Chandrayaan-3: How ISRO Built The Cryogenic Engine That Will Put Lunar Module In Orbit Around Earth

When the 26-hour-long countdown for the launch of Chandrayaan-3 ends later today, the Geosynchronous Satellite Launch Vehicle (GSLV) Mk-III, the Indian Space Research Organisation's (ISRO's) heaviest rocket, will lift off from the Satish Dhawan Space Centre (SDSC) on the barrier island of Sriharikota and roar skyward to put the lander and rover, mechanically interfaced as one, on their journey towards the moon.

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, followed by the shutdown and detachment of the liquid core stage.

And then, the cryogenic upper stage, powered by CE-20 — the 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 until 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.

Propellants 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, the government 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 US's General Dynamics to sell its RL-10 cryogenic engine to ISRO, a version of which is still used on Atlas-V and Delta-IV rockets.

However, the engines came with a hefty price tag of $800 million, and the US government had the power to veto technology transfer.

As a result, the offer was rejected. Subsequently, France's Arianespace made an offer to supply its HM7 cryogenic engines along with the necessary technology for $1.2 billion. The exorbitant price proved to be a deterrent for India, and thus, the deal fell through.

But then, around 1989, Glavkosmos, the commercial arm of the Soviet Union's space agency Roscosmos, made an offer to supply India with its 12-tonne 11D56 cryogenic engines and accompanying technology for a mere $132 million (approximately Rs 240 crore at the time).

ISRO eagerly seized the opportunity. However, after just 15 months, the US raised concerns, citing a violation of the Missile Technology Control Regime.

This group was established in 1987 to restrict the spread of missile technology.

Ironically, the West, which had offered the cryogenic engine itself, suddenly realised that India could potentially employ the cryogenic technology for the development of Intercontinental Ballistic Missiles (ICBMs).

It was an absurd argument — “armageddon-hogwash", some said.

Cryogenic engines are not typically used in ICBMs due to the operational challenges they present. Cryogenic engines require a lengthy fuelling process, often taking several hours, which make them unsuitable for ICBMs that need to be prepared for launch on short notice.

ICBMs are designed for rapid deployment and quick response times, and thus they usually employ solid or liquid-fuelled engines that can be launched quickly.

Moreover, India had already 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.

Citing MTCR was merely a way for the US to justify its objection to the deal.

The reality was that the US had strong commercial interest in denying India the technology — Glavkosmos was demanding nearly 400 per cent less than the price quoted by US’s 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.

The US may have perceived this as a potential threat to its own launch market and, in response, imposed sanctions on both ISRO and Glavkosmos in 1992.

However, these measures failed to deter India's ambitions.

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).

Mastering Cryogenic Technology

As the drama unfolded, the then prime minister P V Narasimha Rao made an announcement during a public meeting in Gorakhpur: India would embark on building its own cryogenic engines, Raj Chengappa of India Today reported at the time, adding, the Americans responded by indicating that the ban on the sale of space components to ISRO might remain in place.

But developing a cryogenic engine was not going to be an easy task.

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 kilometre per second.

However, 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 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 under the deal had been used. However, the Russian engines were not only difficult to get, but they were also difficult to reproduce.

"In the two years, Russia has transferred some technology," Chengappa writes in his August 1993 story, quoting a senior space scientist, "It's like getting about 60 per cent of the pieces in a 1,000-piece jigsaw puzzle. You still need the rest to get anywhere".

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 cancelled.

When tested by ISRO, the efficiency of the engines was found to be below the predicted level. Moreover, 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 tonne to a little over eight tonne.

To reproduce the engines, ISRO had to learn new manufacturing methods and work 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.

When working on the indigenous cryogenic engine in the 1980s, 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.

To make the thrust chamber of the engine, ISRO technicians had to learn a new technique called vacuum brazing. These challenges caused delays.

Moreover, the Russians 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, developing new 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 Bengaluru-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 1980s 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.

Not There Just Yet

The GSLV Mk-III, equipped with the C25 engine, represents a notable advancement in India's space capabilities.

But despite weighing several tonnes more than certain expendable rockets used by other countries like Russia, China, Japan, France, and the US, its carrying capacity is relatively low.

The GSLV Mark-III, weighing 640 tonnes, can carry a payload of up to 4 tonnes to geostationary transfer orbit (GTO).

In comparison, Russia's Proton rocket (693 tonne) lifts 6.3 tonne, Japan's H-IIB rocket (531 tonne) carries 8 tonnes, China's Long March rocket (879 tonne) delivers 14 tonne, and France's Ariane 5 rocket (777 tonne) transports 10.9 tonne to GTO.

Furthermore, when compared to the SpaceX Falcon 9, which can lift 23 metric tonne to low-Earth orbit, the GSLV Mk-III's payload capacity of around 8 metric tonne for low-Earth orbit deployment pales in comparison.

ISRO is trying to overcome this limitation with a semi-cryogenic stage, named SCE-200, which will utilise kerosene and liquid oxygen as fuel and oxidiser, respectively, to generate two mega-newtons of thrust.

This high-thrust engine has the potential to be employed in a clustered configuration, eliminating the need for the L110 liquid-fuelled core and significantly enhancing payload capability. It is envisioned that this engine will power future super heavy-lift launch vehicles.

However, until that happens, ISRO wouldn't be counted among the big boys of space exploration.