The Great Leap Out of the Cradle: The Human Exploration of Our Solar System

Abstract

Moving human space exploration beyond the Moon is the next challenge. Mars, the most Earth-like planet in the solar system, is the next target. Table 4.1 summarises the main physical characteristics of the Earth and Mars.

Appendix: Various Propulsion Systems Proposed for a Manned Mars Mission

4.1.1 Chemical Propulsion

Several proposals³⁶ are under consideration for manned Mars missions based on liquid-propellant rockets. The most efficient fuel is liquid hydrogen, while combustion of hydrogen and oxygen is effective, which allows for a high specific impulse (around 450 s) compared to other propellant types, a technology that is already mastered. The Apollo missions used this technology for the trans-lunar injection. There are three challenges in using this cryogenic propulsion for a manned Mars mission:

• Keeping a spaceship’s initial mass low. If used for the mission’s primary propulsion phases, the total mass in low Earth orbit is around 1000 tonnes, assuming a crew of six astronauts. To reduce the initial mass, propellant for the return trip must be produced on Mars. Alternatively, fuel must be sent in advance and stationed in Mars orbit.

• Mastering the propellant’s evaporation. Today we can store liquid hydrogen and oxygen in a spaceship for a few weeks. But several months of storage are needed for a Mars mission. Space agencies (NASA, CNES) are working to improve the tank’s insulation and active cooling systems.

• Primary rockets are lost after every launch. If plans entail several missions to Mars, non-reusable rockets affect overall costs and are an inefficient solution.

In summary, chemical propulsion consumes large amounts of fuel and has low-cost efficiency. Other technologies are being looked at, such as nuclear fission that uses 50 % less propellant than the best chemical engine.

4.1.2 Nuclear Propulsion

Nuclear-fission propulsion is a broad topic that includes several engine designs, which range from the thermal-nuclear engine to nuclear-electric engines and more advanced designs such as the Rubbia engine.

Thermal-Nuclear Propulsion

In thermal-nuclear propulsion, a nuclear reactor heats a coolant to high temperatures and expels it out through a nozzle. The most common type is the NERVA (Nuclear Engine for Rocket Vehicle Application), developed and tested in the 1960s. Their specific impulse is around 900 s, double that of chemical engines.

The main advantage of thermal-nuclear propulsion is its advanced stage of development (TRL). The United States and the then Soviet Union tested thermal-nuclear propulsion between 1960 and 1971. A few engines have operated for up to one hour with a thrust of 330 kN. Nuclear-thermal propulsion is still considered to be one of the key technologies by NASA and several related research activities within NASA and the DOE laboratories are ongoing. The Russians built and ground-tested several nuclear-thermal engines, which operated on a variety of propellants, including hydrogen, ammonia, and alcohol. Although their work continued until the mid-1980s, no evidence exists to confirm whether the Russians flight-tested any of these engines.

Nuclear-thermal propulsion offers other advantages: (i) shorter mission time (mission time to Mars is around 300 days, around half the time using chemical propulsion, due to the increased thrust); (ii) lower operating costs because of reduced propellant requirement; and (iii) efficiency of operations because the spaceship could be used in several missions to Mars, contrary to spaceships powered by chemicals.

The main challenges associated with the use of nuclear-thermal propulsion are:

• Radiation, which a nuclear engine gives off in large amounts. Before considering these engines, effective shielding mechanisms must be developed. The goal is to limit the crew’s exposure to 10 REM. By comparison, the limit for civilians is 150 REM, and for military personnel 500 REM. The bad news is that this radiation shielding adds to the spaceship’s mass.

• Testing a nuclear-thermal propulsion device is like testing a nuclear-power plant with the primary circuit open. Today, nuclear systems must have at least three confinement barriers: fuel cladding, a primary circuit envelope, and a confinement building for the reactor. Nuclear-thermal propulsion has a single confinement barrier: fuel cladding separating the uranium and fission products from the environment. To provide an environmentally safe system is thus a complex problem. Complexity could constitute a barrier to the development of these engines if not handled properly.

Nuclear-Electric Propulsion

Nuclear-electric propulsion is composed of two main parts: a fission-based power-generation unit and an electric propulsion module. A variety of thrusters use the electricity generated by the nuclear reactor. Ion thrusters use electric fields to accelerate ions to high velocities. In principle, the only limit on the specific impulse achievable with ion thrusters is the operating voltage and the power supply. Hall thrusters use magnetic fields to ionise the propellant gas and to create a net axial-electric field accelerating ions in the thrust direction. MPD thrusters use either steady-state or pulsed-electromagnetic fields to accelerate plasma in the thrust direction. To get a high thrust density, ion thrusters use xenon, while Hall and MPD thrusters can work well with argon or hydrogen.

One advantage of electric-propulsion technology is its steady development since the 1950s. Ion and Hall-thruster technologies have matured to the point that they are now being used on commercial and military satellites with photo-voltaic power sources. Magnetic Plasma Dynamcs (MPD)-thruster technology is still under development. Another advantage is the higher specific impulses of the electric propulsion. In contrast to a chemical rocket or a nuclear-thermal rocket, they can work continuously for days, weeks, and even months. The main problem is that they have a low power-to-weight ratio and a low thrust density. This makes nuclear electric propulsion infeasible for missions where high accelerations are required. Nuclear-electric propulsion is well suited for unmanned cargo missions between the Earth, Moon, and the other planets.

Advanced Nuclear Propulsion

Advanced nuclear propulsion is under study both in the United States and in Europe. The Idaho National Engineering Laboratory and the Lawrence Livermore National Laboratory advanced one proposal for a fission-fragment engine, which is an engine design that directly harnesses hot nuclear-fission products for thrust as opposed to using a separate fluid as the working mass. In a conventional nuclear reactor, the high kinetic energy of the fission fragments is dissipated by collisions to generate heat, which is then converted to electrical power with efficiencies of only 50 %. Alternatively, the fission fragments produced in the plasma reactor can be used directly for providing thrust. This design can, in theory, produce high specific impulses, while still being well within the abilities of current technologies. In Fig. 4.2, A are the fission fragments ejected for propulsion; B is the reactor; C are the fission fragments decelerated for power generation; d is the moderator, either beryllium or lithium; e is the containment field generator; and f is the induction coil.

Fig. 4.2

Fission-fragment plasma-bed reactor (Source: Credit:http://en.wikipedia.org/wiki/File:Dusty_plasma_bed_reactor.svg)

In this engine, fuel is placed into several very thin carbon bundles, each one sub-critical. Bundles are collected and arranged like spokes on a wheel, and several wheels are stacked on a common shaft to produce a single large cylinder. The entire cylinder rotates so that several bundles are always in the reactor core where the additional surrounding fuel make the bundles go critical. Fission fragments at the bundles’ surface break free and are channelled for thrust. Lower-temperature unreacted fuel rotates out of the core to cool. The engine thus automatically selects the most energetic fuel to become the working mass. The engine’s efficiency is high; specific impulses greater than 100,000 s are possible using existing materials. But the reactor-core’s mass makes the overall performance of these engines lower.

In Europe, Rubbia designed an advanced nuclear engine between 1998 and 2002. This engine is powered by americium 242,³⁷ a fissionable material hungry for neutrons and capable of absorbing them when bombarded by neutrons. The fissionable material is coated in a thin layer (less than a micron) on the inner walls of a hollow cylinder. This tube is filled with normal gas (hydrogen) at pressures of one atmosphere. Thanks to this geometry, when fission occurs, a high probability exists that one fission fragment flies into the gas and deposits its kinetic energy there. In the nozzle, a small miracle of physics happens. Inside the tube, the super-heated gas moves in all directions with a chaotic motion. In the nozzle, the gas moves in one direction and pushes in the opposite direction. Newton is vindicated. The spaceship, anchored to the engine, moves in the opposite direction of the gas (Fig. 4.3).

Fig. 4.3

Scheme of The Rubbia engine (Source: Bignami et al. 2011)

Temperature in the gas’s central region can reach around 10,000 °C. With this heating method, a large saving of fissionable material is possible. The spaceship needs just a few pounds of plutonium or americium for a round trip to Mars. This mass is equivalent to two large cups (1 litre of plutonium weighs around 20 pounds). The specific impulse is estimated at 4000 s. Average exhaust speed is estimated at 40 kilometres per second. But a few unsolved problems remain. The first one concerns the material that can withstand the high temperatures of the super-heated gas. Research is ongoing to develop materials that can do this. The second problem concerns the dissipation of the heat generated, which can be done by equipping the spaceship with large heat-exchange surfaces facing the void of space. Because the temperature of space is around 2.7 degree above absolute zero, it has an infinite capacity to absorb heat.

In summary, nuclear propulsion is the best near-term method for powering spaceships directed at Mars. It offers lower costs and shorter travel times. Among possible nuclear engines, we have chosen the more advanced nuclear propulsion, called the Rubbia engine. It enables a shorter time of travel, less use of propellant, and more payload compared to other nuclear-propulsion systems. The major drawback of advanced propulsion is that they are today only at the concept stage. They need extensive engineering work and testing before spaceships powered by them are ready to fly. But our scenario foresees an exploratory Mars mission in the course of the 2040s. This timetable provides sufficient time for the development and testing of advanced nuclear engines.