Studies on Aerodynamic Characteristics of Next Generation's Vertical Take-off and Landing Reusable Rocket in Relation to Safety at Landing

垂直起降火箭是解决低成本、可重复使用的空间运输系统需求的方案之一。这种飞行器的垂直软着陆是通过火箭在地面附近进行反向燃烧来实现的。本文对火箭喷管喷流的地面效应所产生的气动力进行了实验和数值研究。主要结论如下：1.当地面到车座的测量高度较小时，底座压力减小，向下作用力作用于车身。它随着海拔的降低而增加。在非常低的高度，它变得大于喷嘴的推力，车辆被拉向地面。当高度较高时，撞击地面的射流形成“吸力模式”，将环境空气吸入底部区域，使底部压力略有下降。当高度较低时，流型转变为“涡流模式”，在射流周围形成一个涡环，使底部压力显著降低。通过对车辆底座外缘进行修整，并对地面进行粗化处理，可以减小向下的作用力。在多喷嘴的情况下，由于来自喷嘴的射流的相互作用，向下的力变得很弱。在塞式喷嘴的情况下，向下的力小于钟形喷嘴的情况，因为由于地面效应，塞式喷嘴底部的压力增加。为了确保车辆安全，不仅需要在着陆阶段仔细控制车辆，还需要通过第三项所述的方法减少向下的力。

A vertical take-off and landing rocket is one of the solutions to the need for low cost, reusable space transportation system. Vertical soft landing of such vehicle is achieved by retro-burn of the rocket near the ground. In this study, the aerodynamic force induced by the ground effect of the jet from the rocket nozzle is experimentally and numerically investigated. The major conclusions are as follows :1. When the altitude measured from the ground to the vehicle base is small, the base pressure decreases and the downward force acts on the body. It increases with the decrease in the altitude. At very low altitude, it becomes larger than the nozzle thrust and the vehicle is drawn toward the ground.2. When the altitude is relatively high, the impinging jet upon the ground induces the flow pattern classified into the "suction mode", where the ambient air is sucked into the region under the base and the base pressure is slightly decreased. When the altitude is low, the flow pattern is changed into the "vortex mode", where a vortex ring is generated around the jet and the base pressure is significantly reduced there.3. The downward force can be reduced by modifying the outer edge of the vehicle base and roughening the ground surface.4. In the case of multi-nozzles, the downward force becomes weak due to interaction of the jets from the nozzles.5. In the case of the plug nozzle, the downward force is smaller than in the case of the bell nozzle, since the pressure at the plug base increases due to the ground effect.6. To ensure vehicle safety, not only careful control of the vehicle during the landing phase but also reduction in the downward force by the methods as pointed out in item 3 are necessary.