Comment There are fundamental differences (Score 2, Informative) 209
Chem rockets can't achieve the efficiency of jet engines because they carry their own fuel and oxidizer. Jets only carry fuel and thus need to propel less weight. Rockets also must generate enough thrust to support the entire vehicle weight. Jets normally fly at thrust-to-weight ratios below one, by having wings that rest on the surrounding medium (air, lift). Rockets must also propel their payloads under these conditions to ~330,000 ft. Commercial airliners reach cruising altitude at 35-40,000 ft. The climb gulps fuel, but the following cruise sips it; rockets are climbing the entire time. This is all scraped from undergrad propulsion, but I think it's right.
One solution is to combine propulsion methods, to use airbreathing propulsion for atmospheric flight and rockets beyond. This could be either a combined-cycle engine (turbine with a rocket in the spindle), or something like SpaceShipOne/White Knight, where a jet-powered platform brings a rocket-ship to altitude. Chemical rocket costs aren't just limited by rocket makers trying to maximize profits on limited launches. They're inherently less efficient than airbreathing propulsion, but aren't limited by the atmosphere.
