(Amazon)

By now, you've probably heard that Amazon has unveiled a new type of delivery drone. It looks nothing like the little octocopter the company debuted two years ago. And it won't be bringing packages to your doorstep anytime soon, for a number of reasons. But the announcement does offer a few insights into Amazon's broader vision for unmanned vehicles.

This vision is shaped by three things: Data, physics and policy. Together, these factors have contributed to a decision by Amazon to all but rule out delivering heavier, bulky packages. And that, in turn, will shape the future of Prime Air.

Let's start with the policy angle. The most important limitation Prime Air faces right now lies in Washington, where federal officials have been developing rules allowing businesses to fly drones in U.S. airspace. The proposed rules from the Federal Aviation Administration would cover unmanned aircraft that weigh up to 55 pounds.

This weight limit effectively sets a design ceiling for any company that wants to start using drones as soon as possible. Rules for heavier drones are likely to come later, but there's no telling how long that might take; the FAA is already behind schedule on rolling out its rules for "small" drones.

Amazon's newest drones come in at 50 pounds, company spokeswoman Kristen Kish confirmed. That leaves Amazon with 5 pounds of wiggle room for a customer package.

This is where the sales data come in. Five pounds is not an arbitrary figure: As many as 86 percent of all Amazon packages weigh 5 pounds or below, said chief executive Jeff Bezos when he first unveiled the Prime Air program. (Disclosure: Bezos owns The Washington Post). The more products Prime Air can possibly deliver, the better — because a costly new service like drone delivery really only works at scale.

So why not try carrying heavier packages? What if they could ship 100 percent of Amazon packages by drone?

Well, for one thing, you probably don't want your new 50-inch flatscreen TV flying through the air, relatively exposed to the elements. What if something happened to it on the way?

Aside from risks to the product itself, there's one relevant constraint we haven't discussed yet: Physics. The heavier your package, the more powerful the aircraft must be to lift it. And if it's flying to rural or suburban destinations — as Amazon's newest drone is meant to do — it'll need lots of energy to cover great distances very quickly. And that means huge, honking batteries.

"I hate batteries," said Marshall Kaplan, a professor of aerospace engineering at the University of Maryland. "They're heavy. When you build a [space] launch vehicle or a satellite, it's the same problem — they're the biggest single weight item."

Counterintuitively, battery issues become even more significant in urban areas, Kaplan added. Even though Amazon intends to fulfill city deliveries with its smaller octocopters, those unmanned aircraft will have to use lots of energy just to lift off and fly around.

"They can't take advantage of aerodynamic lift," said Kaplan, "because they don't have a wing."

Amazon's larger suburban drone has more lift surfaces and a rear propeller that can push it forward. But that still doesn't mean it can support larger, heavy packages. Remember, there's that 55-pound cap to worry about. The only way to meet the FAA's policy and make room for a heavier package is to cut back on the drone's own weight. Even if you use super-light materials for the frame, you're still left with the need for a heavier battery.

No doubt Amazon will be trying to improve on its design. For now, though, the company is only developing drones under 55 pounds — not heavier unmanned aircraft, according to a person familiar with the matter.

That Prime Air will be limited to delivering 5-pound packages raises important questions for consumers. Even if we take Bezos' statistics at face value and the vast majority of the company's parcels fall within that weight class, are the items inside really ones that would truly benefit from half-hour drone delivery? And even if the answer were yes, are they worth paying non-bulk prices for?

This is really Prime Air's make-or-break hypothesis, which will be proven right or wrong, one way or another. Amazon is betting that consumers will fall in love with half-hour delivery just as they did with two-day delivery, one-day delivery and same-day delivery. Given the company's track record here, that could well be a reasonable expectation. Many common household items — toilet paper, laundry detergent, cooking spices — would be even more convenient to buy on Amazon if you could get them on short notice. Imagine all the sudden emergencies you could avert with half-hour delivery. (I can never remember if I have enough cumin at home for my chili.)

At the same time, you only get Amazon's best prices when you buy in large amounts. In the market for Bounty paper towels? That'll be $2 a roll if you buy 24 at once, but a whopping $4.75 per roll if you buy even a fourth as much.

Worse, neither size would be eligible for Prime Air; the smaller package still weighs nearly a pound too much. If you need your paper towels delivered 10 minutes ago, you're better off going with an even smaller package, and probably paying a little more for it.

Of course, Amazon could do what it does for same-day delivery and ship certain items by drone for free to certain markets. But short of that, the forces shaping Prime Air are pushing it to become, essentially, the anti-Costco — delivering individual products at non-bulk prices.