The third and final X-43A hypersonic test was a success, but will it help ensure NASA's air-breathing, high-speed propulsion research?
In a little over 20s it was all over. High over the Pacific, NASA's Hyper-X programme had achieved its goal and demonstrated successful supersonic- combustion ramjet (scramjet) operation at close to Mach 10 - the fastest speed ever achieved by an air-breathing vehicle.
The record-setting test was the third and final flight of the eight-year, $230 million X-43A scramjet effort and effectively marked the culmination of 40 years of hypersonic research at NASA. Yet, even as the spent vehicle and its booster were hurtling to a watery grave in the Pacific 1,570km (850nm) away from the launch point, urgent behind the scenes work was already under way to help ensure the X-43A marks the start, rather than the end, of the hypersonic story.
Faced with the switch in priorities from aeronautical research to the return to space exploration ordained by the Bush administration, hypersonic and supersonic and propulsion research is under direct threat. Hyper-X had recovered smartly from the expensive failure of the first X-43A in 2001 to notch up a record-breaking Mach 6.83 air-breathing flight with the number two vehicle in March, and a second success was considered vital to prove the feat was both repeatable and beatable. By the time the 16 November attempt came around, budget requests had been submitted for 2006 funding and the event took on even more significance than usual. It was with considerable tension, therefore, that onlookers watched relayed TV images from the NASA control room as the X-43A, mounted on its Orbital Sciences Pegasus booster, dropped from the right wing of the Boeing B-52B "mothership" to begin the test. After the climb through the jetstream, the booster was dropped at just over 40,000ft (12,200m), and ignited after 5s before rapidly accelerating through Mach 1. At around 32s after launch, the "stack" passed 55,000ft at Mach 3 and pushed over to a negative angle-of-attack trajectory.
Hot spot temperatures
Eleven seconds later, at 64,000ft and Mach 4, coolant flow to the 4m-long X-43A was switched on. Coolant was critical to the test as the maximum "hot spot" frictional temperatures at Mach 10 were expected to be around 1,980°C (3,600°F), or around 540°C more than experienced during the previous flight. To cope with these temperatures the vehicle was also changed slightly with added carbon-carbon thermal protection system (TPS) to vertical tails that were now solid instead of hollow, extra carbide coating on the leading edges of the tails, thicker TPS treatment around the engine as well as round the nose, which was also slightly blunter.
At around 90s after launch and at close to 111,000ft and M10, the booster engine burned out and within 7s the X-43A was ejected forward off the stack by a pair of pistons and a set of explosive bolts mounted on the nose of the booster. With separation occurring at around M9.65, the critical few seconds of the entire test occurred as the cowl to the engine was opened to allow air to enter. Freestream flow into the inlet travels at around 2,133m/s (7,000ft/s), but is slowed within milliseconds to around 915m/s as it enters the constricted throat where it was mixed with hydrogen and ignited by injecting silex.
The mixture burned for 10s and produced enough thrust to balance aerodynamic drag - a dramatic demonstration of the scramjet concept over an unparalleled time period and at never-before-seen speeds. During the 10s of fuelled flight, the vehicle travelled around 32km, representing a speed of approximately 5,650kt (10,500km/h). Some 20s after the cowl doors were opened roughly 2min after launch, and following the expenditure of the fuel, the doors were closed.
Aerodynamic heating
The team opted to close the doors after burn out on the higher speed test as the aerodynamic heating would have begun to melt the internal systems and structure, preventing collection of more valuable aerodynamic and loads data taken during the descent. This included a pre-determined set of manoeuvres as the vehicle slowed through M6 down to M2 on the way to splashdown.
So with two successful hypersonic tests under its belt, what is next for NASA? "The next step is to take a turbine and ram/scramjet, and combine these cycles and put them on an aircraft. There are a lot of paths forward from this point," says X-43 project manager Joel Sitz. "We've accomplished a lot, and given government and industry a lot of confidence going forward in hypersonic propulsion. We've definitely opened the door," he adds.
However, in recognition of the tight budget situation and the driving need for routine space access, the Hyper-X team believes the best option is to shape hypersonic research towards "fundamental technologies" that will support a possible Mach 7 vehicle capable of sending up to 9,000kg (20,000lb) payloads into low-Earth orbit (LEO) by around 2015 (Flight International, 16-22 November). The vehicle could incorporate a turbine-based combined cycle (TBCC) engine and "spiral" out from a broad-based set of technologies being proposed as part of a plan being put forward to NASA's Aeronautics Mission Research Directorate by an inter-centre NASA team from Ames, Dryden, Glenn and Langley.
"With such a vehicle we could move right into development, although there are still some big issues such as the integration of the TBCC, and the development of reusable cryogenic tanks," says Hyper-X programme manager Vince Rausch. The study TBCC configuration will probably use a turbojet, based on a current military turbofan such as the Pratt & Whitney F-135, burning hydrocarbon for the jet phase and liquid hydrogen for the ramjet/scramjet phase of the flight. Such a vehicle could also therefore be feasible for near-term use around 2015, he adds. A subscale demonstrator could use a smaller powerplant such as a Williams International FJ33/FJ44-based engine currently being offered for an active missile programme.
In the forefront of the movement to keep the research momentum going is Hyper-X technology manager Charles McClinton, who now acts as lead for policy in hypersonics at NASA headquarters. "Turbojets were the new technology that came out of the Second World War, and the scramjet is the new technology that came out of the Cold War. At the end of the Second World War we had to decide where to put our dollars. We put them into turbojets and that gave us the lead in aerospace. Now we have to make that sort of decision again."
Budget request
The plan, although relatively modest in terms of overall funding with a request for around $50 million over five years, could be adopted in the 2006 budget, but is more widely expected to slide to 2007. "It would keep us alive, and allow us to finish all the basic technology tests already under way," says McClinton. The aim would be integrate a turbojet and scramjet in a "flight-weight" hardware configuration for endurance tests over "thousands of cycles. Nobody believes these things can last for thousands of flights, so if we get the money then we would like to build some sort of test to do that - either as part of a system or just a combination of demonstrations."
Another key aspect will be the transition from low to high speed, and a focus on the inlet design and operation. "What will happen when we open and shut them. That's the sort of thing we need to nail down. The next key thing is the turbojet, and we could build an RTA [revolutionary turbine accelerator] engine for a reusable hypersonic vehicle in six years. The step afterwards is to combine this with a scramjet." McClinton adds that "we need a reusable, reliable vehicle from Earth to LEO, and we think with this technology we could do it."
GUY NORRIS / LOS ANGELES
Source: Flight International
