Current space propulsion systems based on rocket technology alone place limits on the performance of launch vehicles, which leads to multi-stage vehicle configurations that are either expendable or only partly reusable. This results in complex launch systems that are both high maintenance and expensive to operate. REL’s Skylon space plane, a fully reusable single-stage launch vehicle, addresses these drawbacks and could revolutionise future access to space by providing more efficient and reliable launch operations.
Fundamental to its design is the propulsion system that powers it from a horizontal runway launch through to Earth orbit (out to 600km). The Synergetic Air-Breathing Rocket Engine (SABRE) is a new type of rocket engine that incorporates elements of a jet engine into its design. The SABRE, and in particular its advanced heat-exchanger technology, provides a breakthrough in propulsion technology that offers greatly improved reliability, a tenfold reduction in cost, and responsive access to space.
Whilst conventional rocket engines require vast amounts of liquid oxygen as fuel, the SABRE engine takes advantage of the oxygen in the surrounding atmosphere, which reduces the required oxidizer by over 250 tonnes. Once outside of the atmosphere, the rocket phase requires onboard oxygen. During take-off and initial ascent the SABRE engine runs in jet engine mode so it can use the oxygen from the air. Until REL’s breakthrough technology, this configuration was not possible.
As the air intake speed increases, the air intake temperature also increases. By the time it is travelling at Mach 5.5 the air intake temperature is around 1,000°C. At this temperature conventional jet engines cannot operate—materials begin to fail and the air cannot be effectively compressed. REL’s solution to this problem is to super cool the air from 1,000°C to cryogenic temperatures (-150°C) in less than a hundredth of a second with a pre-cooler.
Flight weight is the point at which a component’s performance justifies its weight. The pre-cooler required the development of a new generation of heat exchangers that not only transfer 400MW of heat, but can do it at flight weight (that is, 100 times lighter than conventional heat exchangers).
Validating the enabling technology
To validate the pre-cooler technology, REL built and tested a representative section of a full-size pre-cooler comprising about 27km of 1mm diameter tubes with wall thickness less than that of a human hair. The construction of this pre-cooler module demonstrated the level of engineering precision required, including the attachment of the 1mm diameter tubes with over 15,000 precision-brazed joints.
After demonstrating the manufacturing technology, the next task was to prove the pre-cooler was an efficient heat exchanger and was robust enough for its challenging operating environment. This required demonstrating the pre-cooler in actual working air flows and cryogenic conditions.
The required air mass flows are immense and cannot be generated with electric fans. Instead, a Viper 522 jet engine is deployed to pull a high air flow mass through the pre-cooler. In the actual SABRE engine, liquid hydrogen provides the cooling, but to simplify the test conditions, liquid nitrogen substituted the liquid hydrogen as the cooling source.
On the SABRE engine there are many complexities to pass the liquid hydrogen through the pre-cooler so a secondary cooling circuit of helium exchanges the heat between the precooler and the liquid hydrogen. Similarly, on the test rig a secondary cooling circuit of helium exchanges the heat between the precooler and the liquid nitrogen. Though the test rig only represented a fraction of the SABRE pre-cooler, the heat transferred was such that it would boil over a ton of liquid nitrogen every five minutes.
Validation system implementation
REL instrumented the whole test system using NI LabVIEW system design software and NI CompactDAQ measurement hardware. The modular nature of the NI CompactDAQ platform enabled quick and easy customisation of the data acquisition system, which included a combination of analogue and digital modules to take in a range of signals such as engine speed, air and component temperatures (between -196°C and 1,000°C), as well as the air pressure (up to 250bar) and flow throughout the system.
For safety reasons, the technicians could be in close proximity to this system during the test. Flexible NI-DAQmx drivers were implemented so that acquisition devices could be used over a variety of communication buses with the same application programming interface. For the main pre-cooler diagnostics, a local 8-slot NI cDAQ-9188 chassis communicating via Ethernet was used. A four-slot NI cDAQ-9174 chassis communicating over USB was located within the control room to interface with the plant control system.
LabVIEW helped bring together all the different system components under one application. The NI LabVIEW Datalogging and Supervisory Control Module provided seamless integration with the system controller - a Mitsubishi PLC - and also helped the development of the SCADA user interface (UI). The team took advantage of flexible UI development tools offered by LabVIEW to create one that spanned four monitors and displayed over 150 signals on 28 live charts.
All the data acquired needed extensive analysis, so logging was deemed important. In order to achieve the incredibly fast write speeds necessary to capture all the data, NI’s Technical Data Management Streaming (TDMS) was implemented. In the end, the test team took advantage of only a small fraction of the TDMS maximum write speed of 400Mb/s, but as REL embarks on the testing of larger systems that integrate other technologies, it is confident that the software application can scale to fit.
The future
The European Space Agency has evaluated the SABRE engines pre-cooler heat exchange on behalf of the United Kingdom Space Agency and has given official validation of the test results. Now that the pre-cooler has been successfully validated, the test bed can be reconfigured to test other subassemblies before all the subsystems are brought together into a full-size demonstration SABRE engine to commence ground testing.
Michael Hood is with Reaction Engines
REL was winner in the Physical Test & Measurement category of the National Instruments Engineering Impact Awards 2014.