Single-stage-to-orbit

A single-stage-to-orbit (SSTO) vehicle is a special kind of spacecraft. It can reach space from the ground using only its own fuels and fluids. It does not need to throw away big parts like tanks or engines during its trip. This idea is usually about vehicles that can fly again and again. So far, no Earth launch has used this kind of vehicle. Most space launches from Earth have used rockets with several stages that are thrown away after use.

The big advantage of SSTO vehicles is that they could save money. They would not need to replace parts after each flight. But making these vehicles is very hard and costs a lot of money. They might also need more regular checks and repairs.

It is thought to be just possible to send a chemically fueled SSTO spacecraft from Earth into space. But there are big challenges. The spacecraft needs to go very fast—over 7,400 meters per second—and deal with Earth’s gravity and air.

Recent advances in rocket technology have made it cheaper to send things into space. This reduces the main reason why people thought SSTO vehicles would be better. Some famous ideas for SSTO vehicles include Skylon, DC-X, Lockheed Martin X-33, VentureStar, and Roton SSTO. But none of these have successfully reached space yet.

It is much easier to achieve single-stage-to-orbit on places like the Moon and Mars. These places have weaker gravity and less air than Earth. The Apollo program’s Lunar Module and several other missions from the Moon have done this.

History

Early concepts

Before the mid-1900s, not much research was done on space travel. In the 1960s, some of the first ideas for this kind of spacecraft began.

One of the earliest ideas was the One stage Orbital Space Truck (OOST), proposed by an engineer named Philip Bono. A reusable version called ROOST was also suggested.

Another early idea was a large reusable spacecraft named NEXUS, proposed by Krafft Arnold Ehricke in the early 1960s. It was planned to be very big, with a diameter of over 50 metres, and could carry up to 2000 short tons into Earth orbit, for missions to places far out in the Solar System like Mars.

From 1965, Robert Salkeld looked into different single stage to orbit winged spaceplane ideas. He suggested a spacecraft that would use hydrocarbon fuel in the atmosphere and then switch to hydrogen fuel once in space for better efficiency.

Further examples of Bono's early ideas (before the 1990s) that were never built include:

• ROMBUS (Reusable Orbital Module, Booster, and Utility Shuttle), another design by 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 idea designed to carry soldiers and military equipment to other continents via a sub-orbital path.
• Pegasus, another adapted ROMBUS idea designed to carry passengers and cargo long distances quickly through space.
• Douglas SASSTO, a 1967 launch vehicle idea.
• Hyperion, yet another Philip Bono idea which used a sled to gain speed before takeoff to save fuel.

Star-raker: In 1979, Rockwell International showed 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 parts to satellites to a place in a 300 nautical mile Earth orbit to help build Space-based solar power satellites in geosynchronous orbit. Star-raker would have had 3 x LOX/LH2 rocket engines (based on the SSME) + 10 x turboramjets.

Around 1985 the NASP project wanted to launch a scramjet vehicle into orbit, but funding stopped and the project was cancelled. At about the same time, the HOTOL tried to use precooled jet engine technology, but it did not show big benefits over rocket technology.

DC-X technology

Main article: McDonnell Douglas DC-X

The DC-X, short for Delta Clipper Experimental, was an uncrewed one-third scale test vehicle for a proposed SSTO. It is one of only a few prototype SSTO vehicles ever built. Several other test vehicles were planned, including the DC-X2 (a half-scale test vehicle) and the DC-Y, a full-scale vehicle which would be able to reach orbit on its own. 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 test vehicle.

Roton

Main article: Rotary Rocket

From 1999 to 2001 Rotary Rocket tried to build a SSTO vehicle called the Roton. It got a lot of media attention and a working small version was finished, but the design was not practical.

Approaches

There have been many ideas for single-stage-to-orbit (SSTO) vehicles. These include rockets that launch and land vertically, special planes that launch and land like airplanes, and even vehicles powered by nuclear energy.

For rocket-powered SSTO vehicles, the biggest challenge is carrying enough fuel to reach orbit while also carrying a payload. One idea is to give the rocket a boost using a space gun, like in the Quicklaunch project.

For vehicles that use special engines to breathe air, the challenges include complexity, cost, and needing special materials to survive high speeds. These vehicles usually fly at supersonic or hypersonic speeds and often need a rocket engine for the final push into orbit.

Both types of vehicles must be strong enough to make many trips into space without becoming too heavy or needing too much maintenance. They also need to return safely without damage.

While single-stage rockets were once thought to be impossible, new materials and techniques suggest they could work. For example, the Titan II rocket’s first stage, if used alone, had enough fuel and engine power to reach orbit.

Design challenges inherent in SSTO

Designing a vehicle that can reach space from the ground using only one stage is very hard. Experts say that a two-stage vehicle will usually carry more cargo than a single-stage one. Only if the vehicle is very light can a single-stage vehicle do as well. Small mistakes can mean the vehicle carries no cargo at all, making it difficult to succeed.

The performance of a rocket stage depends on its ability to change speed, called delta-v. This is affected by how much fuel the rocket uses and how heavy the rocket is when it starts. The heavier the rocket compared to its fuel and cargo, the harder it is to reach space. For a single-stage vehicle to work, it needs to be almost perfectly designed, which is very challenging with today’s technology.

Examples

It is easier to build a vehicle that can reach space from places with weaker gravity, like the Moon or Mars. The Apollo Lunar Module went from the Moon's surface to orbit around the Moon in one step.

In the early 1970s, Chrysler Corporation studied a big vehicle for space travel. Their idea, called the Shuttle SERV, was very large and could carry a lot of cargo. It would use jet engines to land. But the military wanted wings on the spacecraft, which led to the Shuttle we know today.

The uncrewed DC-X was a test spacecraft made by McDonnell Douglas. It showed that keeping such a vehicle could be simple, even after landing quickly. However, the project ended when the spacecraft had a small accident during a test.

The Aquarius Launch Vehicle was designed to carry large amounts of supplies into space at a lower cost.

Current development

Today and in the past, projects working on this idea include the Japanese Kankoh-maru project, ARCA Haas 2C, Radian One, and the Indian Avatar spaceplane.

Skylon

Main article: Reaction Engines Skylon

In 2010, the British Government worked with the ESA to support a spaceplane idea called Skylon. This design was started by Reaction Engines Limited (REL). After testing parts of the engine in 2012, the team moved forward to build a full engine for testing.

Alternative approaches to inexpensive spaceflight

Many studies show that using many rockets at once can save money on space travel. This is like how making lots of cars can lower prices for everyone. Some experts think this is better than trying to build one rocket that can fly many times.

In the past, some countries tried building many rockets quickly to save money. Today, companies like SpaceX and Blue Origin work on making rockets that can be used again, which also helps save money.