There are three major launch types for spacecraft: vertical takeoff, horizontal takeoff, and air launch. The cost of launching into space is often measured by the change in velocity required to reach the destination orbit, known as delta-v or Δv. The amount of Δv required for a mission depends on the desired orbit. The change in velocity is dependent on Specific impulse, measured in seconds, which is the amount of time that a unit weight of a propellant can produce a unit weight of thrust. A useful characteristic by which to compare spacecraft is their propellant mass fraction. The propellant mass fraction is the amount of propellant with respect to the overall vehicle mass. Aerodynamic properties of launch vehicles also affect the vehicle’s trajectory. The four forces of flight acting on a vehicle during the atmospheric portion of ascent are the weight, lift, thrust, and drag.
Vertical take-off is the traditional and by far the most popular launch mode. Vertical takeoff rockets typically are able to achieve necessary propellant mass fractions with enough margin to permit the use of relatively low technology pressure fed rocket motors. Additionally,
vertically-launched vehicles are exposed to less bending and no twisting moments during launch, unlike vehicles that are launched horizontally or in the air. Launch site location is extremely important for vertical launches. Vertically-launched spacecraft can only be directly launched into an orbit with an inclination at least as large as the latitude of the launch site.
For example, a vertical launch at Kennedy Space Center in Florida cannot put a satellite into an equatorial orbit because the launch site is at a 28.5 degree latitude, as opposed to the 0.0 degree latitude at the equator. In order to transfer into an orbit with a different inclination, an expensive plane change maneuver must be performed. Additionally, to avoid dropping vehicle parts on populated areas, vertical launches are performed over large bodies of water or deserts. Since spacecraft are almost always launched eastward (to gain a Δv assist from the earth’s rotation), this limits launch site locations to East coasts (or deserts) at the lowest possible latitude.
Vertical launches require a launch pad. Depending on the size of the launch, the launch pad can be extremely large and complicated. For example, the launch pad for the space shuttle had launch towers, movable arms, and a water sound suppression system. Vertical launches also have significant gravity and drag losses. These losses contribute to the large amount of propellant used up in the initial stage of the trajectory. For example, the space shuttle burned 25% of its propellant by the time it reached air launch altitude, while only adding 0.16% of the required kinetic energy to the system.
Horizontal take-off vehicles launch from a runway, similar to an aircraft. This eliminates the need for an expensive launch pad. It also allows the vehicle to fly to any latitude before entering space, which eliminates the need for costly inclination change maneuvers. Horizontal launch vehicles may be powered by rocket engines only or by a combination of jet engines and rocket engines.
While horizontal take-off vehicles are similar to aircraft, they need larger empty mass fractions and lower propellant mass fractions than regular aircraft. The vehicles must be sturdy enough to endure intense bending modes, turbulent air, high aerodynamic pressures, and supersonic flight.
Of the three launch modes, horizontal launch has the highest drag loss, since it consists of a winged body for the entire flight duration. It also takes the most Δv to reach orbit, because it must accelerate horizontally then pitch up toward the vertical to achieve orbit.
Air launches typically involve two vehicles: a winged carrier aircraft and an upper stage spacecraft. There are numerous advantages of air launch systems over the other two launch modes.
While vertical launches are constrained to a launch azimuth of the launch site’s latitude or higher, air launch systems (like horizontal launch systems) can be flown to any latitude before launch. This opens up any orbital inclination to the spacecraft, including equatorial orbits. Since plane changes from American launch sites to an equatorial orbit are very expensive, the capability to launch into any azimuth is a significant advantage of air launch systems.
Additionally, for rendezvous with a current satellite, such as the ISS, air launches permit the spacecraft to be launched directly into a rendezvous orbit immediately, without waiting for the satellite’s orbital plane to pass over the launch site.
The need to launch over a desert or source of water is eliminated, since the carrier is a reusable vehicle that does not drop parts. While a horizontal launch takes the most Δv since it accelerates horizontally then must pitch vertically, an air launch vehicle can be released from the carrier at a pitched angle, reducing the Δv required as compared to a horizontal release
Air launch to orbit (ALTO)
Air launch to orbit (ALTO) is the method of launching rockets at altitude from a conventional horizontal-takeoff aircraft, to carry satellites to low Earth orbit. It is a follow-on development of air launches of experimental aircraft that began in the late 1940s. This method, when employed for orbital payload insertion, presents significant advantages over conventional vertical rocket launches, particularly because of the reduced mass, thrust, cost of the rocket, geographical factors and natural disasters.
The principal advantage of a rocket being launched by a high flying airplane is that it needs not fly through the low, dense atmosphere, the drag of which requires a considerable amount of extra work and thus mass of propellant. Higher densities at lower altitudes result in larger drag forces acting on the vehicle. In addition, thrust is lost due to over-expansion of the exhaust at high ambient pressure and under-expansion at low ambient pressure; a fixed nozzle geometry cannot provide optimal exhaust expansion over the full range of ambient pressure, and represents a compromise solution. Rockets launched from high altitude can be optimized for lower ambient pressure, thus achieving greater thrust over the entire operating regime.
Propellant is conserved because the air-breathing carrier aircraft lifts the rocket to altitude much more efficiently. Airplane engines do not require on-board storage of an oxidizer and they can use the surrounding air to produce thrust, for example with a turbofan. This allows the launch system to conserve a significant amount of mass that would otherwise be reserved for fuel, reducing the overall size. A larger fraction of the rocket mass can then include payload, reducing payload launch costs.
Air launch to orbit offers the potential for aircraft-like operations such as launch on demand, and is also less subject to launch-constraining weather. This allows the aircraft to fly around weather conditions as well as fly to better launch points, and to launch a payload into any orbital inclination at any time. Insurance costs are reduced as well, because launches occur well away from land, and there is no need for a launch pad or blockhouse.
Air launch to orbit also works well as part of a combination launch system such as a reusable air-launched single stage to skyhook launch vehicle powered by a rocket or rocket/ramjet/scramjet engine.
An additional benefit of air launch to orbit is a reduced delta V needed to achieve orbit. This results in a greater payload to fuel ratio which reduces the cost per unit mass to orbit. To further leverage the Delta V advantage, supersonic air launch to orbit has been proposed.
Air launch to orbit also served as alternative if conditions do not allow launching a rocket vertically from ground to orbit due to certain reasons, such as natural disasters (earthquakes, tsunamis, floods and volcanic eruptions).
Air launches do not need fancy, expensive launch pads with launch towers, sound suppression systems, etc. (The rocket launch takes place at a high altitude, where sound does not reflect off the ground and the atmosphere is thinner.) Instead, the carrier can take off from a normal runway. Not only does this make air launches more secure – if something such as a hurricane, earthquake, or terrorist attack were to destroy a launch pad, upcoming vertical launches would be pooched – but it also makes scheduling launches easier since there are more runways available than launch pads.
Additionally, military operations can be more covert with air launches, since they can use any
runway rather than a publicized launch pad. The runway launch also makes air launches a safer option for crew than a vertical launch in cases of an emergency abort.
NASA Armstrong develops tech to bring space launch to any airport
The Towed-Glider Air Launch System, or TGALS, is a low-cost, flexible approach for putting satellites and other payloads into space. Developed at NASA Armstrong Flight Research Center in Edwards, Calif., the innovative TGALS technique uses a low-cost glider to carry rockets and release them at the optimum place in the sky.
The TGALS technique uses a business jet-class aircraft to tow a remotely piloted glider with a launch vehicle mounted underneath it. Once released at about 40,000 feet, the glider uses its own small rocket motor to execute a pull-up maneuver, releasing the launch vehicle for ignition at an elevated flight path angle. After release, the glider returns to the airfield to be stored for the next mission.
“I think one of the big selling points is the flexibility for launch windows and launch locations around the world,” said Brian Boogaard, Technology Transfer Administrator at NASA Armstrong. “There’s only a handful of rocket pads where you launch a rocket, but you could fly the TGALS system anywhere there’s an airport. There’s a lot of flexibility that comes with it.”
In addition to the launch flexibility, TGALS can carry launch vehicles that are 30% heavier compared to air-launched vehicles and 70% heavier than those using ground-based rockets. The system offers improved safety by not having an on-board aircrew in an aircraft attached to or near a potentially explosive rocket.
NASA Armstrong researchers conducted proof-of-concept demonstration flights using radio-controlled one-third scale models of both glider and rocket. The tests included using a 27-foot- wingspan, twin-hulled glider home-built at NASA Armstrong and towed by the small DROID—for Dryden Remotely Operated Integrated Drone—unmanned aircraft. Researchers also conducted studies and simulations of a glider capable of carrying an 80,000-pound rocket.
Transferring technology to American industry
One company, Fenix Space, Inc. in San Bernardino, signed a licensing agreement with NASA to use the TGALS technology. NASA Armstrong is in talks with a second company also interested in the technology.