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 EADS-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.