I'm not an exert in this of course, and what follows may well contain errors. But here's what I've come up with, after a little research:
It is the first time a supersonic-combustion ramjet, or scramjet, which uses air for fuel, had traveled so fast, flight engineer Lawrence Huebner told reporters.
I suspect this is a misquote. Hueber probably just told them it was the first time a scramjet had traveled so fast, and the reporter inserted "which uses air for fuel", apparently based on a misunderstanding of what "
air-breathing engine" meant.
The basic function of an aerospace engine - propeller, jet, or
rocket - is to generate forward thrust by throwing some other mass in the opposite direction of travel. For this you need two things: something to throw, and a source of energy to throw it with. In the case of rockets, the rocket fuel provides both - it's a source of energy, and the fule itself is the mass that gets thrown backwards. This allows a rocket to operate in vacuum, which is cool, but it means that in order to have a lot thrust, you need to carry a large mass of fuel. Thus rockets tend to have big bulky fuel tanks associated with them. This is a major limitation as you try to go faster and faster with rockets - to go faster, you need more fuel, which icreases your mass, which means the acceleration you get by burning that fuel will decrease, since you're pushing more mass. You get a bad case of diminishing returns.
In the case of propellers and jets, the thing you throw backwards is
air, which is readily available as long as you're not trying to go into space.
But you still need a source of energy; that's where the fuel comes in. Since you don't need the fuel to have much mass, you can use fuel with higher energy/mass ratio than you would for a rocket; hydrogen seems to be the preferred choice here. Here's
NASA's press release about the latest X-43A flight. Note it says nothing about using air for fuel, and does say that hydrogen is used as fuel. So, quoting from the original CNN article here, "The "air-breathing" jet was not bogged down with heavy fuel tanks." That doesn't mean there was no fuel, just that there was much less of it than there would have been for a rocket.
Most puzzling, why are we still talking about the record high gas prices, dependence on Middle East, and pollution? Well there are people working on figuring out how to use hydrogen as an alternate fuel for, say, an automobile. But they're not there yet. It's expensive to create (electrolysis is fairly simple, but it takes a lot of electricity to get a little hydrogen). And it's difficult to store safely; as was discovered in the
past. It would certainly be cool if we can move away from using fossil fuels, but there are still a few details to work out it seems...
[ March 28, 2004: Message edited by: Jim Yingst ]