As early as 1986 federal officials tried to replace the shuttle with a spacecraft called the National Aerospace Plane. This sleek vehicle was supposed to look like a plane and fly into orbit powered by an air-breathing engine called a supersonic combustion ramjet, or scramjet. That effort was canceled in 1993 when it became clear that the decision to pursue scramjet technology was a bit premature. Since then, there have been a number of other ideas under consideration (see sidebar, page 81) but SLI produced a series of designs that offered for the first time a fully formed picture of what the future shuttle is likely to be. Concepts were designed by Lockheed Martin, Northrop Grumman, Boeing and Orbital Sciences, among others. All of them use boosters to get payloads into orbit, but that's where the similarities to the shuttle end. For instance, an intriguing design was the Boeing "bimese" reusable launch vehicle (RLV), which has two nearly identical winged rockets stacked together with the orbiter or payload container perched on top of the upper rocket. The reusable launch vehicle takes off vertically with all engines firing. When the fuel runs out in each booster rocket, it separates from the RLV and glides back to Earth to land on an airstrip. The advantage of the bimese approach is that because the two rockets are virtually indistinguishable, it's only necessary to build, test, operate and maintain one winged booster design. With a single set of engines, landing gear, controls and other systems, it's a much simpler spacecraft to manage than the shuttle.
Northrop Grumman was particularly busy during SLI. It proposed several vehicles, including a launcher that uses an enormous jet, equipped with six engines and an oversize wing, to carry the booster to 40,000 feet, where it would disengage and fire its rockets for the final hop into orbit. And Northrop's collaboration with Orbital Sciences produced an RLV with two identical tank-shaped boosters seated below an orbiter-carrying, upper stage space plane. These so-called flyback boosters are fitted with jet engines that power them back to Earth after the boosters rocket the upper stage above the atmosphere.
The vehicle shown on our cover, Andrews Space & Technology's Gryphon concept, is farther reaching. A plane-like launcher accelerates an orbiter to Mach 6 before a high-altitude release. Gryphon's innovative propulsion system uses liquid oxygen, drawn and compressed from the air, as fuel.
What's most significant about the SLI contenders is that the developers have engineered out all of the most gaping shortcomings in the current shuttle design. For one thing, the outdated solid rocket boosters—which require intensive reconstruction after each use—are gone, replaced by modern liquid-fueled rocket engines that mainly operate on hydrogen or kerosene; they're much safer because they can be shut off or throttled back after they ignite. Another long overdue improvement: SLI engineers have eliminated the delicate and precarious heat-resistant tile system. Instead, they use lightweight "shingles" with nickel-alloy skins over ceramic-fiber insulating blankets to protect the orbiter and upper-stage boosters from the dangers of reentry. Unlike brittle tiles, the metal-skinned shingles can be bolted securely to the airframe.
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.