Aerospace giant Boeing just snagged a $6.6 million contract to design a cheap, reusable spaceplane for the U.S. military. The idea: to equip America’s space forces with an airplane-like vehicle that can fly to the edge of Earth’s atmosphere and quickly boost small satellites into orbit, and then land, refuel, load up another satellite, and take off again within 24 hours.

The so-called XS-1 program—short for “eXperimental Spaceplane 1”—isn’t a space weapon. Instead, it’s a sort of defense against space weapons—specifically, the growing fleets of killer spacecraft and satellite-destroying rockets that China and Russia are deploying.

U.S. military planners fully expect that, in any future conflict between major world powers, Earth’s orbit will become a battleground as laser-armed satellites stalk each other across orbital planes and ground- and ship-launched rockets lance into space to smash enemy spacecraft.

The country that can recover fastest from the initial orbital carnage stands to dominate space, the ultimate high ground in any high-tech battle. “In an era of declining budgets and adversaries’ evolving capabilities, quick, affordable, and routine access to space is increasingly critical for both national and economic security,” DARPA stated in a press release.

That’s where the XS-1 comes in. DARPA wants the new spaceplane to be able to boost a two-ton satellite into space every day for 10 days straight for less than $5 million per flight.

That’s a hell of a lot faster, and cheaper, than today’s launches, which can cost hundreds of millions of dollars and take years of planning. XS-1 could “create a new paradigm for more routine, responsive and affordable space operations,” according to DARPA.

Three XS-1s carrying a single satellite per trip and working at max speed could, in theory, replenish practically the Pentagon’s entire satellite constellation in a couple of weeks—and faster if each spaceplane carries more than one satellite at a time.

Of course, that assumes that contractors can build fresh spacecraft fast enough to keep up with the XS-1s’ busy launch schedules. To that end, the military is also working hard on simpler, smaller satellites that it can produce quickly and cheaply.

Conceptually, the robotic XS-1 is elegant in its simplicity. It’s basically just a high- and fast-flying drone that can lend a single-stage rocket speed and altitude, making it easier for the rocket (and its satellite payload) to escape Earth’s gravity.

“Our design would allow the autonomous booster to carry the second stage and payload to high altitude and deploy them into space,” Will Hampton, Boeing’s XS-1 program manager, said in a company press release. “The booster would then return to Earth, where it could be quickly prepared for the next flight by applying operation and maintenance principles similar to modern aircraft.”

In effect, the XS-1 replaces the biggest, priciest main stage of a single-use rocket, while saving money by being reusable. You buy the XS-1 once and use it over and over, paying only for fuel and spare parts for each flight.

Boeing’s concept art depicts a cigar-shaped airframe featuring a bulbous nose, tiny wings, and big engine nozzles for a powerful motor. In its basic outline, the XS-1 could wind up looking a lot like a miniature version of the Space Shuttle, which NASA retired in 2011, or a bigger take on the X-37B robotic spaceplanes that Boeing built for the Air Force a few years ago.

The X-37B—the objective of years’ worth of conspiracy theories—is positively diminutive at just 29 feet in length. DARPA has compared the XS-1 to an F-15 fighter, which is 64 feet long.

But the XS-1 wouldn’t just be bigger than today’s tiny spaceplane. While the X-37B features internal cargo bays with hinging doors, the XS-1, by contrast, could carry its payload—a single-stage rocket with a satellite attached—on its back. The X-37B, like the Space Shuttle before it, is an orbiter that boosts into space atop a rocket. In function, the XS-1 is more akin to the rocket than the orbiter—and only to the rocket’s initial stage, as it would only ever climb to a height of 70 miles or so, still within the atmosphere.

Which is not to say the XS-1 is any less sophisticated than the Space Shuttle and X-37B are. What the government is asking the XS-1 to do is hard. Especially doing it safely and cheaply. To give its payload the energy it needs to escape gravity, the XS-1 will need to accelerate to Mach 10—“hypersonic” speed.

By comparison, aerospace mogul Richard Branson’s SpaceShipTwo, a rocket-powered suborbital spaceplane that could also fly nearly 70 miles high, topped out at Mach 2 before its fatal crash in October 2014. No fewer than half of the hypersonic drones that the Pentagon has tested in recent years have also crashed. (And those have only gone Mach 4 or 5.) Russia and China have had even less luck developing aircraft that can withstand the stresses of hypersonic flight. “The work is not easy,” Boris Obnosov, then the head of Russia’s missile programs, said of his country’s high-Mach efforts in 2013.

Boeing beat out two other companies to snag the recent XS-1 contract. Starting in the summer of 2014, Masten Space Systems and Northrop Grumman had also drawn up XS-1 blueprints. DARPA awarded the three firms $4 million apiece to do that preliminary design work. But Boeing’s success building X-37Bs for the Air Force apparently helped the Chicago-based planemaker win the follow-on contract.

And it didn’t hurt that Boeing enlisted Washington State-based rocket start-up Blue Origin to help with the XS-1’s motor. Founded by Amazon billionaire Jeff Bezos, Blue Origin is working on reusable space rockets that take off and land vertically. It appears Boeing wants to modify Blue Origin’s BE-4 to power the XS-1. Capable of producing more than half a million pounds of thrust, the BE-4 is a monster of an engine.

The next step for Boeing is to complete its XS-1 design and test its basic technologies—all before August 2016. DARPA wants an XS-1 prototype to perform a realistic trial mission no later than 2019. After that, the Pentagon could decide to build XS-1s for regular use.

It’s not clear how much the spaceplanes might cost. The two X-37Bs set the government back around a billion dollars apiece.

A billion bucks or more per XS-1 might seem like a lot, but it’s a small part of what the United States spends in space every year. Counting NASA’s $18 billion budget, the roughly $8 billion the Pentagon drops on rockets and satellites plus space spending by private companies, America invests $40 billion a year in orbit, more than the rest of the world combined. The United States’ more than 400 satellites and spaceplanes represent nearly half the world’s active spacecraft.

“The U.S. has much more invested in space and depends on it for communications, economic and military dimensions much more than everyone else,” said Dr. Laura Grego, a space expert with the Union of Concerned Scientists.

And that investment is fragile because satellites are fragile, Grego wrote in a recent blog post. “The truth is that it is much easier to attack [satellites] than to defend them.”

With its spaceplanes, maneuvering satellites, and surface-launched missiles, the United States is by far the world leader when it comes to destroying orbiting spacecraft, but that offensive capability doesn’t actually do much to protect America’s own satellites from rival space weapons.

In 2007, China blasted one of its own defunct satellites with a rocket, proving it could do the same to another country’s spacecraft. And over the past 18 months, Russia has sneaked three small, highly-maneuverable spacecraft into low orbit aboard rockets carrying communications satellites. “There’s some possibility it’s an anti-satellite system,” said Anatoly Zack, a space historian who closely tracks Russian activity in orbit. Equipped with lasers or explosives, the nimble little spacecraft could sneak up on and disable American satellites.

And if that happens, the Pentagon will have to scramble to restore its orbital infrastructure. XS-1s could take off every 24 hours, boosting a fresh satellite—or many satellites—into orbit at the apogee of each flight. To cut down on the time and money it takes to build new spacecraft, in 2007 the Pentagon established a new “Operationally Responsive Space” organization, now headquartered at an Air Force base in New Mexico.

ORS spends roughly $100 million a year designing comparatively inexpensive satellites—and helping other military organizations do the same. In November 2013, ORS launched a Minotaur rocket from Virginia containing a record-setting 29 satellites in its nose cone. Each of the small “CubeSat” spacecraft, named for their 4-inches-cubed dimensions, weighed just 3 pounds and cost no more than $100,000.

“Take the same microprocessors, GPS units, cameras, modems and radio equipment that we use in smartphones and put them in a satellite body instead,” wrote Major Ethan Mattox, a U.S. Special Operations Command space official. “Add the appropriate software, boost it into orbit and voila—you’ve built a satellite tailored for a specific mission fast and cheap.” Sure, a CubeSat lasts only three or four years, but even with such a short lifespan, the tiny satellites are a still a bargain compared to billion-dollar spacecraft that last much longer.

Combine CubeSats with XS-1s and you’ve got a great way of putting satellites into orbit quickly and cheaply, preserving America’s foothold in space even if an enemy is shooting down spacecraft. It’s not for no reason that Jess Sponable, DARPA’s XS-1 program manager, called the spaceplane a “game-changer.” (PDF)