by G Hari Kumar

In 1992, the US under President George Bush had slapped sanctions on Indian Space Research Organisation (ISRO) and prevented Russia from sharing cryogenic engine technology with the Indian space agency so as to check India from making missiles.

The United States crippled India’s cryogenic engines program in the late 1980s, just as a deal was about to be completed with Glavkosmos, an official space affairs entity of the former Soviet Union. If it had gone through, ISRO would have received two cryogenic engines, technology transfer and a skill-development program for only Rs 230 crore. But because it didn’t, the GSLV was not ready to fly until 2010 instead of sometime in the early 1990s. This incident has had a cascading effect on a range of issues since then – from trade deal negotiations to politics writes Vasudevan Mukunth.

The vacuous US position was that the cryogenic engines could be used by India to power military ballistic missiles. This was a malicious contention as ballistic missiles have to be deployed at very short notice - whereas fuelling a cryogenic engine would take days. Despite the US sanction, ISRO was able to procure 2 mock-up and 5 fully qualified engines from the Soviets. These engines were used to power the GSLV MK-I missions. The Russian engines KVD-1 (RD-56), were later dropped in favour of home-grown technology (CE-7.5) thus proving the point that the American sanction was indeed a blessing in disguise.

As a corollary, during this period, the notorious ISRO espionage scam involving ISRO scientist Nambi Narayanan who was then in-charge of the cryogenics division, also unfolded. In 1994, he was falsely charged with espionage and arrested. The charges against him were dismissed by the CBI in 1996, and the Supreme Court of India declared him not guilty in 1998. Detractors attempted all tricks of the trade in the book to sabotage India's efforts to develop indigenous cryogenic technology.

Why Cryogenic Engines?

Most rocket propulsion is achieved through chemical propellants, where chemical energy is converted into the kinetic energy of hot gases that are expelled from the combustion chamber. A propellant is composed of two parts, a fuel that burns and an oxidiser that aids its burning. The chemical energy is first converted into heat energy through combustion. Because of the heat energy that is released, the gaseous products of the combustion expand. As the hot gases escape through the flared-up geometry of the nozzle at the aft end of the combustion chamber, they gain kinetic energy and exit at a very high velocity. The greater the chemical energy content of the propellant, the higher the exit velocity of the hot gases and the resulting thrust.

Cryogenics is the science and technology of temperatures below 120 Kelvin (−153° Celsius), the limit being defined by the boiling point of methane, a principal component of natural gas. However, cryogenic stage is technically a very complex system compared to solid or earth-storable liquid propellant stages due to its use of propellants at extremely low temperatures and the associated thermal and structural problems. 

Thus, all cryogenic rocket engines are liquid engines but they should be distinguished from rocket engines that use earth-storable liquid propellants that are liquids at ordinary temperatures and can, therefore, be stored as liquids easily. The most common cryogenic propellants used in rocket engines are liquid hydrogen (LH2), which liquefies at −253° C, as the fuel and liquid oxygen (LOX), which liquefies at −183° C, as the oxidiser.

Cryogenic propellants are preferred as rocket propellants when rockets have to carry payloads of high mass because they have the greatest efficiency in terms of thrust generated. This efficiency is measured by what is called “specific impulse”. It is defined as the thrust generated per unit mass of propellant consumed per unit time or, equivalently, the rate of mass ejected from the rocket nozzle. It is measured in units of seconds.

Mark To Indigenisation

ISRO's Cryogenic Upper Stage Project (CUSP) envisaged the design and development of the indigenous Cryogenic Upper Stage (CE-7.5) to replace the stage procured from Russia and used in GSLV MK-I flights, The CE-7.5 engine was developed from scratch by ISRO.

ISRO began experimenting with cryo engines back in the 80s. A small 1-ton pressure-fed cryo engine was developed in 1986 to gain hands-on experience with cryo fuels and technology, writes former ISRO chairman Prof UR Rao in his excellent book "India's Rise As A Space Power".

In 1988, General Dynamics of USA offered their RL-10 engine at a price of $800 million for two engines without technology transfer. And, in 1989, Arainespace offered their HM7 engine with technology transfer for $1200 million. Both deals didn't go through as they were expensive. So, under the direction of then ISRO Chairman Prof UR Rao, ISRO decided to develop their own 12-ton cryo engine with a 8-year timeline based on their experience with 1-ton experimental cryo engine and Vikas liquid engine, even though it meant a delay in GSLV project.

In 1990, Glavkosmos of USSR offered their KVD-1 (RD-56) engine and CS-12 upper stage with technology transfer at an attractive price. ISRO decided to buy these to expedite GSLV project (and thereby shelving their 12-ton engine project!!). So, GSLV was designed to have Russian cryogenic upper stage and engine. ISRO ended up getting fully-qualified KVD-1 engines in 1997 and no further technology transfer.

ISRO restarted the indigenous cryo stage and engine project in 1997. Since GSLV was already designed with Russian cryo stage and engine in mind, ISRO decided to develop an identical stage and engine that can be a "drop-in" replacement in GSLV (otherwise, the whole of GSLV need to be redesigned if upper stage changes which would mean further delays and cost overruns). This in-house engine was named CE 7.5 and indigenous stage was called the CS-12 (a drop-in replacement for Russian CS-12). ISRO had to establish new facilities at ISRO’s Liquid Propulsion Systems Centre, in Mahendragiri in Tamil Nadu state and also at other locations. Over the years, ISRO completed several successfully full duration engine level and stage testing qualifications on more than one hardware.

Needless to say, the CE-7.5 is indeed identical to KVD-1 in terms of specifications. However, it is not a blatant copy of the KVD-1 as only the overall design philosophy was adopted and not reverse engineered which is sadly thought to be.

Actually, a lot of iterative improvements were done on KVD-1 itself to reduce its weight, and increase payload capacity. Similarly, a lot of improvements were done on CS-12 and CE-7.5 to enhance thrust, payload capacity, and payload fairing diameter extensions. (Text extracted from the excellent book "India's Rise as a Space Power", by Prof UR Rao)

Former ISRO Chairman AS Kiran Kumar remarked, "But here, it’s a completely independent system, with a different design, and the advantage of that is that you know the ins and outs of it."

After the success of the CE-7.5 engine, there was the need to develop a higher thrust cryogenic engine vital to ferrying heavier payloads in the 4-ton class into orbit. It also makes possible a host of other missions, including crucial part of India's manned mission. Consequently, ISRO developed a high thrust cryogenic engine the CE-20 stage to be used for the upper stage of its heavy lift launch vehicle GSLV Mk-III. This high thrust cryogenic engine produces a nominal thrust of 196.5 kN in vacuum with a specific impulse of 434 seconds. The engine works on “Gas Generator Cycle” which has flexibility for independent development of each sub-system before the integrated engine test, thus minimising uncertainty in the final developmental phase and reducing development time. This engine generates nearly 2 MW power as compared to 1 MW generated by the engine of Cryogenic Upper Stage (CUS) engine of GSLV. The high thrust cryogenic engine is one of the most powerful cryogenic upper stage engines in the world.

It is worthy to note that successful development of cryo technology has taken an average of 15 years to develop which is a nominal for other countries as well. ISRO has also done a commendable job to develop these hi-tech engines considering the sanction hurdles, budget, resource crunch they faced to develop this exclusive and crucial technology.

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