The specific impulse of most of the jet engines is far lower than it should be. Turbofans should be around 6,000 seconds, for example. Here's a chart showing real-world numbers for various kinds of jet engines: http://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Specific-impulse-kk-20090105.png/700px-Specific-impulse-kk-20090105.png