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LARGE “FUROSHIKI” NET EXTENSION IN SPACE – SOUNDING ROCKET EXPERIMENT RESULTS
LARGE “FUROSHIKI” NET EXTENSION IN SPACE – SOUNDING ROCKET EXPERIMENT RESULTS Shinichi Nakasuka1, Hironori Sahara1, Yuya Nakamura1, Ryu Funase1, Masaki Nagai1, Norihide Miyamura1, Akito Enokuchi1, Yoichi Hatsutori1, Mitsuhito Komatsu1, Yoshiki Sugawara1, and Nobuyuki Kaya2 1
2
Department of Aeronautics and Astronautics, University of Tokyo Department of Computer and Systems Engineering, Kobe University
Abstract: University of Tokyo and Kobe University conducted a sounding rocket experiment to deploy large “Furoshiki” net in space, which is a promising candidate for the future large antenna or solar power satellites. The experimental system consisting of mother and three daughter satellites as well as a folded net was separated from S-310 sounding rocket of JAXA/ISAS at the altitude of 110km, where the daughter satellites, after separated from mother satellite with 1.2 m/s velocity, deployed a large net of 14msized triangle. The deployment was quire successful without any tangling and the dynamics during deployment has been captured by cameras, INS and radar distance measurement system. Retro-directive antenna micro wave transmission experiment using 4 antennae on the bottom of satellites were conducted also successfully. Copyright © 2007 IFAC Keywords: Space Structure, Large Net, Solar Power Satellite, Phased Array Antenna, Dynamics and Control , Formation Flying
1. INTRODUCTION University of Tokyo has been proposing a large membrane space structure named ”Furoshiki Satellite,'' as a promising candidate for the future space structure for those missions requiring large area in space such as a solar cell or a large communication antenna (Motohashi, et.al. 1998a, b, Nakasuka, et.al. 2001, 2006). This membrane is folded in a very small volume during launch and is deployed and controlled by a set of several satellites at its corners (such as in Fig.1) or using centrifugal force generated by rotating the central satellite. "Furoshiki" naming comes from the traditional Japanese very light square-shaped lapping cloth, which can be easily folded and deployed with hands. It is expected that such a structure will reduce the weight per area of the space structure and, if the control technology is properly applied, it can be efficiently folded to be launched and easily deployed. One of the promising applications of Furoshiki Satellite will be solar power generation. The Furoshiki
membrane may be applied to the large solar cells or large solar light reflector. Another promising application is a large phased array antenna. It is difficult to control the position and attitude of the RF transmitters on Furoshiki precisely, but using the "retro-directive" method, the tolerance against such disturbances will be relaxed by large (Kaya, et.al. 1996, 2000, 2005).
Fig.1 Concept of "Furoshiki Satellite"
These two applications can be combined; i.e., if we implement on one surface of the Furoshiki satellite large solar cells and on another surface microwave transmitting antennae, then we can configure a large solar power satellite. This is one of promising systems' concept of the future large solar power satellite, because quite a large area can be obtained without any hard structure, and the weight will not depend very much on the size. If we place many panels on Furoshiki type net, not Furoshiki membrane, which have solar cells on one surface and microwave transmitters on the other surface, then we can configure a “distributed” solar power satellite. “Distributed” means that the power generated at each panels can be transmitted to the ground without concentrating it to a certain element, which can reduce the heat problem of the solar power satellite by large. The technical issue of this configuration is how to place panels at different part of the large net. One solution to do so is to use a robot which can move on the net to carry a panel to the designated points. We started the feasibility study for this “crawling robots” in collaboration with ESA and Technical University of Vienna. To demonstrate the feasibility of the extension of large net structure and phased array antenna performance, micro-gravity experiment was conducted on January 22, 2006, using a sounding rocket named “S-310” of JAXA/ISAS, Japan. In the experiment, one “Mothersatellite” and three “daughter-satellites” as well as a folded net were launched altogether to the altitude of 110 km by a sounding rocket S-310, where the Daughters satellites were released from mother satellite towards three different directions with 120 degree separation, while extending the net, and finally a large regular triangle-shaped net with 14 m side was configured. The motion during the extension is measured by the INS implemented on Mother and Daughter-satellites as well as images of video cameras on the Mother-satellite. The four satellites have transmitters on their bottoms which transmit 2.4GHz microwave towards the ground using retro-directive phased array method. Finally when a net movement is stabilized, small moving robots were released from Mother-satellite, which move on the net for a certain distance. The experiment was performed very successfully and the obtained data has been analyzed. This paper mainly describes the experiment to evaluate the net deployment and satellite-net dynamics during deployment, and the retro-directive antenna experiment was reported in Kaya, et.al. 2006. This paper describes the objectives, setup, system descriptions and the results of this sounding rocket experiment.
2. SOUNDING ROCKET EXPERIMENT 2.1 Objectives of the Experiment To demonstrate the feasibility of the extension of large net structure and phased array antenna perfor- mance, micro-gravity experiment project using a sounding rocket named “S-310” of JAXA/ISAS, Japan started in 2004. The objectives of the experiment are; 1) to evaluate the new net folding and extension method which reduces the possibility of tangling. 2) to grasp the dynamics of the satellites and net during deployment 3) to evaluate the RF transmission performance of the retro-directive phased array antenna on the Furoshiki type large net structure 4) to evaluate the feasibility of a robot crawling on a net structure deployed in this way Figure 2 shows the concept of the experiment, and the experiment scenario is shown below. 2.2 Scenario of the Experiment 1) Mother satellite, three daughter satellites as well as a net is stored in the fairing of the sounding rocket during launch. 2) The rocket releases the whole experimental system just before the apogee (at the altitude of 100150km) in spinning condition. 3) The mother satellite stops the rotation by absorbing the angular momentum using its momentum wheel. During this phase, the momentum wheels of the daughter satellites are also actuated to generate the angular momentum. With this, the mother and daughter satellites become bias momentum status to be tolerant against disturbance. 4) The daughter satellites are separated from the mother satellite with a certain relative velocity, which extends the net initially folded on the top of the mother satellite. 5) During the extension of the net, the attitude dynamics of the satellites are measured using inertial sensors on the four satellites, and the dynamical behavior of the net is captured by the camera on the mother (two cameras) and daughter satellite (one camera). 6) Phased array antenna is configured using the RF transmitter and patch antenna on the bottom of the mother and daughter satellites. During the extension of the net, the performance of the phased array antenna is measured from the ground station.
Daughter satellite separation direction
Inside Fairing (during launch)
Daught er sat
Mother Sat Daughter sat
Daughter sat
of the daughter satellites after the net is deployed to its maximum length as well as for keeping the tension of the net when the robots crawl on the net. The upper part holds INS system and a momentum wheel to keep the attitude of the daughter satellites against disturbances. Fig.5 shows the S310 sounding rocket in its launch tower. This experiment was the 36th launch of this rocket. Net case Robot “garage” Cameras
Three Daughter Satellites T,L,R Elec. Box
Rocket Outer Surface to be Opened before Deployment
Deployment Main Tether Daughter Satellite
Wire Cutter Rocket Top
Structure
Cold gas tank Interface Point with Rocket
Fig.3 Experimental System on a Sounding Rocket Mother Satellite
8.8 m
z-axis
Fig. 2 Image of the Sounding Rocket Experiment 2.3 Description of Subsystems Fig.3 shows the experimental system to be mounted within the fairing of the S310 sounding rocket. The uppermost flat panel mounts a net case in which the net is stowed. This case is released just before the net deployment starts. The second flat panel mounts three cameras seeing towards the daughter satellite release directions. The third and fourth panels are robot “garages,” which hold the crawling robots and release them after the net is deployed and stabilized. The central part of the mother satellites hold three daughter satellites “T”,”L” and “R”. The connection is made by metal wires, which will be cut by wire cutters when the deployment start signal comes from the rocket sequencer. The daughter satellites, pushed by separation springs, fly towards three directions with 120 degree separation, and the separation speed was planned as 1.2m/s. Fig.4 shows the daughter satellites mounted on the main structure (left figure), and its side view (right figure). Two of the three daughter satellites, L and R have 250mN N2 cold gas thrusters consisting of small off-the-shelf gas bombes usually used for “beer servers,” elecro-magnetic valves and a small nozzle. These thrusters are used for suppressing “bound-back”
Fig.4 Daughter Satellite, Mounted on Main Structure (left) and Side view (right)
Fig.5 S-310 Sounding Rocket
3. FLIGHT RESULTS 3.1 Overview of the flight experiment Results The flight experiment took place at Uchinoura Space Center of JAXA/ISAS on January 22, 2006. S-310 rocket was launched successfully at 13:00pm which lifted the experimental system up to 110 km where the system was released from the upper part of the rocket. Soon after the separation, we could obtain Ku-band image telemetry, which showed that the system was rotating at about 0.4 Hz after despun operation using Yo-Yo despinner. The first operation of the system was to stop the rotational motion using the reaction wheel. This operation was conducted at X(launch time)+84.5 to X+120sec, during which the image telemetry showed that the motion was gradually stabilized and finally the almost still image of the Earth was shown. Then the wheels of daughter satellites are spun-up and at X+130, the net case was separated using a wire cutter, and at X+133, the three daughter satellites were kicked off towards three direction with 120 degree separations. Figure 6 shows the camera image just after daughter satellites release. The net deployment was conducted very smoothly, and there are no tangling phenomena. The daughter satellites are in bias momentum situation with spinning wheels inside, and their attitudes were showing nutational motion and could keep the body zaxis (in Fig.4) within a certain deviation from the realistic direction.
that the triangle shape as in Fig.5 was successfully configured at around X+140 sec (see Fig.7). Then “bound-back” occurred, in which daughter satellites moved back towards the mother satellite. Daughter satellite “L” and “R” have cold-gas jet to prevent the bound back, but due to the ill performance of the attitude control around z-axis, the bound back phenomenon could not be stopped.
Fig.7 Camera Images when the Triangle Shape of the Net was configured. Daughter L and crawling net is also shown in upper left figure. During this deployment, retro-directive antenna experiment was conducted. For the detail of the results, please see Kaya, et.al (2006). At around X+158 sec, two crawling robots came out of “the robot garage” of the experimental system, and moved a little over the net, whose image was captured by the camera. (Summerer, et.al., 2006) 3.2 Daughter Dynamics during Deployment Fig. 8 and 9 show the rotational velocity (gyro output) and acceleration of Daughter L. We can observe the nutation motion just after release. Z-axis was not successfully controlled and deviated with time. At around 140 sec, a large tension was exerted which shows the bound-back phenomenon. To prevent this bound-back, in Daughter L and R, the cold gas jet was activated which shows the counter force in y-direction.
Fig.6 Camera Images just after Daughter Satellites’ Release (Cameras 1 and 2 are on the mother satellite towards L and R daughter satellites release direction and Camera 3 is on the T-daughter satellite towards the mother satellite direction. Camera 1 and 2 captured images of daughter L, R and nets, and Camera 3 captured images of the mother and daughter satellites L and R) About 8 seconds later, the daughter satellites reached the end, where the main tether in Fig.2 becomes the longest. All of the three daughter satellites could reach this point almost at the same time, and so we can guess
After detailed analysis, it was found that the separation speed was 1.2 m/s (+-0.1m/s), 1.3 m/s (+-0.1m/s), 1.2m/s (+-0.5m/s) for Daughter T, L and R respectively, while the designed speed was 1.2m/s. The maximum distance of each daughter satellite from the mother was estimated from INS as 8.1m (RF measurement’s estimation was 8.6m), 6.8m (7.3m), and 8.2m (9.3m) for Daughter T, L and R respectively, while the designed distance was 8.8 m. These data indicate that the daughter satellites were released almost with the intended speed and no large tangling occurred during the deployment.
The largest problem was the disturbance on the daughter satellites’ attitude at the timing of release from the mother satellite. In the initial plan, the deviation of z-axis direction of each daughter satellite from the nominal direction should be less than 20 deg in order for the phased array antenna to work effectively, but the actual result shows that the deviation became as much as 70 degree. This seems to come from collisions of the daughter satellite’s bottom against the shelf of the mother satellite which holds the daughter satellites. We should have accommodated an enough gap between these two structures, but could not because we made last minute addition of a certain part into this space in order to improve the performance. In addition, the angular momentum of the daughter satellites was too small to suppress such disturbances. These results showed that more precise modelling of these disturbances and the analysis of required angular momentum based on this modelling should be done. Rotational speed (deg/s) Daughter release
130
Time from launch (s)
Blue: x-axis angular velocity pink: y-axis yellow:z-axis
150
Fig.8 Attitude Rotational Motion of Daughter L Acceleration (m/s2)
Daughter release 130
At maximum distance Time from launch (s)
Blue: x-axis angular velocity pink: y-axis yellow:z-axis
150
Fig.9 3-axis Acceleration of Daughter L 3.3 Net Deployment
We have developed a unique net folding method, in order to fold the large triangle-shaped net in a small net case and to deploy it without tangling. The release speed of the daughter satellite has been decided so that the daughter may reduce speed by the friction force and be stopped at the maximum distance without boundback. Based on the ground test, this friction was estimated as 10 gf in average with which the release speed was determined as 1.2 m/s so that the daughter satellites will stop at the distance of 8.8 m, which is the length of the main tether. The flight experiment indicated that this friction was much smaller than expected (less than 5gf) and so the daughter satellites did not reduce speed much until they came to the maximum distance, which resulted in strong bound-back. On the other hand, if this friction had been estimated too small in design phase, then in the actual deployment the daughter satellite might have stopped far before it comes to the maximum distance. So we should find out an appropriate method to precisely estimate the friction during net deployment based on ground tests. Based on the disturbance estimation obtained in this experiment, a new net dynamic model is now being developed, in which we hope to estimate the friction more analytically. 3.4 Control of Daughter Satellite Position/Attitude and Cold Gas-Jet Experiment The daughter satellites can control its orientation around z-axis so that a certain surface (the surface where the thruster nozzle is attached) always targets towards the mother satellite using a momentum wheel along z-axis. The orientation information is provided by the integral of the z-axis gyro readout plus images taken by the camera which see the mother satellite’s direction. In addition, the L and R daughter satellites have cold-gas jets to generate a force to prevent the bound-back and keep the main tether’s tension. These two controls were tried as advanced experiments. The results showed that the daughter satellites could not get orientation information, because due to the sudden change of its attitude at the timing of release, its camera lost the image of the mother satellite. Therefore the daughter satellites rotated around z-axis and the orientation of the thruster nozzle towards the mother satellite could not be maintained. The thrusters fired normally, triggered by the sequence timer at X+155 sec, but as the nozzle orientation was not maintained, the bound-back phenomenon could not be suppressed. 3.5 Educational Outcomes One more significant factor we should add is that all of the experimental system design, fabrication, ground test and operation were mainly performed by a student
team, with strong guidance from the professional engineers and professors in space field. The whole project over 2 years contributed a lot to students’ training for developing reliable space systems as well as for project management and team works.
array antenna, and so on. We plan to further improve the system’s concept and component technologies and hope to perform another “Furoshiki” experiment for a solar power satellite in Earth orbit in near future. ACKNOWLEDGMENT
3.6 Summary of Outcomes and Lessons Learned The flight experiment can be evaluated as “success” in terms of the “nominal” experiment but “not success” in the “advanced” experiment. We could obtain valuable data on the combined dynamics of net and daughter satellites during deployment including the friction force, which will be utilized for more precise modelling of the net’s dynamics. Several technologies concerning “Furoshiki” deployment systems could be verified and demonstrated such as how to fix and kick daughter satellite with a certain relative speed, how to fold and deploy net without tangling, very small INS and a momentum wheel subsystem for daughter satellites, camera image processing, and cold gas jet subsystem, etc. Systems integration and management expertise for this kind of multi-satellite system is another important technology which we could obtain throughout this experiment. On the other hand, we should admit that we got many “lessons learned” from failures or deficiencies in the advanced experiment. These include collisions of the daughter satellites with the mother during separation, not enough angular momentum of the daughter satellites, not-so-robust attitude control capability of the daughters, etc. We are now developing an improved net model based on the obtained camera images and daughter satellites’ navigation data during the deployment. Especially, the mutual friction of the net as well as bending stiffness of the strings should be carefully modelled.
5. CONCLUSIONS This paper shows the flight experiment for “Furoshiki Satellite” on the S-310 sounding rocket. From the data, it was estimated that one “Mother satellite” and three “Daughter satellites” deployed a triangular net with 14 m-size in space with micro-wave transmission capability, which can be said the first milestone towards the practical research for future solar power satellites. Though not all of the experiments were performed successfully, we could obtain various technologies and valuable data through this experiment concerning large net deployment in space, mother and daughter satellite system, and retro-directive phased
We are very grateful to JAXA/ISAS for giving us this valuable opportunity to do flight experiment and fund for experimental system development.
REFERENCES Kaya, N. (1996), A New Concept of SPS with a Power Generator/Transmitter of a Sandwich Structure and a Large Solar Collector, Space Energy and Transportation, Vol.1, No.3, pp.205-213. Kaya, N. (2000), Development and Demonstration of New Retro-directive Antenna System for Solar Power Satellite of Sandwich Structure, Proc. International symposium on Antenna and Propagation, Vol. 3, pp.1419-1422. Kaya,N., Iwashita,M., Nakasuka, S., Summerer, L., Mankins, J.(2005), Crawling robots on large web in rocket experiment on furoshiki deployment, Journal of the British Interplanetary Society, Vol.58, No.11/12, pp.403-406. Kaya, N., Iwashita, M., Tanaka, K., Nakasuka, S. and Summerer, L. (2006), Rocket Experiment on Microwave Power Transmission with Furoshiki Deployment, IAC-06-C.3.3.03. Motohashi, S. and Nakasuka, S. (1998), On-orbit Dynamics and Control of Large Scaled Membrane with Satellites at its Corners, Proceedings of IFAC Symposium on Aerospace Control, pp. 146-151. Motohashi,, S., Nakasuka, S., Aoki, T., Narusawa, Y., Nagashima, R., Kawakatsu, Y., and Kinoshita, T (1998), On-orbit Dynamics and New Control Scheme for Large Membrane "Furoshiki" Satellite. Proceedings of 21st ISTS, 98-e-20. Nakasuka, S., Aoki, T., Ikeda, I., Tsuda, Y. and Kawakatsu, Y. (2001), "Furoshiki Satellite"-A Large Membrane Structure as a Novel Space System, Acta Astronautica, Vol.48, No.5-12, pp.461-468. Nakasuka, S., Funane, T., Nakamura, Y., Nojiri, Y., Sahara, H., Sasaki, F. and Kaya, N. (2006), Sounding rocket flight experiment for demonstrating ”furoshiki satellite” for large Phased Array Antenna, Acta Astronautica, 59, pp.200-205. Summerer, L. Kaya, N., Putz, B., and Kopachek, P.(2006), First Results of robots crawling on a loose net in microgravity during a sounding rocket experiment, IAC-06-C3.
2
Department of Aeronautics and Astronautics, University of Tokyo Department of Computer and Systems Engineering, Kobe University
Abstract: University of Tokyo and Kobe University conducted a sounding rocket experiment to deploy large “Furoshiki” net in space, which is a promising candidate for the future large antenna or solar power satellites. The experimental system consisting of mother and three daughter satellites as well as a folded net was separated from S-310 sounding rocket of JAXA/ISAS at the altitude of 110km, where the daughter satellites, after separated from mother satellite with 1.2 m/s velocity, deployed a large net of 14msized triangle. The deployment was quire successful without any tangling and the dynamics during deployment has been captured by cameras, INS and radar distance measurement system. Retro-directive antenna micro wave transmission experiment using 4 antennae on the bottom of satellites were conducted also successfully. Copyright © 2007 IFAC Keywords: Space Structure, Large Net, Solar Power Satellite, Phased Array Antenna, Dynamics and Control , Formation Flying
1. INTRODUCTION University of Tokyo has been proposing a large membrane space structure named ”Furoshiki Satellite,'' as a promising candidate for the future space structure for those missions requiring large area in space such as a solar cell or a large communication antenna (Motohashi, et.al. 1998a, b, Nakasuka, et.al. 2001, 2006). This membrane is folded in a very small volume during launch and is deployed and controlled by a set of several satellites at its corners (such as in Fig.1) or using centrifugal force generated by rotating the central satellite. "Furoshiki" naming comes from the traditional Japanese very light square-shaped lapping cloth, which can be easily folded and deployed with hands. It is expected that such a structure will reduce the weight per area of the space structure and, if the control technology is properly applied, it can be efficiently folded to be launched and easily deployed. One of the promising applications of Furoshiki Satellite will be solar power generation. The Furoshiki
membrane may be applied to the large solar cells or large solar light reflector. Another promising application is a large phased array antenna. It is difficult to control the position and attitude of the RF transmitters on Furoshiki precisely, but using the "retro-directive" method, the tolerance against such disturbances will be relaxed by large (Kaya, et.al. 1996, 2000, 2005).
Fig.1 Concept of "Furoshiki Satellite"
These two applications can be combined; i.e., if we implement on one surface of the Furoshiki satellite large solar cells and on another surface microwave transmitting antennae, then we can configure a large solar power satellite. This is one of promising systems' concept of the future large solar power satellite, because quite a large area can be obtained without any hard structure, and the weight will not depend very much on the size. If we place many panels on Furoshiki type net, not Furoshiki membrane, which have solar cells on one surface and microwave transmitters on the other surface, then we can configure a “distributed” solar power satellite. “Distributed” means that the power generated at each panels can be transmitted to the ground without concentrating it to a certain element, which can reduce the heat problem of the solar power satellite by large. The technical issue of this configuration is how to place panels at different part of the large net. One solution to do so is to use a robot which can move on the net to carry a panel to the designated points. We started the feasibility study for this “crawling robots” in collaboration with ESA and Technical University of Vienna. To demonstrate the feasibility of the extension of large net structure and phased array antenna performance, micro-gravity experiment was conducted on January 22, 2006, using a sounding rocket named “S-310” of JAXA/ISAS, Japan. In the experiment, one “Mothersatellite” and three “daughter-satellites” as well as a folded net were launched altogether to the altitude of 110 km by a sounding rocket S-310, where the Daughters satellites were released from mother satellite towards three different directions with 120 degree separation, while extending the net, and finally a large regular triangle-shaped net with 14 m side was configured. The motion during the extension is measured by the INS implemented on Mother and Daughter-satellites as well as images of video cameras on the Mother-satellite. The four satellites have transmitters on their bottoms which transmit 2.4GHz microwave towards the ground using retro-directive phased array method. Finally when a net movement is stabilized, small moving robots were released from Mother-satellite, which move on the net for a certain distance. The experiment was performed very successfully and the obtained data has been analyzed. This paper mainly describes the experiment to evaluate the net deployment and satellite-net dynamics during deployment, and the retro-directive antenna experiment was reported in Kaya, et.al. 2006. This paper describes the objectives, setup, system descriptions and the results of this sounding rocket experiment.
2. SOUNDING ROCKET EXPERIMENT 2.1 Objectives of the Experiment To demonstrate the feasibility of the extension of large net structure and phased array antenna perfor- mance, micro-gravity experiment project using a sounding rocket named “S-310” of JAXA/ISAS, Japan started in 2004. The objectives of the experiment are; 1) to evaluate the new net folding and extension method which reduces the possibility of tangling. 2) to grasp the dynamics of the satellites and net during deployment 3) to evaluate the RF transmission performance of the retro-directive phased array antenna on the Furoshiki type large net structure 4) to evaluate the feasibility of a robot crawling on a net structure deployed in this way Figure 2 shows the concept of the experiment, and the experiment scenario is shown below. 2.2 Scenario of the Experiment 1) Mother satellite, three daughter satellites as well as a net is stored in the fairing of the sounding rocket during launch. 2) The rocket releases the whole experimental system just before the apogee (at the altitude of 100150km) in spinning condition. 3) The mother satellite stops the rotation by absorbing the angular momentum using its momentum wheel. During this phase, the momentum wheels of the daughter satellites are also actuated to generate the angular momentum. With this, the mother and daughter satellites become bias momentum status to be tolerant against disturbance. 4) The daughter satellites are separated from the mother satellite with a certain relative velocity, which extends the net initially folded on the top of the mother satellite. 5) During the extension of the net, the attitude dynamics of the satellites are measured using inertial sensors on the four satellites, and the dynamical behavior of the net is captured by the camera on the mother (two cameras) and daughter satellite (one camera). 6) Phased array antenna is configured using the RF transmitter and patch antenna on the bottom of the mother and daughter satellites. During the extension of the net, the performance of the phased array antenna is measured from the ground station.
Daughter satellite separation direction
Inside Fairing (during launch)
Daught er sat
Mother Sat Daughter sat
Daughter sat
of the daughter satellites after the net is deployed to its maximum length as well as for keeping the tension of the net when the robots crawl on the net. The upper part holds INS system and a momentum wheel to keep the attitude of the daughter satellites against disturbances. Fig.5 shows the S310 sounding rocket in its launch tower. This experiment was the 36th launch of this rocket. Net case Robot “garage” Cameras
Three Daughter Satellites T,L,R Elec. Box
Rocket Outer Surface to be Opened before Deployment
Deployment Main Tether Daughter Satellite
Wire Cutter Rocket Top
Structure
Cold gas tank Interface Point with Rocket
Fig.3 Experimental System on a Sounding Rocket Mother Satellite
8.8 m
z-axis
Fig. 2 Image of the Sounding Rocket Experiment 2.3 Description of Subsystems Fig.3 shows the experimental system to be mounted within the fairing of the S310 sounding rocket. The uppermost flat panel mounts a net case in which the net is stowed. This case is released just before the net deployment starts. The second flat panel mounts three cameras seeing towards the daughter satellite release directions. The third and fourth panels are robot “garages,” which hold the crawling robots and release them after the net is deployed and stabilized. The central part of the mother satellites hold three daughter satellites “T”,”L” and “R”. The connection is made by metal wires, which will be cut by wire cutters when the deployment start signal comes from the rocket sequencer. The daughter satellites, pushed by separation springs, fly towards three directions with 120 degree separation, and the separation speed was planned as 1.2m/s. Fig.4 shows the daughter satellites mounted on the main structure (left figure), and its side view (right figure). Two of the three daughter satellites, L and R have 250mN N2 cold gas thrusters consisting of small off-the-shelf gas bombes usually used for “beer servers,” elecro-magnetic valves and a small nozzle. These thrusters are used for suppressing “bound-back”
Fig.4 Daughter Satellite, Mounted on Main Structure (left) and Side view (right)
Fig.5 S-310 Sounding Rocket
3. FLIGHT RESULTS 3.1 Overview of the flight experiment Results The flight experiment took place at Uchinoura Space Center of JAXA/ISAS on January 22, 2006. S-310 rocket was launched successfully at 13:00pm which lifted the experimental system up to 110 km where the system was released from the upper part of the rocket. Soon after the separation, we could obtain Ku-band image telemetry, which showed that the system was rotating at about 0.4 Hz after despun operation using Yo-Yo despinner. The first operation of the system was to stop the rotational motion using the reaction wheel. This operation was conducted at X(launch time)+84.5 to X+120sec, during which the image telemetry showed that the motion was gradually stabilized and finally the almost still image of the Earth was shown. Then the wheels of daughter satellites are spun-up and at X+130, the net case was separated using a wire cutter, and at X+133, the three daughter satellites were kicked off towards three direction with 120 degree separations. Figure 6 shows the camera image just after daughter satellites release. The net deployment was conducted very smoothly, and there are no tangling phenomena. The daughter satellites are in bias momentum situation with spinning wheels inside, and their attitudes were showing nutational motion and could keep the body zaxis (in Fig.4) within a certain deviation from the realistic direction.
that the triangle shape as in Fig.5 was successfully configured at around X+140 sec (see Fig.7). Then “bound-back” occurred, in which daughter satellites moved back towards the mother satellite. Daughter satellite “L” and “R” have cold-gas jet to prevent the bound back, but due to the ill performance of the attitude control around z-axis, the bound back phenomenon could not be stopped.
Fig.7 Camera Images when the Triangle Shape of the Net was configured. Daughter L and crawling net is also shown in upper left figure. During this deployment, retro-directive antenna experiment was conducted. For the detail of the results, please see Kaya, et.al (2006). At around X+158 sec, two crawling robots came out of “the robot garage” of the experimental system, and moved a little over the net, whose image was captured by the camera. (Summerer, et.al., 2006) 3.2 Daughter Dynamics during Deployment Fig. 8 and 9 show the rotational velocity (gyro output) and acceleration of Daughter L. We can observe the nutation motion just after release. Z-axis was not successfully controlled and deviated with time. At around 140 sec, a large tension was exerted which shows the bound-back phenomenon. To prevent this bound-back, in Daughter L and R, the cold gas jet was activated which shows the counter force in y-direction.
Fig.6 Camera Images just after Daughter Satellites’ Release (Cameras 1 and 2 are on the mother satellite towards L and R daughter satellites release direction and Camera 3 is on the T-daughter satellite towards the mother satellite direction. Camera 1 and 2 captured images of daughter L, R and nets, and Camera 3 captured images of the mother and daughter satellites L and R) About 8 seconds later, the daughter satellites reached the end, where the main tether in Fig.2 becomes the longest. All of the three daughter satellites could reach this point almost at the same time, and so we can guess
After detailed analysis, it was found that the separation speed was 1.2 m/s (+-0.1m/s), 1.3 m/s (+-0.1m/s), 1.2m/s (+-0.5m/s) for Daughter T, L and R respectively, while the designed speed was 1.2m/s. The maximum distance of each daughter satellite from the mother was estimated from INS as 8.1m (RF measurement’s estimation was 8.6m), 6.8m (7.3m), and 8.2m (9.3m) for Daughter T, L and R respectively, while the designed distance was 8.8 m. These data indicate that the daughter satellites were released almost with the intended speed and no large tangling occurred during the deployment.
The largest problem was the disturbance on the daughter satellites’ attitude at the timing of release from the mother satellite. In the initial plan, the deviation of z-axis direction of each daughter satellite from the nominal direction should be less than 20 deg in order for the phased array antenna to work effectively, but the actual result shows that the deviation became as much as 70 degree. This seems to come from collisions of the daughter satellite’s bottom against the shelf of the mother satellite which holds the daughter satellites. We should have accommodated an enough gap between these two structures, but could not because we made last minute addition of a certain part into this space in order to improve the performance. In addition, the angular momentum of the daughter satellites was too small to suppress such disturbances. These results showed that more precise modelling of these disturbances and the analysis of required angular momentum based on this modelling should be done. Rotational speed (deg/s) Daughter release
130
Time from launch (s)
Blue: x-axis angular velocity pink: y-axis yellow:z-axis
150
Fig.8 Attitude Rotational Motion of Daughter L Acceleration (m/s2)
Daughter release 130
At maximum distance Time from launch (s)
Blue: x-axis angular velocity pink: y-axis yellow:z-axis
150
Fig.9 3-axis Acceleration of Daughter L 3.3 Net Deployment
We have developed a unique net folding method, in order to fold the large triangle-shaped net in a small net case and to deploy it without tangling. The release speed of the daughter satellite has been decided so that the daughter may reduce speed by the friction force and be stopped at the maximum distance without boundback. Based on the ground test, this friction was estimated as 10 gf in average with which the release speed was determined as 1.2 m/s so that the daughter satellites will stop at the distance of 8.8 m, which is the length of the main tether. The flight experiment indicated that this friction was much smaller than expected (less than 5gf) and so the daughter satellites did not reduce speed much until they came to the maximum distance, which resulted in strong bound-back. On the other hand, if this friction had been estimated too small in design phase, then in the actual deployment the daughter satellite might have stopped far before it comes to the maximum distance. So we should find out an appropriate method to precisely estimate the friction during net deployment based on ground tests. Based on the disturbance estimation obtained in this experiment, a new net dynamic model is now being developed, in which we hope to estimate the friction more analytically. 3.4 Control of Daughter Satellite Position/Attitude and Cold Gas-Jet Experiment The daughter satellites can control its orientation around z-axis so that a certain surface (the surface where the thruster nozzle is attached) always targets towards the mother satellite using a momentum wheel along z-axis. The orientation information is provided by the integral of the z-axis gyro readout plus images taken by the camera which see the mother satellite’s direction. In addition, the L and R daughter satellites have cold-gas jets to generate a force to prevent the bound-back and keep the main tether’s tension. These two controls were tried as advanced experiments. The results showed that the daughter satellites could not get orientation information, because due to the sudden change of its attitude at the timing of release, its camera lost the image of the mother satellite. Therefore the daughter satellites rotated around z-axis and the orientation of the thruster nozzle towards the mother satellite could not be maintained. The thrusters fired normally, triggered by the sequence timer at X+155 sec, but as the nozzle orientation was not maintained, the bound-back phenomenon could not be suppressed. 3.5 Educational Outcomes One more significant factor we should add is that all of the experimental system design, fabrication, ground test and operation were mainly performed by a student
team, with strong guidance from the professional engineers and professors in space field. The whole project over 2 years contributed a lot to students’ training for developing reliable space systems as well as for project management and team works.
array antenna, and so on. We plan to further improve the system’s concept and component technologies and hope to perform another “Furoshiki” experiment for a solar power satellite in Earth orbit in near future. ACKNOWLEDGMENT
3.6 Summary of Outcomes and Lessons Learned The flight experiment can be evaluated as “success” in terms of the “nominal” experiment but “not success” in the “advanced” experiment. We could obtain valuable data on the combined dynamics of net and daughter satellites during deployment including the friction force, which will be utilized for more precise modelling of the net’s dynamics. Several technologies concerning “Furoshiki” deployment systems could be verified and demonstrated such as how to fix and kick daughter satellite with a certain relative speed, how to fold and deploy net without tangling, very small INS and a momentum wheel subsystem for daughter satellites, camera image processing, and cold gas jet subsystem, etc. Systems integration and management expertise for this kind of multi-satellite system is another important technology which we could obtain throughout this experiment. On the other hand, we should admit that we got many “lessons learned” from failures or deficiencies in the advanced experiment. These include collisions of the daughter satellites with the mother during separation, not enough angular momentum of the daughter satellites, not-so-robust attitude control capability of the daughters, etc. We are now developing an improved net model based on the obtained camera images and daughter satellites’ navigation data during the deployment. Especially, the mutual friction of the net as well as bending stiffness of the strings should be carefully modelled.
5. CONCLUSIONS This paper shows the flight experiment for “Furoshiki Satellite” on the S-310 sounding rocket. From the data, it was estimated that one “Mother satellite” and three “Daughter satellites” deployed a triangular net with 14 m-size in space with micro-wave transmission capability, which can be said the first milestone towards the practical research for future solar power satellites. Though not all of the experiments were performed successfully, we could obtain various technologies and valuable data through this experiment concerning large net deployment in space, mother and daughter satellite system, and retro-directive phased
We are very grateful to JAXA/ISAS for giving us this valuable opportunity to do flight experiment and fund for experimental system development.
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