Deployable boom for Mars Orbiter Magnetometer onboard Tianwen-1

Graphical abstract The deployable boom functioned normally and is consistent with the design expectations after long storage from Earth to Mars. Abstract: A more than 3 m-long deployable boom is an essential component of the Mars Orbiter Magnetometer (MO-MAG) onboard the orbiter of Tianwen-1. The boom was developed to place fluxgate magnetometer (FGM) sensors away from the satellite to reduce the influence of the satellite magnetic field. It was designed as an articulated spring-driven deployable mechanism for single-shot deployment. Functionality, reliability and system constraints are fully considered in the boom design. Mechanical analyses and proof tests show that the boom has sufficient safety margin to withstand environmental conditions, even in the worst cases. After a long voyage from Earth to Mars, the boom was deployed successfully on May 25, 2021. A full deployment was performed in about 4.6 s, sending the two sensors to distances of 3.19 m and 2.29 m respectively, away from the orbiter. After deployment, the field from the orbiter decreased from 1250 nT to less than 6 nT at the sensor mounted at the tip of the boom. The MOMAG boom provides valuable engineering ex-perience for the development of deployable structures stowed for long periods in cold temperatures in space missions.


Introduction
China's first Mars exploration mission -Tianwen-1, was launched on July 23, 2020 in Wenchang, Hainan Province. The probe consists of an orbiter, a lander and a rover. After more than six-month journey, the probe was captured by Mars on February 10, 2021 and on May 15, 2021, the lander and rover successfully landed on Mars. Exploration of Mars is one of China's deep space exploration plans and will become a significant landmark project. It will advance our understanding of the origin, evolution and characteristics of the planet, and promote the development of space science and technology [1,2] .
The Mars Orbiter Magnetometer (MOMAG) is one of the seven scientific payloads onboard the Mars orbiter of Tianwen-1, and serves the mission of analyzing Martian ionosphere, plasma environment and atmospheric escape processes by measuring the ambient magnetic field in the near-Mars space [2−4] . It consists of two fluxgate magnetometer (FGM) sensors, an electronic box and a deployable boom. Unlike Earth, Mars has no global magnetic field [5,6] and magnetic cleanliness programs on the orbiter are somewhat limited because of spacecraft operational considerations and other instrument needs. However, the influence of satellite magnetic fields can be weakened if the sensors are placed away from the spacecraft [7] . Deployable structures have been developed for various planetary magnetic-field investigation missions [8−11] . In this study, a deployable boom was developed to place the FGM sensors away from the orbiter. It is installed on the deck outside the orbiter, providing mechanical interfaces for FGM sensors and carrying cables from the sensors to the electronic box inside the orbiter. Fig. 1 shows the deployed state of the MOMAG boom on the orbiter.
An overview of the MOMAG boom is presented in this study. An analysis of the boom mission is described first. Then the boom design is described in detail, followed with the verification of proof and ground tests. In the end, the inorbit operation shows that the boom functioned normally and is consistent with the design expectations.

Boom mission analysis
MOMAG will not start working until the boom deploys. The boom is folded in launch and transfer phase from Earth to Mars. After the target orbit is reached, the boom will not deploy until the lander successfully separates from the orbiter and lands on Mars. Fig. 2 illustrates the mission profile of MOMAG. A stowage of more than 300 days in a folded state in cold temperatures is a great challenge for the boom, requiring higher reliability, particularly in MOMAG mission where the successful boom deployment is a critical event.
A variety of mission requirements and system constraints were considered in the boom design. The mass of the boom has to be strictly no more than 4 kg and the boom size in folded state must be within 1.2 m long and 0.3 m high to avoid interference with the launch vehicle fairing and descent module in which the Mars rover and landing infrastructures are carried. Interference with other instruments on the same deck must also be avoided during deployment. Thus, the deployed boom length is constrained. In addition, because solar arrays are mounted on the nearby deck, the pointing of the boom must be designed to ensure that the FGM sensors could be placed as far away as possible to minimize the influence of satellite magnetic fields.
Additionally, the boom must survive the mechanical loads from launch, orbit transfer and satellite attitude control. Installed outside the orbiter, the boom must also survive thermal cycling between +80 ℃ and −180 ℃.

Boom design
The boom for MOMAG was developed as an articulated, spring-driven deployable mechanism for single-shot deployment. It consists of three boom limbs (inner, middle and outer limbs), three pairs of hinges (two pairs of elbow hinges and a pair of root hinges), a base assembly and two sets of holddown and release systems. As illustrated in Fig. 3, the outer limb is attached to the middle limb by a pair of elbow hinges, and an identical attachment lies between the middle and inner limbs. The inner limb is hinged to the base assembly via a pair of root hinges. In the folded configuration, the boom is 1.11 m long. Once deployed, the total length of the boom is as long as 3.19 m from the tip to the base assembly, and the angle between the boom and the deck is 135° with an error within ±1° to ensure that the FGM sensors are placed farthest away from the solar array and the orbiter. The outboard FGM sensor (OB Sensor) lies at the tip of the deployed boom and the inboard FGM sensor (IB Sensor) is positioned 0.9 m along the boom away from the OB Sensor. The outputs of the two sensors function as a gradiometric method to subtract unwanted magnetic disturbances from the ambient field [12−14] . A summary of MOMAG boom specification is presented in Table 1.

Boom limbs
The boom limbs are fabricated via moulding, filament winding and tube rolling processes with carbon fiber reinforced   plastics (CFRP) MJ55 and T700S owing to their high stiffness and strength-to-weight ratio. The high stiffness of CFRP easily satisfies the mechanical requirements, and its low density minimizes the mass budget of the boom. Each limb could reach as long as 1.01 m long with an outer diameter of 30 mm. Owing to weight concern, the inner diameters of boom limbs are designed with respect to the stiffness and frequency requirements. The inner and middle limbs provide interfaces for hinges at both ends while the outer limb provides interfaces for both FGM sensors and hinges, as presented in Fig. 4. In addition, a connector bracket made of aluminum alloy is also fixed on the outer limb, so that the FGM sensors could be easily removed for testing. Two bushings made of reinforced aluminum matrix composites are distributed along each limb to provide access for hold-down titanium rods.

Hinges
The hinges are spring-driven with a latch mechanism for single-shot deployment, which are mainly made of aluminum and titanium alloys. In the folded configuration, the energy from the shape change of the volute spiral spring is stowed and a torque of 0.7 N·m is delivered by a single hinge. When fully deployed, the hinges are latched against an end stop, providing sufficient stiffness and preventing back-driving. The root hinges are integrated into the base assembly which provides interfaces for installation on the deck. In addition, two sets of closed cable loops (CCLs), which are derived from the solar arrays' synchronization system, are used to avoid uncontrolled release of spring energy into the system during deployment. Thus, the three pairs of hinges are opened in a synchronized manner until the end stops are reached and the hinges are latched. Therefore, there will be no variation in the deployment trajectory. A typical deployment process is illustrated in Fig. 5.

Hold-down and release system
The hold-down and release system consists of a holding bracket, a hold-down titanium rod, an initiating explosive device (IED) and a shock absorber cap (SAC). Fig. 6 is the CAD model of a set of hold-down and release system. Two sets are required for the deployment of the boom.
The holding brackets are made of magnesium alloy type ZK61M because of weight concern and mounted on the deck of the spacecraft. First, each IED is mounted on a holding bracket. Then a hold-down titanium rod with a diameter of 3.5 mm is passed through the bushings and IED. Finally, the titanium rod is screwed to the bottom of the holding bracket. Thus, the boom is held down in folded configuration, as shown in Fig. 3. IED is a release mechanism based on the principle that a cutting knife is pushed out by the explosion inside the shell to cut the titanium rod. The cutting is   executed within 40 ms and SAC is used to release the shock caused by the cutting. The boom will be deployed as soon as the cutting is done.

Mechanical analysis
Regarding weight concern and high reliability, CFRP, aluminum, titanium and magnesium alloys are the main materials chosen for the boom and its structure design should ensure that the boom is robust enough to withstand mechanical loads in harsh conditions. The finite element method is applied in the boom mechanical analysis. Fig. 7 presents the finite element models (FEM) of the MOMAG boom and hinges in deployed state. The analytical results listed in Table 2 show that both the boom limbs and hinges (elbow and root hinges) have sufficient safety margins of strength to provide high reliability for the boom even in the worst mechanical environments after deployment. The MOMAG goes through several phases during flight. Among them, the greatest mechanical load could reach no more than 2.5 m/s 2 caused by a propulsion force of 3000 N during the Earth-Mars orbit transfer. However, the boom won't deploy until the spacecraft reaches the scientific exploration orbits around the Mars, in which case the propulsion force could be just about one or several hundred newtons. A ground-verification experiment is conducted to test the pointing stability of the boom. The test scheme is illustrated in Fig. 8. After the deployment of the boom, a mechanical load is applied along the X or Z direction with an acceleration of 2.5 m/s 2 , simulating the greatest mechanical load in orbit. Position 0 represents the designed pointing position of the boom, whereas position 1 and 2 represent the oscillation of boom caused by the mechanical load. The results show that the boom could stop at position 0 within 10 s, and the oscillation amplitude is less than 10 cm.

Thermal and cable design
The boom limbs, holding brackets and IEDs are all protected with multilayer insulation (MLI) blankets to reduce thermal gradients. A small, 2-watt heater is situated on each holding bracket to improve the thermal stability of the IEDs. And a thermistor is also attached in the vicinity of each IED to monitor its temperature. To reduce the thermal conduction between the boom and FGM sensors, a 5 mm-thick heat-insulated pad is placed between each sensor and the boom. MLI blankets are also used to cover the sensors and cables for thermal protection.
The FGM sensors and electronic box are connected by two long cables which are protected by a woven sleeve and braid shield for electromagnetic interference and durability. Each cable is more than 4 m long and routed along the boom as shown in Fig. 9. For the benefit of test and operation, each   cable is divided into 2 parts: A pig tail fixed into the sensor and a connecting cable as long as 3.5 m with connectors at both ends. Cable supports are used at both the elbow and root hinges to allow free hinge rotations and to avoid unexpected interference with other parts of the boom during deployment.

Proof tests
As the hinges and IEDs play a key role in the boom deployment, proof tests are necessary to provide adequate reliability for boom deployment after long-term storage in cold environments. Stowed hinges were soaked in liquid nitrogen (LN) for one year and function test was conducted afterward. The test showed that the hinges could deploy and be locked normally after one year of storage in LN, as shown in Fig. 10. And a long-term storage test was also conducted for the hold-down and release system. Two sets of hold-down and release systems were stored in vacuum state at certain temperatures to simulate the environment in orbit for more than 10 months. The hold-down and release systems functioned normally: both IEDs cut titanium rods within 40 ms, as shown in Fig. 11. Further function test for IED was also conducted by keeping five IEDs stored at a temperature lower than −180 ℃ for about one year. All the IEDs worked normally, cutting the titanium rods within 40 ms. A picture of IED transections is presented in Fig. 12.
Because the FGM sensor cable was protected with a woven sleeve, braid shield and MLI blanket, each cable could reach 17.5 mm in diameter. The rigidity of cables, especially in cold environment, and its influence on the boom deployment are critical issues because the successful deployment is a vital mission event in our case. A verification test of sufficient torque margin for the hinges was conducted by deploying a pair of elbow hinges with cables of three times by soaking the whole apparatus in LN (−196 ℃), as presented in Fig. 13. The hinges deployed and locked normally within 1 s, thereby verifying the torque margin of the hinges.

Ground tests
After assembly of the boom, FGM sensors and cables, ground tests must be performed. When testing a deployable boom, the most critical issue is to prove that the boom will deploy successfully. However, it is not easy to test large deployable booms in a simulated space environment on the ground. The deployment test must verify the functionality of the boom, while wear and degradation brought up by the test should also be concerned. A horizontal deployment test scheme was developed, reviewed and implemented for our boom, as presented in Fig. 14.
The test was conducted in room ambient conditions. The boom was installed on a breadboard fixed on a truss. Floata-     tion components were attached to boom limbs. Pressurized air through the floatation components creates a uniform air gap between the components and marble platform, offsetting the weight of the boom, sensors and cables. The air gap allows the floatation components to travel with little friction on the marble platform, and a constant, stable force is maintained for continuous weight offset during deployment. The repeatability of pre-launch deployment tests shows that the angle between the boom and deck lies within 135°±0.1°, and the deployment time is 4.3 s in average.
After the deployment test, the boom was folded and mounted on a vibration table and all three major directions were tested under sinusoidal and random vibrations. Following this, a thermal cycle test between −195 ℃ and +95 ℃ and a thermal vacuum test were performed. Then a post-test deployment was performed to verify the functionality of the boom. Fig. 15 shows the boom during environmental tests.

In-orbit operation
The boom was stowed at low temperature for more than 300 days since its launch. After separation of the orbiter and lander, the boom was deployed successfully on May 25, 2021. The boom deployed the full 3.19 m in approximately 4.6 s as predicted and tested. High-resolution time (32 Hz) data were recorded by both IB and OB sensors throughout the deployment. Fig. 16 shows the total magnetic field data output of each sensor throughout the deployment. In the folded configuration, the field is dominated by the spacecraft field. As the boom deploys and places the sensors away from the spacecraft, the magnetic field at the sensors decreases as expected. The measured field strength in the IB sensor decreased from 1360 nT to less than 163 nT, and the field strength in the OB sensor decreased from 1250 nT to less than 6 nT. The field magnitude is consistent with expectations for the near-Mars space. Field data observed later showed that the boom properties were consistent with the mechanical design expectations.

Conclusions
The MOMAG boom was developed to place the FGM sensors away from the Mars orbiter to minimize the influence of the satellite magnetic fields. It was designed, fabricated and  tested under the requirements of functionality, reliability and system constraints. The boom was controlled within 4 kg and reaches 3.19 m once deployed. Proof and ground tests showed that the boom could function normally after long-term storage in a cold environment. It completed acceptance verification for use in flight in 2019 and deployed successfully on May 25, 2021. The field at both FGM sensors decreased as expected, and the boom appeared to be in good accord with the design expectation. It was predicated as a high-risk task with a short development and fabrication cycle. The MO-MAG mission offers a unique opportunity to advance the functionality and reliability of deployable booms stowed for long periods in cold environments, providing reliable engineering technology of deployable booms for China's future space missions.