Access to space is very expensive and it is one of the main reasons that keeps us from further exploring the solar system. With the arrival of partially reusable launchers, it may start to move. However, we are very far from the dream of a space vehicle that is as easy to use as a car or an airplane. This would imply a fully reusable vehicle and especially able to reach the Earth’s orbit without dropping its stages.
Multi-stage rockets have proved effective since the beginning of the space age
All rockets have a multistages design because the dead mass is the sworn enemy of access to the orbit. To have a chance to be orbited, the vast majority of the mass of a launcher must consist of propellants. By dropping the dead mass as and as the climb, we make sure to preserve this ratio and therefore the performance of the rocket. There are many ways to do this. We can stack stages, each with its own engines, one above the other or even next to each other.
With an architecture that uses boosters, we get two- or three-stage rockets that can additionally use several releasable boosters. It works quite well even if necessarily the possibilities of reuse of all the elements are limited. This design was quickly imposed in the first launchers of the space age. Some engineers have nevertheless tried to imagine single-stage launchers (SSTO), which is perceived as the key to total reuse and therefore a reduced cost of access to space.
Single stage launchers concepts (SSTO) face many technical challenges
If none of these machines are flying today, it is because this type of architecture imposes radical constraints. How to keep the mass of the launcher at minimum without throwing anything during the trip ? How to build an efficient engine at all stages of the flight, with a strong take-off power and a strong specific impulse once out of the atmosphere ? And especially how to insert a payload when the mass of the vehicle already poses many problems ? This is the puzzle that SSTO (Single Stage To Orbit) launchers are trying to solve, space vehicles with necessarily lower performances but which would be made up by a total reuse. There have been many study concepts on the subject and some of these projects are still in development.
As early as the 1960s, Philip Bono, an engineer at Douglas, imagined a series of concepts that were more and more advanced. The first concepts cheat a little by using jettisonable fuel tanks but very quickly it returns with architectures really SSTO, and especially reusable. His early work highlights the key technologies of such an architecture. For Philip Bono, an SSTO launcher must be able to take off and land vertically (VTVL = Vertical Takeoff and Vertical Landing). The launcher must take the form of a rocket and not a plane because the wings are a dead weight too important. Only the hydrogen-oxygen pair can offer the power and the specific impulse required for all phases of flight.
The Delta Clipper SSTO demonstrator was tested in the 1990s
Philip Bono’s ideas made a big impression at McDonnell Douglas which started developing the Delta Clipper demonstrator in the early 1990s. This demonstrator offers a good, small-scale overview of what this type of architecture would look like : conical shape of the rocket, hydrologic engines, modular thrust, lightweight materials and the need for thermal protection for atmospheric reentry.
The Delta Clipper made 12 flights. It has never been beyond 3 kilometers but this demonstrator has been able to demonstrate the high potential of reuse of this architecture with sometimes just more than 24 hours between two flights. The Delta Clipper program never led to an orbital model but some of the engineers who worked on the project are now at Blue Origin.
A space plane concept was developed by Lockeed Martin before being canceled
Although Philip Bono did not believe it, the idea of a vehicle in the shape of a plane seduced other teams. At about the same time that McDonnell Douglas was testing the Delta Clipper, Lockheed Martin was starting to work on the X-33. It was a demonstrator to test all the technologies needed to develop a SSTO vehicle.
Lockheed Martin and NASA had identified a set of technologies that could make possible such a project : a shape to facilitate atmospheric re-entry, a vertical take-off and a horizontal landing, aerospyke nozzles adapted to all the phases of the flight, the use of the hydrogen and liquid oxygen pair. The X-33 was largely to use composite materials to minimize the weight off fuel. For NASA, these were the keys to designing a vehicle that could flight again in a few days and require less maintenance.
The US Space Agency saw in this program the chance to make access to Earth’s orbit ten times safer and ten times cheaper. But that did not go so well. The project was canceled in 2001 due to technical difficulties. In particular, the manufacture of fuel tanks made of composite materials. It’s a shame when we know that cryogenic carbon fiber tanks have since made enormous progress.
The SSTO model does not interest many people because other proven solutions exist
Today, we can not say that SSTO vehicles generate a great deal of interest. Partial recovery seems to have been paying off since SpaceX proved that the technological leap is not so important. There is still a team in the UK who wants to believe in developing the Skylon space plane.
To really make SSTO architectures interesting, we may have to study other solutions than chemical propulsion. With nuclear energy, we can have both a high power and a large specific impulse. Nuclear engines reached advanced stages of development as early as the 1960s. Thermal nuclear propulsion would provide thrust by relaxing hydrogen through the heat of a reactor. This would result in a strong thrust, but which remains insufficient to propel a first stage or a SSTO vehicle.
No space vehicle SSTO will be launched before long
John Bucknell, a former engineer on SpaceX’s Raptor engine, imagined in 2015 a way to improve the power and efficiency of such a system. In the manner of the Skylon space plane, it would be a matter of drawing air into the atmosphere to add a combustion cycle to thermal nuclear propulsion. In such an engine, the hydrogen is first expanded by being heated by a nuclear reactor and then it is injected into a combustion chamber where it burns in contact with atmospheric air. This results in a large increase in the thrust and the specific impulse during the first phase of the flight.
John Bucknell believes that such a system would produce a truly powerful SSTO rocket capable of delivering a large payload into orbit and beyond. It should obviously be reusable to be economically viable because the applied technologies are very complex. Such an engine will probably not be developed for a long time, nor an SSTO orbital vehicle. The constraints still seem much too important compared to the expected gain.
So far another track is envisaged to allow the total reuse of space vehicles. The SpaceX Starship and its booster keep the architecture in two stages but we try to recover them separately. No need for a fundamental technological break. If the Starship manages one day to land on the Moon or on the planet Mars, it will have to take off again to land on Earth, like a real SSTO vehicle.
Images by A. Mann / Glenn Research Center [Public domain] / NASA/MSFC [Public domain]