It is long past time for NASA to develop a new launch system, and I mean NEW. Step rockets have been around for a long time, and there is little in the way of new technology involved in building them, apart from refinements in turbine technology, avionics, and composite construction. Isp values have not been substantially improved in years, as far as I know, in respect to chemical engines. The space shuttle was the last major breakthrough in technology to access space.
We know that a lifting body re-entry vehicle will work, reliably, and that it can be flown to a landing on a runway. We also have learned that our rockets cannot take-off straight up and accelerate as fast as possible, because they encounter the atmospheric turbulence known as max Q, or Max Q, or Max Headroom, (No, wait a minute...) Anyway, the space shuttle actually throttles the engines back as soon as it clears the tower, and runs them at about 60 percent thrust until the vehicle is about 70, 000 feet high, when they are returned to full power. It is at about this altitude that the space shuttle curves over to where it is heading towards the horizon, not straight up, because the air is thin enough at that altitude that max Q is no longer an issue.
Tremendous advances have been made in aerodynamics and engine technology in the last 30 years, and what was once thought impossible is now being considered, or achieved. A wing capable of lifting 1.5 million pounds to 50,000 feet is now within our reach, which would be like lifting our launch pad to a high enough altitude that our rockets would no longer have to take off straight up, but could launch heading towards the horizon, using every bit of the energy in their tanks to accelerate towards the goal necessary to reach orbit, about 5 miles per second, or around 17,500 miles per hour.
If you are interested in finding out more about such a launch system, please see my thread "A cheap and easy way into space" in this forum.