Handsome, cute and cool, there are many ways to keep spacecraft warm

How can spacecraft maintain stable heat preservation when facing extreme temperatures? In fact, high-end spacecraft often only need the simplest heat preservation methods, such as "warm baby", "yellow robe" and "warm grass blanket" and other cool and cute methods, you may wish to learn more.

The trick is not simple

On the morning of May 5, the Long March 2D carrier rocket successfully launched eight satellites into space at the Taiyuan Satellite Launch Center. What was striking was that something fell down when the rocket was launched. Netizens joked that this was to highlight the rocket's "coolness". In fact, what fell was a "warm baby" specially designed by researchers for the rocket - thermal insulation plastic foam.

Thermal insulation plastic foam dropped during rocket launch (Photo: Zheng Taotao)

It turns out that special plastic foam sheets are gently glued around the fairing to ensure that the temperature of the rocket propellant before launch is not too high or too low, so as not to affect the propulsion efficiency or even cause an accident. Since the foam sheets are not firmly fixed to the fairing, they will fall off as soon as the rocket is launched, and the propellant will be consumed quickly at that time. The falling foam sheets can also reduce the burden on the rocket.

my country launches many satellites and deep space probes every year. I believe everyone has noticed that many spacecraft are covered with a golden or silver "coat", and the golden color is particularly dazzling. Netizens nicknamed them "yellow robes". In fact, the "yellow robes" of spacecraft are not to show "nobility", but more like the thick bulletproof armor equipped on tanks, which can be understood as "heat-proof armor" that effectively resists thermal radiation. The scientific name is multi-layer thermal insulation material.

Multi-layer insulation materials are like thick, multi-layer blankets, mainly composed of reflective screens (surface layer) and spacers (inner layer). The reflective screens on the surface can effectively reflect back cosmic radiation, mainly solar radiation, just like a superhero's bulletproof cape; while the reflective screens on the inner layer can prevent the heat inside the spacecraft from escaping too much and too quickly into the vacuum, thereby playing a role in heat preservation.

Specifically, the thermal insulation material commonly used for reflective screens is polyester film or polyimide film coated with metal foil (such as gold foil, copper foil, etc.), which is the common golden "coat", and sometimes it is silver; and the spacer is generally a loose and porous material, such as glass fiber, quartz fiber, etc., which can further prevent excessive heat exchange between the spacecraft and the vacuum environment.

Sometimes, you will find that the spacecraft is covered with "straw". In fact, it is a special "warmth felt", which looks like a soft straw mat on the surface, but is essentially a multi-layer insulation material.

Although it may feel a bit "earthy", it is also an effective way to keep spacecraft warm.

For example, the commonly used second-generation flexible thermal insulation material is made of quartz fiber cotton sandwiched in woven quartz fiber cloth, which is precisely sewn and is also called "flexible thermal insulation felt". It is a quilt-type thermal insulation device with low thermal conductivity, which can effectively reduce the efficiency of heat conduction, with a maximum operating temperature of 750 degrees Celsius, light weight, good heat and shock resistance, and low price. It is naturally the "darling" of the current spacecraft thermal protection system.

Precision control with broad ideas

The three cool and cute spacecraft insulation methods are only a small part of the "Houhou Secret Book". If you want to fully understand the spacecraft insulation methods, you must carefully classify them. Generally speaking, there are two types of spacecraft insulation control methods: passive control and active control.

Passive control, as the name implies, is a temperature control method with a "defensive" nature. The above-mentioned "warm baby", "yellow robe" and "warm grass blanket" all belong to this type. Figuratively speaking, passive temperature control requires the spacecraft to be covered with effective "heat-proof armor", so multi-layer insulation materials are its core components and have the widest range of applications.

Heat pipes installed inside spacecraft are also a very common passive temperature control method. The working fluid (such as ammonia) stored inside the heat pipe can quickly absorb the heat emitted by the heat source at one end, and then dissipate the heat at the other end through heat conduction, thereby controlling the internal temperature of the spacecraft, and the whole process consumes almost no energy.

In the process of absorbing and dissipating heat, the working medium inside the heat pipe will change its physical state: first it evaporates from liquid to gas, and then condenses from gas to liquid. Therefore, this working medium is also called "phase change medium". The application of phase change materials in the aerospace field also includes space suits, return capsule shells, etc.

The Chang'e 5 returner is covered with "warm packs" (Photo by Li Shuheng, a reporter from China Aerospace News)

In addition to multi-layer insulation materials and heat pipes, spacecraft also use a variety of passive temperature control devices such as insulators, thermal insulation coatings, and passive thermal radiators.

The advantages of passive temperature control are simplicity, convenience and minimal energy consumption, but it also has a significant disadvantage, which is poor temperature control accuracy. In order to achieve precise temperature control of various areas inside the spacecraft, an active temperature control system is generally required.

In layman's terms, the working principle of an active temperature control system is similar to that of a household air conditioner. Taking the commonly used working fluid circulation loop thermal control system as an example, it uses a circulating pump as a power source to drive the working fluid to circulate, collect heat at the designated cooling end, and release heat at the other end. This system may seem to be just an "advanced version of a heat pipe", but it can achieve precise temperature control and is an indispensable part of a spacecraft.

The future is smarter

The current spacecraft insulation methods are mature and reliable, but with the advancement of science and the development of demand, aerospace personnel will not be satisfied with the current technology. As the types of spacecraft become more and more diverse, the technical requirements are also getting higher and higher, and intelligent thermal control coating insulation methods have emerged.

Thermal control coatings generally stabilize the temperature by adjusting the surface sunlight absorption rate and reflectivity of the spacecraft, while intelligent thermal control coatings can more sensitively sense the space environment temperature, change the sunlight reflectivity of the spacecraft, and realize autonomous control of the spacecraft temperature system. When the ambient temperature of the spacecraft is significantly different from the temperature required to ensure the normal operation of the spacecraft, the intelligent thermal control coating can increase the sunlight reflectivity and speed up the dissipation of excess heat in the spacecraft. Conversely, the intelligent thermal control coating can also effectively reduce the heat loss of the spacecraft, helping the various instruments and astronauts of the spacecraft to maintain a suitable temperature.

Compared with traditional thermal control coatings, intelligent thermal control coatings have many advantages, such as small size, light weight, sensitive and precise control effect, etc. According to the extreme environment and working conditions encountered by the spacecraft, it can automatically and in real time adjust its own temperature, and avoid the hidden dangers of remote control command intervention faced by passive thermal control means as much as possible.

Intelligent thermal control coating is an emerging technology that has attracted widespread attention in recent years. I believe that my country will launch a spacecraft with intelligent thermal control coating in the near future. Stay tuned.