## 1 Introduction
[20] proposed a lever-type QZS vibration isolator, using magnetic springs to achieve QZS characteristics. It was found that reasonable configuration of structural parameters could greatly improve vibration isolation performance. The aforementioned studies indicate that the design of QZS isolators utilizes the nonlinear force-displacement relationship, which can significantly reduce the amplitude of vibrations transmitted to the payload, thereby protecting sensitive equipment from vibrational damage during the in-orbit and launch process. Furthermore, nonlinear characteristics can provide new ideas for in-orbit launch design. Traditional linear launch system designs are often limited by their dynamic performance and cannot achieve significant launch speed improvement without sacrificing system stability and safety. In previous studies, nonlinear characteristics (especially the negative stiffness phenomenon) have been shown to significantly enhance the dynamic response of the system. The relationship of the enhancement mechanism of the negative stiffness mechanism, consisting of mechanical spring [21; 22], buckled beam [23; 24], magnetic structure [25; 26] and bionic structure [27; 28], to the dynamic response of nonlinear system has been widely studied. For example, [PERSON] et al. [29] replaced the traditional negative stiffness spring pair with super-elastic springs and found that the negative stiffness mechanism presented a dynamic amplification effect. [PERSON] et al. [30] constructed a negative stiffness mechanism based on magnetic springs, which was sensitive to micro-displacement and could amplify the response in the resonance region. [PERSON] et al. [31] proposed a completely symmetrical cross-type structure with equal rod length to achieve negative stiffness characteristics. Results showed that the subharmonic response could be enhanced by adjusting the structural parameters of the cross-type structure. Therefore, compared with the traditional linear design, a nonlinear configuration can achieve higher system flexibility and adaptability under certain conditions, thereby improving the overall launch performance. To address the evident limitations of traditional linear launch system designs in terms of vibration isolation and launch performance, this paper presents a novel aircraft in-orbit launch design utilizing nonlinear dynamic characteristics to enhance vibration isolation and launch performance. The design configures four springs, two of which are orientedperpendicularly to the launch direction to produce negative stiffness, thereby providing additional kinetic energy at critical moments to increase launch speed. The other two springs are designed to balance the aforementioned negative stiffness when the launch outlet is closed, achieving quasi-zero dynamic stiffness to enhance vibration isolation performance. By integrating quasi-zero stiffness isolation technology with nonlinear dynamics in the in-orbit launch system, this research not only overcomes the limitations of traditional linear design methods but also demonstrates how sophisticated nonlinear design can enhance launch speed and achieve efficient vibration isolation. These findings provide valuable insights for the further development of in-orbit launch technology and offer new ideas for future system design and optimization. This paper is organized as follows. Section 2 presents the research model of the proposed aircraft in-orbit launch design. The governing equations for vibration isolation and in-orbit launch are derived in Section 3. Section 4 performs the performance comparison with linear design. Parametric study is conducted in Section 5. The influences on the vibration isolation and launch speed performance of the structure parameters are discussed in detail. All the findings are summarized in Section 6.