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Launch system that only uses one rocket stage

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 term usually, but not exclusively, refers to reusable vehicles. To date, no Earth launched SSTO launch vehicles have ever been flown; orbital launches from Earth have been performed by either fully or partially expendable multi stage rockets. The main projected advantage of the SSTO concept is elimination of the hardware replacement inherent in expendable launch systems. However, the non recurring costs associated with design, development, research and engineering (DDR&E) of reusable SSTO systems are much higher than expendable systems due to the substantial technical challenges of SSTO, assuming that those technical issues can in fact be solved. SSTO vehicles may also require a significantly higher degree of regular maintenance. It is considered to be marginally possible to launch a single stage to orbit chemically fueled spacecraft from Earth. The principal complicating factors for SSTO from Earth are: high orbital velocity of over ; the need to overcome Earth's gravity, especially in the early stages of flight; and flight within Earth's atmosphere, which limits speed in the early stages of flight due to drag, and influences engine performance. 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, 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. One of the earliest SSTO concepts was the expendable One stage Orbital Space Truck (OOST) proposed by Philip Bono, an engineer for Douglas Aircraft Company. A reusable version named ROOST was also proposed. Another early SSTO concept was a reusable launch vehicle named NEXUS which was proposed by Krafft Arnold Ehricke in the early 1960s. 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. The North American Air Augmented VTOVL from 1963 was a similarly large craft which would have used ramjets to decrease the liftoff mass of the vehicle by removing the need for large amounts of liquid oxygen while traveling through the atmosphere. From 1965, Robert Salkeld investigated various single stage to orbit winged spaceplane concepts. He proposed a vehicle which would burn hydrocarbon fuel while in the atmosphere and then switch to hydrogen fuel for increasing efficiency once in space. Further examples of Bono's early concepts (prior to the 1990s) which were never constructed include:
ROMBUS (Reusable Orbital Module, Booster, and Utility Shuttle), another design from Philip Bono. This was not technically single stage since it dropped some of its initial hydrogen tanks, but it came very close. Ithacus, an adapted ROMBUS concept which was designed to carry soldiers and military equipment to other continents via a sub orbital trajectory. Pegasus, another adapted ROMBUS concept designed to carry passengers and payloads long distances in short amounts of time via space. Douglas SASSTO, a 1967 launch vehicle concept. Hyperion, yet another Philip Bono concept which used a sled to build up speed before liftoff to save on the amount of fuel which had to be lifted into the air. 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. Star raker would have had 3 x LOX/LH2 rocket engines (based on the SSME) + 10 x turboramjets. At around the same time, the HOTOL tried to use precooled jet engine technology, but failed to show significant advantages over rocket technology.

The DC X, short for Delta Clipper Experimental, was an uncrewed one third scale vertical takeoff and landing demonstrator for a proposed SSTO. It is one of only a few prototype SSTO vehicles ever built. Several other prototypes were intended, including the DC X2 (a half scale prototype) and the DC Y, a full scale vehicle which would be capable of single stage insertion into orbit. Neither of these were built, but the project was taken over by NASA in 1995, and they built the DC XA, an upgraded one third scale prototype. This vehicle was lost when it landed with only three of its four landing pads deployed, which caused it to tip over on its side and explode. The project has not been continued since.

From 1999 to 2001 Rotary Rocket attempted to build a SSTO vehicle called the Roton. It received a large amount of media attention and a working sub scale prototype was completed, but the design was largely impractical.

There have been various approaches to SSTO, including pure rockets that are launched and land vertically, air breathing scramjet powered vehicles that are launched and land horizontally, nuclear powered vehicles, and even jet engine powered vehicles that can fly into orbit and return landing like an airliner, completely intact. For rocket powered SSTO, the main challenge is achieving a high enough mass ratio to carry sufficient propellant to achieve orbit, plus a meaningful payload weight. One possibility is to give the rocket an initial speed with a space gun, as planned in the Quicklaunch project. For air breathing SSTO, the main challenge is system complexity and associated research and development costs, material science, and construction techniques necessary for surviving sustained high speed flight within the atmosphere, and achieving a high enough mass ratio to carry sufficient propellant to achieve orbit, plus a meaningful payload weight. Air breathing designs typically fly at supersonic or hypersonic speeds, and usually include a rocket engine for the final burn for orbit. It has a sufficiently efficient engine to achieve orbit, but without carrying much payload.

Some SSTO concepts use the same engine for all altitudes, which is a problem for traditional engines with a bell shaped nozzle. Depending on the atmospheric pressure, different bell shapes are required. Engines designed to operate in a vacuum have large bells, allowing the exhaust gasses to expand to near vacuum pressures, thereby raising efficiency. Due to an effect known as Flow separation, using a vacuum bell in atmosphere would have disastrous consequences for the engine. Engines designed to fire in atmosphere therefore have to shorten the nozzle, only expanding the gasses to atmospheric pressure. The efficiency losses due to the smaller bell are usually mitigated via staging, as upper stage engines such as the Rocketdyne J 2 do not have to fire until atmospheric pressure is negligible, and can therefore use the larger bell. One possible solution would be to use an aerospike engine, which can be effective in a wide range of ambient pressures. In fact, a linear aerospike engine was to be used in the X 33 design. Other solutions involve using multiple engines and other altitude adapting designs such as double mu bells or extensible bell sections. Still, at very high altitudes, the extremely large engine bells tend to expand the exhaust gases down to near vacuum pressures. As a result, these engine bells are counterproductive due to their excess weight. Some SSTO concepts use very high pressure engines which permit high ratios to be used from ground level. This gives good performance, negating the need for more complex solutions.

Due to weight issues such as shielding, many nuclear propulsion systems are unable to lift their own weight, and hence are unsuitable for launching to orbit. However, some designs such as the Orion project and some nuclear thermal designs do have a thrust to weight ratio in excess of 1, enabling them to lift off. Clearly, one of the main issues with nuclear propulsion would be safety, both during a launch for the passengers, but also in case of a failure during launch. As of February 2024, no current program is attempting nuclear propulsion from Earth's surface.

Because they can be more energetic than the potential energy that chemical fuel allows for, some laser or microwave powered rocket concepts have the potential to launch vehicles into orbit, single stage. In practice, this area is not possible with current technology.

It is easier to achieve SSTO from a body with lower gravitational pull than Earth, such as the Moon or Mars. The Apollo Lunar Module ascended from the lunar surface to lunar orbit in a single stage. A detailed study into SSTO vehicles was prepared by Chrysler Corporation's Space Division in 1970–1971 under NASA contract NAS8 26341. Their proposal (Shuttle SERV) was an enormous vehicle with more than of payload, utilizing jet engines for (vertical) landing. While the technical problems seemed to be solvable, the USAF required a winged design that led to the Shuttle as we know it today. The uncrewed DC X technology demonstrator, originally developed by McDonnell Douglas for the Strategic Defense Initiative (SDI) program office, was an attempt to build a vehicle that could lead to an SSTO vehicle. The one third size test craft was operated and maintained by a small team of three people based out of a trailer, and the craft was once relaunched less than 24 hours after 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. The Aquarius Launch Vehicle was designed to bring bulk materials to orbit as cheaply as possible.

Current and previous SSTO projects include the Japanese Kankoh maru project, ARCA Haas 2C, Radian One and the Indian Avatar spaceplane.

The British Government partnered with the ESA in 2010 to promote a single stage to orbit spaceplane concept called Skylon. This design was pioneered by Reaction Engines Limited (REL), a company founded by Alan Bond after HOTOL was canceled. The Skylon spaceplane has been positively received by the British government, and the British Interplanetary Society. Following a successful propulsion system test that was audited by ESA's propulsion division in mid 2012, REL announced that it would begin a three and a half year project to develop and build a test jig of the Sabre engine to prove the engines performance across its air breathing and rocket modes. In November 2012, it was announced that a key test of the engine precooler had been successfully completed, and that ESA had verified the precooler's design. The project's development is now allowed to advance to its next phase, which involves the construction and testing of a full scale prototype engine.

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. However he concedes that if this was done, there would be no appreciable mass left for a heat shield, landing legs, or fuel to land, much less any usable payload.

Many studies have shown that regardless of selected technology, the most effective cost reduction technique is economies of scale. 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. Using this concept, some aerospace analysts believe the way to lower launch costs is the exact opposite of SSTO. Whereas reusable SSTOs would reduce per launch costs by making a reusable high tech vehicle that launches frequently with low maintenance, the "mass production" approach views the technical advances as a source of the cost problem in the first place. By simply building and launching large quantities of rockets, and hence launching a large volume of payload, costs can be brought down. This approach was attempted in the late 1970s, early 1980s in West Germany with the Democratic Republic of the Congo based OTRAG rocket. This is somewhat similar to the approach some previous systems have taken, using simple engine systems with "low tech" fuels, as the Russian and Chinese space programs still do. 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.