Scramjet power? Simple: Keep a match lit in a 7,000-mph wind.
- By Michael Milstein
- Air & Space magazine, July 2005
(Page 4 of 5)
Instead, it became the X-43A. The razor-like X-43A is the spitting image of the National Aerospace Plane, but less than a tenth the original size. A 1995 NASA competition made it next in the line of experimental X-planes. The details of its engine chamber remain highly guarded by the military—so much so that while it’s on the ground, padlocked covers on both ends of the chamber shroud it from anyone without clearance. Its scaled-down size was matched to a mission that was narrowed to focus on one basic goal: Prove a scramjet can power an airplane, and do it quickly and cheaply.
That meant keeping it simple. The X-30 was to have a human crew; the X-43A would be automated. It would fly only as long as it took to burn the hydrogen fuel it could carry. Engineers thought about trying to land it on the Navy’s San Nicolas Island, off the coast of California, but gritted their teeth and decided it would be simpler for it to transmit data and ditch in the ocean.
The SR-71 lifts off with a turbojet before accelerating to ramjet mode. The National Aerospace Plane was also intended to shift gears in flight. The X-43A had to rely on a Pegasus rocket to get up to speed.
That raised a question: How do you push the 3,000-pound X-43 off a rocket going thousands of miles an hour? Earlier scramjet experiments in Australia and Russia had attached scramjets to rockets to prove the engine would function at high speeds. They did not try to power an airplane with it. The X-43A had to leave the rocket and fly on its own. Finally engineers found a technology to push the two apart fast enough so that they would not collide: the piston system that B-1 bombers use to eject bombs.
The X-43A was packed to the gills with instruments, fuel lines, and controls. Its budget did not allow for miniaturizing, and its size wouldn’t allow for backup systems. Technicians shaved its battery to the hundredths of an inch. “There was no room to drop a marble inside,” says Castrogiovanni.
Everything was enclosed within a body that would endure the highest temperatures and pressures an airplane had ever faced. Stainless steel and carbon fibers formed the wings’ leading edges, sharpened like razor blades. The aircraft’s nose was made from an enormous piece of tungsten to take the heat and balance the heavy back end of the aircraft. Its engine chamber was crafted from copper and cooled by water lines to control temperatures not much lower than those on the surface of the sun. Its fuselage was covered with the same sort of thermal tiles that cover the space shuttle. But instead of crafting each tile to precise dimensions, as NASA does for the shuttle, X-43A designers kept costs down by first cementing the tiles in place and then machining them to a smooth surface.
Even the airplane’s fins were designed with a small gap so that when they expanded from the heat of high-speed flight, they would not bind with adjacent parts. Designers were careful not to make the gap too large, though, or the airflow would become turbulent and slow the vehicle down.
To avoid another catastrophe, engineers beefed up the Pegasus’ fins and rudders and removed 3,300 pounds of the rocket’s propellant. By eliminating some fuel, the engineers could launch at the higher, thinner altitudes the rocket was designed for without worrying that it would loft the X-43A too high.