Pre-cooler concept gains credibility as tests of flight-weight unit progress
Seeing is believing, as the saying goes. That's certainly what Reaction Engines hopes as it moves deeper into advanced tests of a novel pre-cooler that it believes could revolutionize air-breathing access to space.
Designed to chill engine inlet air by more than 1,150C (2,100F) in less than one hundredth of a second, the pre-cooler is one of the key enabling technologies at the heart of the Sabre (synergetic air-breathing rocket engine). The hybrid propulsion system is designed for either atmospheric transport at speeds around Mach 5 or as the air-breathing element of a single-stage-to-orbit spaceplane dubbed Skylon.
Having fought for credibility for more than two decades, the small U.K.-based team behind the development of Skylon and its combined-cycle rocket engine are seeing growing interest in the technology, as crucial tests of the pre-cooler element continue at its site near Oxford, England.
The pre-cooler functions as a heat exchanger, cooling air from the atmosphere to cryogenic temperatures as it enters the engine via an axisymmetric inlet. The denser air passes through a turbo-compressor and into the rocket combustion chamber where it becomes an oxidizer to be burned with sub-cooled liquid hydrogen fuel. A closed-cycle helium loop forms part of the heat exchanger system and uses hydrogen fuel as a heat sink before it enters the combustion chamber.
“We breathe air for as long as we can up to M 5.5,” says Reaction Engines' Roger Longstaff. After this point, the Sabre transitions to pure rocket mode, burning a combination of liquid hydrogen and liquid oxygen from internal tanks. Throughout most of the atmospheric flight, the intake captures more air than required. Excess air passes down a spill duct that incorporates a burner to recover some of the drag losses.
While the concept itself is not new, no one has so far succeeded in implementing a rocket-based combined-cycle propulsion system that derives the oxidizer from the atmosphere. This is why the main focus for the initial testing is on the pre-cooler and anti-frost systems; the basic feasibility of both was confirmed during independent audits carried out in 2011 by experts from the European Space Agency.
“To date, we have undertaken over 200 test runs of varying duration and purpose,” says Longstaff, who adds that earlier issues with the test rig that slowed the pace of work this summer appear to be resolved. “I can confirm that the test rig is performing nominally and testing continues.” Initial tests of the pre-cooler to date “have confirmed aerodynamic stability and uniformity, structural integrity, freedom of vibration across a wide range above and beyond the flight envelope.”
Following preliminary cooling runs, the current test phase aims to evaluate the system at much colder inlet temperatures down to around -150C and beyond. “Tests of the full-scale pre-cooler unit will demonstrate absolutely everything at proper Reynolds numbers, flow rate—the works. Frost prevention was demonstrated at laboratory scale and is now at full scale,” says Longstaff, who adds that additional work includes evaluation of a contra-rotating turbine for the helium loop.
The pre-cooler on test is made up of 21 helical sections nested together. Made from Inconel 718, the module contains more than 30 mi. of very fine tubes 1 mm in diameter and with walls around 27 microns thick. Despite the number of tubes, the assembly weighs less than 110 lb.
“Completion of the pre-cooler testing will wrap up the most significant part of 'Phase 2,' although there are some additional development activities that have been ongoing alongside the pre-cooler testing,” says Longstaff, who describes the heat exchanger testing as “the headline item.”
Other technology demonstrations either underway or completed include wind-tunnel tests of the engine intake aerodynamics and development tests with-Astrium and German space agency DLR of combustion chamber cooling using liquid oxygen and hydrogen film cooling. Work is also underway on development of other lightweight heat-exchanger systems needed in the Sabre, new types of engine igniters and low-nitrous-oxide combustors capable of emissions below 100 ppm for atmospheric cruise applications.
The cooler work “effectively marks an end to Phase 2 as we make a soft start to Phase 3 over the 2012-14 period,” Longstaff explains. This is planned to culminate with a sub-scale ground engine demonstrator around 2015. “It will be a boiler-plate simulation of all the components but with a flight-weight pre-cooler,” he adds.
The test is expected to take the engine to a higher, pre-flight test technology readiness level, and will include feeding vitiated air at temperatures up to 1000C into the inlet to simulate hypersonic airflow conditions.
Reaction Engines says the sub-scale ground demonstrator “will be used to provide data for the first full-scale Sabre engine. “Flight testing of engine systems will be undertaken in parallel with the ground testing in order to provide, for example, data on the nacelle supersonic and hypersonic aerodynamics,” adds the company. Ground- and flight testing of the first prototype Sabre engine would take place two to three years after the sub-scale testing periods commence.
The exact nature and scale of flight testing remains to be determined, but could include evaluations of modified scaled nozzles on sounding rockets, or even on a specially developed sub-scale vehicle. “While not baselined for the Sabre engine, altitude-compensating nozzles offer the possibility to increase the performance of a rocket engine that has to operate from sea level to the vacuum of space. Reaction Engines has undertaken research in this area, and we are exploring both internally and with ESA, future development options for this technology which could include ground and flight testing,” Longstaff adds.
The engine work is ultimately aimed at powering the Skylon D1 spaceplane, which is designed to deliver 15 metric tons of payload to low Earth orbit. One of the key design challenges of the vehicle is the high speed required for takeoff, and some work is focused on potential improvements to lower liftoff speed.
“The vehicle has a 'straight delta' wing,” says Longstaff. “Various suggestions have been made to enhance takeoff performance including the use of high lift-wing devices, such as gurney flaps. Initial low-speed wind-tunnel tests have been promising, but need further work. In practice, we would expect an airframe prime developer to investigate such enhancements, based upon our design,” he adds.
Given the current development schedule, Longstaff says “we think we could be flying by 2020.”