Single-stage-to-orbit

A single-stage-to-orbit (SSTO) vehicle reaches orbit from the surface of a body using only propellants and fluids and without expending tanks, engines, or other major hardware.

The main projected advantage of the SSTO concept is elimination of the hardware replacement inherent in expendable launch systems.

[4] Advances in rocketry in the 21st century have resulted in a substantial fall in the cost to launch a kilogram of payload to either low Earth orbit or the International Space Station,[5] reducing the main projected advantage of the SSTO concept.

Notable single stage to orbit concepts include Skylon, which used the hybrid-cycle SABRE engine that can use oxygen from the atmosphere when it is at low altitude, and then using onboard liquid oxygen after switching to the closed cycle rocket engine at high altitude, the McDonnell Douglas DC-X, the Lockheed Martin X-33 and VentureStar which was intended to replace the Space Shuttle, and the Roton SSTO, which is a helicopter that can get to orbit.

However, despite showing some promise, none of them have come close to achieving orbit yet due to problems with finding a sufficiently efficient propulsion system and discontinued development.

[1] Single-stage-to-orbit is much easier to achieve on extraterrestrial bodies that have weaker gravitational fields and lower atmospheric pressure than Earth, such as the Moon and Mars, and has been achieved from the Moon by the Apollo program's Lunar Module, by several robotic spacecraft of the Soviet Luna program, and by China's Chang'e 5 and Chang'e 6 lunar sample return missions.

[6] One of the earliest SSTO concepts was the expendable One stage Orbital Space Truck (OOST) proposed by Philip Bono,[7] an engineer for Douglas Aircraft Company.

It was one of the largest spacecraft ever conceptualized with a diameter of over 50 metres and the capability to lift up to 2000 short tons into Earth orbit, intended for missions to further out locations in the Solar System such as Mars.

[12][13][14] Further examples of Bono's early concepts (prior to the 1990s) which were never constructed include: Star-raker: In 1979, Rockwell International unveiled a concept for a 100-ton payload heavy-lift multicycle airbreather ramjet/cryogenic rocket engine, horizontal takeoff/horizontal landing single-stage-to-orbit spaceplane named Star-Raker, designed to launch heavy Space-based solar power satellites into a 300 nautical mile Earth orbit.

[26] The DC-X, short for Delta Clipper Experimental, was an uncrewed one-third scale vertical takeoff and landing demonstrator for a proposed SSTO.

Air-breathing designs typically fly at supersonic or hypersonic speeds, and usually include a rocket engine for the final burn for orbit.

[1] Whether rocket-powered or air-breathing, a reusable vehicle must be rugged enough to survive multiple round trips into space without adding excessive weight or maintenance.

[citation needed] While single-stage rockets were once thought to be beyond reach, advances in materials technology and construction techniques have shown them to be possible.

[citation needed] This inefficiency indirectly affects gravity losses as well; the vehicle has to hold itself up on rocket power until it reaches orbit.

Less horizontal thrust results in taking longer to reach orbit, and gravity losses are increased by at least 300 metres per second (1,100 km/h; 670 mph).

[32] Operational experience with the DC-X experimental rocket has caused a number of SSTO advocates to reconsider hydrogen as a satisfactory fuel.

[citation needed] Still, at very high altitudes, the extremely large engine bells tend to expand the exhaust gases down to near vacuum pressures.

[citation needed] Some designs for SSTO attempt to use airbreathing jet engines that collect oxidizer and reaction mass from the atmosphere to reduce the take-off weight of the vehicle.

[35] Some of the issues with this approach are:[citation needed] Thus with for example scramjet designs (e.g. X-43) the mass budgets do not seem to close for orbital launch.

[42] This growth factor sensitivity is shown parametrically for both SSTO and two-stage-to-orbit (TSTO) vehicles for a standard LEO mission.

[43] The curves vertically asymptote at the maximum structural coefficient limit where mission criteria can no longer be met: In comparison to a non-optimized TSTO vehicle using restricted staging, a SSTO rocket launching an identical payload mass and using the same propellants will always require a substantially smaller structural coefficient to achieve the same delta-v.

Given that current materials technology places a lower limit of approximately 0.1 on the smallest structural coefficients attainable,[44] reusable SSTO vehicles are typically an impractical choice even when using the highest performance propellants available.

Their proposal (Shuttle SERV) was an enormous vehicle with more than 50,000 kilograms (110,000 lb) of payload, utilizing jet engines for (vertical) landing.

Although the test program was not without mishap (including a minor explosion), the DC-X demonstrated that the maintenance aspects of the concept were sound.

That project was cancelled when it landed with three of four legs deployed, tipped over, and exploded on the fourth flight after transferring management from the Strategic Defense Initiative Organization to NASA.

[citation needed] The Aquarius Launch Vehicle was designed to bring bulk materials to orbit as cheaply as possible.

[citation needed] The British Government partnered with the ESA in 2010 to promote a single-stage to orbit spaceplane concept called Skylon.

[52][53] Elon Musk, CEO of SpaceX, has claimed that the upper stage of the prototype "Starship" rocket, currently in development in Starbase (Texas), has the capability to reach orbit as an SSTO.

[citation needed] Merely launching a large total number reduces the manufacturing costs per vehicle, similar to how the mass production of automobiles brought about great increases in affordability.

[citation needed] An alternative to scale is to make the discarded stages practically reusable: this was the original design goal of the Space Shuttle phase B studies, and is currently pursued by the SpaceX reusable launch system development program with their Falcon 9, Falcon Heavy, and Starship, and Blue Origin using New Glenn.