The X-51A has been under development for approximately six years.
So far the testing of this device has been hit-or-miss. The first one flew 200 seconds. The second only a few seconds. The final of the first three ordered will fly today. Hopefully the third test will be successful.
The X51-A uses a scramjet engine. This is basically a highly optimized tube with few or no moving parts. As air enters its accelerated to supersonic speed. Fuel is injected and ignition takes place.
As you can see from this diagram its an extremely simple device.
Below is a test flight video of the first flight:
According to this the X-51A is powered by an "SJY61 Pratt & Whitney Rocketdyne scramjet engine, which is capable of producing between 400 to 1000 lbs of thrust."
So if you had something like this what could you do with it?
DARPA says that this is the precursor to a mach 20 craft capable of traveling over 15,000 miles per hour. Since the earth is only 25,000 miles in diameter this means that such a craft can arrive anywhere in the world in less than an hour.
You could make it a cruse missile or attach a manned capsule for trips into space.
One thing that's important to realize is that US Defense programs like this allow the United States to leapfrog other countries in terms of technology. Rather than building me-too kinds of craft we can actually lead.
I think this comes from the countries focus on science and engineering in the 1960's.
Unlike the social upheavals of the same era the focus on technology was an investment that still pays dividends today for our country.
Sadly I think that this focus is now dying.
Just look at the commercials for technology on TV today - mostly old geezers telling how they were "inspired" years ago to do something.
No young people.
And if there are some, they seem technologically illiterate.