PICA is the abbreviation of Phenolic Impregnated Carbon Ablator. The Chinese name is Phenolic Impregnated Carbon Ablator. It is currently one of the most important thermal protection materials in the NASA interplanetary exploration project in the United States. From the name, we can roughly understand the basic information of the material, that is, it is a material that achieves thermal protection through ablation, and the main components include phenolic and carbon materials. In fact, the most common material composed of carbon and phenolic is the US Department of Defense (DoD) carbon/phenolic (Carbon Phenolic) composite material, and its birth period is much earlier than the PICA material, usually the heat of the intercontinental missile warhead Protective materials. Carbon/phenolic can be obtained by RTM and other processes. The material has a high density, and the density is about 1.4g/cm3, which is very reliable under high heat flow and high pressure. The density of PICA material is much lower than that of carbon/phenolic. The density of the material can be adjusted according to the use environment, usually in the range of 0.25-0.6g/cm3. The density of another material SLA-561 is only about 0.22g/cm3. The following figure shows the thermal environment and the application of thermal protective materials during the landing/reentry process of some spacecraft in the United States. The abscissa in the figure is the stagnation point pressure, and the ordinate is the peak heat flow. It can be seen from the figure that SLA is mainly used in the case of low heat flow, and PICA is capable of higher heat flow conditions.
PS: The surface of the aircraft will be aerodynamically heated during atmospheric flight. This degree of aerodynamic heating can usually be evaluated by heat flow, enthalpy value and pressure. These three parameters have a certain matching relationship, so the heat flow and pressure can basically characterize the heat. Environmental characteristics.
When designing and selecting materials for thermal protection systems, first of all, we must ensure that the materials can withstand the harsh aerodynamic thermal environment without direct failure, secondly, the material's thermal insulation performance is better, and then the material is preferably lighter. Generally, the instruments and equipment inside the spacecraft cannot tolerate too high a temperature, so a sufficient thickness of thermal protective material is required to ensure the normal operation of the equipment. If the thermal insulation of the material is poor, then the thermal protection material must become thicker, resulting in a very bulky outer casing. In the case of unchanged rocket thrust, it will inevitably compress the mass and space of other scientific instruments or equipment inside the spacecraft, greatly reducing the payload.
From this, it can be seen that it is a very effective method to greatly reduce the material density while ensuring that the material can resist high heat flow. Although reducing the density of the material will lose some mechanical properties, for the spacecraft landing/reentry, the benefits of this material being lighter are more impressive.
【R&D of materials】
Since the 1960s, the United States and the Soviet Union have opened the road to Mars exploration. During that period, the thermal protection systems of the Pirate and Pathfinder all used SLA materials. This material has been introduced to the heat flow at 300W. /cm2 or less. With the further development of interplanetary exploration, the thermal environment faced by the aircraft is more severe, and the demand for lightweight and ablation-resistant new heat-resistant materials is more urgent. The NASA Arms material research and development team opened PICA materials in this context. R&D. PICA material is composed of low-density carbon fiber preform and new impregnation process, in which the fiber is Fiberform carbon fiber preform produced by Fiber Materials Corporation (FMI), and the impregnation material is phenolic resin. The density of the fiber preform used in PICA material is in the range of 0.152~0.176g/cm3. This kind of fiber preform material is widely used in industrial fields such as boiler insulation; the resin used is SC1008 resin provided by the British Borden company, which is also commonly used in composite materials. Resin. The NASA material team conducted in-depth research on the impregnation technology, and finally realized the technology of adjusting the material density and ensuring the uniform distribution of phenolic resin by controlling the impregnation process parameters, and applied for a patent for this.
【Material ablation process】
After completing the research and development of PICA materials, NASA conducted a large number of wind tunnel ablation tests. It not only made an in-depth analysis of the ablation characteristics and mechanism of the material, but also explored its failure mechanism and boundary conditions. By comparing and analyzing a large number of test data, they believe that the PICA material ablation process can be divided into three stages.
The first stage occurs when the heat flow conditions are in the range of 425 ~ 570 W/cm2. The ablation performance is mainly controlled by the oxidation rate. The ablation amount of the material is also mainly caused by oxidation. The ablation amount increases with increasing pressure. This is mainly because the increase in surface pressure increases the oxygen atom concentration at the surface of the stagnation point.
The second stage occurs when the heat flow is in the range of 570~1900 W/cm2. The ablation of the material is controlled by diffusion. The heat dissipation of the material is mainly achieved by surface radiation. Instead, the amount of ablation of the material decreases as the pressure increases.
The third stage occurs when the heat flow is above 2000W/cm2. The ablation mechanism of the material is mainly controlled by the sublimation of carbon. In this stage, the surface temperature of the material is very high, so that the carbon in the material directly sublimates, which leads to an increase in the heat protection efficiency of the material. .
The first application of PICA materials was selected as the material of the bottom thermal protection system of the Stardust return module, which was used to protect the return module from landing safely during the re-entry process. Stardust is NASA's interplanetary spacecraft. Its main goal is to detect the composition of comet 2 and its coma components. The so-called coma is a cloud that surrounds the nucleus of the comet. The comet is orbiting the sun. When approaching the sun, the sun's heat will melt the comet nuclear material and sublimate into gas, thus forming a comet. Stardust was launched on February 9, 1999, and the journey reached 4.6 billion kilometers. On January 15, 2006, it returned to the cabin and landed in Utah, United States. Stardust's return cabin was the fastest reentry vehicle (135km altitude reentry speed 12.4km/s). The spacecraft's speed was reduced from Mach 36 to subsonic speed within 110s. During the return process, the cabin surface The highest temperature is over 2900 degrees Celsius. It is said that huge fireballs and sonic booms can be observed in western Utah and eastern Nevada.
Another application of PICA materials is the bottom thermal protection system material of Mars Science Laboratory (Curiosity). Curiosity is NASA's new generation of nuclear-powered Mars rover. Its main task is to investigate the possibility of Mars' past or present life support, and to analyze the composition of the surface and rocks. It was launched on November 26, 2011, and landed on the Gaelic crater on Mars on August 6, 2012. During the landing, the bottom thermal protection system with a diameter of 4.5m was mainly used to directly bear the aerodynamic heating. This system was the largest thermal protection system in the spacecraft at that time. When the probe enters the Martian atmosphere, the speed decelerates from 5.8km/s to 470m/s, the parachute is opened and then further decelerated, and then the thermal protection system is thrown away to facilitate the entire subsequent landing. The maximum surface temperature of the aircraft during the entire process of passing through the Martian atmosphere Up to 2090 degrees Celsius. The Curiosity probe's landing requirements are very high. NASA scientists have likened the landing process to similar to the launch of a golf ball in Los Angeles to hit a golf hole in St Andrews, Scotland. The bottom right of the first picture below is the thermal protection system described in this article, and the second picture is the surface of Mars captured by the probe.
[Key invention figures]
In the development of NASA thermal protection materials, many scientists have made tremendous contributions. What I have to mention here is Ms. Huy Tran, the main inventor of PICA materials in this article. The world-class aerospace engineering designer was born in a small village in Vietnam in 1963. At a young age, she showed great interest in aerospace. Her father worked in the navy of South Vietnam when he was young, but then there was a civil war in Vietnam. North Vietnam defeated South Vietnam and unified Vietnam. After the civil war, Chen Hui's family fled to Indonesia and arrived in the United States after spending a year in a UN refugee camp. Among the nine children of the family, Chen Hui ranked first, shouldered the study of eight younger brothers and sisters in the family, and still admitted to Deansa College, one of the largest and best community colleges in the United States under difficult conditions. During her studies at Deansa College, she successfully applied for an internship at NASA and came to the NASA Arms Research Center. At that time, she was assigned to a heat-resistant material laboratory and received high praise from the laboratory for her hard work. After the one-year internship, she continued to work with several researchers. After receiving a bachelor's degree from San Jose State University in 1990, she returned to the Thermal Materials Laboratory at NASA Arms Research Center and became A real aerospace engineering designer.
In NASA's Arms Research Center, as the main inventor, she has successively developed the phenolic impregnated carbon ablation material PICA and the silicone impregnated ablation resistant material SIRCA. In 2003, she won the NASA Exceptional Engineering Achievement Medal; in 2007, she and the team invented PICA won the NASA 2007 Government Invention of the Year; in 2008, she Appointed Deputy Director of Aeronautics at NASA Arms Research Center. From traditional carbon phenolic to new PICA, Chen Hui's R&D experience once again shows that innovative thinking and perseverance are still the key to promoting the development of science and technology.