Human susceptibility to the harsh space radiation environment has been identified as a major hurdle for exploration beyond low-Earth orbit (LEO). High-energy protons and nuclei ions from Solar Energetic Particles (SEPs) and Galactic Cosmic Rays (GCRs) can result in radiation doses that are dangerous to astronaut health and even survivability if the astronauts are not adequately shielded. These high-energy particles also cause significant amounts of secondary radiation when they impinge on the spacecraft structure. Hydrogen or hydrogen-rich materials are ideal materials for radiation shielding because hydrogen does not easily break down to form a secondary radiation source.
A concept study of a Cryogenic Hydrogen Radiation Shield (CHRS) was conducted in 2008 and results showed that liquid hydrogen is the most mass-effective material for protecting a spacecraft if proper design is implemented. A secondary benefit may be that the hydrogen can also be used as fuel for a final burn that could help capture a spacecraft into low-Earth orbit or allow reentry into Earth's atmosphere while on a return trajectory from other planetary bodies such as Mars or asteroids, or the liquid hydrogen can be used for a burn on a lunar or Martian ascent vehicle. The required areal density of the cryogenic hydrogen depends on the duration and the trajectory of a mission and the intensity of the space radiation. The previous study also showed that in order to protect the astronauts from space radiation with an annual allowable radiation dose less than 500 mSv, 140 kg/m2 of polyethylene is necessary. For a typical crew module that is 4 meters in diameter and 8 meters in length, the mass of polyethylene radiation shielding required would be more than 17,500 kg. The same study found that the requirement for hydrogen shielding is 40 kg/m2 surface area.
Liquid and solid hydrogen at cryogenic temperature have higher densities and require smaller storage tanks. However, the CHRS needs a delicate thermal system that can protect cryogenic hydrogen from evaporation during the mission. This study includes a trade study to compare liquid and solid hydrogen, and a preliminary thermal design for the CHRS cryogenic thermal system. The trade study showed that subcooled solid hydrogen is a better choice than liquid hydrogen. The reasons are as follows. First, solid hydrogen absorbs more heat than liquid hydrogen before it evaporates. Second, solid hydrogen needs a smaller tank than liquid since solid is denser than liquid. Finally, it is easier to manage solid than liquid in space. The design temperature of solid hydrogen is around 10K, which allows solid hydrogen to absorb more heat before changing phase to liquid. The design also includes a ground cooling system that can freeze and subcool liquid hydrogen.
The CHRS crew module is surrounded by an annular hydrogen tank with a bore in the middle. The ends of the capsule are protected by toroidal tanks that allow for axial access. The centers of the toroid can be plugged with polyethylene caps. Thermal and fluid design requirements can be met by using proper cryogenic thermal management techniques such as freezing and subcooling the cryogen, selecting appropriate tank materials and thickness, adding MLI, using aluminum foam inside of the tank to enhance thermal conduction, using a 90K radiator to remove heat at higher temperature, and employing a 10K cryocooler. This study performed parametric thermal analyses for different combinations of passive and active thermal systems. The design was optimized to minimize the system mass and meet the thermal requirements. Load-responsive MLI can be used to reduce parasitic heating at the launch pad. The preliminary thermal design showed that the areal density of CHRS with a cryogenic thermal system is around 70 kg/m2, which is 50% of the density of polyethylene shielding. With CHRS, the mass of the crew module with radiation shielding decreases from more than 26,500 kg to less than 17,800 kg. CHRS saves nearly 8,800 kg from a 4-meter-diameter and 8-meter-long cylindrical crew module, and halves the required shielding mass when compared with polyethylene shields. It saves close to $44 million in launch cost based on $5,000 per kg estimated by SpaceX. With a 10K cryocooler, the system can support missions of any length. A 50-watt heater can maintain the crew module at room temperature.