The us Army is about to embark on a landmark mission: to launch a missile fuelled not by the usual solid fuel, but by a toothpaste-like gel. The gel will help them control the missile better, will improve the missile's range and even allow it to attack more than one targets on its flight path. The change from regular solid fuel was imminent. Like any vehicle, a missile, too, burns more fuel when travelling at high speeds. A fuel-efficient missile would have to travel slowly for a greater part of its journey, accelerating only when it nears its target. But achieving fuel efficiency with conventional missiles is difficult since it involves varying the rate at which the fuel is burnt and this is nearly impossible with solid fuels. "With a solid, the only thing you could do is light it and let it go,' said Jerrold Arszman, propulsion technology manager for the us Army's Future Missile Technology Integration project. "You turn the thing on, it does what it's going to do and then that's it.' On the other hand, it's easy to change the rate at which a missile burns a liquid fuel