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USERS : Unmanned Space Experiment Recovery System

1. Introduction

Manned and unmanned space experiments have their advantages and disadvantages for the particular on-orbit space experiment and space usage. Both experiment means are expected to complement each other to satisfy entire needs for the space experiment demands, but due to the lack of the return capabilities, very few long duration unmanned self return space experiment have been tried in the past. Unmanned Space Experiment Recovery System (USERS) Project has been promoted by the Ministry of Economy, Trade and Industry (METI) and the New Energy and Industrial Technology Development Organization (NEDO), and developed by the Institute for Unmanned Space Experiment Free Flyer (USEF ;presently J-spacesystems), to establish an unmanned on-orbit experiment infrastructure.

The USERS Spacecraft was launched at 17:20 JST on September 10, 2002 by H-IIA launch vehicle with dual launch configuration with DRTS. After approximately 8.5 months of on-orbit operation, REM has been separated from SEM, de-orbited and splashed down at 6:23 JST on May 29, 2003 at open sea east of Ogasawara Islands with the results of processed materials. After the REM return, SEM remains on orbit and not only its performances as the LEO Spacecraft standardized bus but also five experimental advanced bus equipments onboard with commercial parts and technologies are being tested.

This paper first explain about the results of the unmanned self-return space experiment infrastructure including the on-orbit and the reentry operation, and then briefly introduce about the aspect of the standardized bus system development.

2. Expectation for the Unmanned Space Experiment Infrastructure

The reasons for the demand for the unmanned space experiment system are 1) Clean micro-gravity environment with long duration, 2) Little constraints from safety constraints and 3) Operational flexibility. The manned space experiment advantages, on the other hand, are direct Human involvement for the experiment operation, which enables the restoration of the experiment in case of trouble and precise human observation of the experiment. Due to the progress in the communication, computer and control technologies, the shortcoming of the unmanned system are becoming less important. For example, for furnace experiments which require up to several month of crystal growth at the temperature around 1000 degree C, where the material includes some chemically active material, those experiments may not be allowed to be installed and executed on the manned spacecraft due to the standpoint of both micro-gravity environment quality and safety constraints towards astronauts. The unmanned system, on the other hand, has the limitation in its operation and observation because the human capabilities are so great in the precise control and record of the experiments. However, the progress in the autonomous and automatic operation technology with computers and communications may change the situation and the disadvantage of the unmanned system will become less governing in comparison with its inherent advantage. Moreover, unmanned system has flexibilities in the on orbit operation, to change the experiment profile as necessary, in case of contingency, while manned system may not able to do the same because the procedures which were not verified prior to the launch is not usually allowed on the manned system from the standpoint of the safety.

Considering the already established technologies, 1) Long duration micro-gravity environment., 2) Orbit control system, re-entry and recovery system and their operation., 3) Thermal protection system.. 4) Search and recovery operation.. 5) Experiment operation management, were identified as the technical hurdles to be overcome for the realization of the practical unmanned space experiment infrastructure.

3. USERS Project

The USERS Project has been proposed and authorized on following objectives on JFY 1996 based on the experience gained through the Space Flyer Unit (SFU) Project and the EXPeriment RE-entry Space System (EXPRESS ) Project to which the USEF has been participated.

1. To establish the unmanned space experiment system with self return capability.

2. To perform Super-conductor Material Processing Experiment on the orbit, and return the results.

3. To verify commercially available parts and technologies on the low earth orbit

The USERS Spacecraft consists of SErvice Module (SEM) and RE-entry Module (REM), and the return of the capsule part of the REM called REcovery Vehicle (REV) to the earth has been set to achieve first objective. Fig. 1 shows the configuration of the USERS Spacecraft. Super-conductive bulk MAterial Processing experiment (SMAP) has been selected as the first payload of the infrastructure to fulfil the second objective. Large size bulk super-conductive magnet is expected to be applicable for various systems such as energy storage devices and magnetically levitated train. Large size high quality crystal can be realized under micro-gravity environment by the contamination free crystal growth with single point support at the seed crystal. These experiments will be performed by the Super-conductive bulk processing Gradient Heating Furnace (SGHF) on the REM.

Third objective are going to use SEM for the experiment platform. Five advanced bus equipment development components with commercial parts and technology are conducted on the SEM. These experiments with bus system test commercially available off the shelf technologies used for ground commercial equipments. Once their performances are verified on the orbit, they will be applicable for the next generation spacecraft to achieve lower cost and will advance the space system. These experiment started their operation after the REM return in real. The performance of the SEM system as the standardized bus system has been under verification throughout the entire mission. Fig.2 shows the mission scenario of the USERS Project and Fig.3 shows the major part of the experiment accommodation.


Fig.2 USERS Project Mission Scenario

Fig.3 Major part of the experiment accommodation

4. Spacecraft System

The functions of the USERS Spacecraft are to provide mechanical support, electrical power and data interface to the mission payloads. The SEM provides the entire spacecraft with electrical power, attitude and orbit control, data handling and communications to/from ground during the SEM and REM are connected. The SGHF is installed in the REV, which is recoverable portion of the REM and protected by thermal protection material against the heat during atmospheric reentry phase. The PM includes the Retro Boost Motor (RBM), boosted REM into reentry orbit, and be separated from the REV and destructed by the atmosphere. Just prior to the SEM and REM separation, the independent REM control function is turned on and sequential control to perform the reentry operation will be initiated.

The USERS Project policy about the distribution of the various functions between the REM and SEM is to minimize the allocated functions for the REM as much as possible. The REM's functions were strictly restricted to essential for the payloads return. The REM communication subsystem was turned off during the SEM and REM connected operation period, and command control and telemetry data transmission of the entire system was performed through the SEM communication subsystem.

The SEM structure will hold the REM weight during the Launch. The SEM and the REM will be separated prior to the REM return by the SEM ISC control. The SEM and the REM are required to be thermally isolated to simplify the system verification.


Fig.4 On-orbit Configuration of the USERS Spacecraft

All of the Furnace experiments will be performed at the Sun Pointing Normal Mode (NM) Attitude without any Reaction Control Subsystem(RCS) operation to maintain good micro-gravity conditions. Two kind of environment monitor are installed. One is the micro-gravity sensor on REM, and the other is the radiation level sensor on SEM. Mono propellant hydrazine system with 23 N and 1 N thrusters are used in the RCS. The 1N thruster is used for the attitude control and the 23N for the orbit change. Fig. 4 shows the on-orbit configuration of the Spacecraft.

The REM is blunted cone type reentry capsule (Recovery Vehicle: REV) with abrasion type heat shield. The heat flow from three SGHF furnaces were dissipated through the heat shield via radiation coupling between the furnaces and the inner surface of the shell. The Recovery Subsystem, which is consisted of the Parachutes, Location Aids and the Floatation bag are installed in the REM.

The SEM system is designed for not only the service module for the REM, but also as the standardized bus system such as to be applicable for LEO communication and earth observation mission satellites. Maximum weight and power capabilities for the payloads for SEM (REM portion in case of USERS) are 850kg and 700Watts. Various kinds of the communication and earth observation missions may be installed instead of the REM with a little modification. One of the features of the SEM System is the Integrated Spacecraft Controller (ISC). The ISC consist of three voting and one waiting 32 bit processors and related failure avoidance circuits to avoid the single point failure. The system fault detection, payload power monitor, automatic thermal control, battery charge control, etc. are the examples of the ISC system automatic control functions. The advantages gained by the ISC are weight reduction and the flexibility as the standardized bus. Fig.5 indicates how the various function of the control and data handling have been integrated into the ISC and applied to the following spacecraft project as the standardized bus.


Fig.5 The ISC and the Standardized Bus System Application

5. Operation System and the On-orbit Operation

USERS Ground Operation and Control System consist of the USEF Space Operation Center (USOC) and the JAXA's New Ground Networks. USOC is located in Tokyo and consist of the Spacecraft Command and Status Monitor, the Spacecraft Operation Planning Support, the Orbit Determination and Propagation and the Gate Way to connect JAXA's New Ground Network (GN) through Tsukuba.

The various operation support tools, experiment data distribution function, and other support equipments such as the spacecraft simulator to initiate simulated data at the operation training were equipped in the USOC. Fig.5 indicated the USERS Operation System Block Diagram. USOC has the capabilities to both critical operation phase such as the Launch and Reentry operation and normal experiment phase.

The USERS Spacecraft has been launched at 8:20 UT on Sept. 10, 2002 as planned, and the spacecraft reached the normal mode attitude control status as planned within approximately 12 Hrs after the launch. After the system and experiment initial check out has been completed on Oct. 1, 2002, SMAP operation started as planned using the SGHF. Three SGHF furnace operations have been completed by March 31, 2003. The on-orbit operation summary is shown in the Fig.6. During entire SGHF operation period, sun pointing attitude has been maintained without any thruster operation. The sun pointing attitude has been maintained only by reaction wheels and magnetic torquers. Quick evaluation results of the micro-gravity data showed that the intended less than 10 micro-G level of the micro-gravity environment has been achieved.


Fig.6 On-orbit Operation Summary

6. Re-entry and Recovery Operation

Preparation for the re-entry was started on April 1, 2003. The re-entry window was set to open fron May 26, and would last 10 days. Finally, due to the Typhoon, the renetry and recovery operation has been performed on May 29,2003 (UT).

Non guided re-entry system was adopted for the USERS REM to realize simple return system. The REM attitude was kept as it was at the SEM/REM separation by the MW in the REM, which was turned on prior to the separation. After one orbit revolution, the sequential operation of the spin up, RBM ignition, the de-spin and the PM separation have been performed and wait for the atmospheric reentry. Fig 7 showed the final reentry trajectory and the splashed down point. Table 1 indicates the actual time of the major event of the re-entry. The Beacon Signal Transmitter with GPS receiver and the ARGOS system transmitter are used as the location aids. The SEM/REM separation, RBM ignition, parachute opening, floatation bag function, location aid signal operation and retrieval operation into the recovery ship performed completely as planned and the REV was successfully recovered at the expected open sea area east of the Ogasawara Island. Fig.8 showed the recovery operation.


Fig.7 Final Reentry Trajectory and the Splash Down Point

Fig.8 REV Recovery Operation
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