Other experts said the nation would be better served by relying more on unmanned vehicles for launching commercial, scientific and military payloads.
They had hoped to provide launch services for launching payloads into orbits around the Earth.
If they are ever successfully developed, scramjets could make for much faster aircraft and also be used to launch small payloads into orbit.
A Falcon 5 plans to launch 8,000-pound payloads for $18 million, a third of the price of competitors.
Quicklaunch is a company attempting to use a type of space gun to launch payloads into low earth orbit.
These experts argue that it would make more sense to launch military payloads on both shuttles and rockets.
Small companies, like ours, will be launching payloads in the 400 to 4,000 pound range.
They say this Spacelifter system could cut by half the cost of launching payloads into space.
The costs of using a well-tested system to launch payloads are high.
The rockets are meant for launching unmanned payloads such as satellites.