MOJAVE, Calif. – When you walk into the place where Seattle software billionaire Paul Allen’s Stratolaunch Systems is building the world’s biggest airplane, it feels as if you’re stepping into the Starship Enterprise’s construction zone.

“It’s jaw-dropping when you walk into that hangar,” said Chuck Beames, Stratolaunch’s executive director and president of Vulcan Aerospace, during a rare tour last week.

The plane’s wing, taking shape inside a 103,000-square-foot hangar at the Mojave Air and Space Port, stands three stories off the ground and measures 385 feet from tip to tip. That’s three times longer than the distance of the Wright Brothers’ first powered flight in 1903. If the Enterprise is ever built to its “Star Trek” TV dimensions, now or in the 23rd century, the starship would be only a few dozen feet wider.

It doesn’t take long for the numbers – and the view – to boggle the mind. But there’s another side to the Stratolaunch saga: What’s Paul Allen up to? Stratolaunch is designed to serve as a flying platform for sending satellites into orbit, but who will provide the air-launched rockets? What niche will Stratolaunch fill alongside SpaceX, United Launch Alliance, Blue Origin, Virgin Galactic and other space companies?

Like the plane, Paul Allen’s vision isn’t quite ready for its full reveal. But five years after its founding, Stratolaunch Systems is providing glimpses behind the veil.

In a statement, Allen said Stratolaunch and other efforts to expand access to low Earth orbit hold “revolutionary potential” similar to that brought by the PC revolution in the 1980s and the rise of the Web and smartphones in the 1990s.

“When such access to space is routine, innovation will accelerate in ways beyond what we can currently imagine,” the Microsoft co-founder said. “That’s the thing about new platforms: When they become easily available, convenient and affordable, they attract and enable other visionaries and entrepreneurs to realize more new concepts.”

Beames emphasized that Stratolaunch is meant to be a money-maker as well as a manifestation of Allen’s technological vision. He said the air-launch system would be “especially valuable” for sending up hundreds of small networked satellites for communication or Earth imaging.

“It’s not a vanity project for a billionaire,” he told GeekWire after last week’s tour. “We didn’t invent air launch, right? Orbital did air launch, Pegasus, years and years ago.”

Orbital Sciences Corp. (now known as Orbital ATK) pioneered the air-launch system for sending up relatively small satellites, including NASA’s NuSTAR and IRIS probes. Its carrier airplane, a modified Lockheed L-1011 TriStar, is based just down the road from Stratolaunch in Mojave.

“They have a great technical track record of success. But what this does is, it opens it up to more than just that very small payload class,” Beames said.

Stratolaunch’s plane would follow a flight plan similar to Orbital’s: It’s meant to fly over the ocean to an altitude of about 30,000 feet, then release a rocket that would be fired up to send its payload the rest of the way to space. The high-altitude launch would give the rocket a head start to orbit. But perhaps more importantly, the air-launch plane could take off on a mission with little advance notice, fly around bad weather when necessary, and point the rocket toward any orbital inclination.

Beames said Stratolaunch’s plane could launch multiple satellites, and even multiple rockets, during a single flight. “You could go launch, go launch, go launch, turn around and fly home,” he told GeekWire.

The bigger the plane, the bigger the payload capability. Stratolaunch’s twin-fuselage plane, which is currently nicknamed the Roc after a mythical giant bird of prey, is arguably the biggest.

When the plane is loaded up with 550,000 pounds of payload, it’s projected to weigh in at 1.3 million pounds. That’s close to the weight of the world’s most massive airplane, the Ukrainian-made Antonov An-225 cargo transport. And Stratolaunch trumps the An-225 when it comes to wingspan, thanks to its heavy use of lightweight graphite composites.

Stratolaunch purchased two used Boeing 747 jets for additional parts, including six engines (plus two spares), eight sets of landing gear, the seats, avionics and other cockpit equipment.

Some rocketeers, including SpaceX founder Elon Musk, say the advantages of air launch aren’t worth the cost. Paul Allen is betting that they are, to the tune of hundreds of millions of dollars.

Beames declined to say how much Allen is investing in Stratolaunch. But he did say it’s more than the estimated $28 million that Allen put into the SpaceShipOne rocket venture more than a decade ago. When Allen unveiled Stratolaunch in 2011, he said he was willing to spend at least 10 times as much as he did on SpaceShipOne. That suggests an investment of $250 million or more.

What has all that investment bought? Beames and other Stratolaunch team members showed journalists from GeekWire and other media outlets around the Mojave facilities last week for a sneak peek.

More than 300 people work inside the hangar and a composite fabrication building next door. Most of them work not for Stratolaunch, but for Scaled Composites, the Mojave-based venture that built SpaceShipOne. Scaled is in charge of building and testing the airplane for Stratolaunch – just as they built and tested SpaceShipOne with Allen’s backing, and just as they built and tested the first SpaceShipTwo for Virgin Galactic.

By weight, half of the plane’s structure is made of graphite composite, which is Scaled’s specialty. The other half, including the engines, is coming from the 747s.

Stratolaunch says all of the composite pieces have now been fabricated, and assembly is 76 percent complete. During the tour, one team of workers was installing composite side panels on the plane’s left fuselage. Other workers were getting ready to put together the tail for the right fuselage. Still others were grinding away on composites, creating a din that sounded like a dentist’s office with the volume knob turned up.

Much of the electrical and hydraulic installation still has to be done. By the time the plane is finished, workers will have hooked up 100 miles of wiring and a mile’s worth of control cables.

So when will it be finished? That’s a tricky question.

When the project was announced, Stratolaunch said the plane would start flight tests in 2015 and start launching rockets in 2016. Today, Beames shies away from giving a schedule, other than to say that Allen wants to have Stratolaunch sending payloads to orbit by the end of the decade.

“Come hell or high water, you can be sure that we’re going to do that. We’re on track to do that,” Beames said.

During the first phase of operations, Stratolaunch plans to execute one mission per month from Mojave. But the plane is designed to operate from any airport with a 12,500-foot runway, which should dramatically widen the options for air launch.

Beames acknowledged that some have wondered whether the plane would ever enter service. “The skeptics will be pleasantly surprised when we roll this thing out,” he told GeekWire.

Much of the skepticism focuses on whether Stratolaunch can recruit the partners and customers it needs for success.

When the project was announced, the company joined forces with SpaceX. The plan was to use a scaled-down version of the Falcon 9 rocket, with five or six rocket engines on the first stage rather than the usual nine. The launch vehicle could have sent 13,500-pound payloads to low Earth orbit, or 4,500-pound satellites to geostationary transfer orbit.

SpaceX dropped out a year later, saying that the partnership didn’t fit its strategy. Then Stratolaunch teamed up with Orbital ATK and Sierra Nevada Corp. Last year, those partnerships were put on hold as well.

Beames acknowledged that Stratolaunch’s business strategy has been affected by the rapid miniaturization of satellite technology.

We have a business plan, a strategy that Paul has approved.

Today, 20-pound nanosatellites like the ones being built by Planetary Resources can take on jobs that once required much heavier spacecraft. For Allen and the rest of the Stratolaunch team, that’s a good thing. “Paul’s whole thing is to nourish the mavericks,” Beames said. But the shift has required some rethinking, and that has contributed to Stratolaunch’s stretched-out schedule.

During last week’s tour, Beames signaled that the rethinking is finished. “We have a business plan, a strategy that Paul has approved,” he said.

The plan calls for a variety of launch providers and business partners that are suited for a wide range of space services. Beames declined to name any partners, but he said as much as half of Stratolaunch’s business could come from the military, NASA and other federal agencies.

The business plan also relies on synergies with other parts of Paul Allen’s holding company, Vulcan Inc. For example, Vulcan Capital is investing in Seattle-based Spaceflight Industries, which manages payload logistics for multiple launch vehicles and has its own Earth-imaging satellite subsidiary. That puts Spaceflight high on the list of Stratolaunch’s potential partners.

Beames said Stratolaunch and its partners are likely to synergize with Allen’s other ventures, running the gamut from wildlife monitoring to the fight against Ebola and other infectious diseases. “There’s an aerospace component to all of them,” he told GeekWire.

So what’s the bottom line? The way Beames sees it, Stratolaunch Systems has to be about more than just building the world’s biggest airplane.

“I have to remind the boss about this sometimes, too,” he said. “It’s more than just an airplane. It’s more than just an airplane and a rocket. It’s a very complex economic system, if you will.”

Yes, the nearly built airplane looks impressive. But if Stratolaunch builds a business to make use of it, that will be at least as impressive a feat.

Vulcan Aerospace’s Chuck Beames is due to give one of the keynotes at this week’s NewSpace 2016 conference in Seattle. Check back with GeekWire for coverage of NewSpace 2016.

Update for 4:20 p.m. PT June 20: An earlier version of this story reported incorrectly that the Antonov An-225 was Russian-made rather than Ukrainian-made.