As the hard-won progress in hypersonic propulsion technology can attest, the road to sustained air-breathing engines that can operate above Mach 5 is littered with five decades worth of failed tests, some more dramatic than others.
Australia's ambitious Scramspace project is the latest pioneering hypersonic project to fall victim to a flawed rocket booster that had nothing to do with the payload. The 2.5-year project ended in bitter frustration for the team on Sept. 17 at the remote northern Norwegian Andoya rocket range when the nozzle of the S30 first-stage booster disintegrated on launch, taking the guidance fins with it. From that second on, the scramjet payload was doomed and, after a short, but wildly dynamic flight, it splashed down into the sea without reaching anything near its intended test conditions.
“We feel robbed,” says Russell Boyce, director of the Scramspace project and lead for the University of Queensland-led flight. Yet, aside from the misfortune of launching on a flawed version of the normally dependable Brazilian-made S30, the Scramspace team is able to take a surprising number of positives from the flight, the vehicle, its various experiments and the program experience as a whole.
“The payload collected data from launch to splashdown,” says Boyce. Shortly after the rocket nozzle disintegration at launch, the vehicle went out of control. “As the thing ascended at around 200 meters (656 ft.) or so we saw a fireball come out of the motor, which was the nozzle throat coming out. The upper stage fell off the front and began a violent flight. The upper stage immediately went into a corkscrew and made it up about 5 km (3.1 mi.) before coming down 100 meters off the breakwater [of the local town of Andennes]. The dynamic pressure the payload was experiencing was extreme, but it remained intact,” he adds.
Positioning data from the onboard sensors, GPS and navigation unit subsequently helped divers locate the remains of the payload in 8 meters of water. “The carbon-carbon fins were smashed to bits, but they were all there. Everybody is calling it an engineering success because it functioned and it entered the water intact. It was ready to do a scramjet test,” Boyce says.
Immediately after the failure, an international investigation board began looking into the cause; all pending launches using the S30 were suspended until further notice. The solid-fueled S-30 motor (also known as the VS-30) is developed by Centro Tecnico Aerospacial (CTA) in Sao Jose dos Campos, Brazil, and has become a popular sounding rocket. Suspension of its use, albeit temporary, also threatened to have a major impact on the coming launch schedule at the range, which includes several VS-30-powered flights between now and late 2014. One of these is a repeat test of the U.S. Air Force/Australian Defense Science and Technology Organization (DSTO) hypersonic international flight Research Experiment (Hifire) Flight 5, which will use the same booster combination as the failed Scramspace test.
The hydrogen-fueled vehicle was intended to be boosted by a two-stage S30/Improved Orion sounding rocket to an apogee of 200 mi. and reoriented exoatmospherically by a cold gas reaction control system, before reaching a gravity-assisted speed of around Mach 8 on descent toward the Norwegian Sea. The aim was to test the operation of the scramjet for just over 2 sec. as it descended through the upper atmosphere at an altitude between 16-19 mi. Despite its seemingly short duration, the test would have provided a wealth of data, according to the team, which included DSTO, Australia's University of New South Wales, DLR of Germany, CIRA (Italy's aerospace research center), Japan's, , Teakle and Aimtek. By comparison, approximately only 30 sec. of total hypersonic test time have been accumulated during 11,000 shock tunnel tests run by Australia's University of Queensland over the past 23 years.
The axisymmetric inlet-injection Scramspace was to be the first free-flight test of a hypersonic combustion concept called “radical farming.” This is considered a key Australian-developed technology with a potential application to higher Mach number propulsion concepts such as multi-staged space access vehicles, or future transports. A brass-board, captive-carry version of the concept was also successfully tested during the Hifire HF-3 flight from Andoya in September 2012.
Unlike the recent U.S. Air Force-led X-51A hypersonic tests, which were powered by a scramjet burning hydrocarbon fuel injected into the combustor section of the flow path, Scramspace burns hydrogen fuel injected through holes in the inlet rather than in the combustion chamber. Leading-edge shocks are deliberately ingested via the main inlet and elongated starter doors, where they interact with other shock waves and expansion flows in the constant-area combustor. The combination produces an area of localized higher pressure and temperatures where “radicals” are formed. This not only helps accelerate the ignition process, but achieves combustion at lower mean static temperatures and pressures than would be created in a larger combustion chamber. As radical farming injects fuel in the intake, this reduces the mixing length needed in the combustor, therefore enabling the use of a smaller chamber, which also reduces skin-friction drag.
The flight was also due to test more than the scramjet. Secondary and tertiary experiments included a tunable-diode laser absorption spectroscopy (Tdlas) flight instrument to measure flow, temperature and possibly angle-of-attack data, an integrated inertial navigation system processor (INSP) and ultra-high-temperature ceramic materials made by CIRA. Both the laser and navigation system provided data which are still undergoing analysis, says Boyce. “We did detect inlet pressure, which means the nose cone came off. Even though the laser did not go hypersonic, it got data at about 200 meters per second,” he adds.
The Tdlas, developed at the University of New South Wales-Canberra, was integrated into the inlet and projected a beam across the flow to measure velocity and temperature. Sven Wittig, a laser system engineer involved in the Tdlas development, says: “Part of the issue is that a pitot is either going to melt or is bad for the flow, so the idea is to use light instead of matter and put a laser in front of the flow.” The system works by measuring the Doppler shift in the peak of the absorption spectrum of oxygen. “Most people use water vapor because there is always enough in the air to do this, but for hypersonic conditions at higher altitudes, there is less water vapor in the air, so oxygen is used. However, because the signal is weak and hard to read, the Tdlas incorporates multiple reflectors to produce a parallel flightpath across the flow which is read by a photo diode. The system was tested at Mach 6 at the University of Southern Queensland before installation in Scramspace.
The INSP was designed to provide angle-of-attack and side-slip angle data out of the atmosphere using two Kalman filters to fuse together information from a GPS, accelerometer, magnetometer, gyroscope and aerodynamic database. Starting with the de-spin maneuver, which was to have steadied the vehicle when it exited the atmosphere, the processor was designed to start tracking Scramspace state by using a combined inertial measurement unit/GPS merged and filtered with magnetometer data to determine attitude. On reentry, “we would use GPS and a vehicle dynamics-state estimator to 'bound up' the attitude,” says co-developer Michael Creagh. By integrating off-the-shelf sensors, Creagh says, the small package could be put on any flight vehicle as a back-up processor for a fraction of the usual cost. “You are replacing a $100,000 system with a $500 sensor.”
The CIRA-developed ultra-high-temperature ceramic fins were to have been the first small sharp-edged fins of their type to fly hypersonically. “Sharp geometry is one of the keys to better performance on reentry,” says Antonio Del Vecchio, a quality assurance and project manager for CIRA. Connected to thermocouples and pressure sensors, the flight was to provide the first hypersonic-flight-conditions experience for the material. Roberto Gardi, a CIRA project engineer for space hot structures, explains that proving the viability of hotter sharp-edged structures could lead to their use in vehicles with “increased down- and cross-range, which means lower loads for the structure and safer conditions.” An identical fin to the one lost on Scramspace is set to undergo testing in the agency's Sirocci plasma wind tunnel in 2014.
As the primary industry contributor, BAE Systems Australia played a key role in developing hardware-in-the-loop systems tests for the sensors, actuators and all systems on board that were relevant to flight control. “We tricked the vehicle into thinking it was flying,” says the company's hypersonics technical lead, Andrew George. The work, which included failure mode checks, helped BAE develop new models and components “and focused me on a software reconfigurable system,” he adds. From a company perspective, George says, “we want to extend our knowledge of the hypersonic regime, because we've come from a heritage of developing high-speed aircraft. It is part of developing our capability, and a lot [derives from] partnering with DSTO and the universities. It gives us a head start over other industries.”
Although it seems unlikely a reconstituted Scramspace will fly, the effort has already achieved a significant program objective, says Boyce. “We had the talent pool, the training and development. We demonstrated we could build a payload that was performing and functioning. It was as strong as an ox and it was ready to fly.” The effort has therefore boosted Australia's credibility in the international hypersonics and space arenas, despite the let-down, he adds.