The SABRE engine must be able to function as a turbojet engine, a ramjet engine or a rocket engine. There are still many technical challenges to solve in order to make this engine work. In particular, it needs a high-performance heat exchanger that can cool incoming air at a temperature of 1,000 degrees Celsius in a twentieth of a second.
The SABRE engine heat exchanger has been successfully tested in a wind tunnel located in Colorado. The device designed by Reaction Engine Limited has safely beared a hot air flow to Mach 5. It has cooled enough the air to be used by the SABRE engine. It is therefore one of the most critical elements of the Skylon space plane project that has just demonstrated its viability. There is still a lot of work to do before seeing an SSTO aircraft reach Earth’s orbit, but the British company is clearly moving in the right direction.
Why does the SABRE engine has a curved shape ?
– News of March 24, 2019
The shape of the SABRE engine can be surprising. Over its entire length, it has a total inclination of 14 degrees. In reality, the reactor does not need to be curved. This, however, is necessary for the operation of the Skylon spaceplane which will fly with a small angle of incidence to maintain its lift. In this configuration, it is necessary that the air inlets of the SABRE engine are in front of the air flow, while the output propulsion must be in the direction of the center of mass of the aircraft.
This is also why the Lockheed SR-71 Blackbird reactors are pointing slightly downwards. At high altitude, it also had to fly with a small angle of incidence. In general, we can also observe these small angles on commercial aircrafts. The curvature of the SABRE engine seems very great because it has to fly at a very high speed and very high altitude.
Once the reactor is developed, there will still be a lot of work to be done to move from the SABRE project to the Skylon spaceplane. The spaceplane is for example designed to enter the atmosphere without thermal tiles. Its shape aims to reduce temperatures on hot spots. The outer surface of Skylon will be designed in a special ceramic. The Skylon spaceplane will need a runway of at least 5 km to land, which will probably need to be built.
ESA has validated the preliminary design of the SABRE engine
– News of March 19, 2019 –
The objective of the Skylon space plane is to reach the orbit without dropping anything from space. This type of flight plan has never been finalized. This would however greatly facilitate the full reuse of orbital launchers. To achieve this, Skylon wants to use a unique engine named SABRE.
The SABRE engine is designed to use the Earth’s atmosphere at take-off and during the start of the space plane’s ascent. The engine takes oxygen directly into the atmosphere. Once at an altitude of 25 kilometers and at Mach 5,4 the Skylon space plane tilts on internal tanks and finishes its journey like a classic rocket.
This should allow Skylon to save a few tons of propellant and thus make the SSTO (Single-Stage-To-Orbit) strategy viable. However this is very difficult to develop. Reaction Engines, the company working on SABRE and Skylon, tries to develop this technology since a long time. The main difficulty is to design a very efficient heat exchanger. It must be able to cool the air entering the reactor at -150 degrees in a fraction of a second.
But SABRE and Skylon are of interest to many people, including big investors like Rolls Royce and Boeing, who poured $ 38 million into Reaction Engines a year ago. The European Space Agency is also following the development of the engine closely. ESA and the British space agency have validated the preliminary design of SABRE.
Mark Ford, head of ESA’s propulsion section, spoke of a major milestone in engine development. The European Space Agency has been involved in the project since 2010. It helped finance heat exchanger tests in 2012. By validating the design of the engine, ESA is paving the way for the construction of its first prototype.
But before that, there are still two major tests to perform. In about a month, the heat exchanger will be put on a test site in the United States. To simulate the conditions encountered at Mach 5 in the upper atmosphere, it will cool 1000 degrees incoming air in one twentieth of a second. To achieve this, the heat exchanger is traversed by thousands of small tubes which offer a large heat exchange surface in a very small mass.
The reactor core will be tested in the United Kingdom. This is the central part of the engine, without the heat exchanger and without the nozzle. During a series of tests scheduled to start this year until 2020, the reactor core will have to prove that it is able to accelerate from 0 to Mach 5 using only ambient air.
If Reaction Engines manages to develop the SABRE engine, it could have many applications. In a space plane, the SABRE engine could pave the way for a fully reusable and one-piece orbital vehicle. Maybe this will greatly reduce the cost of access to space. It could also be used as a suborbital propulsion mode.
Having a reactor capable of handling all the propulsive phases from 0 to Mach 5 could also be of great interest to civil and military aviation. But before we get there, there is still a lot of work. Developing the SABRE engine is only part of the problem in developing an easily reusable orbital plane.
Image by Science Museum London Picture Library