Radiation protection methods for suborbital aircrafts

This paper presents the outcomes of a study conducted by SALTO team in the framework of the Student Aerospace Challenge, focusing on designing structures to optimize passenger protection against radiation in a suborbital vehicle. The study delves into space radiation characteristics and their potential impact on human health during suborbital flights. Through comprehensive research and the utilization of CARI-7A software, the team evaluated radiation exposure and explored both passive and active shielding solutions. The passive shielding solution involved the design of a multifunctional sandwich panel and a shielding layout for windows, minimizing weight penalties on the vehicle’s structure. Additionally, an active shielding concept utilizing high-temperature superconductor material and solenoid layouts was investigated. The study underscores the benefits and challenges associated with both passive and active shielding methods, emphasizing the importance of further exploration to enhance suborbital flight safety and pave the way for future human space exploration endeavors.


The project
The project finds its origin in the Student Aerospace Challenge, which allows university students from all over Europe to participate to the study of a manned suborbital vehicle.The aircraft needs to be reusable and can be designed either for local suborbital flights, that reach Mach 3.5 and 100 km of altitude, or for hypersonic long-range flights, dedicated to point-to-point transportation.Different topics, from propulsion, structure and avionics to medical and legal aspects, can be investigated by the student teams.Specifically, from November to May the teams have to submit several reports, that are reviewed by experts; the last delivery includes a final report, a poster and a bilingual summary suggesting new and innovative solutions, considering both their economic feasibility and sustainability.On the Suborbital Day in June the teams present their final project orally to representatives of the sector.This day is a special event that gathers students and partners at the French Air and Space Museum in Paris and ends the annual edition of the Student Aerospace Challenge [1].In this occasion, the best-quoted projects are awarded with prizes from the partners, which include: the ESA Grand Prix, which offers the best student team the chance to showcase their project to space professionals during a space-related event in Europe; the ArianeGroup Prize; the Dassault Aviation Prize; the Communication Prize, sponsored by the Astronaute Club Européen; the Suborbital Day's Special Prize, sponsored by the French Air and Space Museum.
Our team, SALTO, comprises five students from Politecnico di Torino, each hailing from different branches of Engineering: Quintilla Berti (Master's student, specializing in Space Engineering), Juliane Coutinho (Master's student, specializing in Mechatronics Engineering), Laura De Zotti (Master's student in Materials Engineering, focusing on structural materials), Alessio Fanni (Bachelor's student in Aerospace Engineering) and Mauro Antonio Murrone (Master's student in Aerospace Engineering, focusing on Aerostructures and composite materials).SALTO is the Structure & Materials subteam of PoliTOrbital, a student team of Politecnico di Torino with a mission to design a suborbital aircraft for space tourism flights.During the 17 th edition of the Student Aerospace Challenge, SALTO team chose Work Package 8 -Structures suited to suborbital flight, to study components built to withstand the unique forces, stresses and environmental conditions and requirements associated with suborbital flight.Within the WP, teams could choose between three topics, focusing on the vehicle's structure, materials and radioprotection methods; SALTO investigated Structures which optimize passengers' protection against radiations for local suborbital flights.The team was awarded with the most coveted prize of the challenge, the ESA Grand Prix.

Our work 2.1. Introduction
Suborbital flights can serve many functions, like providing a peculiar environment for scientific experiments, sub-orbital transportation and paving the way to space tourism.Thus, it is essential to understand the type and entity of space radiation humans may be exposed to during such missions and working on possible solutions.The aim of SALTO team's study was indeed to analyze possible radiation protection systems to shield a suborbital aircraft, ensuring the safety and comfort of crew members and passengers while minimizing the weight penalty on the vehicle's structure.

Radiation environment
Deep research on space radiation, their intensity and penetrating power was carried out, so that these values could be compared with the recommended dose for the human body.To understand the radiation to which passengers and members of the crew are exposed to during the flight, it was necessary to discuss how radiation is distributed according to altitude.Given that suborbital flights usually spend ≤15 minutes at high altitudes in space, Galactic Cosmic Rays (GCRs) and Solar Particle Events (SPEs) should be the primary source of ionizing radiation exposure, if the flight path is chosen to avoid the trapped radiation of the South Atlantic Anomaly.Other mitigation strategies were discussed, such as timing to avoid radiation peaks.For a conservative approach, the radiation rate at the peak of the trajectory (100 km of altitude) was considered as worst case scenario and it was assumed that passengers are exposed to such high radiation rates for most of the flight.Leveraging CARI-7A software [2], the team better evaluated the radiation environment humans could be exposed to during a 90 minutes flight.The settings the team selected are summarized in Table 1.The latitude and longitude values are the magnetic north pole's ones: since at this location the lines of Earth's magnetic field are almost perpendicular to the terrestrial surface, the blockade of charged particles is much lower than that at the equator or at lower latitudes [3].The altitude is a conservative estimate based on the mission profile.Different time inputs were considered as solar activity varies with time and the value chosen for the study was 2022 average dose rate, the worst scenario among the evaluated ones.Even if the aircraft will spend around 5 minutes at the highest altitude, it will be subjected to space radiation along the whole trajectory.Both in the early and the latest phases of the mission the radiation dose should be quite low [4], therefore it was decided to have a whole body absorbed dose equal to 11.51 µSv for the whole flight, which is computed by considering the average 2022 dose rate for one hour (conservative estimate).This lead to the conclusion that the whole body absorbed dose could exceed conservative estimations of the recommended limit, equal to 0.0053 mSv [5](the Gray, Gy Absorbed Radiation dose, is equivalent to Sievert, Sv Dose Equivalent Radiation).To address this, passive and active shielding solutions were separately explored.The two methods are at different stages of development in existing literature: while for passive shielding the main point was finding the best possible trade-off between radiation attenuation, lightweight and structural strength, active shielding demanded innovative research to pioneer a solution that has never been implemented yet.

Method
For the passive solution, a multifunctional sandwich panel was designed to shield the structure and withstand the flight loads.In this way it was possible to have a solution with no weight penalty on the vehicle with respect to configurations that employ additional protective layers on top of the structure itself.Radioprotective materials were investigated, with a particular focus on hydrogen-rich materials because they are electron-dense, light nuclei materials are much better neutron moderator than heavy nuclei ones and, lastly, hydrogen cannot itself fragment into neutrons so when it interacts with particles there are very few ways to produce secondary particles [6].Nevertheless, it was essential to look for materials that can offer some sort of structural properties, necessary for the spacecraft: this was a very challenging point as hydrogenrich materials often do not possess this quality.To develop the panel the team studied different materials for both the faces and the core, which have been evaluated from different points of view to get to the final choice, based on a trade-off between low density, stiffness and high radiation absorption.The last property is described by the following equation: Where: I 0 is the dose rate that affects the aircraft; I is the dose rate which remains after traversing the material; µ is the linear attenuation coefficient, which depends on the type of the material; r is the thickness of the material.The authors modelled the sandwich panel as made of three layers, namely the faces and its core.The thickness combination of the sandwich materials was evaluated by using a MATLAB code based on the MATLAB function fmincon.
The material choice for each layer was made with several trials of different combinations.The considered materials are reported in Table 2 and Table 3.The optimization constraints were set up based on equation (1) and the optimized function is the mass per unit area1 , equation (2).
Where: ρ i and t i are respectively the density [g/cm 3 ] and the thickness [cm] of i-th layer, and the layer number is 3.The same search and optimization method was applied to the spacecraft windows, which has the same number of layer as the sandwich panel.Furthermore, a thermal protection system was designed for the external layers, which might not be able to withstand typical re-entry temperatures otherwise.Finally, the whole passive system was validated by OLTARIS software [11], a web-based tool that uses HZETRN (High Charge and Energy Transport) transport code to analyze the effects of space radiations on humans and electronics for different spacecrafts and scenarios.For this study, the chosen materials were uploaded on OLTARIS, which computes their shielding properties based on the input composition.Then, the vehicle was simplified and studied as a three-layers sphere, whose materials and layers' thickness were defined according to MATLAB results.In this way, the team was able to insert computerized male and female anatomies inside the spheres and evaluate the effective dose equivalent (E).It's worth noting that this study focuses on medium energy radiations (the reference energy for the materials' linear attenuation coefficient is 0.1 MeV), which are the most harmful to humans; higher energy ones are omitted.However, the same procedure applied for medium energy radiation can be used to evaluate higher energy radiation shielding.In parallel, the team researched about active shields, that leverage an electromagnetic field to deflect charged particles from the spacecraft.This alternative shows different advantages such as long-lasting effects, excellent protection against charged particles, adaptability, and low maintenance cost.In particular, magnetic shielding is likely the most effective but technologically challenging active protection method.Indeed, reflection preserves radiation composition: no new particles are created, while absorption alters radiation composition and spectrum (secondary particles).Thus, since magnetic fields just divert charged particles without changing their energy, the radiation penetrating the habitable zone never exceeds incident radiation.Three different configurations for this type of shield were defined: toroidal, coupled coils and concentric solenoids.For those layouts, the installation was thought to take place between the passenger's cabin and the skin of the spacecraft.After a careful examination, the solenoid layout was selected, as it appeared to be a more reliable and simpler solution, while the other configurations showed some problems regarding their weight, the non-uniformity of the produced magnetic field and their cost.The chosen active shield configuration consists of two concentric solenoids, depicted in Figure 1, in which the electrical current is running in the opposite direction in order to leverage an electromagnetic field only outside the cabin without affecting the passengers and crew.Besides that, the team also studied which materials would properly fit the shield.The permanent magnets in space can only be based on superconducting materials, due to basic power and mass considerations.Moreover, to generate a big electromagnetic field and to resolve the "high energy consumption" problem, normal conductors do not behave as desired, so resorting to superconductors is necessary.Therefore, type I, type II, organic and high-temperature superconductors were studied.After the suitable material had been selected the team proceeded with the sizing of the active shield.The active shield must generate a magnetic field having a magnitude (B) of 5 T, so the length of an External and an Internal spire, respectively indicated with L ESpire and L ISpire , were evaluated through equations ( 3).Where d stands for the cable diameter.

Property
Then the electrical current which must flow in the solenoid to leverage a magnetic field having the required magnitude is calculated by: In conclusion, the total weight m T ot and L T ot length of the solenoid configuration are evaluated, and then its cost is estimated.
Lastly, some requirements for the cooling system of the shield were defined.

Results
Regarding the passive shield, the studied configurations are reported in Table 4.The chosen final solution is Solution 3. Specifically, a composite material with epoxy resin as matrix and long carbon fibres was selected for the external faces (each 0.1 cm thick) of the sandwich panel and a hydrogen-rich benzoxazine composite reinforced with UHMWPE was chosen for the core (2.26 cm thick), the properties of both materials are given in Table 2.This solution stands out as the lightest combination of thickness and materials found by the authors, having a mass per unit area of 2.58 g/cm 3 and a total thickness of 2.46 cm.The faces stack up is [0/90/+45/-45] to have a quasi-isotropic laminate, so it has isotropic properties in the fibre plane.A triple-pane configuration with quartz fused (outside layer, 0.5 cm thick), borosilicate (middle layer, 0.9 cm thick) and aluminosilicate glass (internal layer, 0.5 cm thick) was chosen as shielding layout for the windows.In particular, the outer and inner layers were selected to withstand mechanical and thermal loads, while the goal of the middle layer, an advanced glass enriched with boron, is to improve shielding capabilities.In between the layers a vacuum and an argonfilled gap (both 0.2 cm thick) were inserted, to reduce heat propagation.The physical and thermal properties of the selected material for windows are shown in Table 3.The areal weight of the windows is 4.44 g/cm 2 .The output of OLTARIS analysis shows that the passive solution would effectively protect the passengers and the crew, as summarized in  Finally, the team chose a radiative Thermal Protection System (TPS) as it does not degrade and is characterized by a certain value of attenuation coefficient to partly exploit it as (additional) passive shield.Specifically, a thermal barrier coating based on aluminium oxide was selected to mitigate the thermal degradation of the underlying panel by providing a steep thermal gradient through its thickness.A recent study [12] showed that TBC/CFRP composites can be fabricated using vacuum-assisted resin transfer molding (VARTM) method, in which carbon fabrics are used as substrate and epoxy resin was used as matrix, and they can resist temperatures much higher than 624 K (maximum re-entry temperature of our study, output of CFD analysis).Regarding the active shield, among the high-temperature superconductors taken into account, Yttrium Barium Copper Oxide (YBCO) showed a promising behavior for the active shielding needs, being easy to fabricate in thin films and having a high critical current density [13].Indeed, being a type II superconductor, it is able to carry much higher current densities than type I superconductors, which is important for generating strong magnetic fields in the solenoids [14].Additionally, it can withstand stronger magnetic fields before it loses its superconducting properties, which is essential for maintaining the magnetic field in the solenoids.Another important advantage of YBCO is being able to operate at relatively high temperatures (critical temperature of around 93 K), which makes it easier to cool down using liquid nitrogen or other cryogenic fluids [15].The chosen YBCO cable has a diameter of 3 mm and a density ρ of 6300 kg/m 3 [16].From the geometry of the cabin, given by Table 6 on the left, and selecting the number of coils N (500) and the magnitude of the magnetic field (5 T), the sizing parameters of the active shield are evaluated by using Equations ( 3)-( 5).The evaluated geometrical parameters for the active shield are reported in Table 6  The found configuration requires 8.653 km of cable and weighs 384.5 kg.Considering that YBCO cables cost 500-1000 EUR/m (Granta Edupack) this solution would be very expensive.

Conclusion
Delving into innovative radiation protection methods, the study focuses on novel passive shielding techniques and explores the potential of active shielding, enhancing the safety and feasibility of suborbital flights.The findings highlight that, due to the relatively low radiation levels and short duration of suborbital flights, passive shielding currently represents the optimal solution.It offers a lighter, more reliable, and structurally advantageous approach.However, it is essential to acknowledge the advantages of active shielding for future space exploration.These methods demonstrate a range of benefits, including a long-lasting effect, adaptability to varying radiation environments, and low maintenance costs.Notably, this solution avoids generating secondary particles and preserves the energy of incoming particles, simply diverting them away from the spacecraft.However, several challenges must be addressed: this approach carries additional weight requirements, necessitates the implementation of a cooling system, and does not provide effective neutron shielding.Despite these limitations, SALTO team emphasizes the significance of further exploring active shielding possibilities, recognizing their potential as the optimal solution for human space exploration and interplanetary missions.

Figure 1 :
Figure 1: Concentric solenoid configuration and its side view on the right, where H is the cabin's height, R is the external radius, and r1 is the cabin's radius.

Table 1 :
Settings used for CARI-7A (on the left) and radiation dose rate at 110 km altitude over the magnetic north pole computed with CARI-7A (on the right).

Table 2 :
[7]perties of Epoxy/CF and Aluminium foam given from Granta Edupack and properties of hydrogen-Rich Benzoxazine/UHMWPE taken from paper[7].The Young moduli of Epoxy/CF and HRB/UHMWPE are evaluated in the fibre direction.

Table 3 :
Properties of studied glasses.

Table 5 .
Moreover, cost estimations were carried out for the sandwich panel and the windows, which would cost respectively 505.9 [EUR/m 2 ] and 225.04 [EUR/m 2 ].The prices per unit volume of the selected materials are taken from GRANTA.

Table 6 :
on the right.Cabin dimension (on the left) and active shielding sizing (on the right).