Activities in the Past
HYFLEX    Objectives and Overview
   Guidance and Control
   Thermal Protection System
   Acquisition of Flight Data
   HYFLEX Vehicle Configration
   Conclusion

  The H-II Orbiting Plane-Experimental (HOPE-X) will be an unmanned re-usable spaceplane that will be launched on conventional booster, work in low-earth orbit, for example to deliver a payload or to conduct scientific experiments, re-enter the atmosphere and land horizontally on a normal runway. To reduce the cost and risk of developing HOPE-X, NAL and the National Space Development Agency of Japan (NASDA) have been carrying out a series of experiments using small vehicles to acquire and demonstrate key technologies. These experiments are similar in concept to the US National Aeronautics and Space Administration's (NASA) " X " series of experimental vehicles.

  The first in this series of experiments was the Orbital Re-Entry Flight Experiment (OREX), which was conducted in 1994 to evaluate autonomous de-orbit technologies and vehicle thermal protection systems and materials to protect from aerodynamic heating during high-speed atmospheric re-entry.

  During its return flight from orbit, a reusable spaceplane must generate lift in order to be able to change its trajectory while traveling at hypersonic speed in the upper atmosphere. The Hypersonic Flight Experiment (HYFLEX), the second experiment in the series, was conducted in February 1996 to demonstrate the hypersonic flight performance of a lifting vehicle, and launched a lifting body on a sub-orbital trajectory by a J-1 booster.


  Objectives and Overview

  Unlike capsule-type expendable re-entry vehicles, reusable space transportation vehicles must generate lift during re-entry. This primarily for two reasons: to generate force to change trajectory, and to reduce aerodynamic heating rate during re-entry First, unlike expendable reentry vehicles such as capsules which touch down within a large designated area, either at sea or on land, and are retrieved by mobile recovery teams, reusable space vehicles must reach a specific point, such as a runway. To reach a destination that does not lie directly beneath the orbital path, it is more efficient to change the flight trajectory of the vehicle in the upper atmosphere during re-entry using aerodynamic forces than to use rocket engines to change the orbital plane before re-entry, which requires a lot of propellant. Trajectory change using aerodynamic forces is achieved by banking the vehicle left or right and using some of the lifting force generated by the flow of air over the wings and body to change direction, like an airplane. The trajectory change capability depends upon the value of lift divided by the drag. A reusable space transportation vehicle therefore needs to have a relatively high lift to drag ratio at the hypersonic speeds at which it will be traveling in the upper atmosphere.

  Secondly, the thermal protection system of a reusable space transportation vehicle must also be reusable. Current thermal protection materials, however, can only withstand low aerodynamic heating rates compared to expendable ablators (thermal protection materials which sublimate to carry away heat) used by capsule-type spacecraft, so the aerodynamic heating rate during re-entry flight must be reduced. The rate of aerodynamic heating is proportional to the square root of air density, so reusable space vehicles must fly at higher altitudes for a given speed than a purely ballistic trajectory would afford and must generate lift to do so. It is therefore important that lift surface loading (that is, the ratio of mass to the product of lift coefficient and reference area) is small at hypersonic speed. For a vehicle to fly suspended by lift, the generated lift force must be greater than the force of gravity on the vehicle (strictly, the effect of centrifugal force due to the curvature of the Earth must also be taken into account).

  Thus, the vehicle's body and wings must generate a lifting force at hypersonic speed that exceeds the gravitation force to allow the vehicle to remain at higher altitudes for longer to reduce heating, with sufficient additional margin to be used for trajectory change.

  As with airplanes, the amount of lift generated at low speeds as well as the lifting surface loading are important factors affecting landing. While there are vertical landing concepts that involve the use of rocket engines instead of wings to generate lift force for landing, such concepts still need to generate lift to change trajectory and reduce the aerodynamic heating rate in the upper atmosphere.

  The purpose of the HYFLEX experiment was to acquire fundamental technologies for lifting re-entry vehicles and to demonstrate them in flight. Compared to OREX, HYFLEX has a more complex shape to enable it to generate lift and has aerodynamic control surfaces for attitude control. The primary goal of HYFLEX was to demonstrate guidance and control technology to control the attitude of the vehicle during flight and to effect changes of trajectory. Another major objective was to demonstrate hypersonic vehicle design and thermal protection systems. Further objectives were to acquire aerodynamic and aerothermodynamic data in hypersonic flight, as with OREX. The configuration of HYFLEX is that of a lifting body, a vehicle that is shaped to generate lift over the whole body area. For attitude control, the vehicle has a reaction control system (RCS) at the base of the fuselage and two elevons. Like OREX, the vehicle's surface is covered by a thermal protection system (TPS). HYFLEX was launched on a sub-orbital trajectory by a J-1 booster, and needed a relatively narrow shape to fit inside the payload shroud.

  The Illustration below shows the HYFLEX flight profile. The vehicle was launched from Tanegashima Space Center on a trajectory with a maximum altitude of 110km. It was released from the launch vehicle while traveling at a speed of approximately 3.9km/s and performed a gliding right turn around Chichi-Jima Island in the Ogasawara Islands group while flying at maximum Mach number of 15. It finally splashed down in the Pacific using a parachute northeast of Chichi-Jima (see the flight results).

HYFLEX Mission Profile

  Guidance and Control

Trajectory in the D-V plane
  There are two major objectives in guidance and control: To reach to the destination such as a runway (it is also important for kinematic energy to be almost zero when the destination is reached), and to maintain the aerodynamic heating rate, dynamic pressure, etc. within tolerable limits.
  " Guidance, Navigation and Control " technology is necessary to achieve this. Navigation measures the position and speed of the vehicle relative to the destination. Based on this information, guidance computes the trajectory to reach the destination while staying within flight limits. Control then controls the vehicle to follow this trajectory while maintaining stable flight.

  The HYFLEX vehicle has an autonomous automatic flight control system; that is, it uses onboard systems for guidance and control rather than being controlled remotely. The vehicle's trajectory and attitude are regulated by guidance and control laws programmed into an onboard computer, based on the vehicle's attitude, position and velocity measured by an Inertial Measurement Unit (IMU). Attitude control commands from the computer are used to drive to the elevons and RCS. Guidance commands are computed once per second, and control commands 20 times a second. The relationship between velocity and drag acceleration is shown on the right.

  Guidance that satisfies all flight limits, such as aerodynamic heating rate and dynamic pressure, and depletes the kinetic energy exactly at the destination, is performed on this plane. It can be seen that guidance was performed properly in the experiment. Moreover, HYFLEX demonstrated its ability to accurately reach a destination by the fact that it splashed down only around 3km from the planned point.

Flight result

  Thermal Protection System

  Since re-entry vehicles must assume high angles of attack to generate high lift at hypersonic speeds, the rate of aerodynamic heating on the lower surface of the fuselage is high around the nose and along the wing leading edges, but relatively low on the upper fuselage.

  The HYFLEX vehicle has thermal protection system that is much more representative of an operational vehicle compared to that of OREX.

  The nosecap and the elevons, where angles to the free stream are high depending on the pitch angle, are made of carbon/carbon composite material. The curvature of the nosecap is greater compared to OREX and is similar to that of HOPE. The lower surface of the fuselage is covered with ceramic tiles and the upper surface is covered with relatively soft flexible insulation made of ceramic fibers. This flexible insulator, which is easier to handle than rigid tiles, can be used for the upper surface since the aerodynamic heating rate is low and there are less stringent requirements for surface smoothness. Ceramic fiber seal tubes are installed in the gaps between the elevons and fuselage to prevent flow from the lower surface leaking through. The exposed surfaces of the ceramic tiles are colored black to increase their emissivity, which suppresses temperature rise by increasing thermal radiation, while the flexible insulation on the upper surface is white is to reflect heat from solar radiation, considering that an operational vehicle would stay on orbit for several days.

HYFLEX Thermal Protection System

  Acquisition of Flight Data

An examole of heating nate history
  Flight data on aerodynamic and aerothermodynamic phenomena were obtained in addition to the objectives mentioned above. The parameters measured onboard are listed in the table below and, as an example, the time history of the aerodynamic heating rate of the lower surface of the fuselage is shown on the right. It is well known that a boundary layer, the flow adjacent to a surface, has two states: Laminar and turbulent. Since the aerodynamic heating rate is much greater for a turbulent boundary layer, it is important to estimate the conditions in which the boundary layer becomes turbulent. In the figure on the right, you can see a rise in heating rate between 130 to 150 seconds after separation of the HYFLEX vehicle from the booster. This was caused by the boundary layer transitioning from a laminar state to turbulent. Valuable data on boundary layer transition was therefore obtained.

  Onboard Measurement Items of HYFLEX Flight Experiment Aerodynamic

Heating Rate Measurements Temperature Measurements for TPS Evaluation
Temperature Measurements for Whole Vehicle Thermal Analysis
ADS (Air Data Sensor) Pressure Measurements for RCS Interaction
Surface Pressure Measurements Elevon Hinge Moment Measurement
Strain Measurements for Health Monitoring of the Vehicle Structure
Reflectometer (Measurements on Ionized Electron Density around the Vehicle)
Navigation Measurement (Position, Velocity, Attitude)
Maximum Temperature Measurement by Heat-Sensitive Label

  HYFLEX Vehicle Configration

HYFLEX

HYFLEX Vehicle Configuration

  Conclusion

  HYFLEX was Japan's first hypersonic flight with a lifting vehicle, and demonstrated fundamental technologies. Unfortunately, the vehicle could not be recovered after splash down, because a riser connecting the vehicle to a floatation bag had worn out. Nevertheless, the HYFLEX experiment acquired valuable data and experience for reentry-vehicle technology.