What about the possiblility of having a first stage that uses a couple of conventional jet engines strapped to the side of the core rocket. The idea would be that for the first minute or two of flight the jet would provide thrust. In an atmosphere, the ISP of a jet far exceeds any other chemical rocket and it can do it with good thrust to weight ratio. The down side is that after leaving the atmosphere, the jet is just dead weight. But by that point, it's already paid for itself. It can be jetisoned and recovered with the rest of the first stage.<br /><br />Any thoughts?