Why do rockets look white?
Rockets are predominantly white for several key reasons, primarily related to the thermal management of their cryogenic propellants.
Thermal Protection
The main reason rockets are white is to minimize the absorption of heat from solar radiation. Cryogenic propellants, such as liquid hydrogen and liquid oxygen, must be kept at extremely low temperatures to remain in liquid form. For example, liquid hydrogen needs to be cooled to below -253°C (-423°F), and liquid oxygen to below -183°C (-297°F)14.
White is the most effective color for reflecting sunlight and heat, rather than absorbing it. This helps to slow down the warming of these cryogenic propellants, reducing the boil-off and venting that occurs as they warm up14.
Historical Lessons
The use of white coloration was learned from early experiences, such as with the Saturn V rocket. Initially, the Saturn 1 rocket had black and white stripes, but the extensive use of black paint led to heat spikes in the fuel tanks. This issue was addressed by reducing the amount of black paint and using white predominantly, especially on the cryogenic upper stages12.
Additional Techniques
While paint is often used, the white appearance of rockets can also come from other materials like white beta cloth blankets and low-temperature reusable surface insulation (LRSI) tiles, which provide thermal insulation. Additionally, ice formation on the surface of the rocket due to the reaction of cryogenic propellants with moist air can also contribute to the white appearance1.
Weight Considerations
In some cases, the use of paint was found to be unnecessary and was abandoned to save weight. For instance, the external fuel tank of the Space Shuttle was initially painted white but later switched to foam insulation, saving about 272 kg (600 pounds) of weight12.
In summary, the white color of rockets is a deliberate design choice to protect the cryogenic propellants from heating up due to solar radiation, thereby ensuring the stability and efficiency of the launch vehicle.