Understanding the Smoke from Rocket Launches: Beyond Engine Exhaust

When watching a rocket launch, one of the most observable phenomena is the cloud of smoke or vapor that often appears. However, most people are puzzled by the plumes originating from rockets beside the main engine exhaust. This article aims to elucidate the fascinating details of what those smokes truly are.

What is the Smoky Plume from Rockets?

While the primary source of propulsion for a rocket is indeed its engine exhaust, the smoke or vapor you observe during a launch is a combination of various gases and condensates released from different parts of the rocket. Most notably, it’s crucial to note that the plume observed is not solely the engine exhaust but a mix of water vapor, gases, and sometimes solid propellant products.

Condensation of Water Vapor

Rocket engines burn a variety of fuels, many of which produce significant amounts of water vapor as a byproduct. When this water vapor mixes with the atmosphere encountered at high altitudes, it condenses into minute droplets. This condensation forms a visible plume commonly known as a vapor trail or snowstorm exhaust. Similar to the condensation trails (contrails) formed by aircraft, the sudden expansion and cooling of the exhaust gas result in a similar visible cloud effect in the atmosphere.

Solid Rocket Boosters

During launches that utilize solid rocket boosters, such as those employed in the Space Shuttle program, the situation is a bit different. The combustion of solid propellant can produce a thick, white smoke primarily composed of water vapor and other combustion byproducts. The white color is due to the high amount of water vapor, as solid propellants typically consist of a mixture of oxidizers and fuel. This smoke is visible as long as the gases remain hot and can continue to release significant water vapor into the atmosphere.

Atmospheric Conditions and Visibility

The overall visibility and density of the smoke or vapor plume are influenced by atmospheric conditions, specifically humidity, temperature, and altitude. In humid environments or at lower altitudes, the plume might be more visible, while at higher altitudes, the trails may dissipate more quickly, making them less noticeable.

cryogenic Propellant Venting

One of the most common sources of the visible smoke on a launch pad is not from the engine exhaust but rather from venting of the rocket's cryogenic propellant tanks.

Cryogenic propellants, such as liquid oxygen and liquid hydrogen, are stored at extremely low temperatures. As the rocket rises, the pressure inside the propellant tanks decreases, causing the fuel to boil and vaporize. This process, known as cryovaporization, releases gases into the atmosphere. Liquid oxygen, being extremely dense and cryogenic, can have a dramatic effect: as it passes through the vent, the cold oxygen can cause water vapor in the surrounding air to condense, creating the white cloud.

Examples of Rocket Venting and Launches

To provide a better visual understanding, some examples of rocket venting before launch and typical rocket launches will be provided. These visual aids will help illustrate the points mentioned in the text.

Cryogenic Propellant Venting

The cryogenic propellant venting seen before launch involves liquid oxygen and sometimes liquid hydrogen. The following video showcases the venting process:

The venting process for an Atlas rocket before it reaches its launch state can be seen here:

Atlas Rocket Launch

To observe a typical launch involving an Atlas rocket, here is the video:

SpaceX Falcon 9 Venting and Launch

A SpaceX Falcon 9 also experiences venting before launch, as the following video demonstrates:

And the launch itself:

These visual aids provide a clear demonstration of the venting process and the resulting plume observed during a launch.

Conclusion

The smoke or vapor plume seen during a rocket launch is a fascinating combination of several elements, including water vapor condensation, cryogenic propellant venting, and the products of solid propellant combustion. Understanding these phenomena is key to appreciating the complex science behind rocket launches and the engineering that goes into safely sending payloads into space.

For further details and visual demonstrations, check out the provided video links and continue to explore the wonders of rocketry.