The world’s largest airplane recently made a test flight. This plane, named after the mythical bird Roc, will launch spaceships from high altitude for the American company Stratolaunch. Roc is not alone in this, earlier this year Virgin Orbit launched a Boeing 747 rocket, while sister company Virgin Galactic’s White Knight launched Spaceship 2 to the edge of space. What makes airplanes so attractive for launching spacecraft?
With a length of 73 m and a width of 117 m, the Roc is the world’s largest aircraft and also has two fuselages that are connected to each other via a central wing. The same goes for Virgin Galactic’s White Knight. Both aircraft are specially developed for their task as flying launchers. Virgin Orbit’s Boeing 747, called Cosmic Girl, is an exception. However, the first launch from this existing aircraft was also preceded by years of development, conversion and testing before the aircraft launched its LauncherOne in October.
Objectively, the plane only bridges the first 10-20 kilometers of a hundred kilometer long orbital flight. How can such a launch, with the added weight of an airplane, which is also much less aerodynamic than a rocket alone, be more efficient than a launch from Earth?
Before answering the above question, first an explanation of a traditional launch from a fixed location on the surface of the earth. “When a rocket is launched from the ground, it will travel faster and faster through the lowest parts of the atmosphere, where the density of the air is highest,” says Niels Bernving, R&D space systems engineer at the Netherlands Aerospace Center. “During launch, the rocket is exposed to acoustic forces and resistances that increase until the moment when the maximum dynamic load is. During a launch from the ground, these forces and vibrations are greatest, and the construction of the rocket must be able to withstand them.”
With increasing vertical speed, the rocket must not only gain altitude, but also horizontal speed. An orbit around the earth is, after all, a horizontal movement, parallel to the earth’s surface. For that reason, every rocket leans almost immediately after launch. “Most of the lateral velocity is then provided by gravity. It pulls on the tilted launch vehicle and converts part of the vertical velocity into horizontal motion,” says Bernving. “We call this a gravity orbit.”
For a launch, the latitude of the launch site limits the trajectory that can be reached. A rocket taking off from the US Kennedy Space Center in Florida (28.5ISLAND north latitude), can achieve an orbit with an inclination to the equator of no more than 28.5ISLAND. To achieve a smaller angle of inclination (more towards the equator), the rocket can perform a steering trick, the so-called plane change maneuver. Bernving: “You want to perform all these maneuvers immediately after take-off, when the speed is still relatively low. The greater the velocity vector in a particular direction, the more difficult it is to bend it. The first phase of the launch from the ground is therefore a fight against gravity and high air resistance. It requires a lot of energy.” To illustrate: the shuttle used 25% of its fuel to get to an altitude of 10 kilometers. The shuttle kinetic energy required increased by only 0.16%.
An airplane gets part of its lifting capacity from the shape of its wings, which create lift or lift. “Furthermore, aircraft engines get their oxygen from the air,” says Bernving. “A rocket takes oxygen with it. When launching from under an aircraft, less oxygen and fuel are therefore required. Moreover, the rocket engines only start at an altitude of 10-20 km, where the density of the atmosphere is lower than on the Earth’s surface. The maximum dynamic pressure and thus the vibrations on the structure are then much lower. The rocket can do with a less robust construction.”
The aircraft eliminates the need for the fuel-guzzling flight change maneuver because it can fly to the latitude corresponding to the bank angle of the desired trajectory.
Flying to the correct latitude has another advantage. Rockets are always fired in an easterly direction, which is the direction of the earth’s rotation. The rocket thus receives the earth’s horizontal rotation speed as a gift. At the equator, the Earth rotates on its axis at about 430 m/s. This speed decreases as the width increases by a factor equal to the cosine of the latitude. At the Kennedy Space Center at 28.5ISLAND north latitude, the rotation speed is only 430 * cos 28.5 = 378 m/s. By flying the plane towards the equator, the rocket gains ‘free’ speed. In addition, the rocket also receives the horizontal speed of the aircraft (more than 200 m/s).
Although launching by plane has many advantages, there are of course also disadvantages. For example, the carrying capacity of the aircraft limits the weight of the rocket, which in turn has consequences for the amount of fuel that can be carried and the height that the rocket can reach. Typically, aircraft launches reach a low earth orbit of 400-800 km.
“Such a launch brings its own problems with gravity,” says Bernving. “Between the moment the aircraft is released and the rocket engine starts, the rocket is in free fall. The liquid rocket fuel tends to bubble up in such a weightless state. It is then very difficult to get the fuel for the engine.” Solid fuel does not have this problem, but is not an option because solid fuel does not provide enough momentum for the rocket to reach space.
Another disadvantage of high-altitude launch is the high cost of developing the aircraft. It must be able to take off and land from airports and fly over larger areas of inhabited areas than a rocket. Therefore, it must meet all very strict aviation safety requirements and also safely carry a rocket full of fuel.
“Despite the disadvantages and the high development costs, a business case can be made,” says Bernving. “At least in the US, where several commercial parties compete on the market for launches. No other country puts as many satellites into space as the United States.” Of the 5,465 satellites now in operation, the United States launched 3,433.
Many of the commercial satellites today are like sardines in the rocket can. Space X, for example, simultaneously launches dozens of satellites for Internet communications. “If one of these satellite networks breaks, it is very expensive to replace it with another Space X rocket. A launch by plane is definitely cheaper.”
There is another advantage to the aircraft that is of particular interest to the US military. It can be deployed faster and more flexibly than a rocket that takes off from a fixed base. The military has been working for years with mysterious unmanned mini-spacecraft such as the X40 and its successor the X-37B. Until now, the Air Force has launched these shuttles with a rocket, but recently the X-37B flew as a passenger under Virgin Galactic’s White Knight for a test flight in the atmosphere.
Stratolauncher will launch the Talon-a, a test aircraft for hypersonic flight at mach 5-6, in early 2023. The company is also working on a space variant. After the first successful test flight earlier this year, Virgin Orbit will use its Launchers for commercial launches. These companies are not the first to reach space by plane. Since 1990, the American Northrop Grumman has launched a Pegasus rocket 45 times from under the Stargazer, a converted Lockheed Tristar.
The future offers even more perspectives air launch into orbit systems. For example, NASA is working on a twin-hull glider. While floating on the thermals, such an aircraft can reach the desired launch altitude with almost no fuel. The Dutch-New Zealand based Dawn Aerospace goes one step further. It is developing a spaceship that independently, like an airplane, takes off and flies to space. The first scale models made test flights while the company was also testing its rocket engines.
Flying into space, once science fiction, is slowly but surely losing its fictional character. Who knows, this way space will one day be accessible from Dutch soil.
Thanks to Barry Zandbergen from the Faculty of Aerospace Technology at TUDelft, who provided valuable information for this article.