Working with Sandia—Avocet and SHIRP

Low-cost RPRVs have contributed to the development of hypersonic vehi­cle concepts and advanced cruise-missile technology. The first such proj­ect undertaken at Dryden originated with the Sandia Winged Energetic Reentry Vehicle (SWERVE).

Sandia National Laboratories developed the SWERVE under an exploratory tactical nuclear weapon program. With a slender cone-shaped body and small triangular fins that provided steering, the SWERVE was capable of maneuvering in the range from Mach 2 to Mach 14. Several flight tests in the late 1970s and early 1980s demonstrated maneuver­ability at high speeds and high angles of attack. Three SWERVE vehi­cles of two sizes were lofted to altitudes of 400,00 to 600,000 feet on a Strypi rocket and reentered over the Pacific Ocean. The SWERVE 3 test in 1985 included a level flight-profile segment to extend the vehicle’s range. Because technologies demonstrated on SWERVE were applica­ble to development of such hypersonic vehicles as the proposed X-30 National Aero-Space Plane (NASP), Sandia offered to make a SWERVE – derived vehicle available to defense contractors and Government agen­cies for use as a hypersonic testbed.[972] During the early 1980s, NASA’s Office of Aeronautics and Space Technology (OAST) began studying technologies that would enable development of efficient hypersonic aircraft and aerospace vehicles. As part of the program, OAST officials explored the possibility of a joint NASA-Sandia flight program using a SWERVE-derived vehicle to provide hypersonic entry and flight data. Planners wanted to use the capabilities of both NASA and Sandia to refine the existing SWERVE configuration to enable data measurement in specific flight regimes of interest to NASA engineers.[973] The SWERVE shape was optimized for hypersonic performance, but for a transatmo­spheric vehicle to be practical, it had to be capable of subsonic opera­tion during the approach and landing phases of flight. In 1986, Sandia and NASA officials agreed to participate in a joint project involving an unpowered, radio-controlled model called Avocet. Based on the SWERVE shape, the model retained the slender conical fuselage but featured the addition of narrow-span delta wings. It was approximately 9 feet long and weighed about 85 pounds, including instrumentation. For flight tests, the Avocet vehicle was dropped from a Piper PA-18-150 Super Cub owned by Larry G. Barrett of Tehachapi, CA. The test plan called for 30 to 40 flights to collect data on low-speed performance, handling qualities, and stability and control characteristics.[974] Dryden engineers Henry Arnaiz and Robert Baron managed the Avocet project. R. Dale Reed worked with Dan Garrabrant and Ralph Sawyer to design and build the model. Principal investigators included Ken Iliff, Alex Sim, and Al Bowers. Larry Schilling developed a simulation for pilot training. James B. Craft, Jr., and William Albrecht served as systems and operations engineers, respec­tively. Robert Kempel and Bruce Powers developed the flight control sys­tem. Eloy Fuentes provided safety and quality assurance. Ed Schneider served as primary project pilot, with Einar Enevoldson as backup.[975] All tests were conducted at the China Lake Naval Weapons Center, about 40 miles northeast of Edwards. The model was carried to an altitude of about

8,0 feet beneath the wing of the Super Cub and released above a small dry lakebed. Schneider piloted the vehicle from a ground station, using visual information from an onboard television camera. After accomplishing all test points on the flight plan, Schneider deployed a parachute to bring the vehicle gently to Earth. Testing began in spring 1986 and concluded November 2. Results indicated the configuration had an extremely low lift-to-drag ratio, probably unacceptable for the planned National Aero­Space Plane then being considered in beginning development studies.[976] In 1988, Sandia officials proposed a follow-on project to study the Avocet configuration’s cruise and landing characteristics. Primary objectives included demonstration of powered flight and landing characteristics, determination of the long-range cruise capabilities of a SWERVE-type vehicle, and the use of Avocet flight data to determine the feasibility of maneuvering and landing such a vehicle following a hypersonic research flight. The new vehicle, called Avocet II, was a lightweight, radio – controlled model weighing just 20 pounds. Significant weight reduction was made possible, in part, through the use of an advanced miniature instrumentation system weighing 3 pounds—one-tenth the weight of the instrumentation used in Avocet I. Powered by two ducted-fan engines, the Avocet II was capable of taking off and landing under its own power.

NASA Dryden officials saw several potential benefits to the projects. First was the opportunity to flight-test an advanced hypersonic config­uration that had potential research and military applications. Second, continued work with Sandia offered access to a wealth of hypersonic experience and quality information. Third, Avocet II expanded the NASA – Sandia SWERVE program that had become the heart of NASA’s Generic Hypersonic Program, a research project initiated at Dryden and managed by Dr. Isaiah Blankson at NASA Headquarters. Finally, the small-scale R/C model effort served as an excellent training project for young Dryden engineers and technicians. Moreover, total costs for vehicle, instrumen­tation, flight-test operations, miscellaneous equipment, data analysis, and travel were estimated to be $237,000, truly a bargain by aeronauti­cal research standards.[977] In 1989, a team of researchers at Dryden began work on Avocet II under the direction of Robert Baron. Many of the orig­inal team members were back, including William Albrecht, Henry Arnaiz, R. Dale Reed, Alex Sim, Eloy Fuentes, and Al Bowers. They were joined by engineers Gerald Budd, Mark Collard, James Murray, Greg Noffz, and James Yamanaka. Charles Baker provided additional project man­agement oversight. Others included ground pilot Ronald Gilman, crew chief David Neufeld, model builder Robert Violett, and instrumentation engineer Phil Hamory. James Akkerman built and supplied twin ducted – fan engines for the model.[978] For flight operations, the team traveled to the remote test site in a travel trailer equipped with all tools and supplies necessary for onsite maintenance and repair of the model. After setting up camp on the edge of a dry lakebed, technicians unloaded, preflighted, and fueled the model. If the configuration had been changed since the pre­vious flight, an engineer performed a weight-and-balance survey prior to takeoff. When the crew chief was satisfied that the vehicle was ready, the flight-test engineer reviewed all pertinent test cards to ensure that each crewmember was aware of his responsibilities during each phase of flight. The ground pilot followed a structured sequence of events outlined in the test cards in order to optimize the time available for research maneuvers.

Typically, the pilot flew a figure-eight ground track that produced the longest-possible steady, straight-line flight segment between turns at each end of the test range. The ground pilot controlled the Avocet II using a commercially available nine-channel, digital pulse-code modu­lation radio-control system. Since loss of the vehicle was considered an acceptable risk, there was no redundant control system. Software per­mitted preprogrammed mixing of several different control functions, greatly simplifying vehicle operation. After landing, recorded test data were downloaded to a personal computer for later analysis.[979] Initial taxi tests revealed that the model lacked sufficient thrust to achieve takeoff. Modifications to the inlet solved the problem, but the model had a very low lift-to-drag ratio, which made it difficult to maneuver. The turn­ing radius was so large that it was nearly impossible to keep the model within visual range of the ground pilot, so the flight-test engineer pro­vided verbal cues regarding heading and attitude while observing the model through binoculars. The pilot executed each research maneu­ver several times to ensure data quality.[980] The first flight took place November 18, 1989, and lasted just 2 minutes. Ron Gilman lost sight of the model in the final moments of its steep descent, resulting in a hard landing. Over the course of 10 additional flights through February 1991, Gilman determined the vehicle’s handling qualities and longitudinal sta­bility, while engineers attempted to define local flow-interference areas using tufts and ground-based high-speed film.[981] The instrumentation system in the Avocet II vehicle, consisting of a Tattletale Model 4 data logger with 32 kilobytes of onboard memory, provided research-quality quantitative analysis data on such performance parameters as lift-curve slope, lift-to-drag ratio, and trim curve. An 11-channel, 10-bit analog – to-digital converter capable of operating at up to 600 samples per sec­ond measured analog signals. The 2.2-ounce device, measuring just 3.73 by 2.25 by 0.8 inches, also featured a 128-kilobyte memory expansion board to increase data-storage capability.

The pilot quantified aircraft performance by executing a quasistatic pushover/pull-up (POPU) maneuver. Properly executed, a single POPU maneuver could simultaneously characterize all three of the desired flight-test parameters over a wide angle-of-attack range. Structural vibra­tion at high-power settings—such as those necessary to execute a POPU maneuver—caused interference with onboard instrumentation. Attempts to use different mounting techniques and locations for both engines and accelerometers failed to alleviate the problem. Eventually, engineers developed a POPU maneuver that could be flown in a steep dive with the engines at an idle setting. In this condition, the accelerometers pro­vided usable data.[982] Researchers at Dryden teamed up with Sandia again for the Royal Amber Model (RAM) project, later renamed the Sandia Hybrid Inlet Research Program (SHIRP). This project included tests of subscale and full-scale radio-controlled models of an advanced cruise missile shape designed by Sandia under the Standoff Bomb Program. The goal of the SHIRP experiments was to provide flight-test data on an experimental inlet configuration for use in future weapons, such as the Joint Air-to-Surface Standoff Missile, then under development. Sandia engineers designed an engine inlet to be "stealthy”—not detect­able by radar—yet still capable of providing good performance charac­teristics such as a uniform airflow with no separation. Airflow exiting the inlet and entering the turbine had to be uniform as well. The design of the new inlet was complex. Instead of a standard rectangular chan­nel, the cross-sectional area of the inlet varied from a high aspect ratio V-shape at the front to an almost circular outlet at the back end.[983] Sandia funded Phase I flight tests of a 40-percent-scale RAM from August 1990 through August 1991. Because the project was classified at the time, flight operations could not take place at Dryden. Instead, the test team used secure range areas at Edwards Air Force Base North Base and China Lake Naval Weapons Center.[984] The first flight took place in August 1990 at China Lake. Typically, the model was released from the R/C mother ship at an altitude of about 600 feet. The ground pilot performed a series of gliding and turning maneuvers, followed by a controlled pullup prior to impact. Results from the first four flights indicated good longitudinal and directional stability and neutral lateral stability.

The next three flights took place in February 1991 at North Base, just a few miles northeast of Dryden. During the first of these, a recov­ery parachute deployed at 150 feet but came loose from the vehicle. The ground pilot made a horizontal landing on the runway centerline. On the next flight, the vehicle exhibited good controllability and stability in both pitch and yaw axes at airspeeds between 35 and 80 miles per hour (mph). The pilot elected to land on the runway rather than use the recovery parachute. The final 10 flights took place at China Lake, ending July 13, 1991.[985] During fall 1991 and early 1992, researchers proposed tasks and milestones for the second phase of testing, and in February 1992, RAM Phase II was reorganized as the unclassified SHIRP proj­ect. During spring 1992, however, conditions arose at both Sandia and Dryden that required modification of the proposed schedule.

In support of a Sandia initiative to conduct a prototype flight dem­onstration program, the stabilizing and lifting surfaces for the baseline Standoff Bomb were reevaluated based on the most recent wind tunnel data and taking into account the current mass properties and flight pro­files. This revised geometry was used for the definition of wind tunnel models to collect data on static aerodynamics, diffuser distortion, and total pressure loss. In order to use the revised definition for the SHIRP flight-test models, the schedule had to be compromised.[986] An initial flight-test series in December 1992 involved launching a subscale model called Mini-SHIRP from the R/C Mothership. The team also constructed two full-scale vehicles, each 14 feet long and weighing about 52 pounds. SHIRP-1 was uninstrumented, unpowered, and lacked inlets. SHIRP-2 featured the experimental inlet configuration and was pow­ered by two electric ducted-fan engines to extend the glide range and provide short periods of level flight (10-15 seconds). The ground pilot controlled the vehicle through a fail-safe pulse-code modulation radio­uplink system. The test vehicles were equipped with deployable wings and pneumatically deployable recovery parachutes. The two full-scale vehicles, tested in 1993, were launched from the modified Rans S-12 (also known as "Ye Better Duck”) remotely piloted ultralight aircraft.

Flight operations began with takeoff of the mother ship from North Base followed by launch and landing of the test article in the vicinity of Runway 23 on the northern part of Rogers Dry Lake. The SHIRP flights demonstrated satisfactory lateral, longitudinal, and directional static and dynamic stability. The vehicle had reasonable control authority, required only minimal rudder deflection, and had encouraging wing-stall char­acteristics.[987] NASA project personnel included Don Bacon, Jerry Budd, Bob Curry, Alex Sim, and Tony Whitmore. Contractors from PRC, Inc., included Dave Eichstedt, Ronald Gilman, R. Dale Reed, B. McCain, and Dave Richwine. Todd M. Sterk, Walt Rutledge, Walter Gutierrez, and Hank Fell of Sandia worked with NASA and PRC personnel to analyze and document the various test data. In a September 1992 memoran­dum, Gutierrez noted that Sandia personnel recognized the SHIRP effort as "an opportunity to learn from the vast flight-test experience avail­able at Dryden in the areas of experimental testing and data analysis.”

In acknowledging the excellent teaming opportunity for both Sandia and NASA, he added that, "Dryden has an outstanding rep­utation for parameter estimation of aerodynamic characteristics of flight-test vehicles.”[988]