Activities in the Past
OREX    Objective and Overview
   Navigation system and De-orbit
   Thermal Protection System Evaluation
   Other Onboard Measurements
   OREX 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 Orbital Re-Entry Flight Experiment (OREX), the first in this series of experiments, was conducted in 1994 to demonstrate technologies for autonomous de-orbiting and to evaluate thermal protection systems and materials to protect a vehicle from aerodynamic heating during high-speed atmospheric re-entry.

  Objective and Overview

  The OREX flight experiment had two goals to demonstrate re-entry technologies. The first goal was to demonstrate autonomous " de-orbit " technology. To make it re-enter the atmosphere, an orbiting vehicle is decelerated by firing rockets opposite to the direction of travel (" retro-rockets "). Accurate performance of this retrograde " deorbit burn " is essential not only to reach the target landing point but also to keep aerodynamic heating and aerodynamic loads during re-entry within tolerable limits. The second goal was to evaluate heat-resistant materials. During re-entry, the surface of a vehicle is exposed to severe aerodynamic heating and its temperature reaches more than 1400 Celsius. To protect its primary structure and payload, a Thermal Protection System (TPS), consisting of heat-resistant materials that withstand high temperatures and thermal insulators which prevent heat from entering the vehicle, is attached to the vehicle's surface. The OREX experiment evaluated such a TPS in a real flight environment.

  A further objective of OREX was to obtain measurements of aerothermodynamic phenomena around the vehicle during re-entry to acquire fundamental data for research and development of re-entry technology. The OREX vehicle is a small axisymmetric capsule. While such a shape does not require active attitude control during re-entry, allowing the OREX systems to be kept relatively simple, it cannot generate lift to allow the aerodynamic heating rate to be reduced by flying for longer at higher altitudes during re-entry, and so the maximum heating rate experienced tends to be greater than for lifting re-entry vehicles. Although ablative thermal protection materials, which dissipate heat by sublimation, can protect from the very high aerodynamic heating rates associated with ballistic re-entry trajectories and have been used successfully on capsule type re-entry vehicles such as the U.S. Gemini, they are expended during re-entry. On the other hand, reusable thermal protection systems cannot tolerate heating to the same degree as ablative materials, so the maximum heating rate they sustain must be reduced. To reduce the heating rate on a vehicle, the aerodynamic drag force it experiences must be increased, and OREX was designed with a large diameter to achieve this. (Aerodynamic force is the product of the vehicle's surface area and drag coefficient. A large drag coefficient, which depends on shape but not size, is desirable for a high-performance vehicle. Designing OREX with a larger diameter increases drag force by increasing area while preserving drag coefficient.)

  OREX was launched on the first test flight of the H-II launch vehicle in February 1994. After making a single circular orbit with an altitude of approximately 450 km, the OREX vehicle performed a de-orbit burn to re-enter within the coverage of a telemetry ground station in Okinawa. During re-entry, the vehicle acquired various measurement data which were transmitted to the ground station. After it had decelerated to subsonic speed, the vehicle deployed a parachute to slow its descent to ensure continued telemetry transmission. (The parachute was not for vehicle recovery.)

OREX Mission Profile

  Navigation system and De-orbit

  De-orbit was performed using a set of four 150N hydrazine thrusters. A de-orbit burn must be performed at the proper position and in the proper direction, and must give the proper impulse. OREX demonstrated autonomous de-orbit, in which the optimal timing, direction and duration of the de-orbit burn were commanded by an onboard computer based on a predetermined guidance law, with an onboard Inertial Measurement Unit (IMU) used to determine orbital parameters, the position on the orbit and the vehicle's attitude. The de-orbit burn was set to end when the vehicle's angular momentum around the center of the Earth reached a certain value. However, its duration was 12.5 seconds shorter than planned because the thrusters gave approximately 5.5% greater thrust than designed. This demonstrated that the de-orbit navigation and control system worked properly.

  During the flight, experiments on navigation were performed such as updating the inertial navigation data using GPS measurements, and performing IMU-Drag measurement navigation to estimate altitude after atmospheric entry. Inertial measurements are used to obtain drag force, and using this measurement and the vehicle's known aerodynamic characteristics, atmospheric density can be computed. From this, altitude can be deduced.

  Thermal Protection System Evaluation


  Ideally, thermal protection materials for a reusable space transportation system should have adequate heat resistance and thermal insulation while being reusable. However, there is no current technology material available for practical use that has both the required heat resistance and thermal insulation properties. Therefore, combinations of materials must be applied appropriately to different parts of the vehicle according to the conditions they experience.

OREX Vehicle Configuration

  As can be seen in the illustration above, the OREX vehicle is covered with three different types of thermal protection material. Four-millimeter thick Carbon/Carbon (C/C) composite material is used on the nosecap at the center of the vehicle, where the most severe aerodynamic heating occurs. While its thermal insulation properties are poor, this material can withstand large aerodynamic forces and its strength does not degrade even at temperatures of 1600 Celsius.

  Thermal insulation tiles, which are made by hardening ceramic fibers, are used on the outer parts of the vehicle. This material has outstanding heat insulation characteristics of approximately 0.1 W/m K or lower, depending on temperature and ambient pressure, is heat resistant to about 1400 Celsius, and also has low density (about 0.2 g/cm3). The tiles, however, have low strength. They are glued to the honeycomb aluminum alloy skin, and the skin must bear the structural loads.

  The C/C panel is a type of " standoff " TPS. This a panel constructed of C/C composite combined with a heat insulator and supported by heat-resistant metal. Such panels can withstand higher temperatures than ceramic tiles, and were used for the area between the nose cap and the ceramic tiles.
  The fasteners and adhesives used for attaching the C/C panels and ceramic tiles were selected and designed to ease the stresses between the TPS materials and the vehicle's base honeycomb aluminum panels caused by differential thermal expansion due to their different temperatures or thermal properties.
  Analysis of the flight data showed that these TPS performed as expected. The C/C nosecap and ceramic tiles were applied to the subsequent HYFLEX experiment.

C / C - Panel -and Ceraminc Tiles
C / C - Panel -and Ceraminc Tiles

  Other Onboard Measurements

A compavison of electron density based on two different CFD models with the flight result.

A compavison of electron density based on two different CFD models with the flight result.
  Flight data parameters acquired during the experiment, most of which are related to aerothermodynamics, are listed in the table below. OREX was the first Japanese vehicle to achieve atmospheric re-entry from orbital speed, and a velocity of greater than 7 km/sec encountered during re-entry from orbit corresponds to a kinetic energy of approximately 30 MJ/kg. Such extreme conditions cause gas molecules in the atmosphere to dissociate or to be ionized. These conditions can only be partially simulated on the ground, and it is necessary to use flight data to validate the results of ground tests and Computational Fluid Dynamics (CFD) analysis. As an example of flight data analysis, the figure below shows a comparison between measurements of electron density surrounding the vehicle during flight and two CFD computed predictions. Two different chemical reaction models were used in the CFD computations, indicated by the solid and dashed lines. It can be seen that one of these models, indicated by the solid line, matches the measured data well. The data acquired by OREX are quite valuable, and their analysis is continuing not only in Japan but also overseas.

Table : Acquired Flight Data
TPS Temperature Measurements
(Aerodynamic Heating Rate Measurement and TPS Performance Evaluation)
Surface Pressure Measurements
High Resolution Accelerometer Measurements
(Aerodynamic Drag in Rarefied Atmosphere)
Dissociation Recombination Sensor
(Evaluation of Ablator Performance)
Electron Density Measurements by Electrostatic Probe
GPS Experiment
(GPS-IMU Navigation Experiment)
Radio Wave Intensity received at the Ground Station
(Radio Black-out Measurement)

  OREX Vehicle Configration

OREX

  Conclusion

  OREX was the first experiment in a series of flight experiments using small vehicles. Not only did OREX achieve all its objectives, such as the evaluation of heat-resistant materials, but it also has significance as a pioneer of a method of future space programs, in which prototype flight experiments form an integral part of the development program. Finally, through OREX, much knowledge and experience has been gained of flight testing itself.