Radical new rocket.

Goodbye space shuttle; hello RLV. That's "Reusable Launch Vehicle," the new spacecraft that NASA officials say will replace the shuttle early next century.

Engineers are especially fired up because the RLV, called VentureStar, has a radical new type of rocket engine, the aerospike. This month, they'll strap the new engine to the back of a souped-up spy plane and take it for a test flight.

What makes the aerospike so hot? It shoots fiery gases along the outside rim of an inverted cone (see diagram). When the gases push against the curved surfaces, they thrust the rocket upward.

The key benefit of this design is that the gases push with even thrust at all altitudes. In contrast, the space shuttle's engines, which fire hot gases inside a bell-shaped nozzle, don't work as well near ground level. If you ever watch a space-shuttle launch, you'll notice that the engines shake violently during ignition and liftoff.

The engines shake because, under the air pressure found at groundlevel, the gases don't expand enough to fill the nozzle. So they produce an uneven push. At higher altitudes, where air pressure is lower, the gases expand more and the shuttle flies smoothly.

Because the aerospike maintains optimum thrust throughout its flight, designers say it will use less fuel than traditional rocket engines. And that, they say, will make the VentureStar rocket cheaper to fly.

VentureStar will also be cheaper because the aerospike engines are built into the spacecraft. In other words, VentureStar is a single-stage-to-orbit rocket; it doesn't jettison fuel tanks and booster rockets the way the space shuttle does.

Bob Baumgartner, an engineer working on the RLV, says a full-scale spacecraft should be ready for liftoff by 2004. Who knows? If the RLV really does make space flight cheaper, you may one day be able to hitch a ride for a vacation in space.