NASA's X-43A Hyper-X Reaches Mach 10 in Flight Test

Aviation Week & Space Technology 11/22/04
author: Michael A. Dornheim

Mach 10, But Now What?

A number of hypersonic-related programs are drawing confidence from the back-to-back successful flights of NASA's X-43A research craft that show scramjet operation at Mach 7 and Mach 10.

Last week's free flight at Mach 10 was especially important because it's difficult to test on the ground at that high speed. An early look at the data suggests that a series of 0.005-sec.-duration reflected shock tunnel runs did a very good job of predicting the flight, says Robert Bakos, vice president of ATK GASL in Ronkonkoma, N.Y.

Those two results--that the engine/ airframe combinations produced usable thrust, and that their behavior was close to analytical and wind tunnel predictions--should give a boost to other programs that haven't flown yet by increasing confidence in the validity of their designs, says Anthony Castrogiovanni, ATK GASL president. "Hopefully today's flight, showing it can be done twice, will trigger Defense Dept. interest," he said after the Nov. 16 test. ATK GASL, along with NASA Langley Research Center, was responsible for the X-43A "Hyper-X" design, and the company built the 12-ft.-long craft at its facility in Tennessee.

The first 12-ft.-long Mach 7 X-43A displays its lines during ground testing. Copper-colored engine is on bottom, and 800-lb. steel-colored slab of tungsten ballast at nose is surrounded by reinforced carbon-carbon leading edge. The Mach 10 version has more heat protection.Credit: NASA/TOM TSCHIDA

By having an integrated engine and airframe in free flight at operational conditions, Hyper-X has gone "orders" beyond what has been accomplished previously with scramjets, he says. Other tests have been mostly on the ground or, if in flight, fixed solidly to the front end of a booster rocket or were not an integrated vehicle.

Despite the apparent X-43A success, NASA has no funded scramjet follow-on program to capitalize on the new knowledge and retain the expertise. The three-flight Hyper-X program cost $230 million, and that would increase if it were all under the current "full cost" accounting system.

A team of officials from the Langley, Dryden, Glenn and Ames research centers are proposing a "modest but aggressive spiral development approach" of scramjet activities working with the Defense Dept. and universities, says Vincent L. Rausch, the Hyper-X program manager at Langley. Individuals working the final test showed frustration at having many years of work come to a halt as they finally obtained promising flight data. Rausch and others have proposed working toward a scramjet first stage for space launch (AW&ST Nov. 1, p. 56).

Pegasus first stage with X-43A on nose is dropped from B-52B carrier aircraft, and ignites 5 sec. later. The rocket took the X-43A to a 110,000-ft. Mach 9.65 start.Credit: NASA/CARLA THOMAS

The Nov. 16 launch had been delayed several times, most recently on Nov. 15 when malfunctions required rebooting the X-43A computer twice. Each reboot took 40 min., pushing the flight beyond available range time and daytime landing requirements for the B-52B carrier aircraft of NASA Dryden Flight Research Center here. Dryden conducted the flight test, and it was the last research flight for the venerable "008" B-52B, which has been dropping test aircraft since 1959. Boeing Phantom Works did systems engineering, thermal protection, guidance, navigation and control design, flight control software and internal layout and structural design.

The X-43A was mounted on the front of a modified Orbital Sciences Pegasus launcher first stage. The stack, weighing 43,000 lb., was carried under the right wing of the B-52B and dropped westbound at 2:34 p.m. PST over the Pacific Ocean at 40,000 ft. from a point about 50 mi. off the southern California coast. The solid rocket motor took the stack up to the Mach 10 starting condition at 110,000 ft.

At 7-8 sec. after motor burnout, pistons pushed the X-43A forward away from the booster at Mach 9.8, and the X-43A's higher density made it pull ahead. Very little tipoff was observed at separation, unlike the Mach 7 flight on Mar. 27 that had noticeable tipoff, and drag and lift higher than expected (AW&ST Apr. 5, p. 28). Engineers learned from that and were able to correctly adjust for the faster condition.

The engine inlet was closed during the entire boost, but 2.5 sec. after separation the door opened downward to become the lower inlet lip. At 3 sec. after separation, the engine started firing at a speed of Mach 9.65 at 110,000 ft. with a dynamic pressure of 1,050 psf., or 557 kt. equivalent airspeed. It was still in a 2.1-deg. climb from the booster path.

Pegasus first stage with X-43A on nose is dropped from B-52B carrier aircraft, and ignites 5 sec. later. The rocket took the X-43A to a 110,000-ft. Mach 9.65 start.Credit: NASA/CARLA THOMAS

The gaseous hydrogen fuel was on for 10-12 sec., says Randall Voland, the scramjet propulsion team lead at Langley. The first 4 sec. of firing included the pyrophoric chemical silane to ensure ignition, and the hydrogen ran at two flow rates to check different mixture ratios. The flow rates were repeated with silane off for comparison, and the flow then tapered down as hydrogen supply pressure ran low. The engine ran air-only for 8-9 sec. to check fuel-off characteristics, and at 21 sec. past separation the inlet door closed for the rest of the flight.

The craft descended at an approximately constant dynamic pressure and conducted maneuvers at decreasing Mach numbers to compare aerodynamic characteristics against predictions. A Navy P-3 downrange received telemetry for at least 14 min. after separation, which is likely the point at which it hit the ocean after traveling about 850 mi. The craft sank and there are no plans to recover it.

The X-43A was controlled to about 1 deg. angle of attack (AOA) to meet design conditions for the engine run, but the AOA for lift to match weight was about 5 deg., Voland says. A higher angle would have increased drag but also increased shock wave compression and engine efficiency, and good performance is a balance of factors like these, he adds.

Project officials say early data show thrust was about the same as drag, as expected. They note that the X-43A derives from a Mach 10 cruiser design by McDonnell Douglas, implying that this level of "cruise" thrust is what was planned. However, given that the inlet was operating at an AOA that would not support the weight of the craft, countered by the X-43A being an unusually heavy test vehicle, it's hard to translate "thrust equals drag" into anything meaningful for an operational aircraft.

Nevertheless, 3.8 lb. of hydrogen were burned during the 10-12-sec. period at a bulk fuel/air ratio that was 20-30% richer than stoichiometric. Given uncertainties in the airflow, the mixture was intentionally fuel-rich to avoid lean-burning, Voland says. The speed of sound was about 680 mph., and flying at Mach 9.6 for 12 sec. covered 18.9 naut. mi., giving 5 naut. mi./lb. of hydrogen. Since hydrogen has about three times the specific impulse of kerosene, that translates into 1.7 naut. mi./lb. on jet fuel. Gross weight was 2,860 lb., or about the weight of a Cessna 182, which gets about 2 naut. mi./lb. on gasoline.

THE HIGHER SPEEDS compared with the last flight produced a real-gas stagnation temperature of about 5,500F and surface temperatures up to 3,600F on the nose, and heating rates twice as high. The prior flight had a peak of 2,600F on the wing leading edges. Thermal beefup included changing from hollow structure on the vertical tails to solid reinforced carbon-carbon leading edges. A hafnium carbide coating was added to leading edges, and the vehicle nose was made blunter for a more detached bow shock to reduce heating. Thicker thermal protection was used on the engine, and water cooling passages in the cowl leading edge were changed.

Pegasus first stage with X-43A on nose is dropped from B-52B carrier aircraft, and ignites 5 sec. later. The rocket took the X-43A to a 110,000-ft. Mach 9.65 start.

Engine height and flow lines were altered for the different shock wave pattern at Mach 10, and there is new controller software for the engine and airframe.

Ongoing programs that will be looking at Hyper-X results include Navy and Air Force efforts to make a fast strike missile that can cover 600 naut. mi. in 10 min., or an average speed of 3,600 KTAS or about Mach 6. An important difference from Hyper-X is that the military programs use essentially kerosene fuel, which is harder to burn than hydrogen but is much more dense. The Defense Advanced Research Projects Agency and the Office of Naval Research have a HyFly program that's set to flight test in 2005 a dual combustion scramjet engine integrated to an airframe that will free-fly in powered Mach 6 flight for about 30 sec. after being released from a booster.

The Air Force HyTech program plans to demonstrate a flightweight integrated scramjet in a ground test for several minutes next year. The engine will have fuel-cooled structure, and engineers will be checking whether it can survive the minutes of running required to meet the fast-strike goal, as well as performance and ease of operation. Part of the technology is to manage the cooling heat flow to crack the JP-7 fuel into desirable quick-burning gases like ethylene and hydrogen, Castrogiovanni says.

The flight test portion of HyTech is called Scramjet Engine Demonstrator, which may fly in 2008.

The HyFly Program gets a mention in the above article. A lot of excitment in the hypersonics world, but little funding. The priority game must be a real drag with so many things to investigate and so little money to spread around.