NASA Research : ADAMS
Commercial aircraft speeds rarely exceed 600 mph. However, experimental hypersonic flyers currently being developed by researchers at NASA's Langley Research Center (Hampton, VA) and Dryden Flight Research Center (Edwards, CA) will soar up to 10 times the speed of sound, or approximately 6,800 mph. Hyper-X, NASA's multi-year, multi-million dollar hypersonic flight and ground test program seeks to demonstrate key enabling technologies including an airframe-integrated, "air-breathing" engine.
According to Dave Bose, Senior Project Engineer from Analytical Mechanics Associates (AMA) (Hampton, VA) and a critical Hyper-X team partner, "At Mach 10, all vehicle subsystems must work flawlessly, or the consequences could be disastrous. From the launch and separation of the vehicle from the Pegasus booster through the Mach 10 engine experiment, the successful execution of each phase of the test flight is critical to achieve our goals of obtaining aerodynamic and propulsion data."
AMA provides expertise in complex problem-solving through mathematical modeling and immersive engineering tool development. On the Hyper-X project, this expertise was focused on the problem of stage separation of the Hyper-X vehicle from the Pegasus booster. Advanced simulation technology such as ADAMS mechanical system simulation software from Mechanical Dynamics (Ann Arbor, MI) was critical to the effectiveness of AMA's analysis.
ADAMS Reduces the Risks
To understand all of the design uncertainties and their effect on the separation event, the Hyper-X program had two choices: construct multiple system-level tests with prototype hardware to prove out the separation under worst case conditions, or develop realistic physics-based models for simulation. Testing would have required at least $400,000 to run and would have taken up to an additional 12 months or more of the project's schedule - very costly propositions.
Instead, AMA developed an ADAMS model of the separation mechanisms including the ejector pistons, Pegasus booster, adapter, and Hyper-X test vehicle. For simplicity, the ejector pistons were modeled as mass/spring elements. The aerodynamic force data were applied as a multi-dimensional spline. The spline was generated from wind tunnel data taken at the US Air Force Arnold Engineering Development Center over various positions and orientations of the test vehicle. The result was a complete representation of the aerodynamic forces over a variety of separation conditions.
The ADAMS model was parameterized to allow quick turnaround of variable changes. Piston characteristics, mass properties, attachment points, and other system components were parameterized. In all, more than 50 parameters were set-up as ADAMS variables, allowing the model to be mechanized to support batch simulation. Monte Carlo analysis was performed using ADAMS with random variable values for the parameters. For greater processing speed, an SGI R10000 12-processor machine was used, running eight copies of ADAMS. Each case was checked for proper clearance during separation and acceptable recovery from the separation upset. The result was 99.7% confidence that the mission would not fail due to improper separation design.
The Critical Separation Event
The separation between the Hyper-X vehicle and the Pegasus rocket takes place in about 400 milli-seconds - approximately the time it takes to blink an eye.
The Hyper-X vehicle is mounted to the Pegasus rocket via a Pegasus-to-vehicle adapter. The adapter is integrated into the nose of the rocket and attaches the Hyper-X vehicle by a series of explosive bolts that are located on the adapter bulkhead and fore-body – a trapezoidal structure that nestles into the nozzle area of the vehicle. Separation is initiated by two high-pressure, ejection pistons that push the test vehicle forward while triggering the explosive bolts. Simultaneously, the Hyper-X attitude control system starts to command the vehicle to the attitude required by the engine experiment. All of these events occur at a high Mach number with tremendous aerodynamic heating and dynamic pressure.
Uncertainty in the Separation Event
Uncertainty in the design data for the Hyper-X mission is tremendous. Bose adds, "We are seeking answers to questions such as what are the forces under hypersonic conditions? What is the pressure in the pistons as a function of time, stroke length, and load? What are the dispersions of the atmosphere on any given day for altitude, wind, pressure, density, etc? Are there any misalignments? What are the mass properties?
"All of these variables affect the separation sequence. Our challenge is to consider all the uncertainties and provide 3-sigma or better than 99.7% confidence that the separation will be successful."
ADAMS Simulations Prove Proper Design
It definitely takes rocket science to design a separation mechanism for a hypersonic vehicle. At one point in the vehicle development, NASA Langley was investigating an alternative separation mechanism. It was a "drop-jaw" design in which the adapter fore-body, or jaw, dropped away during the separation. But the hypersonic aerodynamic forces on the jaw were so large that a disaster might have occurred. ADAMS simulations indicated that the jaw could strike the booster before sufficient separation was achieved and that the Hyper-X vehicle may diverge under the increased aerodynamic loads resulting from the rotating jaw.
The Hyper-X Program is focused on one of the greatest aeronautical research challenges, that of air-breathing hypersonic flight. A new generation of airplanes and space vehicles could be developed from the research data of the Hyper-X, X-43A vehicles.
Current aircraft are limited to subsonic or low supersonic flight regimes. The SR-71, which cruises at slightly above Mach 3, is the world's fastest air-breathing aircraft. To achieve speeds of up to Mach 10 requires the development of new propulsion technologies, such as the scramjet (supersonic-combustion ramjet). Scramjets are ramjet engines in which the airflow through the whole engine remains supersonic. Only limited scramjet testing can be performed in ground facilities such as wind tunnels. The Hyper-X program is designed to collect sustained high Mach data for the scramjet using an unmanned flight test vehicle.
The Hyper-X vehicle is 12 feet long, five feet wide, and weighs 3,000 pounds. It is equipped with a scramjet engine, guidance and control systems, and onboard computing for navigation, data collection, and telemetry. The vehicle is initially mated to the front of a Pegasus rocket. The combined Pegasus rocket and Hyper-X vehicle are launched from under the wing of a NASA Dryden's B-52 jet.
First, the B-52 carries the Pegasus and Hyper-X vehicle to a launch altitude of approximately 43,000 feet. There, the Pegasus and Hyper-X are released and the Pegasus rocket boosts the Hyper-X vehicle to test conditions. To date, three tests are planned – two at Mach 7 and 95,000 feet, and one at Mach 10 and 100,000 feet. Upon arrival at the test condition, the Hyper-X is separated from the Pegasus rocket, the scramjet engine is started, and the data collection begins.