It appears that BRC Imagination Arts, a Southern California design firm, has a zero gravity roller coaster proposal that's waiting for a US$50 million investment. According to PopSci, BRC's proposed theme-park ride is inspired by NASA's astronaut training aircraft the KC-135 (aka "Vomit Comet") and would give riders the sensation of floating within a stable chamber.
During a 2-3 hour NASA training flight, the KC-135 aircraft performs 30-40 parabolic plunges that create 20-25 seconds of microgravity - the sensation astronauts are expected to experience when traveling in space. In an attempt to recreate this experience, BRC's anti-gravity roll coaster would speed up a track, reaching a top speed of 100 mph (161 km) before it would then suddenly decelerate, jolting the passengers out of their seats and leaving them in suspended air - they would still be loosely belted into their seats, however. This is then maintained for several seconds as the coaster begins it backwards drop from the top of the tracks. A programed computer system ensures that the coaster matches the speed of the falling passengers, thus creating the sensation of weightlessness.
With such a high-speed free falling experience, this would be a ride to be taken on an empty stomach! But just in case it should cause a "Vomit Comet" effect, drains allow the coaster cabin to be hosed down at the end of the ride. As for the passengers - well I guess that's just the price you pay for a space-like experience of this kind.
BRC is the also the design firm behind the Shuttle Launch Experience simulator, which is currently available for passenger rides at NASA's Kennedy Space Center Visitor Complex. BRC interviewed 27 space shuttle astronauts, to create this multisensory thrill-ride that recreates the experience of traveling into space.