How ISRO Built The Cryogenic Engine That Will Put The Lunar Module Of Chandrayaan-2 In Orbit Around Earth

When the 20-hour-long countdown for the launch of Chandrayaan-2 ends later today (22 July), the Geosynchronous Satellite Launch Vehicle (GSLV) Mark-III, the Indian Space Research Organisation’s (ISRO’s) heaviest rockets, will lift-off from the Satish Dhawan Space Centre (SDSC) on the barrier island of Sriharikota, and roar skywards carrying an orbiter, a lander and a rover — all interfaced mechanically in one.

The first to ignite will be the two solid strap-on boosters on both sides of the rocket. Breathing out an orange plume, the boosters give the rocket the thrust required for lift-off and guide it through its brief but majestic vertical ascent. A few seconds later, the liquid-fuelled core, or the second stage, powered by a pair of Vikas engines, will fire, and the strap-on boosters, having served their purpose, will be discarded. After this, the payload fairing, or the covering over the payload, will be jettisoned, and this will be followed by the shutdown and detachment of the liquid core stage.

And then, the cryogenic upper stage, powered by CE-20 — largest cryogenic engine ever made by ISRO — will kick in, driving the lunar module to a highly elliptical Earth Parking Orbit.

CE-20 burns for around 10 minutes. It took ISRO nearly 20 years to make it.

India started working on indigenous cryogenic technology as early as 1971, but the project was shut down after the death of Vikram Sarabhai and not revived till the 1980s, when India felt the need to build launchers that can put heavy communication satellites into Geosynchronous Transfer Orbit (GTO). Using cryogenic engines was the simplest way of transforming its existing capabilities to make a more powerful satellite launcher that it could use to put communications satellites into the GTO.

A cryogenic engine stage offers many advantages: It has a higher efficiency as it produces more thrust for every kilogram of propellant it burns in comparison to the solid and earth-storable liquid propellant rocket stages. Propellents used in a cryogenic engine — liquid oxygen and liquid hydrogen — are relatively environment-friendly, non-toxic, non-corrosive, and, most importantly, economical.

‘Armageddon-Hogwash’

Sometime around 1986, ISRO began developing a one-tonne cryogenic engine to use it on the planned GSLV. However, realising that it would take a very long time to build a functional cryogenic stage indigenously, ISRO decided to look for the technology outside the country which could be imported to meet India’s needs.

In 1988, an offer was made by a US’ General Dynamics to sell ISRO its RL-10 cryogenic engine, a version of which is still used on Atlas-V and Delta-IV rockets. However, the engines were priced at $800 million, and the US government had the option to veto technology transfer. The offer was rejected. This was followed by an offer from France’s Arianespace, which was ready to supply its HM7 cryogenic engines along with the technology for $1.2 billion. The exorbitant price deterred India and the deal did not go through.

But then, around 1989, Glavkosmos, the commercial arm of the Soviet Union’s space agency Roscosmos, offered to supply India its 12-tonne 11D56 cryogenic engines and technology for just $132 million (around 240 crores at the time). ISRO grabbed the deal with both hands. But 15 months later, the US came knocking, citing a violation of the Missile Technology Control Regime, a group which was established in 1987 to limit the proliferation of missile technology. Just a few years after offering cryogenic engine itself, the West suddenly realised that India could use the cryogenic technology for the development of Intercontinental Ballistic Missiles (ICBMs).

It was an absurd argument — “armageddon-hogwash,” some said. Cryogenic engines are not useful for ballistic missiles as they are difficult to operate and it takes several hours to fuel a cryogenic engine and equip a rocket with it. This fact makes cryogenic engines unusable in ICBMs as missiles, unlike satellite launchers, are to be readied for launch at very short notice. Moreover, India had already indigenously mastered solid and liquid-fuel motors which were preferred for missiles. Multiple US experts warned their government about this fact, but all calls fell on deaf ears.

Of course, citing MTCR violation was just a way to legalise its objection to the deal. The truth was that the US had strong commercial interests in denying India the technology— Glavkosmos was demanding nearly 400 per cent less than the price quoted by US’ General Dynamics for its cryogenic engines and the price-per-kg of launching a satellite on the GSLV equipped with a cryogenic engine would have been less than half the price quoted by launch vehicles used by the US. This, the US may have thought, would have a considerable effect on its launch market.

The US imposed sanctions on ISRO and Glavkosmos in 1992.

Under pressure from the US, Russia, which inherited the engine and its technology after the dissolution of the Soviet Union in 1991, dissolved the original deal inked in 1991 for two cryogenic stages and technology invoking ‘force majeure’. After ISRO Chairman U R Rao’s efforts, Glavkosmos offered a new deal with four fully functional engines and two mock-ups without the technology transfer. A deal was reached in 1994 with the option of India buying three more cryogenic engines for $9 million.

“I told them that I had paid them too much money for just two engines. If you are not giving me the technology, give me six more engines,” Rao writes in his book India’s Rise As A Space Power (Cambridge University Press).

No Option But To Develop Indigenously

Thrust, or the propulsive force of an engine, depends on the flow of the fuel through it. The faster the fuel flows through it, the higher is the thrust. Solid fuels don’t flow. Gaseous fuels do flow, although not as well as liquid fuels do, and the latter also have higher energy density. But the fuel that provides the highest exhaust velocity is hydrogen, a gas. When hydrogen burns in the presence of oxygen, the energy released in the process provides an exhaust velocity as high as 4.55 kilometer per second. But as we know, gases barely flow, and the thrust an engine generates depends on how well the fuel flows through it.

Pumping hydrogen through the engine in form of gas is difficult. To make them flow better, both hydrogen and oxygen are liquified — liquids flow better. Liquified hydrogen has both the desirable qualities of a fuel: high flowability and high exhaust velocity. But this has its own complexities. While hydrogen liquefies at 20 kelvin (-253 ºC), oxygen liquefies at 89 kelvin (-184 ºC). As a result, they have to be stored separately and cryo-pumps are required to cool them.

Special igniters and turbo-pumps are required to move the propellants into the combustion chamber while maintaining high pressure. Parts of the engine have to be made up of special alloy because normal metals become brittle in cryogenically low temperatures and can fail. Due to its low density, hydrogen can leak easily. Cryogenically cooled fluids also continuously vaporise. These issues, and many more, make a cryogenic engine complex to build. But the advantages that hydrogen provides over other fuels makes dwarfs these complexities.

A cryogenic engine has tanks to store the fuel and the oxidiser. Liquids are ferried to the thrust chamber in the right proportion through the plumbing, structure to transmit the thrust to the vehicle, a power source, and control devices to initiate and regulate the propellants' flow.

“The thrust chamber is the powerhouse of the engine where combustion of fuel and oxidiser takes place. The burnt gases are ejected through a nozzle, converting the thermal energy of the combusted products into kinetic energy. The cryogenic engine thrust chambers need to be cooled to protect them from high temperatures,” Space India (January-March 2000), an ISRO publication, read.

With little to no access to foreign technology, ISRO started a cryogenic technology programme in April of 1994. The plan, in the short run, was to model the engine on the Russian specifications, configuration and interfaces so that ISRO could continue GSLV flights after all the engines supplied by Russia were used.

However, the Russian engines were not only difficult to get, they were also difficult to reproduce. Built in the 1960s for the mammoth N-1 rocket, the Soviet equivalent of Saturn-V which launched Apollo-11, the engines did not enter regular use and were mothballed after the N-1 programme was canceled due to four successive failures.

When used by ISRO, the efficiency of the engines was found to be below the predicted level. Also, it was found that the engines Russia supplied were heavier than what ISRO had expected. To make it capable of carrying extra load, Russia is believed to have increased the maximum thrust from 7.5 tonnes to a little over eight tonnes.

To reproduce the engines, ISRO had to learn new manufacturing methods and working with unfamiliar materials. Arrangements had to be made so that materials needed to make the engine could be made within the country. Moreover, the Russian engines used a technology called stage combustion which was relatively efficient but complex.

To make the thrust chamber of the engine, ISRO technicians had to learn a new technique called vacuum brazing. All of this delayed the process.

When working on the indigenous cryogenic engine in the 80s, before the signing of the deal with Russia, India was developing an engine using a comparatively simpler and much more flexible gas generator cycle instead of a staged combustion cycle.

Therefore, reproducing the Russian engine was not easy.

Russia only supplied the hardware. ISRO, as journalist R Ramachandran writes, had to develop on its own the control systems required for the management of the cryogenic stage and the mission. This included tank pressure control, thrust and propellant mixture ratio control, gimbal control and post-flight passivation, and the facilities where the stage was to be prepared and servicing of the propellant.

The engine built by ISRO based on the Russian design and specifications, called CE-7.5, “incorporated several changes from the Russian cryogenic stage,” S Ramakrishnan, former Director of Vikram Sarabhai Space Centre, said in a presentation in June 2012, adding, “however, the main stage architecture of fixed main engine with two steering engines was adopted from cryogenic stage as such.”

The first test of the CE-7.5 engine was conducted in 2000, which failed. After a series of tests to rectify the design, the engine was tested successfully in 2002. India used the Russian supplied engines for the GSLV flights in the 2000s, many of which failed. The first flight of the ISRO-built cryogenic engine took place in 2010 and it was not successful. ISRO had to go back to the drawing board. However, three consecutive tests using the ISRO built engine were successful between 2014 and 2016.

ISRO learned much in the process. And it developed modern facilities, including liquid oxygen and liquid hydrogen production plants at Mahendragiri. Under a new contract with Russia’s Glavkosmos, ISRO also set up a facility at SDSC for cryogenic propellant filling, a process that takes days and is much more complex than filling operations for earth-storable liquid propellants.

To fabricate the engine and the stage, ISRO roped in Godrej, Hyderabad-based MTAR Technologies, and Bangalore-based Hindustan Aeronautics Limited. The infrastructure and the partnerships, and the lessons it learned during the development of CE-7.5, came in handy for ISRO in its indigenous cryogenic engine project. Work on the fully indigenous CE-20 engine and the C25 cryogenic stage were ongoing even as ISRO was developing CE-7.5 to match Russian design and specifications.

And the engines are different in two major ways.

One, for CE-20, ISRO used the simpler gas generator cycle that it was developing in the 80s instead of the better performing but very complex staged combustion cycle which was used by the Russian cryogenic engine and the follow-up Indian CE-7.5. India chose the gas generator cycle for its indigenous engine because it gives the flexibility of independent development of each sub-system. As a result, ISRO could work on the engine and stage elements in parallel. Testing of some elements had begun in 2004.

Two, CE-20 uses a gimballing nozzle for controlling the trajectory of the rocket instead of the vernier engines that were used in the Russian engine and CE-7.5 for making fine adjustments to altitude and velocity.

C25 stage has a four-metre diameter and 13.5-metre height. It has separate propellant tanks for storing 27 tonnes of LOX and LH2. Equipped with the C25 stage, which has the CE-20 engine, 640 tonne GSLV Mark-III can carry a payload of up to 4 tonnes to the GTO. The Lunar Module of Chnadrayaan-2 weighs around 3.8 tonnes.

Are We There Yet?

C25 equipped GSLV Mark-III is a marked improvement in India’s capability. However, its carrying capacity is low even as it weighs several tonnes more than some of the other expendable rockets used by Russia, China, Japan, France and the US.

For example, Russia’s Proton weighs 693 tonnes and can lift a payload of 6.3 tonnes to GTO. Similarly, Japan's H-IIB rocket weighs 531 tonnes but can place an 8-tonne payload in GTO. China’s Long March weighed 879 tonnes and can lift a payload of 14 tonnes to GTO. France’s Ariane 5 weighs 777 tonnes but can lift a payload 10.9 tonnes to GTO.

ISRO, reports say, is working on a semi-cryogenic which will use kerosene and liquid oxygen as fuel and oxidiser, respectively, to generate two mega-newtons of thrust. In comparison, CE-20 produces a nominal thrust of 196.5 kN.

Experts believe that this high-thrust engine, referred to as SCE-200, may be used by ISRO in a clustered configuration to eliminate the L110 liquid-fuelled core, significantly enhancing the payload capability of its launch vehicle in turn.

However, until that happens, ISRO can’t be counted among the big boys.