Reusable launch system
A reusable launch system (RLS, or reusable launch vehicle, RLV) is a launch system which is capable of launching a payload into space more than once. This contrasts with expendable launch systems, where each launch vehicle is launched once and then discarded.
No completely reusable orbital launch system has ever been created; however, several partially reusable launch systems have existed. The Space Shuttle was partially reusable: the orbiter, which included the Space Shuttle main engines, and the two solid rocket boosters, were reused after several months of refitting work for each launch. However, the external tank and launch vehicle load frame were discarded after each flight.
The Falcon 9 rocket is designed to have a reusable first stage; several of these stages have been safely returned to land after launch. However, as of 2016, none of these first stages have yet been reused.
Orbital RLVs are thought to provide the possibility of low cost and highly reliable access to space. However, reusability implies weight penalties such as non-ablative reentry shielding and possibly a stronger structure to survive multiple uses, and given the lack of experience with these vehicles, the actual costs and reliability are yet to be seen.
- 1 History
- 2 Reusability concepts
- 2.1 Single stage
- 2.2 Two or more stages to orbit
- 2.3 Biamese & Triamese (Crossfeed)
- 2.4 Horizontal landing
- 2.5 Vertical landing
- 2.6 Horizontal takeoff
- 2.7 Vertical takeoff
- 2.8 Airbreathing
- 2.9 Propellant
- 2.10 Launch assistance/non rocket space launch
- 2.11 Reentry heat shields
- 2.12 Weight penalty
- 2.13 R&D
- 2.14 Maintenance
- 2.15 Manpower and logistics
- 3 Orbital reusable launchers
- 4 Suborbital reusable launchers
- 5 Regulations
- 6 See also
- 7 References
- 8 Bibliography
- 9 External links
In the first half of the twentieth century, popular science fiction often depicted space vehicles as either single-stage reusable rocket ships which could launch and land vertically (SSTO VTVL), or single-stage reusable rocket planes which could launch and land horizontally (SSTO HTHL).
The realities of early engine technology with low specific impulse or insufficient thrust-to-weight ratio to escape Earth's gravity well, compounded by construction materials without adequate performance (strength, stiffness, heat resistance) and low weight, seemingly rendered that original single-stage reusable vehicle vision impossible.
However, advances in materials and engine technology have rendered this concept potentially feasible.
Before VTVL SSTO designs came the partially reusable multi-stage NEXUS launcher by Krafft Arnold Ehricke. The pioneer in the field of VTVL SSTO, Philip Bono, worked at Douglas. Bono proposed several launch vehicles including: ROOST, ROMBUS, Ithacus, Pegasus and SASSTO. Most of his vehicles combined similar innovations to achieve SSTO capability. Bono proposed:
- Plug nozzle engines to retain high specific impulse at all altitudes.
- Base first reentry which allowed the reuse of the engine as a heat shield, lowering required heat shield mass.
- Use of spherical tanks and stubby shape to reduce vehicle structural mass further.
- Use of drop tanks to increase range.
- Use of in-orbit refueling to increase range.
Bono also proposed the use of his vehicles for space launch, rapid intercontinental military transport (Ithacus), rapid intercontinental civilian transport (Pegasus), even Moon and Mars missions (Project Selena, Project Deimos).
In Europe, Dietrich Koelle, inspired by Bono's SASSTO design, proposed his own VTVL vehicle named BETA.
Before HTHL SSTO designs came Eugen Sänger and his Silbervogel ("Silverbird") suborbital skip bomber. HTHL vehicles which can reach orbital velocity are harder to design than VTVL due to their higher vehicle structural weight. This led to several multi-stage prototypes such as a suborbital X-15. Aerospaceplane being one of the first HTHL SSTO concepts. Proposals have been made to make such a vehicle more viable including:
- Rail boost (e.g. 270 m/s at 3000 m on a mountain allowing 35% less SSTO takeoff mass for a given payload in one NASA study)
- Use of lifting body designs to reduce vehicle structural mass.
- Use of in-flight refueling.
Other launch system configuration designs are possible such as horizontal launch with vertical landing (HTVL) and vertical launch with horizontal landing (VTHL). One of the few HTVL vehicles is the 1960s concept spacecraft Hyperion SSTO, designed by Philip Bono. X-20 Dyna-Soar is an early example of a VTHL design, while the HL-20 and X-34 are examples from the 1990s. As of February 2010[update], the VTHL X-37 has completed initial development and flown an initial classified orbital mission of over seven months duration. Currently proposed VTHL manned spaceplanes include the Dream Chaser and Prometheus, both circa 2010 concept spaceplanes proposed to NASA under the CCDev program.
The late 1960s saw the start of the Space Shuttle design process. From an initial multitude of ideas a two-stage reusable VTHL design was pushed forward that eventually resulted in a reusable orbiter payload spacecraft and reusable solid rocket boosters. The external tank and the launch vehicle load frame were discarded, and the parts that were reusable took a 10,000-person group nine months to refurbish for flight. So the space shuttle ended up costing a billion dollars per flight.[better source needed] Early studies from 1980 and 1982 proposed in-space uses for the tank to be re-used in space for various applications but NASA never pursued those options beyond the proposal stage.
During the 1970s further VTVL and HTHL SSTO designs were proposed for solar power satellite and military applications. There was a VTVL SSTO study by Boeing. HTHL SSTO designs included the Rockwell Star-Raker and the Boeing HTHL SSTO study. However the focus of all space launch funding in the United States on the Shuttle killed off these prospects. The Soviet Union followed suit with Buran. Others preferred expendables for their lower design risk, and lower design cost.
Eventually the Shuttle was found to be expensive to maintain, even more expensive than an expendable launch system would have been. The cancellation of a Shuttle-Centaur rocket after the loss of Challenger also caused an hiatus that would make it necessary for the United States military to scramble back towards expendables to launch their payloads. Many commercial satellite customers had switched to expendables even before that, due to unresponsiveness to customer concerns by the Shuttle launch system.
In 1986 President Ronald Reagan called for an airbreathing scramjet plane to be built by the year 2000, called NASP/X-30 that would be capable of SSTO. Based on the research project copper canyon the project failed due to severe technical issues and was cancelled in 1993.
This research may have inspired the British HOTOL program, which rather than airbreathing to high hypersonic speeds as with NASP, proposed to use a precooler up to Mach 5.5. The program's funding was canceled by the British government when the research identified some technical risks as well as indicating that that particular vehicle architecture would only be able to deliver a relatively small payload size to orbit.
The 1990s saw interest in developing new reusable vehicles. The military Strategic Defense Initiative ("Star Wars") program "Brilliant Pebbles" required low cost, rapid turnaround space launch. From this requirement came the McDonnell Douglas Delta Clipper VTVL SSTO proposal. The DC-X prototype for Delta Clipper demonstrated rapid turnaround time and that automatic computer control of such a vehicle was possible. It also demonstrated it was possible to make a reusable space launch vehicle which did not require a large standing army to maintain like the Shuttle.
In mid-1990, further British research and major reengineering to avoid deficiencies of the HOTOL design led to the far more promising Skylon design, with much greater payload.
From the commercial side, large satellite constellations such as Iridium satellite constellation were proposed which also had low cost space access demands. This fueled a private launch industry, including partially reusable vehicle players, such as Rocketplane Kistler, and reusable vehicle players such as Rotary Rocket.
The end of that decade saw the implosion of the satellite constellation market with the bankruptcy of Iridium. In turn the nascent private launch industry collapsed. The fall of the Soviet Union eventually had political ripples which led to a scaling down of ballistic missile defense, including the demise of the "Brilliant Pebbles" program. The military decided to replace their aging expendable launcher workhorses, evolved from ballistic missile technology, with the EELV program. NASA proposed riskier reusable concepts to replace Shuttle, to be demonstrated under the X-33 and X-34 programs.
The 21st century saw rising costs and teething problems lead to the cancellation of both X-33 and X-34. Then the Space Shuttle Columbia disaster and another grounding of the fleet. The Shuttle design was now over 20 years old and in need of replacement. Meanwhile, the military EELV program churned out a new generation of better expendables. The commercial satellite market is depressed due to a glut of cheap expendable rockets and there is a dearth of satellite payloads.
Against this backdrop came the Ansari X Prize contest, inspired by the aviation contests made in the early 20th century. Many private companies competed for the Ansari X Prize, the winner being Scaled Composites with their reusable HTHL SpaceShipOne. It won the ten million dollars, by reaching 100 kilometers in altitude twice in a two-week period with the equivalent of three people on board, with no more than ten percent of the non-fuel weight of the spacecraft replaced between flights. While SpaceShipOne is suborbital like the X-15, some hope the private sector can eventually develop reusable orbital vehicles given enough incentive. SpaceX is a recent player in the private launch market succeeding in converting its Falcon 9 expendable launch vehicle into a partially reusable vehicle by returning the first stage for reuse.
On 23 November 2015, Blue Origin New Shepard rocket became the first proven Vertical Take-off Vertical Landing (VTVL) rocket which can reach space, by passing Kármán line (100 kilometres), reaching 329,839 feet (100.5 kilometers).  Previous VTVL record was in 1994, the McDonnell Douglas DC-X ascended to an altitude of about 3.1 kilometers before successfully landing. 
There are two approaches to Single stage to orbit or SSTO. The rocket equation says that an SSTO vehicle needs a high mass ratio. Mass ratio is defined as the mass of the fully fueled vehicle divided by the mass of the vehicle when empty (zero fuel weight, ZFW).
One way to increase the mass ratio is to reduce the mass of the empty vehicle by using very lightweight structures and high efficiency engines. This tends to push up maintenance costs as component reliability can be impaired, and makes reuse more expensive to achieve. The margins are so small with this approach that there is uncertainty whether such a vehicle would be able to carry any payload into orbit.
Two or more stages to orbit
Two stage to orbit requires designing and building two independent vehicles and dealing with the interactions between them at launch. Usually the second stage in launch vehicle is 5-10 times smaller than the first stage, although in biamese and triamese approaches each vehicle is the same size.
In addition, the first stage needs to be returned to the launch site for it to be reused. This is usually proposed to be done by flying a compromise trajectory that keeps the first stage above or close to the launch site at all times, or by using small airbreathing engines to fly the vehicle back, or by recovering the first stage downrange and returning it some other way (often landing in the sea, and returning it by ship.) Most techniques involve some performance penalty; these can require the first stage to be several times larger for the same payload, although for recovery from downrange these penalties may be small.
The second stage is normally returned after flying one or more orbits and reentering.
Biamese & Triamese (Crossfeed)
Two or three similar stages are stacked side by side, and burn in parallel. Using crossfeed, the fuel tanks of the orbital stage are kept full, while the tank(s) in the booster stage(s) are used to run engines in the booster stage(s) and orbital stage. Once the boosters run dry, they are ejected, and (typically) glide back to a landing. The advantage to this is that the mass ratios of the individual stages is vastly reduced due to the way cross feed modifies the rocket equation. Isp*g*ln(2MR^2/MR+1) & Isp*g*ln(3MR^2/MR+2)[clarification needed] respectively.[original research?] With hydrogen engines, a triamese only needs an MR of 5, as opposed to an MR of 10 for a single-stage equivalent vehicle.
A criticism of this approach is that designing separate orbiter and boosters, or a single vehicle that could do both, would compromise performance, safety, and possible cost savings. Compromising maximum performance to reduce cargo cost however, is the point of the triamese approach. Stacking two or three winged vehicles can also be challenging. Optimistically, the lower mass ratios would translate to lower overall R&D costs, even if two different stage designs. While many aerospace designs have successfully been modified far beyond the original designers intentions (Boeing's 747 is perhaps the best example) the slow and painful birth of the F-35 family demonstrates that it is not always a guarantee of such flexibility.
In this case the vehicle requires wings and undercarriage (unless landing at sea). This typically requires about 9-12% of the landing vehicle to be wings; which in turn implies that the takeoff weight is higher and/or the payload smaller.
Parachutes could be used to land vertically, either at sea, or with the use of small landing rockets, on land (as with Soyuz). McDonnell Douglas DC-X ascended to an altitude of about 3.1 kilometers before successfully landing. 
Alternatively rockets could be used to softland the vehicle on the ground from the subsonic speeds reached at low altitude (see DC-X). This typically requires about 10% of the landing weight of the vehicle to be propellant.
SpaceX's grasshopper rocket, a 10-story Vertical Takeoff Vertical Landing (VTVL) vehicle, became the first reusable rocket designed to test the technologies needed to return a rocket back to Earth intact. While most rockets are designed to burn up in the atmosphere during reentry, SpaceX's rockets are being designed to return to the launch pad for a vertical landing.
SpaceX's Falcon 9 rocket became the first orbital rocket to vertically land its first stage on the ground, after propelling its second stage and payload to a suborbital trajectory, where it would continue on to orbit. <
The vehicle needs wings to take off. For reaching orbit, a 'wet wing' would often need to be used where the wing contains propellant. Around 9-12% of the vehicle takeoff weight is perhaps tied up in the wings.
This is the traditional takeoff regime for pure rocket vehicles. Rockets are good for this regime, since they have a very high thrust/weight ratio (~100).
Airbreathing approaches use the air during ascent for propulsion. The most commonly proposed approach is the scramjet, but turborocket, Liquid Air Cycle Engine (LACE) and precooled jet engines have also been proposed.
In all cases the highest speed that an airbreathing engine can reach is far short of orbital speed (about Mach 15 for Scramjets and Mach 5-6 for the other engine designs), and rockets would be used for the remaining 10-20 Mach into orbit.
The thermal situation for airbreathers (particularly scramjets) can be awkward; normal rockets fly steep initial trajectories to avoid drag, whereas scramjets would deliberately fly through relatively thick atmosphere at high speed generating enormous heating of the airframe. The thermal situation for the other airbreathing approaches is much more benign, although is not without its challenges.
Hydrogen is often proposed since it has the highest exhaust velocity. However tankage and pump weights are high due to insulation and low propellant density; and this wipes out much of the advantage.
Still, the 'wet mass' of a hydrogen fuelled stage is lighter than an equivalent dense stage with the same payload and this can permit usage of wings, and is good for second stages.
Dense fuel is sometimes proposed since, although it implies a heavier vehicle, the specific tankage and pump mass is much improved over hydrogen. Dense fuel is usually suggested for vertical takeoff vehicles, and is compatible with horizontal landing vehicles, since the vehicle is lighter than an equivalent hydrogen vehicle when empty of propellant. Non-cryogenic dense fuels also permit the storage of fuel in wing structures. Projects have been underway to densify existing fuel types through various techniques. These include slush technologies for cryogenics like hydrogen and propane. Another densifying method has been studied that would also increase the specific impulse of fuels. Adding finely powdered carbon, aluminum, titanium, and boron to hydrogen and kerosene have been studied. These additives increase the specific impulse (Isp) but also the density of the fuel. For instance, the French ONERA missile program tested boron with kerosene in gelled slurries, as well as embedded in paraffin, and demonstrated increases in volumetric specific impulse of between 20-100%.
Dense fuel is optimal early on in a flight, since the thrust to weight of the engines is better due to higher density; this means the vehicle accelerates more quickly and reaches orbit sooner, reducing gravity losses.
However, for reaching orbital speed, hydrogen is a better fuel, since the high exhaust velocity and hence lower propellant mass reduces the take off weight.
Therefore, tripropellant vehicles start off burning with dense fuel and transition to hydrogen. (In a sense the Space Shuttle does this with its combination of solid rockets and main engines, but tripropellant vehicles usually carry their engines to orbit.)
As with all current launch vehicles, propellant costs for a rocket are much lower than the costs of the hardware. However, for reusable vehicles if the vehicles are successful, then the hardware is reused many times and this would bring the costs of the hardware down. In addition, reusable vehicles are frequently heavier and hence less propellant efficient, so the propellant costs could start to multiply up to the point where they become significant.
Launch assistance/non rocket space launch
Since rocket delta-v has a non linear relationship to mass fraction due to the rocket equation, any small reduction in delta-v gives a relatively large reduction in the required mass fraction; and starting a mission at higher altitude also helps.
Many systems have proposed the use of aircraft to gain some initial velocity and altitude; either by towing, carrying or even simply refueling a vehicle at altitude.
Various other launch assists have been proposed, such as ground-based sleds, or maglev systems, high altitude (80 km) maglev systems such as launch loops, to more exotic systems such as tether propulsion systems to catch the vehicle at high altitude, or even Space Elevators.
Reentry heat shields
Reentry heat shields on these vehicles are often proposed to be some sort of ceramic and/or carbon-carbon heat shields, or occasionally metallic heat shields (possibly using water cooling or some sort of relatively exotic rare earth metal.)  Some shields would be single-use ablatives, discarded after reentry.
A newer Thermal Protection System (TPS) technology was first developed for use in steering fins on ICBM MIRVs. Given the need for such warheads to reenter the atmosphere swiftly and retain hypersonic velocities to sea level, researchers developed what are known as SHARP materials, typically hafnium diboride and zirconium diboride, whose thermal tolerance exceeds 3600 C. SHARP equipped vehicles can fly at Mach 11 at 30 km altitude and Mach 7 at sea level. The sharp-edged geometries permitted with these materials also eliminates plasma shock wave interference in radio communications during reentry. SHARP materials are very robust and would not require constant maintenance, as is the case with technologies like silica tiles, used on the Space Shuttle, which account for over half of that vehicles maintenance costs and turnaround time. The maintenance savings alone are thus a major factor in favor of using these materials for a reusable launch vehicle, whose raison d'etre is high flight rates for economical launch costs.
The weight of a reusable vehicle is almost invariably higher than an expendable that was made with the same materials, for a given payload.
The research & development costs of reusable vehicle are expected to be higher, because making a vehicle reusable implies making it robust enough to survive more than one use, which adds to the testing required. Increasing robustness is most easily done by adding weight; but this reduces performance and puts further pressure on the R&D to recoup this in some other way.
These extra costs must be recouped; and this pushes up the average cost of the vehicle.
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Reusable launch systems require maintenance, which is often substantial. The Space Shuttle system required extensive refurbishing between flights, primarily dealing with the silica tile TPS and the high performance LH2/LOX burning main engines. Both systems require a significant amount of detailed inspection, rebuilding and parts replacement between flights, and account for over 75% of the maintenance costs of the Shuttle system. These costs, far in excess of what had been anticipated when the system was constructed, have cut the maximum flight rate of Shuttle to 1/4 of that planned. This has also quadrupled the cost per pound of payload to orbit, making Shuttle economically infeasible in today's launch market for any but the largest payloads, for which there is no competition.
For any RLV technology to be successful, it must learn from the failings of Shuttle and overcome those failings with new technologies in the TPS and propulsion areas.
Manpower and logistics
The Space Shuttle program required a standing army of over 9,000 employees to maintain, refurbish, and relaunch the shuttle fleet, irrespective of flight rates. That manpower budget must be divided by the total number of flights per year. The fewer flights means the cost per flight goes up significantly. Streamlining the manpower requirements of any launch system is an essential part of making an RLV economical. Projects that have attempted to develop this ethic include the DC-X Delta Clipper project, as well as SpaceX's Falcon 9 and Falcon 1 programs.
One issue mitigating against this drive for labor savings is government regulation. Given that NASA and USAF (as well as government programs in other countries) are the primary customers and sources of development capital, government regulatory requirements for oversight, parwork, quality, safety, and other documentation tend to inflate the operational costs of any such system.
Orbital reusable launchers
- RLV-TD, (India): On 23 May 2016, ISRO successfully performed test flight of India’s first reusable launch vehicle that operates at hypersonic speed.
- Adeline - Reusable launch system concept developed by Airbus Defence and Space
- Avatar RLV - Under development, first scaled-down demonstration flight planned in 2016.
- Blue Origin is developing a reusable booster system, as of November 2015[update]. Blue Origin New Shepard rocket is the first rocket successfully launched and which is proven to be able to land vertically on earth VTVL after reaching space, by passing Kármán line. 
- As of 2014[update], China is working on a project to recover rocket boosters, using a "paraglider-type wings" approach. Powered flight tests are in the future, and the process is expect to take until approximately 2018.
- Skylon (spacecraft) (proposed airbreathing SSTO spaceplane)
- SpaceX reusable rocket launching system—(currently in development and test)—is planned for use on both the Falcon 9 and Falcon Heavy launch vehicles. A second-generation VTVL reusable design was publicly announced in 2011. The low-altitude flight test program of an experimental technology-demonstrator launch vehicle began in 2012, with more extensive high-altitude over-water flight testing planned to begin in mid-2013, and continue on each subsequent Falcon 9 flight. On December 21, 2015, SpaceX successfully landed a Falcon 9 first stage after it boosted 11 commercial satellites into low earth orbit on Falcon 9 Flight 20.
- Swiss Space Systems is developing launching system including the suborbital spaceplane SOAR. The first 2 stages, an Airbus 300 and SOAR, are completely reusable.
- zero2infinity is developing a launching system called bloostar based on the rockoon system, which consists in elevating to the near space the launcher using a high-altitude balloon and once there launch a multi-stage rocket to put a satellite into orbit.
Proposed and concept vehicles
- SpaceFleet (The EARL Project - a space-capable UAV concept[when?][full citation needed]
- SpaceLiner (a mid-2000s German proposed suborbital, hypersonic, winged passenger transport concept)
- Shenlong (spacecraft) (an early 2000s Chinese proposed, scaled model tested at high altitude in 2005)
- PlanetSpace Silver Dart (a 2000s partially reusable spaceplane concept, based on a hypersonic glider design)
- As of January 2015[update], the French space agency CNES is working with Germany and a few other governments to start a modest research effort with a hope to propose a LOX/methane engine on a reusable launch vehicle by mid-2015, with flight testing unlikely before approximately 2026.
- Sura (proposed by Ukraine)
- CORONA (proposed by Russia)
- Baikal French/Russian early-2000s joint-project concept. Cancelled after "CNES officials concluded that a rocket system with a reusable first stage would need to launch some 40 times a year" in order to make the project economically feasible.
- HOTOL British SSTO.
- Hyperion SSTO 1960s concept HTVL spacecraft.
- Kliper planned Russian partly reusable orbiter, cancelled in 2006.
- Liquid Fly-back Booster proposed design of reusable boosters for Ariane 5 with additional derivatives
- MAKS proposed Russian system of Buran-like smaller winged reusable orbiter on heavy aircraft carrier.
- Spiral cancelled Soviet military system of small winged reusable orbiter on winged hypersonic air-carrier.
- Phoenix SSTO
- X-30 NASP, X-33 and VentureStar proposed SSTO replacement for the Space Shuttle, cancelled in 2001.
- Roton Commercial launch vehicle project, cancelled in 2000 due to lack of funds.
Reusability dropped, flown only as expendable
- SpaceX Falcon 1 was announced as a partially reusable launch vehicle, and the 28 September 2008 test flight reached orbit, but vehicle recovery was never demonstrated and the vehicle was retired after 2009.
Suborbital reusable launchers
|Armadillo Aerospace||Black Armadillo||Planned|||
|Blue Origin||New Shepard||In test|
|Canadian Arrow||Canadian Arrow||Planned|||
|The da Vinci Project||Wild Fire MK VI||Planned|
|Masten Space Systems||XA 1.0||Planned|||
|Pablo de Leon & Associates||Gauchito||Planned|
|Space Adventures||Explorer||Planned|||
|Starchaser Industries||Starchaser V - Thunderstar||Planned|||
|TGV Rockets||Planned|||
|The Spaceship Company||SpaceShipTwo||In test|
|North American Aviation||X-15||Historical|
|Scaled Composites||Tier One (SpaceShipOne)||Historical|
|XCOR Aerospace||XCOR Lynx||Planned|||
|Rocketplane Limited, Inc.||Rocketplane XP||Cancelled|
In 2006, the US Federal Aviation Administration issued a new regulation regarding commercial reusable launch vehicles, both suborbital and orbital, as Part 431. The text can be found under the US Federal Code at 14 CFR Part 431. The new regulation was made in anticipation of planned commercial reusable launch operations including the American companies listed above. FAA regulations only have jurisdiction within the United States and its territories, and to aircraft and spacecraft registered in the United States.
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Both of the rocket’s stages would return to the launch site and touch down vertically, under rocket power, on landing gear after delivering a spacecraft to orbit.
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Several years ago, SpaceX was going to open up the smallsat launch market with the Falcon 1, which originally was to launch about 600 kilograms to LEO for $6 million; the payload capacity later declined to about 420 kilograms as the price increased to around $9 million. Later, the Falcon 1e was to provide approximately 1,000 kilograms for $11 million, but the company withdrew the vehicle from the market, citing limited demand.
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|Wikimedia Commons has media related to Reusable manned spacecraft.|
- Airbus unveils 'Adeline' re-usable rocket concept, BBC, June 2015.
- Illustration of a Space shuttle at takeoff and Orbiter (Visual Dictionary - QAInternational)
- History of the Phoenix VTOL SSTO and Recent Developments in Single-Stage Launch Systems
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