The F-35 joint attack aircraft may acquire the capability of external auxiliary fuel tanks in the fourth batch of modernization projects. Stealth fighter jets can also be equipped with a secondary fuel tank - this has sparked curiosity: under what circumstances do fighter jets need to be equipped with a secondary fuel tank? From the current situation of major countries' fighter jets being equipped with auxiliary fuel tanks, it can be basically divided into the following situations. There are two types of fighter jets that do not have a secondary fuel tank: one is the self-sufficient type, with the typical representative being the Su-27 fighter jet and some derivative models. From the beginning of the design, sufficient oil storage space was left, and it can fly far with only the fuel inside the fuselage, without the need for external fuel supply; Another type is the stealth type, with the more representative being the F-22 fighter jet. In order to maintain stealth, these fighter jets generally do not have external auxiliary fuel tanks and mainly rely on aerial refueling aircraft to expand their range. A high proportion of fighter jets can choose to install auxiliary fuel tanks. Most current fighter jets have attachment points set at the belly centerline, wing attachment points, or under the conformal fuel tank. Generally, 1-3 auxiliary fuel tanks can be configured, and some models can have up to 4-5. For example, the French Rafale fighter jet can mount one on the belly centerline, one on each inner side of the left and right wings, and two in the middle of the wings. These mounting points can be used to mount auxiliary fuel tanks or switch to weapon mounting points according to mission requirements. The main aircraft that require the installation of auxiliary fuel tanks are carrier based fighter jets. Due to the high fuel consumption and limited amount of built-in fuel, in order to ensure that the fighter jet can take off lightly from the aircraft carrier, it is generally necessary to first install an empty auxiliary fuel tank and then refuel it through an aerial refueling aircraft to prevent the aircraft from being affected by insufficient fuel. The F/A-18E/F fighter jet is like this, usually equipped with 2-3 auxiliary fuel tanks, which can be expanded to 5 if necessary to increase range. Installing a secondary fuel tank can effectively compensate for the lack of fuel in fighter jets, expand mission range, combat radius, and idle time, reduce the accompanying support of aerial refueling aircraft, and reduce excessive dependence on frontline airports. In general, during use, fighter jets often use the fuel in the secondary fuel tank first, and then consume the fuel inside the fuselage. In case of enemy or emergency air combat situations, the secondary fuel tank can be abandoned at any time to reduce weight, drag, and improve maneuverability. However, mounting a secondary fuel tank on a fighter jet also has drawbacks, such as increasing resistance, occupying weapon mounting points, damaging stealth appearance, and affecting tactical flexibility. Especially in terms of maneuverability and concealment, fighter jets with external auxiliary fuel tanks are significantly less flexible than those with conformal fuel tanks. For example, after adopting a conformal fuel tank design, the F-15 fighter jet not only reduced the drag by half, but also reduced the radar reflection cross-sectional area. However, using a conformal fuel tank also has its drawbacks, as it is often a semi permanent device that cannot be abandoned in case of special circumstances. This is also an important reason why many fighter jets prefer to retain the option of an external auxiliary fuel tank. From the current development situation, in order to solve the problem of fighter jet endurance, some countries are actively developing aerial refueling aircraft, unmanned refueling aircraft, next-generation stealth refueling aircraft, etc. Some countries are also studying how to make the auxiliary fuel tank play a greater role. The latter mainly focuses on the following aspects: firstly, developing stealth and disposable secondary fuel tanks, using new materials and stealth technologies to design and develop low drag, low-cost, and low detectability secondary fuel tanks. In safe areas, fighter jets can mount secondary fuel tanks to extend their range, and when facing threats, they can abandon the secondary fuel tanks to "restore" the full stealth or high maneuverability state of the fighter jet. The second is to develop an independent intelligent fuel management system that deeply integrates the aircraft fuel supply system with flight control, mission computers, and other systems. In this way, the onboard computer can intelligently determine the timing and timing of abandoning the secondary fuel tank based on mission requirements, threat level, fuel level, etc., and automatically refill the remaining fuel in the secondary fuel tank into the aircraft fuel tank before discarding to avoid waste. It can also call for aerial refueling aircraft or partners to refuel as needed. The third is to optimize the hanging point design of the auxiliary fuel tank. Through modular design, the auxiliary fuel tank can be embedded with miniaturized data links, electronic warfare antennas, or other mission payloads depending on the situation. When the secondary fuel tank is depleted and thrown, it can immediately transform into a reconnaissance, communication, electronic warfare node or attack unit. Of course, some fighter jets choose to abandon the secondary fuel tank. Some experts propose to break the traditional streamlined refueling pod design by integrating distributed conformal fuel tanks and flexible oil bag technology, that is, by using conformal technology integrated with the fuselage contour and setting distributed flexible oil bag storage in some spaces, maximizing the use of the aircraft body space to expand the storage capacity. In short, the installation of auxiliary fuel tanks on fighter jets is gradually moving from simply solving "range anxiety" to diversification, integration, and intelligence, providing greater choices for fighter jets. (New Society)