While observing a rocket launch, it is common to see what looks like white flaky pieces falling off the launch vehicle during liftoff. These “flakes” are pieces of ice, and there is a good reason why they form on a rocket.

Ice primarily forms on a rocket when water vapor in the atmosphere reacts with the subzero temperature of the exterior of the launch vehicle’s uninsulated cryogenic fuel tanks, resulting in the formation of ice through a process called deposition on the spacecraft’s cold exterior surface.

Whenever you pour a cold drink into a glass, it is only a matter of minutes before the glass is covered with water droplets. This is because the temperature of the fluid is colder than the ambient temperature, resulting in the water vapor in the air condensing on the glass.

The ice forming on a rocket’s exterior surface before launch is the result of a very similar process, as the following section will highlight.

Why Ice Forms On A Rocket’s Exterior On A Launchpad

Ice On Rocket Lab Electron
Pieces of ice fall off a Rocket Lab Electron during liftoff.

While a liquid propellant rocket stands on the launchpad, fuel gets pumped into its internal tanks minutes before liftoff. The propellants consist of the fuel and the oxidizer in the form of liquid oxygen.

In order for the oxygen to stay in liquid form, it has to be cooled and stored at a temperature of below -183° Celsius (-297° Fahrenheit). To increase its density, companies like SpaceX also cool the fuel component (RP-1 **) to -7° Celsius (19.4° Fahrenheit).

(Learn more about why rockets need to carry oxygen in addition to fuel in this article.)

On the launchpad, the cryogenic propellants cause the exterior surface of a launch vehicle to cool and rapidly reach subzero temperatures.

All atmospheric air contains some percentage of water vapor. On the coast, however, there is a substantial increase in the amount of moisture present in the atmosphere as a direct result of its proximity to the ocean, with an ample supply of water available for evaporation.

Most major rocket launch facilities throughout the world are located either on or near the coast for a number of reasons. As a consequence, the air on a launchpad is much more humid than in locations situated inland.

When the humid air comes in contact with the icy cold exterior of the rocket, the water condenses almost instantly as ice on the surface. It occurs through deposition, where water vapor is turned directly into its solid state without passing through its liquid form.

During liftoff, the vibration caused by the rocket’s thrusters, combined with the airflow across the vehicle, causes the ice that accumulated on the surface to loosen and fall off. The loose pieces of ice are the “white flakes” one can see falling off a rocket during ascent.

This explanation answers the question as to why ice forms and falls off a rocket during launch but still leaves the question as to why rocket fuels need to be cooled to their liquid state to be used as a propellant.

Ice On Saturn V Rocket
Ice fragments fall from the cold exterior of the liquid oxygen tanks on a Saturn V rocket (which formed part of the Apollo Program) during the vehicle’s liftoff.

(** RP-1, a highly refined form of kerosene, is not the only type of fuel used in modern liquid propellant rockets, but more on the different types of fuels and why many need to be stored at cryogenic temperatures in an upcoming section.)

Why Rocket Propellants Are Stored At Subzero Temperatures

Most liquid rocket propellants, apart from RP-1 (a refined form of kerosene), are stored at cryogenic temperatures for a very specific reason. The fuels remain in a gaseous state at room temperature & have to be cooled to extremely low temperatures to turn into a liquid.

It is simply not practically possible to use chemical compounds in their gaseous state as rocket fuel. Gas is a fraction of the density of its liquid counterpart, and the size of a fuel tank able to hold an adequate amount of fuel in its gaseous state will be colossal.

Falcon 9 Boils Off Liquid Oxygen
A SpaceX Falcon 9 rocket vents excess liquid oxygen as the cryogenic fuel heats up on the launchpad.

As a result, to reduce the density of the propellant, it has to be cooled to cryogenic temperatures to be used as a liquid. It is also much easier to transport & pump fuel in a liquid state within a rocket than moving gas around, which provides a myriad of problems.

Hydrogen, the fuel used by the Space Shuttle and Delta IV rocket, is a gas at normal ambient temperatures and has to be cooled and stored at temperatures of -253° Celsius (-423° Fahrenheit) to turn into and remain a liquid.

Similarly, liquid methane (used by SpaceX’s Starship launch vehicle) also remains a gas at ambient temperatures and has to be cooled to a temperature of -162° Celsius (-260° Fahrenheit) to turn into a liquid.

Apart from hypergolic fuels (highly toxic propellants that spontaneously combust upon contact), all liquid propellants need to combine with an oxidizer in the form of liquid oxygen to be able to combust.

Like hydrogen and methane, oxygen is also a gas at ambient temperatures and has to be cooled and stored at a temperature of below -183° Celsius (-297° Fahrenheit) to remain a liquid.

RP-1 (Rocket Propellant-1 or Refined Petroleum-1) is still the most widely used type of liquid rocket propellant. Although it can easily be stored at room temperature, it is still cooled to subzero temperatures by some space agencies to increase density.

Whenever these cryogenic fuels are pumped into a rocket and directly exposed to the vehicle’s exterior surface, it will lead to rapid cooling of the surface and the inevitable buildup of ice on the structure.

It has to be noted, though, that the fuel tanks of most hydrogen-fueled launch vehicles are insulated to preserve temperatures and protect them from the environment. (As illustrated by the orange foam insulation on the Space Shuttle’s external fuel tank & Delta IV rocket.)

Foam Insulation On A Delta IV Heavy Rocket
The orange foam insulation on a Delta IV Heavy rocket protects the cryogenic hydrogen fuel.

As a result, the exterior surfaces of these propellant tanks never get cooled to the extent that any ice buildup can occur.

(Learn more about the different types of fuel rockets use and their characteristics in this article.)

Conclusion

As this article illustrated, ice forming on a rocket’s exterior while on the launchpad is a perfectly normal occurrence. The cryogenic fuels pumped into the vehicle cool the exterior surface to subzero temperatures, which causes the buildup of ice.

As the rocket lifts off, the vibration of the thrusters, combined with the airflow over the vehicle, causes the ice to loosen and fall off as the rocket ascends.

Although all cryogenic fuels are stored at freezing temperatures inside a rocket, only launch vehicles with uninsulated propellant tanks cause an ice buildup on the exterior. Insulated tanks like the external hydrogen tank of the Space Shuttle prevent this phenomenon.

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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