After more than 22 months, the US Air Force's secret space plane is expected to land in California on Tuesday at Vandenberg Air Force Base. This will mark the completion of the third test flight of the X-37B which, like the Space Shuttle, is a reusable space plane that takes off like a rocket but lands like an aircraft. Unlike the Space Shuttle, the X-37B is quite a mystery.

Space experts and satellite watchers alike have been trying to figure out the purpose of this vehicle and program for years. A few claim it's a bomber. Others have suggested that it's designed to steal or take out enemy satellites. Perhaps a more plausible theory is that it may be a precursor to a specialized kind of reconnaissance satellite. But nobody really knows for sure what it's for.

At the most superficial level, it's straightforward: it's a spaceship that goes into space, orbits, lands, and gets refurbished before launching again. But space flight is such an enormously demanding and expensive endeavor that committing significant resources to projects like X-37B went out of favor decades ago. So why go to all that trouble?

Even more than six decades after launching a satellite into orbit, space flight is constantly pushing our technological limits and is still just barely this side of possible. Launches involve expending colossal amounts of energy in a drawn-out explosion, taunting catastrophic failure. Space environments are utterly unforgiving. Reentry is like trying to bring an out-of-control car to a screeching halt by slamming it into a wall of lava. Then, heaven forbid, after slamming, shaking, baking, freezing, irradiating, burning, and landing a spaceship — a process that tests the limits of its design and materials — somebody decides they want to take that same poor vehicle and run it through the wringer again and again?

At the end of the day, these extreme challenges mean that visiting space is very difficult and expensive. Mission planners are loath to add any requirement that isn't essential. Each new set of tasks incurs a whole new set of complications, challenges, and costs. While there might be an occasion here and there where an already-successful mission might be extended a little bit, or a small bonus sections added, there's absolutely no way that any space program will add an entire new capability on a whim.

It's like major surgery. Even with all the best in medical technology and surgical techniques, operations like heart transplants are still difficult and risky, but feasible. Arbitrarily adding a spurious reusability requirement to a spacecraft is like doctors deciding during a heart transplant that they might as well toss in some freebie, spur-of-the-moment brain surgery, because YOLO.

This is the issue that makes the X-37B such a puzzle for observers. Of the four types of technological hurdles listed above — launch, space operations, reentry, and reuse — the latter is the biggest jump. Making the X-37B reusable seems like an unneeded flourish, while the design of the X-37B as a spaceplane, rather than a ballistic capsule, is an even more vexing addition.

Apparently the Powers That Be have opted for a series of additional requirements, yet nobody really, truly knows why on Earth — or in space — anyone would need to fulfill this specific set of requirements. Nearly everything you can think to do with X-37B can be done more easily and cheaply some other way.

The Air Force will, to its credit, say something about the X-37B: "The primary objectives of the X-37B are twofold: reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth."

But that's just tantalizingly close enough to a big, deep dive into the point of the X-37B that it sends some space-industry watchers into a tizzy of speculation. Of course, this is what it does, but why go to all the trouble and expense to do that?

The US military already sends a whole slew of experiments into space to study the space environment, develop new technologies, and figure out new uses for spacecraft. All this is pretty straightforward and generally makes sense. They want to test space technology, so they do it in the easiest way possible. They only need to clear two major operational hurdles to get what they need: launch and operating in space. No need for reentry because there's no need to retrieve the experiment once the data is sent back.

There are some kinds of experiments where it is very important to get the samples back to Earth. For instance, this series of tests (Materials International Space Station Experiment 5, 6A and 6B, 7, 8, and 8 FSE) which were run aboard the ISS involved long exposures to space and return of samples to Earth.

With the retirement of the Space Shuttle in 2011, the only way for the US to run a long-duration experiment in space and get the sample back to Earth is on the ISS. So it might be that the X-37B is simply another way to run those same kinds of experiments, but do it without letting everyone else know what's going on.

This sounds fairly plausible, but still doesn't explain why the Pentagon would bother doing this with a spaceplane, rather than a capsule. Russia, among other countries, has used uncrewed space capsules for scientific research, like the Bion series for biological experiments. The US has an operational capsule in SpaceX's Dragon. Why should the US screw around with trying to add wings and aerodynamics, when all they do is add weight and complexity?

Well, part of the answer might actually tie back to the fact that doing stuff in space is difficult and expensive. If all that equipment is terribly costly and delicate, throwing it away after a single use seems wasteful. The choice is between spending more money and time up front to make something reusable, versus going for cheaper, disposable usage. Is it better to use a mug or a paper cup? That depends on how much each costs, how much money you have to spend, whether it can be washed, and a whole host of other issues.

The Space Shuttle was an attempt at reusability that didn't work out nearly as well as the earliest, wildly optimistic hopes that it would operate just like a commercial airliner. So there could definitely be something to the Air Force's claims about improving the technology of reusability. Maybe it's just a matter of running two sets of experiments and getting twice as much information from each mission: one set is in the payload bay, while the other experiment is the vehicle itself.

This is starting to get deeper into the realm of plausibility, but it still doesn't really seem to seal the deal. It's nice that the Air Force would want to do all that general technology development, but there currently aren't any big plans to implement that kind of reusability in a spacecraft.

But there is one area in the Air Force that is getting money. It's the new hot and sexy trend for those in love with the technological bleeding edge: hypersonics. As other countries chip away at the stealth advantage, there's been more and more interest in going faster and faster. If stealth technology is based on the idea that you can't hit what you can't see, hypersonics go with the idea that you can't hit what you can't catch. A hypersonic missile could reach out and hit a target very quickly, not only evading defenses but nailing elusive targets before they have a chance to get away.

However, hypersonic (speeds of Mach 5 and higher) is a really difficult operating regime. For instance, a 2011 DARPA test failed because the vehicle's own skin peeled off. There's also the obvious challenge of keeping the flame in a jet engine lit in a 4,000-mph airflow.

For engineers trying to make hypersonic vehicles, it's even more frustrating: it's not just difficult to solve these challenges, it's difficult to even make a wind tunnel that can reach hypersonic speeds. While the world's fastest wind tunnel can generate speeds of up to Mach 30, it only can manage to do that for about 1/500th of a second.

So, maybe the deal sweetener for the X-37B really is what's being advertised: a hypersonic test bed with bonus space experiments thrown in for fun. The things being tested are the things that make it stand out as unusual: wings, flaps, rudders, thermal tiles, cargo bay doors, and all that.

Of course this is all speculation, but it could just be that the secrecy surrounding X-37B is the sneakiest bit at all. It has invited observers to refrain from judging a book by its cover, when that is precisely where the entire story can really be found.

Follow Ryan Faith on Twitter: @Operation_Ryan