。。。海平面比冲。。。第一次听说哈。优势如下，体积小，好看，比冲较高

二楼被吞了?

两个比主帆比冲还大的发动机

简略照搬。因为有一边不是密封的了，比冲稳定而且较高。

RS-68经过大气层优化后海平面比冲应该也能达到这个程度。那种尖引擎可能在优势方面尚没有那么明显，但造价比较高，所以没有被投入使用。要知道航天飞机的主引擎比RS-68的费用要高七倍，而RS-68的造价甚至已经低于航天飞机主引擎的单次发射维护费用，但推力反而更大，所以才把航天飞机淘汰了。

关于楼主的那个问题，wiki上的对aerospike的描述是这样的
In real life, the heavy aerospike nozzle is an advanced alternative to the classic hourglass-shaped De Laval nozzles sported by the LVT engines. The bell of a rocket nozzle contains and accelerates the supersonic exhaust gas, converting exhaust pressure into velocity along its length. The longer the nozzle, the lower the final pressure becomes. However, in an atmosphere the exhaust stream interacts with the air around it, and if its pressure falls too low in comparison the stream can become unstable and inefficient. In a vacuum, an ideal nozzle has an infinite length, but practically nozzle length is a tradeoff - shorter nozzles can handle thicker atmosphere, but don't generate the exhaust velocity of longer ones. An aerospike provides comparable performance to a De Laval nozzle in space, but at lower altitudes it can be 20-30% more efficient.

目前只找到这一篇相关中文资料，发动机专业的可能就多了，国内竟然还有好几本书研究这种结构的发动机，因此相信我国也正在尝试类似结构发动机的研发。
http://baike.baidu.com/view/3832388.htm