<< Page 12 Page 13: Page 14 >>   Exotic Airplane Engines: Rocketplanes, Solar/Electric, Human-Powered Rocket engines: These come in two varieties: The simple solid-fuel engines and the more complicated liquid-fuel ones. Solid-fuel rocket engines were invented by the Chinese thousands of years ago and are used today in everything from fireworks to Estes model rockets to the Space Shuttle boosters to Brazilian Satellite-Delivering rockets. They work like this: A tube is filled with a solid fuel. The bottom is ignited, which makes the fuel at the bottom heat up, burn, and turn into expanding gas, which shoots out the bottom because it has nowhere else to go. The heat from this combustion ignites the fuel right above it, which combusts into hot gas and shoots out the bottom, in turn igniting the next-higher bit of fuel, etc. Very straightforward. However, very dangerous, because once it’s started, this chain reaction cannot be stopped or slowed/throttled until all the fuel is gone. The oxidizer is mixed right in with the fuel. Liquid-fuel rockets (see above) store the fuel – usually liquid hydrogen – and the oxidizer – usually liquid oxygen – in separate tanks. The fuel and oxidizer are pumped into a combustion chamber (like a jet engine’s burner) continuously during operation, where they burn, forming hot gas (usually water vapor), which shoots out of a hole in the bottom of the combustion chamber. The engine can be shut down if no fuel or oxidizer are pumped into this chamber, and it power can be increased or decreased by changing the amount of fuel and oxidizer pumped in per second. Hybrid rocket engines combine some of the simplicity, low cost, and high thrust of solid-fuel engines with the throttle-ability and control of liquid-fuel engines. The solid fuel fills a tube, but holes and tunnels are drilled into the solid fuel. The oxidizer is kept in a separate tank. When the oxidizer is pumped through the tunnels and holes in the fuel, the fuel is ignited and burns. Depending on how much oxidizer is pumped into the tube that holds the solid fuel, the solid fuel will burn more quickly or more slowly (or not at all). This kind of engine is not in common use yet, but most rocket scientists strongly advocate their adoption. SpaceShip One, Rutan’s X-prize contender, runs on a hybrid engine (tire rubber oxidized by nitrous oxide). The Space Shuttle has two solid-fuel boosters, the tall thin white rockets at the sides, and three liquid-fuel engines, at the back of the Shuttle itself. The liquid-fuel engines are fed during launch with O2 and H2 from the huge orange fuel tank, which is released just before the Shuttle reaches orbit. The Shuttle still holds enough fuel and oxygen internally for the de-orbit burn, a backwards burn that slows the Shuttle down, causing it to fall out of orbit and begin re-entry. Possibly the most common use of rockets in aviation (not counting missiles or the space program) is attaching small solid-fuel rocket engines to airplanes to allow them to take off and land in less space. This was typically done with some large cargo planes, many early jets, most target drones, some modern UAVs, and occasionally other aircraft (such as the A-3, the Martin PBM Mariner, the B-26, and Swiss Mirage 3s). This is called JATO or RATO (rocket-assisted takeoff) and was first tried (in 1941) by the US Army on a tiny Ercoupe. The Blue Angels demonstrate a RATO at most airshows, performed with their C-130 support plane. Above: A B-47, an F-5, a Prowler UAV, a C-130, and a Mirage 4, performing RATOs. This C-130 can take off and land on snow, and on its landing gear carries the largest skis in the world. And that B-47 picture has to be one of the coolest aviation photographs ever! In addition to rocket-assisted takeoffs on large cargo planes (and on early jets like the F-84 and B-47), some jet fighters have had rocket engines added to them, so that they could be flown just about out of the atmosphere to train astronauts, or so that they could take off from a platform on the ground. For example, some F-84s, F-100s, F-104s and MiG19s were fitted with rocket engines and a rail that allowed them to be “fired” from a truck like a missile, not requiring a runway. The idea is 1) you “surprise” bombers by having a fighter come at them from an area where no air base is in sight (i.e. you can set-up “hidden” fighter launch stations to complement AAA in areas you think are in danger of being attacked), and 2) if your air bases are destroyed, you can still truck your fighters around, and maybe use them as bombers too. The rocket motor would launch the fighter, and once at flight speeds it would shut down and the jet engine would keep the fighter in the air. (This is called “zero length launch”. Search for this online and you’ll get some wild videos of fighters being rocketed off trucks). The Germans in WW2 were the first to think of this idea, and designed a purely zero-length-launch fighter, the Bachem 349, which was launched vertically up a rail like, well, a rocket. It was not very successful, but the idea is still amazing (and the plane is very weird-looking. Who shoots rockets out of the NOSE? Honestly…) To train astronauts, a few F-104s were fitted with rocket engines, becoming the NF-104 (bottom, left). The engine would be fired up when this F-104 was already near maximum speed and altitude, and it would take the NF-104 to 120 thousand feet. When the engine shut down, the pilot experienced 2 minutes of weightlessness, as the airplane slowed down and fell back into the atmosphere. During this time, no air passed over the control surfaces, so the plane was controlled by gas discharges at the nose, wingtips and tail, just as spacecraft are controlled in orbit (Reaction Control System). There have been very few purely-rocket-powered airplanes. They were mostly experimental prototypes going for speed and altitude records, or rockets designed to go to space. The most famous rocket planes wee the X-1, the first plane to break the sound barrier, and the X-15, which flew to 354200 feet and MACH 6.7. They both currently hang at the Smithsonian. During WW2, the Germans designed and built a rocket-powered interceptor (not the Ba349, but one that takes off and lands horizontally). With an extremely small range, they could only defend the base where they were kept. They would take off from a runway and climb almost vertically just before the arrival of allied bombers, try to shoot them down, and glide back to base. Although built in small numbers and not very successful a fighter, the strange-looking Messerschmitt-163 Komet (left) was the only purely-rocket-powered manned combat aircraft ever to see military service (and was much more successful than the Bachem 349).The Japanese tried imitating the design with the J8M, but did not develop it enough to justify production during the war. - - - - - - - - - - - - - - - - - - Electric Airplanes Electric power systems are heavy. An electric motor cannot provide the power of a turboprop or even a piston engine of similar weight and size, and batteries and fuel cells cannot store as much energy as a comparable weight of hydrocarbon fuel. Given this, electric airplanes have to be very, very light and very, very low-drag (so they can fly with little thrust and little energy, so as to not have to carry big motors and lots of batteries). In other words, their power systems are so heavy and weak that they are designed much like the earliest airplanes: light, flimsy, slow, tons of wing area and not much else. Since the 70’s, Paul MacCready’s AeroVironment has been experimenting with solar-powered aircraft, more successfully than anyone else. They Solar Challenger flew to over 14000 feet, and crossed the English Channel in 1981. NASA is exploring electric airplanes with AeroVironment’s Pathfinder and Helios, unmanned flying wings powered by solar cells and several electric motors. The Helios has about the same wingspan as a 747 (except it is little more than just wing), can fly using no more energy than is provided by the solar cells (but only during daylight hours, currently), and has broken the SR-71’s altitude record for non-rocket airplanes. Fuel cells are being developed for it which will be able to absorb energy during the day and release it at night, allowing the Helios to stay aloft for weeks at a time. These airplanes fly in extremely thin air and can use the energy of the sun to fly, so their designs are actually being studied and developed with the idea in mind that, one day, an airplane much like these could be flown on Mars. Due to the time it takes for radio waves to travel to Mars, the could not be remotely-flown in real time – they would have to be taken there by a probe, assemble themselves, and launch and fly themselves, much like the Spirit and Opportunity rovers. Unless people go to Mars first, in which case they could operate the aircraft in real time. (Note: At the time this text was written, no hydrogen-fuel-cell-powered electric airplanes had flown, but this has since changed - thanks to Paul McCready and Aerovironment, as expected. Please see the addendum for these updates). - - - - - - - - - - - - - - - - - - Human-Powered Airplanes Given the very poor power-to-weight ratio of a human (when compared to an internal combustion engine), human-powered planes also suffer the weight and speed restrictions (and the wing size requirements) of electric airplanes and the earliest airplanes. Although not exactly practical, thanks to Paul MacCready’s pioneering efforts a few human-powered airplanes have flown since the 70’s. They all carried one person – always a very fit bicyclist. The most famous of them, the Gossamer Albatross, crossed the English Channel in 1979. There are many prizes for achievements in human-powered flight. They include prizes for speed, endurance, distance, making figure-8s, etc. Apart from MacCready’s creations, MIT airplanes and the German “Musclair” have also claimed some of these prizes. MacCready’s Gossamer Condor (which first demonstrated sustained maneuverable human-powered flight by taking off and performing a figure-8) hangs in the Smithsonian National Air and Space Museum. Above: the Albatross. Below: the Condor.