic Engine uses liquid hydrogen and liquid oxygen as propellants. A propellant is a chemical mixture which is burned to provide thrust in rockets.

On the other hand, a semi-cryogenic engine uses refined kerosene (civil aviation fuel) instead of liquid hydrogen. It uses liquid oxygen as an oxidiser. Kerosene is much lighter than liquid fuel and can be stored at a normal temperature. The Kerosene combined with liquid oxygen provides more thrust to the rocket. Compressed Natural Gas (CNG) can also be used as fuel.

It also occupies less space which makes it possible to have more propellant in the fuel compartment or tank. The semi-cryogenic engine complies to modern ecological standards, is adequately cost-effective and more powerful compared to a cryogenic engine. It can carry a huge weight to a higher altitude. Moreover, the payload can be increased since the weight of the fuel will come down by 500kg. Under deep throttle, it will provide and work with much higher operability.

The Semi Cryogenic engine will also be used in the Gaganyaan Mission. India’s strategic partner Russia is offering its semi-cryogenic technology along with critical components for this mission. The chairman of ISRO, Sivan has stated that the technology will be transferred under the “Make in India Programme”. The engines will be built in India only.

There are a few disadvantages which come to the forefront regarding cryogenic propellants. The fuel used in the cryogenic engine is easily combustible and ignition can cause explosions. Secondly, cryogenic storage tanks must be developed to withstand extreme temperatures and pressure. The storage tanks will require thicker walls and strong alloys. Thirdly, cryogenic fuel is very toxic in nature and can be fatal if inhaled. It will provide higher levels of asphyxiation.

India is currently working on the Semi-Cryogenic Engine (SCE-200) as India’s Space program needs to move to the next level. The SCE-200 project was approved in 2009 for 1798 Crores. It is being developed by the Liquid Propulsion Systems Centre one of the four main pillars of ISRO. The other three are the Vikram Sarabhai Space Centre (VSSC), ISRO Inertial Systems Unit (IISU) and the Indian Institute of Space Science and Technology (IIST).

According to news reports in Nov 2022, the test facility and stand had been nearly ready for engine as well as SC120 stage test which would upgrade India's existing LVM3 rocket. ISRO outlined that the full development of the semi-cryogenic engine is expected to be completed in the next one or two years and the thrust chamber of the engine is expected to be ready soon.

The development required ten years of research, requiring the mastering of 70 key technologies, the development of 50 new materials and the construction of 61 sets of engines with a combined total of more than 40,000 seconds of ignition time by 2013.

Once the thrust chamber is tested, the next step would be integrating it into an engine and test the engine itself.