Even an Isp of 240 is not all that high. This page: http://www.astronautix.com/props/n2o4mmh.htm http://www.astronautix.com/props/n2o4mmh.htm Lists a bunch o

Even an Isp of 240 is not all that high. This page: http://www.astronautix.com/props/n2o4mmh.htm http://www.astronautix.com/props/n2o4mmh.htm Lists a bunch of engines that have Isp values in the 300 range. SpaceX tends to optimize for total operational cost, rather than performance, so I wouldn't be surprised by the tradeoffs they've made to create a reliable and reusable system.