Indian Space Agency Achieves Engineering Breakthrough

By R Anil Kumar

• ISRO has now mastered the technology.

• On November 29, ISRO successfully hot-tested its CE-20 cryogenic engine for sea-level ignition at ISRO Propulsion Complex, Mahendragiri, Tamil Nadu

• Preparing for inflight re-ignition, full nozzle cryogenic engine (CE20) undergoes test outside vacuum chamber

Bengaluru, December 14. A cryogenic engine (CE) is a rocket engine that uses fuels and oxidizers stored at extremely low temperatures (e.g., liquid hydrogen and liquid oxygen) – to achieve high efficiency and thrust for space missions. These engines are typically used in the upper stages of rockets to place payloads into orbit or beyond.

The immediate payoff of ISRO’s technological breakthrough would be simplified acceptance testing of the CE20 engine.

According to the ISRO press release, “Acceptance tests for CE20 engines are currently being performed at the High-Altitude Test (HAT) facility, thereby adding complexity to the acceptance testing procedure.

“To reduce the complexity related to the testing at HAT, a sea level test utilizing an innovative Nozzle Protection System was devised that has paved the way for a cost-effective and less complex procedure for acceptance testing of the cryogenic engines.”

In the long term, however, the breakthrough is an important step by ISRO towards attaining cryogenic engine capability matching the capability embodied in SpaceX’s Raptor full-flow staged combustion engine designed to operate across environments.

Current ISRO Cryogenic Engine Capability

ISRO’s Liquid Propulsion Systems Centre has developed two cryogenic engines so far: the CE-7.5 engine for the CUS (Cryogenic Upper Stage) of GSLV Mk-II and the CE-20 for the CUS of GSLV Mk-3 (also known as LVM3).

The CE-20 has been qualified to operate at a thrust level of 19 tonnes. “Recently, the engine was qualified for the Gaganyaan mission with a thrust level of 20 tonnes and also to an uprated thrust level of 22 tonnes for the future C32 stage, towards enhancing the payload capability of the LVM3 launch vehicle.”

So far, ISRO cryogenic engines were capable of operating at high altitudes or in space only, not at sea level.

Cryogenic Engine Ignition Start Challenges

The dynamics of starting a CE vary – in space, at high altitudes, or at sea level.

Space starts require advanced propellant management and reliable ignition systems to operate in a vacuum and handle thermal extremes.

At high altitudes or in space, the lack of atmospheric pressure eliminates back pressure on the nozzle, which simplifies ignition and exhaust flow dynamics.

In space, lower chamber pressures can achieve efficient thrust since there’s no opposing atmospheric force.

In space, thermal gradients can be more predictable due to the vacuum environment, whereas at sea level, atmospheric conditions like humidity and temperature variations can complicate ignition.

Sea Level Start challenge

Compared to starting at sea level, it’s technically less challenging to start a cryogenic engine at higher altitudes or in space because of the absence of environmental constraints — such as atmospheric pressure and related flow instabilities.

A CE capable of both sea level and space starts, such as the Raptor, requires daunting engineering complexity. The engine has to deal with the challenges of space start and overcoming atmospheric conditions, and ensuring structural stability.

At sea level, the high atmospheric pressure can lead to combustion instabilities, such as oscillations that could damage the engine or lead to inefficient performance.

Designing injectors and combustion chambers that can handle these instabilities is critical. Ignition at sea level requires reliable systems to initiate combustion, even in potentially humid or varying atmospheric conditions.

Ensuring robust ignition under cryogenic conditions, where fuel like liquid hydrogen is extremely cold, adds complexity.

Handling the extreme temperature gradients between cryogenic fuel and the hot combustion gases is particularly challenging at sea level, where atmospheric cooling is less effective than in space.

At sea level, engines must produce higher chamber pressures to counteract atmospheric pressure and maintain thrust efficiency. This requires robust materials and systems.

ISRO’s Engineering Breakthroughs

According to the press release, ISRO tested “its CE20 Cryogenic Engine featuring a nozzle area ratio of 100 … Performance of a multi-element igniter that is required for engine restart capability was also demonstrated during this test.”

The nozzle area ratio is the ratio of the nozzle’s exit area to its throat. In other words, it is the expansion ratio of exhaust gases.

High nozzle area ratio engines are optimized for operation in the vacuum of space. They maximize thrust efficiency by allowing the exhaust gases to expand fully and smoothly to low pressures.

In a vacuum, no atmospheric pressure exists to resist the expanding exhaust gases.

At sea level, high-area-ratio nozzles face significant challenges. Atmospheric pressure prevents the exhaust gases from expanding fully within the nozzle. This leads to flow separation, where the exhaust detaches from the nozzle wall, causing turbulence and potential structural damage.

The ISRO press release acknowledges the challenge: “Testing the CE20 engine at sea level poses considerable challenges, primarily due to the high area ratio nozzle, which has an exit pressure of approximately 50 mbar.”(A millibar or mbar, is 1/1000th of a bar and is the amount of force it takes to move an object weighing a gram, one centimeter, in one second. Millibar values used in meteorology range from about 100 to 1050. At sea level)

Restartable Engine

Besides simplifying acceptance testing of its CEs, the successful test paves the way for ISRO to field a Raptor-like engine that can operate at sea level or in space while also being restartable.

A restartable engine comes with additional challenges, which the ISRO has overcome.

In the vacuum of space, there’s no atmospheric pressure to contain or stabilize the flow of fuel and oxidizer during ignition. If the nozzle is open to the vacuum during restart, the propellants may not mix properly, leading to inefficient combustion or failure to ignite.

In a vacuum, gases tend to expand rapidly and may not interact effectively without controlled conditions.

In earlier ground tests, ISRO has demonstrated vacuum ignition of the CE20 engine without nozzle closure.

In the recent test, ISRO validated the performance of a multi-element igniter that is required for engine restart capability.

A multi-element igniter uses several small jets or ignition points to evenly distribute the initial combustion energy.

ISRO press release states, “Only the first element was activated, while the health of the other two elements were monitored.

“During this test, both the engine and facility performance were normal, and the required engine performance parameters were achieved as anticipated.”

— (With Inputs from ISRO)