Most liquid rocket propellants like hydrogen, methane & liquid oxygen are cryogenic, meaning they have to be stored at subzero temperatures to remain in a liquid state. This poses a major challenge for launch providers.
At a launch site, cryogenic fuel is kept cold by double-walled storage tanks, with a vacuum between the walls filled with insulation material to prevent heat transfer. In a launch vehicle, a combination of insulation materials and radiation shields protect the fuel from atmospheric and radiant heat.
Most orbital rockets use RP-1 (a highly refined form of kerosene) to power their first-stage boosters. The fuel’s high density and energetic nature, but specifically its ability to be stored at room temperature, makes it the preferred choice for many launch providers.
However, the majority of liquid propellants used in the upper stages of launch vehicles, their oxidizers, and new fuel types like methane are cryogenic substances, which means they have to be kept at temperatures below -153° Celsius (-243° Fahrenheit) to stay in liquid form.
For example, liquid hydrogen used in many upper stages of 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.
(Learn more about the different types of fuel orbital rockets use, their characteristics, and each one’s advantages and drawbacks in this article.)
Keeping these propellants cold is a major challenge during a launch event. As the following sections will illustrate, various techniques are used to keep them at cryogenic temperatures while being stored at a launch site before fueling and during liftoff within the launch vehicle.
Cryogenic Propellant Storage At Launch Site Before Fueling
Unlike the launch vehicle itself, where the propellants are pumped into its internal tanks only a few minutes to a couple of hours before launch, the fuel stored at the launch site itself can be stored for up to several weeks in advance of the actual launch date.
For cryogenic liquids, this means specially designed storage tanks need to be used to keep the fuels cold without necessarily deploying active or permanent cooling (which can be extremely expensive and not always practical at a launch site).
The continuous venting or boiling off of gas evaporating as the cryogenic liquid warms up is not an option since the fuel needs to remain inside the tanks for several days or longer, and continuous replacement of the lost propellant will be extremely expensive & labor-intensive.
As a result, a cryogenic storage tank must be able to keep the propellants in their liquid state by limiting any heat buildup, specifically from external sources. This will limit the amount of evaporation, the resulting pressure buildup, and the amount of venting that is needed.
To achieve this, a typical cryogenic propellant tank situated at a launch facility consists of a double-walled spherical steel structure. The inner wall acts as a pressure vessel that contains the liquid, while the outer wall shields the inner wall from direct heat exposure.
To stop the heat that reaches the outer wall from being conducted through air particles to the inner wall, a vacuum is created in the space between the two so that no particles are present to conduct the heat to the inner structure.
However, by simply having a clear line of sight between the two walls, heat can still reach the inner wall through a process called radiation heat transfer. To prevent this from happening, an insulation material, typically perlite, is placed in the vacuum between the two walls.
(NASA improved on this type of insulation with their latest hydrogen storage tank built at Launch Complex 39B at Kennedy Space Center by replacing the perlite with specially designed glass bubbles that resulted in a 48% reduction in the boiloff of evaporated gas.)
By implementing these techniques in the construction of cryogenic storage tanks, some heating, as well as the resulting pressure buildup and venting, will still occur but will be dramatically reduced to an evaporation rate of only approximately 0.03% per day.
Cryogenic Propellant Storage In Launch Vehicle Before And After Launch
Unfortunately, having heavy double-walled steel tanks inside an orbital launch vehicle is simply not an option, as their mass alone will make it almost impossible to lift off. (Saving unnecessary weight is crucial and one of the first priorities in building an orbital rocket.)
It is just a fact accepted by all launch providers that some evaporation and loss of propellant will occur as soon as it enters a rocket’s internal tanks. This is part reason why fuelling a rocket is postponed to the last possible moment, sometimes minutes, before launch.
As soon as cryogenic propellants are pumped into an orbital rocket, they start to warm up & evaporate or “boil off.” To prevent a dangerous pressure buildup that can rupture its tanks, the rocket vents the boiled-off gas into the atmosphere.
As a result, the fuel inside the vehicle, while still on the launchpad, is constantly being replenished to replace the lost propellant from evaporation and venting. (It also must be noted that the boiling-off process has the benefit of having a cooling effect on the liquid.)
However, this does not mean that a rocket’s cryogenic fuel tanks are not protected against heat buildup. The external hydrogen tank of the Space Shuttle, as well as Delta IV’s hydrogen tank, were covered by insulation foam that was sprayed on the exterior surface.
The familiar orange foam (also used on NASA’s new SLS vehicle) not only helps to protect the propellants from external heat sources on the launchpad but also from the heat generated by friction as the vehicle travels through the dense air of the lower atmosphere.
Another effective form of insulation, cork, is used instead of foam to protect the engine section of NASA’s Space Launch System (SLS) since it provides a stronger, more robust type of protection. It is also used under the solid rocket boosters of the launch vehicle.
In fact, most propellant tanks used by orbital launch vehicles have some kind of insulation material added to their external surfaces (and sometimes internally as well) to protect them from external heat sources for the brief period they are exposed to Earth’s atmosphere.
As most orbital launches take place from regions close to the Equator, which generally experience high temperatures throughout the year, using white as the predominant color on a rocket’s exterior surface is another effective way of keeping propellants cold.
Of all the colors in the visible spectrum, white is the most effective at reflecting the heat from the sun away from the vehicle’s surface instead of absorbing it and causing the propellants in the internal tanks to warm up more quickly.
(Learn more about the different ways in which launch providers use white colors on orbital rockets to keep propellant temperatures from rising too quickly in this in-depth article.)
Once a launch vehicle reaches Space, it still requires some form of protection. Although the temperatures in the vacuum of Space drop to -270° Celsius (-454° Fahrenheit) and there are no air particles to conduct heat, a craft still experiences extreme heat when facing the Sun.
The radiant heat from the sun causes any object directly exposed to it to experience very high temperatures as a result of radiation heat transfer (the same process that causes the inner wall of a cryogenic storage tank at a launch site to be heated by the outer wall).
As a result, the upper stages of rockets, like the Centaur upper stage rocket that is used by many launch vehicles like the Delta IV and Atlas V, use a radiation shield, apart from other forms of insulation like insulation blankets and closed-cell foam to keep propellants cold.
By effectively shielding the propellant tanks of a spacecraft from the Sun’s radiant heat while in Space, the propellant can stay cold for much longer and require a lot less venting. It saves fuel and allows the vehicle to operate in this hostile environment for extended periods.
Cryogenic propellants are dangerous to handle and extremely difficult to keep cold, especially for extended periods without any form of active cooling. Liquid hydrogen with a boiling point of -253° Celsius (-423° Fahrenheit) is especially hard to manage.
(Learn more about liquid hydrogen, why it is so popular for use in the upper stages of orbital launch vehicles, as well as its advantages and drawbacks in this article.)
However, as this article illustrated, by accepting some loss of propellant due to evaporation & venting and deploying a wide variety of different techniques to keep it cold (both at a launch site & in an orbital rocket), cryogenic fuels can be effectively kept cold for extended periods.
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