Propellantless Sail-Craft Design for the Main Belt Asteroid Exploration Mission

Propellantless Sail-craft Design for the Main Belt Asteroid Exploration Mission

By Liu yufei1), 2), Cheng zhengai1), Huang xiaoqi1), Zhou lu1), Wang li1)

1)The Qian Xuesen Laboratory of Space Technology, CAST, Beijing,China 2) State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, Dalian, China (Received 1st Dec, 2016)

Based on the propellantless characteristic, a multiple main belt asteroid exploration mission in which the solar sail is the only propeller is proposed by China academy of space technology (CAST). The mission aims to explore at least three main belt asteroids in seven years. The process of determining probe objects and the main nodes of the mission trajectory are first presented. To realize the mission, the spacecraft is a square solar sail with the areal density less than 12g/m2, the side length of 160m and the total mass of 200kg. Then the main subsystems of the solar sail are introduced. The cutting and splicing scheme, the fold and deployment scheme and the margin strengthen scheme are designed in the sail subsystem. A new four radius lenticular boom with two inflatable tubules is proposed to reduce the mass and improve the mechanical property. The slot and membrane antenna and the wireless network are used in the communication subsystem. Two mass blocks and four roll stabilizer bars are designed to control the attitude and orbit. The distribution installation thin film solar cells are used in the power subsystem, so that each sensor and actuator which is not in the central body could be supplied power by cells on itself. At last, the properties of the solar sail meet the mission design constraints.

Key Words: solar sail, main belt asteroid, mission design, main subsystem

1. Introduction side length of the sail is about 160m and the boom length is In recent decades, solar sails have aroused more and more about 115m. interests in theoretical and experimental research. Since the The main exploration target is the Vesta which is the first successful solar sail mission IKAROS[1] was launched in third huge asteroid and is much nearer the Earth than other big May 2010, it indicates that the solar sail technology has asteroids. The Scientific data in the mission can also be entered into the engineering stage. In fact, many space compared with results in the DAWN mission. The other research institutes, such as NASA, ESA and DLR[2-4] et al. all targets selected in the GTOC(Global Trajectory Optimization have projects to develop the solar sail technology. Competition)[12] asteroid database. In recent years, a mission supported by the solar sail to For optimization of interplanetary transfer trajectory, the explore at least three main-belt asteroids in seven years has force is just the gravity in the Earth influence sphere, and in been designed by China academy of space technology the heliocentric coordinates the forces are the solar gravitation (CAST)[5-11]. The initial idea is that the propellantless solar sail and the solar radiation pressure force. For planetary mission, can fly in the asteroid belt for a long time and there are there are three trajectories satisfied for the seven years abundant valuable targets for the exploration on the small condition. The exhaustion method and the costate initial value celestial bodies. To realize the mission, the basic normalization method are adopted to solve the trajectory consideration for sail-craft is the sail area density, or areal optimal problem. density. Areal density is the total vehicle mass divided by the Table 1. Three chances for the main belt mission. total sail area. The basic parameter defines the capability of Depart time; Arrival The first The second The total the concept as to successfully perform many scientific and time asteroid asteroid time(year) research missions. Some novel design ideas of the solar sail 2020/8/17;2027/8/6 1703 Barry 1831 Nicholson 6.97 include several subsystems such as the sail, the boom, the 2021/12/30;2028/12/7 1089 Tama 1831 Nicholson 6.94 attitude and orbit control, the power systems, and 2021/12/18;2028/9/28 1219 Britta 1831 Nicholson 6.78 communications. The results show that a same second asteroid is used in At first, the process of determining probe objects and the all the three chances and the size of the first asteroid is very main nodes of the mission trajectory are given to introduce the similar, so the last chance will be the final choice. The mission schedule. Then, some special designs in the main sail-craft flies to the asteroid belt in 2021; it meets the first subsystems of the solar sail are described respectively. At last, main-belt asteroid (1219 Britta) in 2024; it meets the second some conclusions of the mission and the solar sail are given. asteroid (1831 Nicholson) in 2026; it meets the third asteroid (Vesta) which is the main science target in 2028. 2. The mission schedule The solar sail is mounted as payload on a launch vehicle In this Main Belt Asteroid Exploration Mission, the areal that is capable of placing it directly into the transfer orbit. The density is necessary to be less than 12g/m2, which means the escape orbit parameters in the geocentric equatorial inertial total mass is 200kg and the area is larger than 16900m2. The coordination are listed in Table2.

Table 2. The escape orbit parameters. Type Value Optimal entry time /UTCG 2021/12/10 19:38:51.463 Injection point radius/km 6700 Injection point velocity/km/s 10.91 Injection point position/(km) [-591.1313; 6440.8255; 1748.2363] Parabolic velocity [-9.7810; -2.0805; 4.3577] component/(km/s) The main time node in the flight process is shown in the following table: Table 3. The main time node in the interplanetary flight. Time node Time(MJD) UTCG Comment Depart time 59566.55 2021-12-18 The distance is less Begin to close 60424.19 2024-04-24 than ten million Fig. 2 The angular velocity curves of the pitch angle and the cone angle. Britta kilometer. Closest to The closest distance is 60606.30 2024-10-23 3. The sail-craft project Britta 4621km. The sail-craft includes the central body, four booms, four Begin to leave 60833.98 2025-06-07 Britta triangular sails, the communication subsystem, the power The distance is less subsystem and other subsystems. The approximate mass Begin to close 61246.11 2026-07-25 than ten million distribution is that the central body is 80kg, the sails are 40kg, Nicholson kilometer. and the booms are 40kg. The approach to increase sail area or Closest to The closest distance is 61393.67 2026-12-13 decrease satellite mass can be adopted to cope with the Nicholson 11337km. challenge of decreasing areal density. Because the wider sail Begin to leave 61549.25 2027-05-24 area will result in more strong booms, more difficult Nicholson The closest distance is deployment and decreasing reliability, the better method is to Rendezvous 62041.23 2028-09-27 125km. Then it begin to decrease the mass. The sail membrane is necessary to be made Vesta fly with the Vesta. of new lighter material or to be thinner. The cross section of The total time is 2474.68 days（6.78 years） the booms should be optimized to decrease the mass. It is The trajectory of the total flight process in the most of importance to reduce the mass of the central body. heliocentric ecliptic coordinate system is given in Fig. 1. The The methods include the integrated design for the antenna, the blue arrows represent normal vectors of the solar sail. Based sails and the booms, and using the wireless network to reduce on the trajectory, the angular velocity curves of the pitch angle the weight of the cables. and the cone angle are given in Fig. 2. The pitch angular The solar sail concept is shown in Fig. 3. velocity is less than 7deg/day，the cone angular velocity is less than 2deg/day。

Fig. 1. The trajectory of the total flight process.

Fig. 3. The fold and deployment solar sail diagram.

3.1. The sails The sails include four triangular sail membrane. The material is 2μm polyimide with 1000A of aluminum in front. The first problem is cutting and splicing the huge membrane. The material film width is only 1.5m, so there are many splicing seams in the sail membrane. The seams will bring wrinkles and performance degradation. Two cutting and splicing schemes are compared. In the first scheme, the splicing seams are parallel to the hypotenuse. There are 54 seams and the angle between the seam direction and force direction is 22.5 degree. In the second scheme, the splicing seams are perpendicular to the hypotenuse. There are 108 seams and the angle is 67.5 degree. The angle is proportional to the tension. Based on the above analysis, the final choice is the first scheme. Fig. 7 Stress and strain curves of 12.5μm polyimide film with a center hole. 3.2. The booms There are four ultra light deployable booms in the sail-craft. One of the best booms for the solar sail is the DLR’s 160m CFRP booms. It consists of two co-bonded omega-shaped 160m carbon fiber half shells with 0.1 mm wall thickness each[13]. At first, the mechanical properties of three different cross-section 67.5 22.5 ° booms with the same material are compared. The first boom cross-section is round, and the radius is 150mm. The second boom cross-section is double “Ω” with two radius 150mm and

100mm. The third is a special design, of which the Fig. 4. The cutting and splicing schemes. cross-section is double “Ω” with four radii 150mm, To ensure the deployment, a new folding method was 41mm,30mm and 12mm. The smooth transition of the researched and it was named as skew leaf-out folding method. connecting points of the variable curvature can come into The mass of the folding sail is relatively uniform. The folding reality with the four radii. The results are listed in Table 4. method is given in Fig. 5.

Fig. 5. The skew leaf-out folding method. It is to use the 12.5μm polyimide film for strengthening the margins and the holes of the sail. The results of tensile tests are shown below. It shows the 12.5μm polyimide conforming with the mechanical requirements. Fig. 8 The three cross-section booms. Table 4. The inherent frequency of former five orders. frequence(Hz) Type 1 2 3 4 5 Round 0.072 0.072 0.469 0.469 1.332 Two radii 0.069 0.096 0.434 0.599 1.214 Four radii 0.074 0.075 0.470 0.475 1.314 The first order natural frequency of the improved four radii booms is higher than the round booms and two radii booms. It increases the frequency by 7.25 percent. Another apparent design change is to place two small inflatable tubules in the boom. At the final phase, all the sail Strain membrane total deployment needs enough force from the Fig. 6 Stress and strain curves of 12.5μm polyimide film. boom. And one hundred meter long booms needs extremely

3 high reliability. The self deployment capability of the double receiver. To reduce the mass, the slot antenna and the omega boom is not enough to secure stability and reliability. membrane antenna can be adopted instead of the traditional The two tubules can improve the reliability and provide low and high gain antennas. Using the aluminum plating and additional deployment force by inflation. The structure of the the etched slots, the slot antenna array can be produced as the new boom is given in Fig. 9. low gain antenna to get the gain about 6 dBi. Putting the circularly polarized membrane antennas on the surface of the booms as the high gain antennas, the gain can be got about 30 dBi. The structure of the antennas sees Fig. 11. The main purpose of the communication system is to periodically transmit the coordinates of the solar sail to the earth. The solar sail will be tracked by the deep space network. One of the advantages of the membrane antennas is wide

azimuth coverage angle. The antenna can cover the earth orbit Fig. 9. The boom with two inflatable tubule. without attitude control when the solar sail faces the Sun in 3.3. The attitude and orbit control subsystem the asteroid belt, sees the below chart. The sail-craft can only use the solar radiation pressure force and solar pressure moment to control the attitude and orbit. The attitude and the orbit are coupled for the solar sail. The attitude change brings the solar radiation pressure and the orbit change, so the core issue is the attitude controller. Two mass blocks moving on the booms and four roll stabilizer bars (RSBs) on the top of the booms are the actuators[14]. When the mass blocks are moved from the equilibrium positions, the center of mass for the solar sail is changed, producing a moment-arm between the resultant force and the center of mass which causes the moment required to stop the low gain antenna high gain antenna unwanted angular velocity. The actuators can handle the yaw Fig. 11 The structure of the low and high gain antenna. or pitch angular velocity. When the RSBs turn a angle, the normal direction of each triangular sail will change, producing a tangent force which causes the moment required to handle the spin angular velocity. The design constraints of the attitude control actuators are as follows, the attitude control precision is better than 0.1degree, the attitude stability is better than 0.01deg/s, the max angular velocity of the RSBs is less than 10deg/s, the mass blocks velocity is less than 0.5m/s. The final parameters Fig. 12 The coverage area diagram. of the mass blocks and the RSBs are listed in Table 5. The plans of the actuators see chart below. In the sail-craft, the sensors, the actuators take the position on the top of the booms, in the central body and on the mass blocks. It will need long cables for the communication among the sensors、actors and the central body. Using wireless communication can reduce the weight of the cables. A communication network was built based on ZigBee protocol. 3.5. The power subsystem Fig. 10 The attitude control actuators diagram Four extendable thin film solar cell, arrays were put on Table 5. The parameters of the attitude control actuators the sail surface to provide the power necessary to drive the Actuator parameter data central body of the sail-craft. On the mass blocks、the RSBs number 2 and the sensors and actuators not in the central body, there are weight 1.5kg mass block also the thin film solar cell arrays on the surface of themselves. limit position 100m The scheme can retrench the cables and reduce the total mass. limit velocity 0.5m/s pole length 1.2m 4. Conclusion pole weight 0.1kg RSBs The most prominent advantage of the solar sail is no limit spin angle ±30° limit spin velocity 21°/s propellant. Make full use of the advantage and the abundant 3.4. The communication subsystem exploration objects in the asteroid belt, a mission is designed to explore the main belt asteroids. The solar sail’s areal The main components in the commuciations system will density is less than 12g/m2, the total mass is 200kg, and the consist of a low and high gain antenna, a transmitter, and a

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