Air-launch-to-orbit

Air-launch-to-orbit (ALTO) is the method of launching smaller rockets at altitude from a heavier 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.
Air launching has also been developed for sub-orbital spaceflight. In 2004 the Ansari X Prize $10 Million purse was won by a team led by Burt Rutan's Scaled Composites, launching the SpaceShipOne from the purpose-built White Knight carrier aircraft.
The first air-launch-to-orbit was a test launch of the ASM-135 ASAT antisatellite rocket, the first commercial air-launch-to-orbit took place on 5 April 1990 with a Northrop Grumman Pegasus.
Advantages
The principal advantage of a rocket being launched by a high-flying airplane is that it need not fly through the lower, denseer atmosphere, whose drag 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, such as 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 jet 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 kilogram to orbit. To further leverage the delta V advantage, supersonic air-launch-to-orbit has been proposed.
Air-launch-to-orbit also serves 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).
Disadvantages
According to Aviation Week and Space Technology, air-launch-to-orbit is limited by aircraft size. Additionally, airplanes may generate large lateral forces which could damage payloads.
SpaceX CEO Elon Musk argued in a Q&A session at the Royal Aeronautical Society that the increase in performance is not worth the additional complexity and limitations:
"…it seems like...you're high up there and so surely that's good and you're going at...0.7 or 0.8 Mach and you've got some speed and altitude, you can use a higher expansion ratio on the nozzle, doesn't all that add up to a meaningful improvement in payload to orbit? The answer is no, it does not, unfortunately. It's quite a small improvement. It's maybe a 5% improvement in payload to orbit...and then you've got this humungous plane to deal with. Which is just like having a stage. From SpaceX's standpoint, would it make more sense to have a gigantic plane or to increase the size of the first stage by five percent? Uhh, I'll take option two. And then, once you get beyond a certain scale, you just can't make the plane big enough. When you drop...the rocket, you have the slight problem that you're not going the right direction. If you look at what Orbital Sciences did with Pegasus, they have a delta wing to do the turn maneuver but then you've got this big wing that's added a bunch of mass and you've able to mostly, but not entirely, convert your horizontal velocity into vertical velocity, or mostly vertical velocity, and the net is really not great."
Air launch systems
Operational:
Northrop Grumman Innovation Systems (originally Orbital Sciences, then Orbital ATK, since 2018 Northrop) Pegasus
Retired:
ASM-135 ASAT (anti-satellite system)
NOTS-EV-1 Pilot
Virgin Orbit LauncherOne
Under development:
Stratolaunch
CubeCab
ARCASPACE
Generation Orbit Launch Services - contracted for NASA NEXT
NASA Armstrong Flight Research Center Towed Glider Air-Launch System
CDTI, CNES, DLR Aldebaran (rocket)
Antonov, Aerospace Industry Corporation of China Antonov An-225 Mriya
Orbit Boy microsatellite air-launch system
Proposed:
Vulcan Aerospace 75-percent-scaled Dream Chaser crew-carrying spaceplane with rocket by Orbital Sciences
(proposed by Ukraine)
(proposed by Ukraine)
Abandoned projects:
DARPA ALASA
AirLaunch LLC
MAKS
Ishim
Orbital Sciences Pegasus II – contracted design/build for Stratolaunch Systems
Swiss Space Systems SOAR
XCOR Aerospace Lynx Mark III
Falcon 9 Air Developed 2011-2012, In partnership between SpaceX and Stratolaunch systems
See also
NOTS-EV-1 Pilot
NOTS-EV-2 Caleb
Buoyant space port
Rockoon
Launch vehicle types by launch platform
Black Horse (rocket)
Rocketplane XS
References
External links
A Study of Air Launch Methods for RLVs (AIAA 2001-4619)
Low Cost Launch of Payloads to Low Earth Orbit
Illini Space Jet
Rocketry
Space access