Biofuel-powered passenger aircraft are becoming more and more common. But what if you mashed that up with a rocket engine? That's essentially what the Zero Emission Hypersonic Transport (Zehst) project by European defense manufacturer EADS is all about. Zehst could be the rocketplane that flies our kids around the world in the same time it takes us to drive to work.
The Concorde was retired nearly eight years ago, and the SR71 Blackbird concluded flights 22 years ago, but if they'd got together in the 1980s and had an airborne offspring they would've produced something very much like Zehst. The aircraft was unveiled yesterday before the influential Paris airshow, and EADS seems intent that it's no pie-in-the-sky experiment: Zehst will hopefully be flying in prototype form by 2020 and in service 30 years after that.
You may be thinking "zero emissions and a rocket plane does not compute" but here's why it kinda makes sense: Zehst has seven engines hidden inside its nacelles, two that breathe air and work like conventional jet aircraft's do, two giant ramjets, and three that burn liquid hydrogen and oxygen like conventional rocket engines do. It also relies on existing well-known technologies, materials, and engine designs. So, while it's going to take 40 years to get off the ground, it's not purely science fiction.
The four air-breathers burn biofuel, which could be based on algae—a technology that's getting more and more plausible. Indeed Boeing just flew its newest aircraft, the 747-8 freighter, to Paris by using biofuel. This gives them an extremely low eco-footprint compared to conventional jet fuel. Meanwhile the rocket engines burn oxygen and hydrogen, like the Space Shuttle, which leaves a polluting wake that consists only of superheated steam (making and storing the LH2 and LOX in the first place does take energy, which currently isn't necessarily the most environmentally friendly practice—but by 2050 it certainly will be).
The idea is the two jet engines propel the vehicle at low speeds on takeoff and landing. When it's accelerating into the air, three rockets take over to push it higher and faster in a boost phase. As the aircraft moves fast enough to ignite ramjets, two of these take over the power production as it continues to rise. After reaching its peak altitude and speed it glides, and then slows as it dives toward a landing.
Since Zehst needs to carry seven engines in various configurations, as well as its rocket fuel inside it (like Concorde or Blackbird and unlike the Space Shuttle), there's reduced space for passengers. But the 50 to 100 folk inside the aircraft will get the chance to fly up to 20 miles in altitude and travel at Mach 4.5 (around 3,000 miles per hour), which is pretty incredible—though will probably cost quite a bit of coin.
Sadly Zehst doesn't fly high enough for its passengers to be called astronauts. But with all that speed—and, if the Concorde is any role model, luxury—it will hardly matter. The one worry is that this complex design will suffer the same bureaucratic and technical screw-ups as its speedy predecessor.