The pilots of passenger planes aren't leaving the cockpit for the foreseeable future, but in other applications, aircraft that fly themselves are on the way, and they're going to be a big deal. For folks like the US military, their potential is nearly limitless. They can fly into nasty areas without endangering a pilot. They don't bring along space-consuming stuff like a seat and life support systems and a canopy. They can stay aloft for hours or even days at a time, since there's no human pilot who wastes valuable time by touching down to do things like eat and rest.

Now the US Navy has removed one of the requirements that bring even human-free aircraft back down to Earth: the need for fuel. Last month, it refueled an autonomous plane in midair for the first time.

This is the latest step in a long and laborious series of tests for the Navy's self-flying X47-B. A few years ago, the sleek stealth aircraft made the first aircraft carrier-based arrested landing and catapult launch on the USS George H.W. Bush.

A ring laser gyroscope Honeywell

In this most recent test, conducted off the coast of Maryland and Virginia, the X-47B autonomously flew up to a standard, human-flown Omega K-707 tanker and maneuvered its refueling probe into the tanker's basket, at the end of a refueling arm.

"It's a great testament to aviation," says Tom Hart, vice president for the defense and space business unit at Honeywell, a contractor on the project. Fueling is "a very difficult thing for a [human] pilot to do." In-air refueling means flying right next to another, much bigger aircraft, at hundreds of miles per hour. That's difficult enough without worrying about hundreds of pounds of explosive jet fuel flowing through a straw. A single mistake and millions of dollars worth of hardware (and the pilot's life) could be at risk.

To help keep the plane in exactly the right spot, Honeywell designed an ultra-accurate inertial guidance system for the plane. It's the same idea as the gyroscopes and accelerometers in your iPhone, only much more accurate and much more expensive.

It allows the plane to take an initial bearing through GPS, then uses something called a ring-laser gyroscope to determine speed, attitude, and direction without needing an ongoing GPS fix. That's useful because a GPS can be jammed or blocked by other aircraft (like the refueling tanker).

It's easy to see why the military wants aircraft that can refuel themselves. The drones could fly into contested areas and operate without any risk to human life, or as a part of a force multiplier with human controlled planes.

"This is the first step of what we're going to see," says Hart. "It'll be a very different Navy and Air Force 20 years from now."