Science Forum

Aireal · Posted: Wed Apr 12, 2006 7:35 am Post subject:

Could Ion Drive technology be the key to radiation shielding, while at the same time improving ion drive efficiency?

The standard ion drive has a stage of operation where it collects excess electrons and injects them into the ion beam to prevent the spacecraft from accumulating a large negative potential.

What if that stage could be eliminated, and the negative potential be used to help shield the crew and equipment. First we must look at the problems of radiation shielding for crews in outer space. The three main schools of thought on these are: Material Shielding, which has the drawback of mass. A major factor for a ship with ion drive. Magnetic Shielding, which is weak at some points, much like the Earth is. It also requires a strong magnetic field of about 20 teslas, which might have its own ill effects, we don't know yet. Electrostatic Shielding, by giving the hull a positive charge of 2 billion volts or so , you can repel cosmic-ray protons. But it creates a bombardment of electrons in the range of its field effect.

The key might be to combine the strengths of each of these, while reducing their drawbacks, with a unique hull concept.

Picture the hull as a giant capacitor, and the ion drive giving it a charge with the electrons that would normally be collected and ejected into the positive ion stream. Each charged layer of the hull would be separated by an insulator material that is also a good radiation shielding material. The most likely bet would be a plastic, or maybe a ceramic. The skin of the hull should have a neutral charge, where as one end of the ship would have a positive charge, and the other a negative, powered by the hull capacitor system. This will generate a magnetic shield around the ship. Now all three of the main shielding systems are in place, let's look at how they interact.

At this time a nuclear electric propulsion (N.E.P.) can deliver power levels of several hundred kilo watts to an ion drive, and may reach the megawatt category in the near future. This means an ion drive could generate a large charge in the hull to create a magnetic shield around the ship. I am not sure what the max. field strength would come to, but most likely not large enough to stop the radiation on its own. The good new is, it does not have to stop it, just redirect a few particles toward the poles of the ship, and slow down the rest.

Once the charged cosmic particles have been slowed down by the magnetic field, they will encounter the hull. Passing through the skin of the outer hull, they will next encounter the material which serves as insulator and shielding. This will slow the particles down even more. Then it will encounter the first charged layer of the hull capacitor system. These charged layers will act as electrostatic shielding within the hull. Then the cosmic particles must repeat the process of going through material and charged layers till it is hopefully stopped. Because our hull has both positive and negative layers in it, it will protect against a greater range of charged particles than the standard electrostatic field design. To a charged particle, this process will act as if the hull was a material shield with electrostatic shields within it, combining these two shielding concepts. As this setup is used to create the magnetic field, all three shielding concepts are now one system. So we now have all three of the main radiation shielding systems in place, all working together. So what are the advantages of this approach, let us look.

The first advantage is an overall reduction in mass needed to protect the crew. Every gram of weight saved in material shielding would be worth its weight in gold for an ion drive ship. With just material shielding alone, it would take about one kilogram of material per square centimeter to protect a crew, that's a lot of mass. Where as magnetic and electrostatic systems have huge energy requirements. By combining the magnetic and electrostatic systems into one, we have already cut the power needed by half. With the system using the potential difference created by the ion drive, the need for its own power supply is reduced or eliminated, another reduction in weight. Because the load of protecting the ship is split between three systems, we do not have to use as much power for magnetic and electrostatic systems as if the were run alone. So the overall energy requirements for the ship is reduced also. Yet every design has some drawbacks, so let's look at how these can be reduced in this system.

Magnetic fields provides little shielding at the poles. As our magnetic field is created by the charged areas at each end of the ship, this problem will be increased from particle bombardment in these regions. The simplest way to solve this is to place the crew area in the center of the ship, away from the poles. Antenna or other extensions from the ship could move the poles even farther away from the crew, for an added measure of safety. As the crew would also need shielding from the reactor, the ship design may look something like this. A long cylinder with a antenna projecting from the front of it, the reactor and ion drive at the rear, with the crew area in the center. Of course other designs are possible with this concept. The standard electrostatic shielding concept has the drawback of causing particle bombardment due to its field radius in space around the ship. By locking these charged areas in the hull with a capacitive design, we limit the range that this field expands into space, thereby reducing the problem of particle bombardment. Next we must pick the shielding/insulator material. As it must fill two jobs, it may take some research to determine the best choices. Plastics like polyethylene might be a good starting point. If a lightweight material for the charged layers could be found, this would help in mass reduction, and make the charged layers better as material shielding. Work is already being done in this area. Adding carbon, graphite or nanotubes to plastics is one approach, polycarbonate might be a good chice for this, as it would add some impact protection also. Interpenetrating Polymer Network ( I.P.N.), and similar work by companies like the Eeonyx Corporation are promising also. Thus we can reduce the disadvantages of each of these systems while taking advantage of their strong points. The next question is, are there any other aspects we need to address with this design concept?

What if the capacitive charge in the hull started climbing too high for some reason? There would need to be a system in place to bleed off excess power and divert it to the ion drive or some other use. In the event of a solar storm, primary power could be diverted to increase the field strength. After the danger has passed, the extra power could be sent to the ion drive. This approach may allow for less than max. protection in the normal state, and protection increased when needed, resulting in an even lighter ship design. Perhaps a Faraday Cage (RF shielding) could be constructed around the crew area to help protect them from the magnetic field effects also. Another area for improvements would be the outer skin of the hull. Could it be made to generate usable power from the constant bombardment of particles. A recent discovery by researchers of the Material Sciences Division of Berkeley and partners show that alloys of indium, gallium, and nitrogen can convert virtually the full spectrum of sunlight, from the near infrared to the far ultraviolet, to electrical current. So even the outer hull of the ship may have more than one use in the future. The hull should be modular in construction and design. This will lessen the chance of system failure if a hull section gets damaged, allow repairs to be conducted in flight, and make construction quicker and more cost effective.

An increase in ion drive efficiency might be obtained by correctly locating the positive pole at the rear of the ship in relation to the ion drive. As the positive ion stream left the ion drive it would encounter the positively charged field at the rear of the ship. The ion stream would be repelled even faster away from the ship, while imparting a slight increase in thrust to the ship. While it may not be much of an increase, every little bit helps over long interstellar trips.

Hopefully this approach would reduce the weight and energy requirement of our spaceship by 2/3 over standard designs, while still protecting the crew and equipment.