Hypervelocity impacts of Micrometeoroid and Orbital Debris (M/OD) seriously threaten the safety of manned spacecraft and astronauts in orbit. At present, M/OD above 10 cm, which can be monitored and predicted in advance, can usually be avoided by orbital maneuver. As for the small-size M/OD, because of the difficulty of monitoring, it is the main impact threat, as well as the main object of impact risk assessment and protection design of manned spacecraft.
The probability of no penetration (PNP) of the sealed cabin under M/OD impact is usually used as the method to assess the probability of no failure of the system in manned space missions. A few of methods have been developed to assess the M/OD impact threat, such as the probability of no penetration (PNP), the probability of no catastrophic failure (PNCF), and the Manned Spacecraft Crew Survivability (MSCSurv), in ascending order of the accuracy of assessing the safety and survivability of manned spacecraft and astronauts.
At present, the PNP of sealed cabins was still used as the method for assessing the probability of one catastrophic failure in the design of manned spacecraft in China, which leads to inaccurate risk assessment of catastrophic failures of manned spacecraft in M/OD environment. Therefore, more studies should be carried out to improve the assessment of the catastrophic failure in sealed cabins of manned spacecraft in China.
In a research paper recently published in Space: Science and Technology, several scholars studied critical perforation diameters and critical crack lengths of sealed cabins in response to typical catastrophic failure modes, developed a failure assessment module to improve the M/OD Assessment and Optimization System Tools which developed by the China Academy of Space Technology, and ballistic limit equations and perforation and crack equations suitable for stuffed Whipple shields were established, providing reference for the design and assessment of long-term on-orbit missions of the ultra large manned spacecraft in the M/OD environment.
First, authors analyze three typical catastrophic failures of manned spacecraft in an M/OD environment. For the astronaut casualty mode caused by low-pressure or hypoxic environment, hereinafter collectively referred to as astronaut hypoxic failure mode. In case of emergencies such as perforated gas leak that occurs in the cabin, the gas pressure control system in the sealed cabin should keep the total pressure not lower than PTE, the oxygen partial pressure not lower than PO2E, and duration not less than the critical escape time TCE to support the astronauts in emergency on-orbit plugging or evacuation. When the perforation diameter Dh greater than the critical perforation diameter DhE, the total pressure is lower than PTE, or oxygen partial pressure is lower than PO2E at TCE, the astronaut hypoxia failure happens.
In the analysis of the critical perforation diameter, variations in the internal pressure of the sealed cabin at a perforation diameter of Dh and the critical perforation diameter DhE of the cabin at a given TCE are obtained. Sealed cabin fracture failure mode refers to the situation where cracks induced by M/OD impact propagate or expand under the internal pressure of the sealed cabin, resulting in depressurization and then catastrophic failure of the cabin. When the stress intensity factor (SIF) at the tip of a crack on the cabin wall is equal to the fracture toughness of the wall material, the corresponding crack length is defined as the CCL of the sealed cabin.
Under M/OD impact, if the crack length is greater than the CCL, SIF at the crack tip exceeds the fracture toughness of the material and, thus, the sealed cabin fractures. The relationship between SIF at the tip of an axial crack on a cylindrical cabin wall and crack length, which proposed by Folias, was used in this paper. Furthermore, spacecraft breakup is the most serious failure mode of manned spacecraft. The M/OD critical size criterion was used for cabin breakup failure mode assessment. When the M/OD particle impacted on the sealed cabin is larger than 3 cm in size, the spacecraft would fail to break up.
Then, authors establish the perforation and crack equations for stuffed Whipple shield of sealed cabin. The ballistic limit equations of the stuffed Whipple shield are derived in advance, which are important bases for studying perforation equations. To obtain the equations, hypervelocity impact tests on 3 types of stuffed Whipple shields were completed first, obtaining the ballistic limits of such shields. Then, on the basis of the genetic algorithms and multiple linear regression method, the coefficients of NASA's Christiansen equation commonly used in the world were corrected, and finally the ballistic limit equations suitable for the stuffed Whipple shield of a specific ultra large manned spacecraft was obtained.
With the above corrected ballistic limit equations of the stuffed Whipple shield and the perforation data of impact tests, the perforation equation for region 1 of the W-S hole equation was corrected, while the perforation equations for regions 2 and 3 and the crack length equation remain are consistent with those in the study of Williamsen and Schonberg in terms of form and coefficients.
On this basis, the equations of perforation diameter and crack length on the sealed cabin were obtained, which are suitable for the stuffed Whipple shield of a specific ultra large manned spacecraft under different debris diameters and impact velocities. Moreover, depending on the structural parameters of 3 types of shield structures for a specific ultra large manned spacecraft and the modified perforation and crack equations, perforation diameter and crack length on the sealed cabin under an impact velocity of 3 km/s and an impact angle of 0 were predicted.
At last, authors conduct catastrophic failure assessment of sealed cabin of ultra large manned spacecraft in M/OD environment. The assessment is based on the MODAOST framework (main modules are seen in Fig. 10), which is developed by the China Academy of Space Technology and has been successfully used for on-orbit PNP assessment of manned spacecraft of multiple models, such as the Tiangong-1 space module and the Tianhe core module. Among the modules in MODAOST, the impact characteristic database is used to describe failure modes of spacecraft and corresponding failure models.
The impact characteristic database was expanded by adding the database modules of ballistic limit equations, perforation diameter calculation, and crack length calculation, as well as corresponding failure criterion modules of the sealed cabin structure that were supplemented. The catastrophic failure assessment of ultra large manned spacecraft is verified via the 3-module assembly of a specific ultra large manned spacecraft had an orbit of 400 km and an orbit inclination of 42 degrees, flying in a triaxial stable attitude.
Results show that among the typical failure modes, the gas-leakage-induced astronaut hypoxia is the primary factor, with the R factor reaching 0.159. Compared with the PNP assess method, the PNCF of the system achieved by the proposed method increases from 0.9970 to 0.9995, followed by the sealed cabin fracture, while spacecraft breakup is of the lowest probability. Under orbital debris impact, quadrants II and IV of the small column segment of the core module are the riskiest zones of perforation failure, while quadrant III of the small column segment is the riskiest zone under micrometeoroid impact. Authors pint out that the quantitatively catastrophic failure assessment of a specific ultra large manned spacecraft in an M/OD environment using MODAOST can provide reference for the design and assessment of long-term on-orbit missions of the spacecraft.