Abstract: To address the buckling phenomena under external pressure for a large expansion ratio nozzle during its high altitude ground test，the steady-state and starting transient flow field of the nozzle has been investigated both numerically and experimentally. The results show that for the upper stage engine with nozzle area ratio 70，chamber pressure 4.5MPa and the ambient pressure of 87kPa，the nozzle bulking will not happen during the steady-state in that the nozzle is subject to internal pressure load. However，the nozzle buckling due to the external pressure is liable to occur during the starting transient especially when the vacuum-pumping was not conducted. This is because the flow-field within the nozzle was established within a very short time of about 0.4s，and the pressure in the test chamber was lower than that at nozzle inner wall until 10s. Thus external pressure was loaded on the nozzle wall during this period. Finally，as the engine was started with vacuum-pumping，the external pressure was dominant between 0.14s and 0.42s，which was at a high level before 0.25s，and this period was very short，so the nozzle buckling will not happen. Therefore，vacuum-pumping before engine start-up is an effective method for avoiding buckling for a large expansion ratio nozzle.

Key words: Large expansion ratio nozzle；Altitude simulation test facility；Starting process；Buckling under external pressure

摘要： 针对上面级发动机大膨胀比喷管高模试车时发生的外压失稳现象，进行了高模试车时的稳定工作和启动过程数值仿真和实验研究。结果表明:对于室压4.5MPa，环境压力87kPa，喷管面积比70的上面级发动机，稳定工作时，喷管承受的是内压载荷，不会发生外压失稳；不预抽真空启动时，喷管内的流场建立过程所需的时间很短约0.4s，而试验舱的压力只能依靠发动机的引射而降低，从0.4s开始喷管承受较大的外压载荷，直至10s左右试验舱的压力低于喷管内壁压力，在较长时间内喷管一直承受外压载荷，很容易发生外压失稳；预抽真空启动时喷管在0.14s到0.42s时间段承受外压载荷，且只在0.14s到0.25s内承受的外压载荷较大，喷管承受外压载荷的时间很短，不会发生外压失稳；启动前抽真空是避免喷管发生外压失稳的一种有效手段。

关键词: 大膨胀比喷管；高空模拟试车台；启动过程；外压失稳