On June 21, the FAA finalized rules for commercial UAVs, or drones. While commercial applications for drones vary from aerial photography to package delivery, especially exciting is Amazon's Prime Air, promising to deliver a five-pound package to your doorstep (or back yard) in 30 minutes or less.

The new FAA regulations currently prevent this application, citing a requirement for line-of-sight control (the operator has to be able to see the drone first-hand throughout the entire flight path). This is a safety requirement created to prevent collisions with airplanes, power lines, or people and property on the ground. In my opinion, this restriction will eventually be lifted as detect-and-avoid, GPS navigation, and remote-control technologies continue to improve to the point we will trust them, just like we have learned to trust auto-pilots and auto-landing systems, and will soon be comfortable with self-driving cars.

However, Amazon's Prime Air drone delivery plan of flying a five-pound package within 30 minutes to your house violates other laws: the laws of physics. Here is what nobody ever talks about.

I've been a drone hobbyist for three decades now, and served in the U.S. Air Force Civil Air Patrol Auxiliary as a transport mission pilot and aerospace education officer. Last year, I applied for (and received) a grant from the Air Force Association to start a UAV/drone program in our squadron. I purchased the drone, and started teaching cadets about it. The drone's specifications indicated that it was good for up to 20 minutes of total flight time. Other drones were rated with shorter flight times. While racing drones can reach speeds of 80 mph (but for only about 2-3 minutes before the batteries die), "payload" drones will rarely reach speeds faster than 40 mph, and even less when they are carrying a payload (such as a package).

My first investigation was aimed at understanding why the drone flight time was limited to 20 minutes. Being an engineer, I developed the math for it. It is based on a few known characteristics of the current state of technology. Most drones use electric motors and batteries. In my research, I found that a battery typically holds a capacity of 65Wh (Watt-Hour) for every 1 pound of battery weight. The "hover" or cruise speed power requirement for a drone is 100W for every 1 pound of overall weight (drone + batteries + payload), while it requires 200 W/lb to climb or fly at speed. Finally, the power system (motor + speed controller) delivers 1,000W for every 1 pound of drone weight (not including batteries or payload). I checked the performance specifications for many different sizes and manufacturers of electric motors and batteries, and found that the numbers above were very consistent.

I don't want to bore you with the math, so I'll skip right to the conclusion. When you do the calculations, you find that it results in the following:

For a 30-minute flight, a drone's overall weight (drone + batteries + package) must be 20 times that of the package alone. The batteries' weight accounts for most of that.

For a five-minute flight, the overall weight has to be only 1.5 times that of the package.

The drone will not be able to fly more than 32 minutes, and at that time, it will not be able to carry any package whatsoever.

So when Amazon tells you they will be able to deliver a five-pound package to you in 30 minutes, they really mean that they will not be able to fly the drone for more than 15 minutes to you, and 15 minutes back (including takeoff and landing time), at speeds of no more than 40 mph, and not until they have a drone that weighs 20 times what the package weighs. Delivering a five-pound package will require a 100-pound drone. The FAA restricts drone weight to 55 pounds, which means one that can fly 15 minutes to you and 15 minutes back with only a 2.75-pound package.

Furthermore, 15 minutes (assuming no time was spent on takeoff or landing) at 40 mph means that the Amazon warehouse location has to be within 10 miles of your home. Currently, Amazon has nine fulfillment centers in California, covering 163,696 square miles. To cover 100 percent of California with centers that are within a 10 miles radius of the customer, they need 521 centers, and 855 centers to cover Texas's 268,597 square miles, up from their current six. Of course, Amazon may decide that the service is available only to much smaller coverage areas, and not to everyone.