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AIAA-2008-2544

Falcon HTV-3X – A Reusable Hypersonic Test Bed

Dr Steven Walker, DARPA TTO Deputy Director Mr Ming Tang, Booz Allen Hamilton

Mrs Sue Morris, CENTRA Technology, Inc. Mr Caesar Mamplata, CENTRA Technology, Inc.

15th AIAA International Space Planes and Hypersonic Systems and

Technologies Conference

28 April – 1 May 2008

Dayton, Ohio USA

15th AIAA International Space Planes and Hypersonic Systems and Technologies Conference28 April - 1 May 2008, Dayton, Ohio

AIAA 2008-2544

Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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Falcon HTV-3X – A Reusable Hypersonic Test Bed

Dr Steven Walker, DARPA TTO Deputy Director Mr Ming Tang, Booz Allen Hamilton

Mrs Sue Morris, CENTRA Technology, Inc. Mr Caesar Mamplata, CENTRA Technology, Inc.

Abstract

The Falcon program is developing a series of incremental hypersonic flight demonstrators or Hypersonic Technology Vehicles (HTVs). The third of this series, HTV-3X, is focused on addressing an extended duration, hydrocarbon-fueled, reusable hypersonic cruise vehicle. Technologies enabling this capability are turbine-based combined cycle propulsion, efficient aerodynamics, high temperature structures, and thermal management techniques. HTV-3X will take-off from a conventional runway under its turbojet power, accelerate to Mach 6 under combined propulsion, decelerate and make a turbojet powered landing. The vehicle will then prepare for its next flight. The combined cycle propulsion system enables this complete transition from low-speed to high-speed flight and enables aircraft-like operations. This paper will discuss the vehicle conceptual design as well as ground demonstration results. It is envisioned that flight demonstration of this vehicle in a realistic environment will enable the future development of reusable high speed operational systems for ISR, strike, or other national need missions.

Nomenclature

AEDC = Arnold Engineering Development Center DARPA = Defense Advanced Research Projects Agency DMRJ = Dual Mode Ramjet FaCET = Falcon Combined-cycle Engine Technology HCV = Hypersonic Cruise Vehicle HiSTED = High Speed Turbine Engine Demonstration HTV = Hypersonic Technology Vehicle HyCAUSE = Hypersonic Collaborative Australia US Experiment ISR = Intelligence, Surveillance, and Reconnaissance ONR = Office of Naval Research RATTLRS = Revolutionary Approach To Time Critical Long Range Strike RCCP = Reusable Combined Cycle Program TBCC = Turbine-based Combined-Cycle

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Background The USAF and the Defense Advanced Projects Agency (DARPA) started the Falcon program in 2003 to develop and demonstrate hypersonic technologies that will enable prompt global reach missions. The future vision for this capability includes a reusable Hypersonic Cruise Vehicle (HCV). Falcon is developing and demonstrating the technologies that will be required by an HCV: high lift-to-drag aerodynamics; high-speed, turbine-based combined cycle propulsion; high-temperature materials; thermal protection systems; and advanced guidance, navigation, and control. The Falcon program addressed the implications of hypersonic flight and reusability by developing a series of hypersonic technology vehicles (HTVs) to incrementally demonstrate these required technologies in flight. See Figure 1 for the program construct.

Figure 1. Falcon Program Overview

The Hypersonic Technology Vehicle 1 (HTV-1) is an unpowered, maneuverable, hypersonic reentry vehicle integrated with state-of-the-art hypersonic technologies to address materials and fabrication challenges. A set of HTV-1 ground tests were conducted to develop and validate the vehicle’s aerodynamic, aero-thermal, and thermal-structural performance as well as to validate advanced carbon-carbon manufacturing approaches. HTV-2 is a second generation design developed under Falcon that incorporates advanced aerodynamic configuration and thermal protection systems, and improved guidance, navigation and control systems for greatly improved performance compared with HTV-1. The HTV-2 detailed design has been completed and an aeroshell prototype fabricated. Two HTV-2 flight tests will be conducted in 2009 launched from Vandenberg Air Force Base using the Minotaur IV Lite launch system. The Falcon program originally planned to develop a third Hypersonic Technology Vehicle (HTV-3) that would focus on reusable materials. However early on in the execution of Phase II of Falcon, DARPA elected to exercise an option to fund development and ground testing of propulsion technology necessary

TTeecchhnniiccaall AApppprrooaacchh

• Aero-Thermal Dynamics

• High-Temperature Materials & Structures

• Navigation Guidance and Control

• Communications through Plasma

• Combined Cycle Propulsion

MMiilliittaarryy UUttiilliittyy

• Prompt Global Reach from CONUS

– Long range strike

– Reconnaissance

– Anti-access capability

• Reusable Space Access

–

HTV-2 HTV-1 HCV

Ground Demonstrations

First Flight Mar 09 Conceptual Design/

Risk Reduction

Vision Vehicle

HHTTVV--33XX

FaCET

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to realize a reusable hypersonic testbed. This task, dubbed FaCET (Falcon Combined-cycle Engine Technology), has the objective of developing a reusable, hydrocarbon fueled, Turbine-Based Combined Cycle (TBCC) propulsion system capable of operating from a conventional runway up to speeds greater than Mach 6. Propulsion advances in this effort coupled with the availability of the High Speed Turbine Engine Demonstration (HiSTED) program high-Mach capable turbojet engine led to the decision to evolve HTV-3 into a testbed that would take off from a conventional runway, cruise at Mach 6, and land back on a runway. This new design was subsequently designated HTV-3X. HTV-3X is a highly integrated testbed that will allow demonstration of key technologies such as efficient aerodynamic shaping for high lift to drag, lightweight and durable (reusable) high-temperature materials, thermal management techniques including active cooling, autonomous flight control, and turbine-based combined cycle propulsion. See Figure 2 for a depiction of the HTV-3X testbed and identification of its key technologies.

Figure 2. HTV-3X Key Technologies

Turbine-Based Combine Cycle Propulsion The propulsion system for a reusable hypersonic vehicle must operate over a very broad environment in terms of freestream dynamic pressure and Mach number. No single family of engines can meet the full range of operational requirements. As a consequence, the propulsion system of choice is a combined cycle in that it consists of a turbojet engine integrated with a scramjet engine (technically a dual mode ramjet/scramjet). This Turbine Based Combined Cycle or TBCC propulsion system can be thought of as a three-stage system, see figure 3. Propulsion from take-off through supersonic flight regimes (and also for landing) is provided by the turbojet engine. Propulsion during a broadly defined high supersonic to low hypersonic flight regime is generated by a ramjet operational mode where this mode is characterized by a subsonic combustion process occurring in the engine. As the vehicle accelerates to yet

Single Fuel (JP-7) for All Propulsion

CG / Fuel Control out to Mach 6

Stored Energy APU for Power After TJ

Shutdown

Over-Under Combined Cycle

Propulsion Engines

Regen Cooled Flight Weight DMRJ, TJ

& Airframe Nozzle

Waverider Integrated Inward Turning Turbo-

Ramjet Inlet

Flight Demo Firsts

“Hot/Warm” Metallic Primary Structure

Low Transonic Drag– High

Hypersonic L/D

Hot Metallic Leading Edges

Hot Metallic Control Surfaces

Demonstrated Technologies

Integrated Dual Turbo- Ramjet SERN

Nozzle

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higher speeds to its cruise condition, the combustion process transitions to supersonic in the engine providing the name to this mode of operation as supersonic combustion ramjet or scramjet.

Figure 3. Turbine-based Combined Cycle Propulsion Concept The physical configuration of this TBCC propulsion system consists of two flowpaths. The upper portion of the figure where the turbojet(s) are located represents the low-speed flowpath. The dual mode ramjet comprises the lower portion and thus represents the high-speed flowpath. In the case of the HTV-3X design, each of the two TBCC propulsion systems share common inlet and exhaust components. Integration of these flowpaths into a single integrated propulsion system represents the greatest technical challenge for the HTV-3X design. Figure 4 is a detailed view of the HTV-3X TBCC propulsion system showing some of the complexities of such an integrated system.

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Figure 4. HTV-3X Turbine-based Combined Cycle Propulsion System

Probably the most demanding aspect of the TBCC’s design and development is the operational phase during which air flows and combustion occurs in both flowpaths simultaneously. The corridor through which the HTV-3X flies as it ascends and accelerates through the atmosphere defines to a large extent the inlet air quality in terms of enthalpy and pressure. This corridor must be selected with care to keep the propulsion system within its operational limits while allowing the vehicle to reach its cruise condition with greatest economy (i.e., least fuel consumed). Variable inlet and exhaust geometry controls air flow split between upper and lower flowpath and backpressure experienced by both. HTV-3X employs a unique inward-turning inlet design that efficiently provides air during each engine cycle. Fuel control in terms of over-all equivalence ratio for each flow path and local fuel distribution within the turbojet and ramjet combustors provides yet another “dial” to the engine designer. These variables must be carefully adjusted until the mode transition is complete and propulsion is provided exclusively by the high speed flow path. Overall, a TBCC engine requires a very high degree of integration and complexity if the engine is to operate efficiently from Mach zero at the initiation of take-off to a cruise Mach number of 6 at altitude. Increased complexity can imply increased engine weight if the engine is not properly integrated. As illustrated there are several actuators needed to control the inlet and exhaust plus boundary bleed and dump doors. An over under engine configuration is illustrated with the turbojet on top of the dual mode ramjet. An alternative configuration could be a side by side configuration with the right selection of engine cycle thrust levels.

Ramjet Scramjet

Integrated Nozzle

Integrated Inlet

Turbojet

Inlet Diverter Flap

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Maturing TBCC Technology Maturing TBCC engine technology has required significant individual engine technology demonstration as a first step to validating overall engine performance and operability. The HiSTED program has been focused to provide ground testing of the turbojet engine technology at the conditions required for HTV-3X operation. The FaCET program is developing and demonstrating the inlet, combustor, and nozzle through individual ground tests, and culminating in an integrated freejet engine test with simulated turbojet engines. While thorough ground testing of the integrated TBCC system is necessitated to develop and demonstrate requisite propulsion system capabilities, current facility limitations will be a challenging factor. HiSTED The HiSTED program falls under the Reusable Combined Cycle Program (RCCP) umbrella and is jointly funded by the USAF and DARPA to develop and demonstrate engine technologies for a turbine-based combined cycle propulsion system. The objective of HiSTED is to ground test a Mach 4 capable turbojet engine. The two HiSTED performers are Williams International of Detroit, Michigan and LibertyWorks (Rolls-Royce North American Technologies) of Indianapolis, Indiana. Williams International will test their engine, designated XTE88 at the Arnold Engineering Development Center (AEDC) facility in Tullahoma, TN, in November 2008. LibertyWorks will test their engine, designated XTE18, in September 2008 at their facility in Indianapolis, IN. The LibertyWorks XTE18 is a derivative of the engine developed for the Office of Naval Research (ONR) Revolutionary Approach To Time-critical Long Range Strike (RATTLRS) air vehicle, a turbojet-powered Mach 3 missile. These tests will demonstrate individual turbine components and integrated engines to support HTV-3X and technologies for scaling to larger, reusable air vehicles. FaCET The FaCET ground test program is being conducted to validate the operability and performance of the components of a TBCC engine flow path, the inlet, combustor, and nozzle. Each of the engine components will be tested over the operational Mach range of the TBCC engine, while focusing on the engine cycle transition Mach range. The components will be integrated and a freejet engine ground tested. Inlet Test: FaCET is addressing the integration of the inward turning inlet in the low speed and high speed flowpaths of the TBCC propulsion system using a sub-scale wind tunnel model of the inlet. The objectives of the test are to demonstrate the mass capture and pressure recovery required to meet propulsion performance requirements and the scheduling of mechanical doors, bleeds, and other internal flowpath hardware needed to keep the overall, turbojet, and ramjet/scramjet inlets operating without un-starting over a wide range of Mach numbers. Figure 5 shows a picture of the sub-scale model installed in Lockheed Martin’s 4’ x 4’ blowdown wind tunnel in Grand Prairie, Texas. The first series of inlet tests validated operability from turbojet-to-ramjet/scramjet mode transition to the beginning of hypersonic speed and identified the need for a rotating cowl flap, similar to the design used in the Hypersonic Collaborative Australia US Experiment (HyCAUSE), to maintain inlet start until the air vehicle reached cruising speed. Results from the first inlet test series also validated analytical prediction tools and were used to match HTV-3X combustor design with inlet exit pressure and flow conditions. This also provided design guidance for the FaCET freejet test integrated engine model rig.

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Additional inlet tests currently being conducted under HTV-3X are addressing propulsion performance enhancements and integration for the updated air vehicle conceptual design.

Figure 5. FaCET Inlet Model Combustor Test: The overall objective of the FaCET Direct Connect Combustor test program is to demonstrate performance, operability and structural durability of a circular combustor fueled with liquid hydrocarbon fuel. The tests conducted included: nozzle calibration, demonstration of pilot ignition, and flame stability. The tests successfully demonstrated stable combustion and were a significant milestone in demonstrating the lower speeds of the dual-mode ramjet needed for transition from turbojet power. Combustor performance and operability at selected flight points validated one-dimensional (1-D) codes and three-dimensional (3-D) Computational Fluid Dynamics (CFD). Figure 6 shows the 44% scale combustor test rig at United Technologies Research Center in East Hartford, Connecticut.

Figure 6. FaCET Direct Connect Combustor Rig Static Nozzle Test: The purpose of the FaCET static nozzle test is to determine the static cold-flow performance for the HTV-3X vehicle. This was achieved by testing a variety of configurations and conditions in the predicted operating range. A variety of trade studies on the baseline nozzle were conducted to study and understand the effect of variable aft cowl flap geometry, effect of variable turbojet nozzle internal area ratio, effect of different mixing plane conditions, and the performance of a refined nozzle loft reflecting lessons learned from previous CFD simulations on the baseline nozzle. Lastly, the test validated CFD methods and results. By building a database of performance values from the test, it will be possible to analyze benefits/impairments of different configurations and analyze nozzle performance up to vehicle free-stream Mach numbers of approximately five. Figure 7 shows the 9% scale model in Lockheed Martin test facilities.

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Figure 7. FaCET Static Nozzle Test Rig Freejet Test: The freejet engine test will integrate the TBCC propulsion component technologies, and demonstrate propulsion system operability and performance. Freejet test objectives are as follows:

Demonstrate the HTV-3X TBCC technology in a high Mach number environment using hydrocarbon endothermic fuel (JP-7).

Explore mode transitions at fixed Mach number

Determine engine performance using thrust as a primary indicator.

Test data is expected to verify scramjet performance and operability including inlet margin, combustion stability and overall system thrust. The current plan is to test an approximately 70% scale propulsion system using the Arnold Engineering Development Center (AEDC) Aerodynamic Propulsion Test Unit (APTU) facility. See figure 8 for a graphical depiction of the freejet test rig. The freejet test is in the critical design phase and testing is scheduled to begin in September 2008. The test will provide a significant milestone in demonstrating the hypersonic engine flowpath and system design methods.

Figure 8. FaCET Freejet Engine Model and Pedestal

HTV-3X Conceptual Design

The objective of HTV-3X is to conduct a number of design trade studies and risk reduction efforts resulting in a conceptual design review of the flight testbed in May 2008. The basic vehicle objectives set-forth were a reusable hypersonic testbed that utilizes an integrated air-breathing propulsion system capable of take-off and landing under its own power using a conventional runway, and acceleration to a max speed of Mach 6.

Inlet

Combustor

Nozzle

Turbojets (simulated flow)

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The basic HTV-3X aerodynamic configuration evolved from the Falcon HCV waverider shape to a higher fineness ratio, slender body to reduce wave drag. The aero shape resulted from the requirement to take-off, accelerate through transonic, efficiently execute mode transition, achieve hypersonic flight, and perform a powered landing. Aero wind tunnel testing was completed to confirm drag predictions and anchor computational fluid dynamic calculations and other prediction tools. This testing also provided data on handling characteristics to ensure adequate stability and control from take-off to landing, including the ability to execute an aileron role at hypersonic speeds, to show capability for aircraft-like operations and maneuvers. Flight to Mach 6 requires careful choice of materials for the airframe, propulsion system, and hot areas of the aircraft such as the nose and leading edges to ensure both high-temperature operation and re-usability. The current HTV-3X approach is a “warm” aircraft structure utilizing primarily metal alloys with active cooling and superalloys and non-metallics where cooling is not easily provided or where weight reduction is needed. Combined thermal/structural analysis of the air vehicle over the flight regime and detailed subsystems internal layout and definition ensured that the vehicle could reach and cruise at hypersonic speeds long enough to obtain engine thermal equilibrium. Results also refined weight estimate and sizing, which also impact future program cost. The far-term Falcon HCV is hydrogen-fueled to satisfy the Falcon program objective of prompt global reach from CONUS, which requires long-range and Mach 9. However, for near-term flight demonstrations, hydrocarbon fuel is easier to handle and will result in a smaller scale vehicle, which greatly reduces program cost. Without the benefit of HCV vision vehicle cryogenic fuel on board to assist with thermal management, fuel cracking in the ramjet/scramjet engine and water and other fluid subsystems on board provide active cooling to allow hypersonic cruise for several minutes. Other subsystems unique to the demo requirements were also defined to support the need to cocoon the turbojet propulsion system at high-Mach and facilitate in-flight re-start. Trajectory analysis rolled up vehicle characteristics and validated that the design closes at Mach 6+ with margin. Approved US hypersonic flight corridors were investigated and flight trajectories were developed to execute a flight envelope expansion flight test approach.

The Next Step

The feasibility of exploring and developing hypersonic flight as exemplified by the HTV-3X concept and the potential this technology offers to substantially enhance future warfighter capability has motivated DARPA and the US Air Force to jointly pursue a hypersonic flight test program called Blackswift. It is envisioned that flying this hypersonic testbed in a relevant flight environment would permit the future development of enhanced-capability reusable high-speed vehicles for intelligence, surveillance, reconnaissance, strike or other critical national missions. The USAF signed a Memorandum of Understanding with DARPA in 2007 for the Blackswift program and a program solicitation was released in March 2008. Program award is expected before the end of fiscal year 2008. A notional schedule for the Blackswift program will achieve first flight in 2012. Program development phases include vehicle preliminary design, critical design, and then a build-test-fly phase.

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Acknowledgements The authors would like to thank Fred Rodgers of CENTRA Technology, Inc. and Ray Chase of ANSER for their contributions to this paper. In addition, many of the graphics and photos found in this paper were provided by Lockheed Martin ADP Skunk Works.