Start with the largest aircraft ever built, with a wingspan longer than a football field and a split fuselage fitted with six Boeing 747 jet engines -- enough thrust to get 1.3 million pounds off the ground, about 425,000 pounds more than a fully loaded 747. Sling a 120-foot, three-stage rocket below the aircraft, and when the plane reaches 30,000 feet, fire the rocket into space. Then the plane flies back to Earth.
Microsoft co-founder Paul Allen calls his newest venture Stratolaunch, a system designed to lift 13,500-pound payloads -- satellites, science experiments, cargo and, eventually, humans -- into low-Earth orbit, where the space shuttle used to fly and where the international space station still dwells. Construction of the aircraft is under way in California, with test flights planned for the end of 2016 and the first mission to occur in late 2017 or early 2018.
"You have a certain number of dreams in your life that you want to fulfill, and this is a dream I am very excited about seeing come to fruition," Allen said at a news conference in late 2011 to announce Stratolaunch. He said he sought to take advantage of "a much-expanded opportunity" for private enterprise now that NASA is focusing on deep space missions. This could lead to "a radical change in the space launch industry."
Paul Ghaffari, chief investment officer for Vulcan Capital, an arm of Allen's Seattle-based firm, said Stratolaunch is a medium-size system that has no real competitors now but even in the future should have "unique advantages" over ground-based rivals, including the ability to launch in inclement weather, to fly without worrying about the availability of launch pads and to operate from different locations. Ghaffari said Stratolaunch hopes ultimately to host six to 10 missions per year.
Allen, like PayPal founder Elon Musk, whose Dragon spacecraft has already docked with the international space station, and real estate developer Robert Bigelow, whose two inflatable Genesis spacecraft have been orbiting the Earth for six and seven years respectively, is hoping to turn low-Earth orbit into a commercial moneymaker, now that NASA is focusing on longer-distance exploration missions.
But will Stratolaunch really take off?
Space is expensive, and all aspiring entrepreneurs must eventually decide whether there will be enough demand for launches to enable them to recoup their investment. Allen, Stratolaunch's sole funder, has not discussed his expenditures.
"The first reaction is skepticism, because this is an immense airplane and a medium-sized rocket, and there will be competitors," said John Logsdon, former director of the Space Policy Institute at George Washington University. "But then I think, 'These are awfully smart people, and they're technically sound, and they must see something.' "
The design and engineering of Stratolaunch relies heavily on two firms -- Dulles, Va.-based Orbital Sciences and Scaled Composites of Mojave, Calif. -- that have outstanding credentials as aerospace innovators.
Ghaffari said Allen nursed the idea for Stratolaunch for nearly two decades and began studying it in earnest in conversations with Scaled Composites founder Burt Rutan around 2000. With Allen as financial backer, Rutan's SpaceShipOne was launched from an aircraft in 2004 to become the first piloted civilian spaceship to make a suborbital flight above Earth's atmosphere.
SpaceShipOne was regarded then primarily as proof-of-concept for the viability of space tourism at relatively affordable prices, and Scaled Composites is building a passenger spacecraft with funding from Virgin Group founder Richard Branson. Ghaffari, however, said Stratolaunch was always foremost in Allen's thoughts: "SpaceShipOne was definitely a predecessor," he said. Rutan has retired from Scaled Composites, but he is a Stratolaunch board member, as is former NASA Administrator Mike Griffin.
Orbital Sciences had been consulting for Stratolaunch since last year and formally joined the project in May. Orbital is the developer of Pegasus, the first air-launched civilian spacecraft able to put small payloads into low-Earth orbit. Orbital Sciences spokesman Barron Beneski said proven expertise in air launch was one of the "connecting fibers" that linked the company to Stratolaunch.
Stratolaunch has only two employees -- Chief Executive Officer Gary L. Wentz Jr. and Chief Operating Officer Susan G. Turner -- but Wentz estimated that close to 200 contractors are working on design and construction.
Wentz said project engineers are "looking at a multitude of potential challenges." The host aircraft, with a 385-foot wingspan, will be made of composite materials, and the spacecraft will have three stages. The plane will drop the spacecraft at 30,000 feet and initiate a banked turn. Rocket ignition will come four to seven seconds later to give the aircraft enough time to get out of the way of the back blast. As for the rocket, the first stage, which is to be reusable, drops into the ocean; the second burns up in the atmosphere; the third puts the satellite into orbit, and eventually that third stage, too, burns up in the atmosphere.
Wentz, speaking from Stratolaunch's headquarters in Huntsville, Ala., said the team has not yet decided whether the spacecraft will use liquid or solid fuel, and planners are still scouting airports for a likely base. Stratolaunch needs about two miles of runway to take off -- about 50 percent more than any other commercial aircraft.
Wentz said the Kennedy Space Center's Shuttle Landing Facility is the leading candidate, in part because infrastructure for both solid and liquid fuel is already in place. Planners are also looking at the Vandenberg and Edwards air force bases in California and at NASA's Wallops Island Flight Facility in Virginia. The aircraft must also have access to other large airports in case of an emergency landing, and the company will need to obtain a commercial launch license from the Federal Aviation Administration.
Stratolaunch's logistics are fairly simple and should offer considerable advantages over ground-launched competitors. Missions need not be automatically scrubbed because of bad weather, and the aircraftwould be able to launch its rocket above the clouds from almost any location. Also, clients would not have to wait in line to fly from oversubscribed terrestrial launch pads.
The physics of airborne space launches, by contrast, are elaborate, with many variables that affect the cost, frequency and reliability of flights. Wentz said the Stratolaunch team has not made all the calculations but expects the company will realize significant savings by "tweaking" the variables. Ground-based launches are just as complicated as air launches, but the science is much better understood.
One of Cape Canaveral's attractions is that it is on the East Coast, next to the Atlantic Ocean. It is always desirable to launch to the east to capitalize on the direction of the Earth's spin. The Earth travels about 1,000 mph west to east at the equator; you need to reach a speed of 17,000 mph to get to low-Earth orbit, so there's no point in penalizing yourself 1,000 mph by heading in the wrong direction. Wentz said the team has also considered alternative sites closer to the equator, where this effect is more pronounced.
Also, a Stratolaunch flight would climb out of the first 30,000 feet of the Earth's gravity well on jet fuel instead of much more expensive rocket fuel, and the spacecraft would be released in rarefied atmosphere with high tail winds to kick it forward.
Finally, Wentz said, the aircraft would be traveling nearly 300 mph when it launches the spacecraft, and "the goal is to maintain as much of the forward velocity as possible." Traveling downrange, the rocket would ignite and rise at an angle of 12 to 15 degrees. The aggregate effect of all these forces is similar to the acceleration a javelin thrower gets by sprinting down the runway before throwing his spear. While the savings in energy over a ground launch is only 5 to 10 percent, Rutan said at the 2011 news conference, "this is huge" for space travel.