Chair Force Engineer

Tuesday, January 24, 2006

Rocket-Powered Rosebud

As jet aircraft made their way onto aircraft carriers, it was soon realized that aircraft carriers would need to grow to support the increasing takeoff lengths for these planes. A solution to longer decks was invented by the British in 1950: the steam catapult.

The Royal Navy quickly adopted the steam catapult, and the US Navy followed suit. Now carrier-based planes could be shot to takeoff speeds on existing decks. However, some people think that similar catapults (electromagnetic catapults or rocket-sleds) can help push rockets into space. While there is some historical precedent for this, you can count me as a skeptic.

Eugen Sanger, the Austrian who designed rocket planes for the Nazis in World War II, originally viewed the rocket sled as a means of takeoff for his "Sanger-Bredt Antipodal Bomber," also known as "Silverbird." After the war, Sanger continued his studies while working for Messerchmidt-Bolkow-Blohm. The Silverbird (which was supposed to skip across the upper atmosphere like a stone skipping across water, except for the heating problems which Sanger apparently didn't know about) evolved into the RT-8-01. This was a two-stage rocket, both stages with delta wings. While the RT-8-01 also used the rocket sled, Sanger's team had settled on vertical takeoff for its final iteration, the Sanger I.

Similarly, the American engineer Phillip Bono wanted to build a rocket sled to launch his Hyperion single-stage spacecraft. However, Bono realized that to take advantage of the speed boost, the rocket sled would have to run vertically with respect to the launch site. He proposed building his rocket sled up the side of a 1.7 km mountain.

One important thing to remember about catapults and rocket sleds is that they accelerate their associated spacecraft at low altitudes. The denser atmosphere near sea level (or even within a few miles of sea level) makes a huge difference in the drag and heating that the spacecraft will undergo.

As an example of the challenges of high-speed, low-level flight, we have the F-111. While "The Vark" was an exceptional plane for flying supersonically at low altitudes, this ability came at the expense of structural weight. As the F-111 was the product of a misguided "joint" effort between the Air Force and Navy, the two services had competing requirements for the F-111. The Air Force wanted it to fly supersonically at low altitudes so it could penetrate under the radar coverage of Soviet anti-aircraft missiles. The Navy was willing to settle for a high subsonic speed at low altitudes, knowing that a lighter structure was needed. The resulting F-111 could break Mach 1 at low altitude, but it was too heavy for safe carrier operations.

Even today, while planes like the the F-14, F-15, and F-16 can break Mach 2 at high altitudes, they can't achieve this same speed at low altitudes. The original B-1A bomber, with movable intake ramps, could make a Mach 2 dash at high altitudes only; the revised B-1B, with fixed inlet ramps, is optimized for flight just over Mach 1 at low altitudes.

Even if a lightweight airframe could withstand the stresses and heating associated with Mach 3+ flight at altitudes of just a few miles above sea level, there is still the problem of controlling the rocket's trajectory. Much like running the Hyperion up the side of a mountain, a sled-launched rocket must transition into a near-vertical trajectory. Not only does this maneuver bleed off precious velocity, but the aerodynamic stresses it imposes dictate a stronger structure lest the vehicle break apart.

While the book isn't entirely closed on rocket-sleds and electromagnetic catapults, there are many reasons for skepticism. There are good reasons why rockets have been launched vertically from the ground (and, less frequently, dropped from the bellies of aircraft.) These methods minimize drag and gravity losses without imposing excessive heating or aerodynamic forces.