When watching a rocket launch, you may have heard the callout “Engine chill has started,” which forms part of a long list of prelaunch procedures during an orbital launch event. It performs a crucial function, though.

An engine chill is a prelaunch procedure where cryogenic propellants are allowed to flow into parts of a rocket engine to cool them down to prevent thermal shock when the main cryogenic propellants are injected under extreme pressure through the turbopumps into the combustion chamber during launch.

An orbital rocket launch is not a simple, straightforward procedure. It never is. It explains why so many steps are involved in a rocket prelaunch sequence. One often hears a long list of callouts and confirmations preceding and during the actual launch of a rocket.

These are all necessary steps that need to be taken to ensure all systems aboard the launch vehicle are prepared and ready for liftoff. It includes pressurization & fueling propellant tanks, weather briefings, transferring to internal power, and several other checks and confirmations.

(Learn more about the primary steps followed during a rocket launch sequence both prior and post liftoff in this article.)

One of these procedures is called an engine chill or prechill. It may sound like just one of numerous routine prelaunch procedures, but as the upcoming section will illustrate, this is a crucial part of preparing a rocket engine to receive propellant and combust.

Falcon 9 First Stage
All nine engines of a SpaceX Falcon 9 rocket’s first stage are chilled down with cryogenic liquid oxygen before launch.

Why A Prelaunch Rocket Engine Chill Down Is Performed

When one thinks of a rocket, the red-hot exhaust plumes of its boosters often come to mind. And inside a rocket engine, the temperatures often reach close to 3 300° Celsius or 6 000° Fahrenheit while the propellants are combusted.

However, many of the liquid propellants used to combust and propel an orbital rocket forward are cryogenic, which means they have to be cooled down to temperatures below -153° Celsius (-243° Fahrenheit) to remain in liquid form.

For example, liquid hydrogen used in the upper stages of many orbital rockets needs to be cooled to temperatures below -253° Celsius (-423° Fahrenheit). Liquid oxygen, the oxidizer used with most liquid fuel types, needs to be cooled to -183° Celsius (-297°Fahrenheit).

Even liquid methane, the fuel that will power the next generation of launch vehicles from SpaceX, Blue Origin, and the United Launch Alliance (ULA), needs to be stored at temperatures of -162° Celsius (-260° Fahrenheit) and below to remain a liquid.

The RS-25 engine used on the Space Shuttle and SLS rocket use liquid hydrogen that flows through the engine at -253° Celsius (-423° Fahrenheit).

While a rocket is on the launchpad and propellants are being loaded before liftoff, the temperature inside a rocket engine is relatively on par with the surrounding air temperature. There are primarily two reasons why an engine chill needs to be performed at this point:

  1. Preventing Thermal Shock
  2. Preventing Evaporation & Bubble Formation

1) Preventing Thermal Shock

Suddenly introducing supercooled cryogenic fuels to materials at these ambient temperatures is a terrible idea. Similar to pouring ice water into a warm glass, pumping cryogenic liquids into a relatively warm engine can result in what is called thermal shock.

Although not as dramatic as a glass shattering when suddenly filled with ice water, cryogenic propellants can still damage the materials in a “warm engine,” possibly causing cracks in the metal or causing sensitive parts like the engine’s turbopumps to not function properly.

By flowing a small amount of cryogenic propellant through the turbopumps and other parts of the engine before launch, the materials are slowly thermally conditioned and cooled down, preventing any thermal shock and the possible resulting damage to the engine.

2) Preventing Evaporation & Bubble Formation

Another danger of pumping cryogenic propellants through a warm rocker engine is that the liquids can warm up and start to evaporate. This can lead to the formation of small gas bubbles, a process known as cavitation.

Gas bubbles can seriously disrupt the flow of propellants and interfere with a smooth and even ignition in the combustion chamber. Chilling the engine down prelaunch prevents or limits the possibility of this dangerous condition from occurring.

How A Rocket Engine Chill Down Is Performed

A rocket engine chills down typically starts several minutes to over an hour before launch, depending on the type of launch vehicle and rocket stage. Timing the engine chill is important, as it can’t take place too long or too shortly before liftoff.

SpaceX Falcon 9 rocket is the perfect example of a typical prelaunch engine chill. Since both the launch vehicle’s first and upper stage uses RP-1 Fuel and liquid oxygen (kerolox) as its main propellants, it used liquid oxygen to chill its engines.

Approximately 7 minutes before liftoff (T-7 minutes), prevalves open to allow liquid oxygen to flow the top of the turbopumps (and other parts of the engine requiring chilling) to start to cool them down.

(In engines like the cryogenically propelled RL-10 used in the Centaur upper stage of the Atlas V rocket, the engine chill starts as early as 90 minutes or T-90 minutes before liftoff.)

This process continues until approximately 1 minute 15 seconds before liftoff. Monitors placed in the turbopumps and at various critical points in the engine carefully monitor the temperatures to ensure critical components are cooled down sufficiently prior to ignition.

RL-10 Hydrogen Rocket Engine
The RL-10 engine used in the Centaur upper stage.

Rocket engines that use hydrogen as their fuel component, like the RS-25 engines that were used on the Space Shuttle and are currently used on NASA’s SLS rocket, use hydrogen to chill the engine down to handle the fuel at temperatures of -253° Celsius (-423° Fahrenheit).


As illustrated, although it forms part of a long list of “normal” prelaunch procedures that need to be completed before a rocket gets the final go-ahead for liftoff, a rocket engine chill is critical in ensuring an orbital rocket engine performs optimally and reliably.

The primary function of an engine chill down is to cool the turbopumps & engine down sufficiently to receive cryogenic propellants while preventing thermal shock. It also prevents evaporation & gas bubbles from forming, which can disrupt the combustion process.

This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.

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