Owing to its potential, organizations around the world are researching scramjet technology. Scramjets will likely propel missiles first, since that application requires only cruise operation instead of net thrust production. Much of the money for the current research comes from governmental defense research contracts.
One issue is that scramjet engines are predicted to have exceptionally poor thrust to weight ratio- around 2 4. This compares very unfavorably with the 50-100 of a typical rocket engine. This is compensated for in scramjets partly because the weight of the vehicle would be carried by aerodynamic lift rather than pure rocket power (giving reduced 'gravity losses'), but scramjets would take much longer to get to orbit due to lower thrust which significantly offsets the advantage. The takeoff weight of a scramjet vehicle is greatly reduced over that of a rocket, due to the lack of onboard oxidiser, but increased by the structural requirements of the larger and heavier engines.
An aircraft using this type of jet engine could dramatically reduce the time taken to travel from one place to another, thus putting any place on Earth within a 90 minute flight. However, there are controversies about whether such a vehicle could carry enough fuel to make useful long trips.
Sources:
http://en.wikipedia.org/wiki/Scramjet
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