Microsoft co-founder Paul Allen raised a lot of eyebrows with his plan to build the largest airplane ever, then use it to launch rockets into space. But as wild as the idea of a six-engine airplane carrying a multi-stage rocket may be, it is evolutionary, not revolutionary.
Air-launched rockets have been around for more than 60 years, and airplanes have been launching payloads into orbit since the 1990s. Even Burt Rutan, the legendary aerospace designer working with Allen on Stratolaunch Systems, has a history with the technique. He designed the wing for an air launched rocket back in the 1980s as well as SpaceShipOne and its mother ship White Knight winning the X-Prize in 2004.
The only difference is the scale. Stratolaunch is taking air launches to a whole new level.
Allen and Rutan have proposed building an aircraft that features six Boeing 747 engines and a wingspan of 385 feet — more than 120 feet wider than an Airbus A380, currently the largest commercial passenger plane in service. It’s nearly 100 feet more wingspan than the Antonov An-225, the world’s largest airplane. The airplane will have a gross weight of 1.2 million pounds, including a 490,000-pound booster rocket being developed by SpaceX. The mothership will fly to an altitude of about 30,000 feet, then release the rocket. The aircraft will be designed and built by Scaled Composites.
Allen, the billionaire co-founder of Microsoft, joins super-wealthy entrepreneurs like Elon Musk, Richard Branson and Jeff Bezos in looking to the heavens for his next venture, as NASA turns to the private sector for help getting to space.
Stratolaunch is easily among the most ambitious proposals. But the idea behind it dates to the early days of aviation, when airships launched biplane fighters toward the end of World War I.
Then, as now, the idea was to maximize range, or payload, while minimizing the amount of fuel needed for a mission. In the earliest days of aviation, airplanes simply could not carry enough fuel for long flights in battle. These days, it’s about needing less fuel and optimizing a design for delivering a payload to orbit.
One of the biggest challenges to putting things into low earth orbit is the amount of energy required to get there. The International Space Station orbits around 200 to 250 miles above the earth. Like the small biplanes of the early 20th century, a space vehicle would need less fuel for its mission if it could be carried even a small percentage of the way to orbit by the relatively more efficient aircraft. Expendable rockets require enormous amounts of fuel in order to put a relatively small payload into low earth orbit — the payload may be as little as 1 to 3.5 percent of the vehicle’s launch weight.
Carrying a rocket to high altitude means it needs less fuel, thereby saving weight and money. Much of the fuel needed to launch a rocket is needed just to get above the dense lower levels of the atmosphere. At 30,000 feet more than half of the density of the atmosphere would be below the rocket. Beyond saving fuel, air-launching a rocket allows engineers to design more efficient rocket nozzles because they’re operating in the thinner parts of the atmosphere.
There also is a slight reduction in gravitational force at higher altitudes, and some of the velocity needed to achieve orbit is provided by the launch vehicle’s forward motion.
Granted, many of the benefits offered by air-launched vehicles are small, but they add up. As a result, getting into orbit is a little easier, and cheaper when you make an airplane the first stage of a multi-stage system to deliver payloads into orbit.
Another big advantage of using an airplane as a launch platform is the ability to launch from almost anywhere. There is no need to build a specialized, and expensive, launch facility with launch pads and other equipment familiar to anyone who’s seen Cape Canaveral. This makes it easier to take advantage of weather or optimal launch sites, such as equatorial locations that can further reduce the energy needed to achieve orbit.
Here, too, Allen and Rutan are looking to the past in building Stratolaunch.
From the earliest days of rocket-powered aviation, the goals have been top speed and highest altitude. To avoid having to carry the extra fuel (aka, weight) needed to achieve the altitude needed for test flights at the dawn of the rocket age, experimental rocket aircraft were carried aloft by larger airplanes. On October 14, 1947 a rocket-powered airplane carried by a Boeing B-29 bomber made history when Chuck Yeager flew the Bell X-1 beyond the speed of sound. Air-launched rockets allowed test pilots to inch their way toward space through the 1940s and 1950s.
By the late 1960s, NASA and the U.S. Air Force were collaborating on the X-15. The X-15 made multiple air-launched flights to sub-orbital space and pushed the boundaries of hypersonic flight. But when the X-15 program ended in 1969, so to did the idea of air-launched vehicles. The Saturn V and Soyuz rockets took over the duties of reaching space, joined later by the Space Shuttle.
During the 1980s, Dr. Antonio Elias began working on a new air launched space vehicle that could use an airliner as its launch platform. The Pegasus rocket was tested in 1990 by the same NASA Boeing B-52, “Balls 8,” that carried the X-15. Pegasus, featuring a delta wing designed by Rutan, could deliver a relatively small payload of about 1,000 pounds to low earth orbit. Once testing was finished, Orbital Sciences used a former Air Canada Lockheed L-1011 to carry Pegasus to altitude and launch it into orbit.
Orbital Sciences’ Lockheed L-1011 drops the Pegasus rocket on an orbital mission.
The L-1011 has flown 33 missions with the Pegasus rockets (the first seven were flown with the B-52). After a few launch failures early on, the system has a perfect record since 1996. It has delivered more than 80 satellites to orbit.
Even now, Allen isn’t alone in pursuing the idea.
DARPA is investigating the possibility of using an off-the-shelf airplane to deliver small payloads to orbit. The idea is to make it far cheaper to put a 100 pound payload in space using something as small as a business jet or fighter jet as the launch platform.
There have been several other ideas over the years, including Boeing’s Air Launch which was to use a 747 as the carrier aircraft. A British concept called Interim HOTOL would use Antonov An-225, currently the world’s largest aircraft, as the carrier aircraft by adding two more engines for a total of eight. There even have been studies looking into the possibility of towing a spacecraft like a glider or even carrying a rocket in the cargo hold of an airplane and pushing it out the back.
But Virgin Galactic may have the most high-profile air-launch. The Scaled Composites follow on to Rutan’s SpaceShipOne is SpaceShipTwo, a sub-orbital spacecraft similar in concept to the X-15.
This time around, Scaled Composites and Rutan are thinking even bigger. In classic Rutan style, Stratolaunch will use engines, landing gear, cockpit items and other parts from a pair of used Boeing 747s it has purchased in order to reduce development costs.
Designed by engineers at Scaled Composites, the massive Stratolaunch carrier aircraft will have a range of 1,300 miles. This will give it some flexibility in being able to take off from different airports around the world and flying to a safe location for launch. That said, the fact it will need a 12,000-foot runway will limit the number of airports able to accommodate the giant.
The booster rocket is based on the Falcon 9 rocket from SpaceX. Once released at approximately 30,000 feet, the rocket will use a two stage booster to deliver a payload of up to 13,500 pounds to low earth orbit.
First flights for the Stratolaunch system are scheduled for 2016.
Images: Stratolaunch, NASA