For the sake of simplicity the forcefields in this blogpost are assumed to have the following characteristics; can be projected in a variety of geometrical shapes, have mechanical strength, repel kinetic and radiant energy. It is also assumed that either the forcefield is invisible when in matter-repelling mode, or it can be tuned to block or admit certain frequencies of light.
- Airlocks: this has been seen in both Star Trek and Star Wars, and might be one of the least stupid of the non-defensive ways that forcefields have been used. The entrance to the hangar bay has physical doors, a forcefield that allows slow moving shuttles to push through is activated when required. As air molecules move quite fast, they cannot penetrate the field, and so pressurisation is maintained. However, everyone had better remain in p-suits; a blown fuse could otherwise result in the inconvenience of explosive decompression. Note that a real world device called a Plasma Window can achieve much the same result.
- Landing Gear: this example comes from the SF comedy Galaxina, and is employed by the spaceship Infinity to overcome the problems with landing on uneven ground. Like the forcefield airlock this is well and goo, until the power fails and several hundred or thousand tonnes of spaceship crashes to the ground. While it would be somewhat foolhardy to equip a normal spaceship with these, they could serve for special landings where the ground is unhardened, say for military or rescue missions, with the forcefield acting like a futuristic giant snowshoe.
- Structural Reinforcement: the ships of Star Trek's Federation are much maligned among engineers and those of scientific bent for their structurally stupid design. The narrow struts connecting the hulls and warp nacelles are under incredible stress in any manoeuvre, and to come with this the ship uses Structural Integrity Fields. Again, good until the power goes out, which is probably when you don't want the ship snapping into three pieces.
- Emergency Containment: although - yes, power again - physical barriers are probably preferable for containing anything from prisoners to poisonous gasses, forcefields might prove indispensable in an emergency. If they can power up in instants they could seal off an area far sooner than ponderous blast or containment doors, felling to seal off of dangerous situation in moments.
- Life Support: the vacuum of space makes simple jobs a nightmare, pushing the costs or orbital construction sky high. If a starship under construction in orbit could be enclosed with a forcefield just strong enough to contain an atmosphere, matters would be much simpler. Even though for safety p-suits would still be worn, the 'air' could equally easily be there to enable plasma cutting as breathing. A forcefield on a pant could be used as an emergency shelter from natural attacks such as volcanos, tsunamis, and hurricanes.
- Power & Drive Reactors: the lower limit to the size and weight of a nuclear fission power supply is the critical mass needed for a sustained reaction. However, if the forcefield is a perfect neutron reflector it is easy to see how it could cause even a few grams of uranium or plutonium to fission. The resulting plasma could be confined by the forcefield and piped through a magnetohydrodynamic generator, the whole package limited in size only by the forcefield generators. Fusion could be treated in the same way, opening up the way to abundant clean energy. The most obtuse of the implications that result from this is the development of torch drive spacecraft, which I will discuss in a future post.
- Energy Storage: antimatter is often proposed as the ultimate starship fuel; a misleading statement. Like the use of hydrogen as car fuel, antimatter acts only as an energy storage device, a battery. Starships need astronomical amounts of energy, so antimatter is used as giving the ultimate power-to-weight. Create a hollow container from a forcefield, fill it with a vapour that absorbs a frequency of light allowed to pass through the field. Energy is added to the 'battery' with lasers, and thanks to the phenomenon of electron shells the light released by the vapour is unable to pass the field, most of it at a different than original frequency. A device such as this could have no limit to the energy contained, making 'nuclear hand-grenades' look like damp firecrackers.
- Directed Energy Weapons: as anyone familiar with the Kzinti Lesson knows, the exhaust plume of a starship or torch drive spaceship is as deadly a weapon as can be found. The Fission Fragment Rocket, possible to build in the real world, has an exhaust velocity of a few % of c, making it a deadly beam weapon at close range. By piping the plasma from a fusion reaction out through a forcefield nozzle a devastating weapon could be created; it disadvantage a fairly short range due to the dispersion of the beam.
- Airships: if a forcefield has no mass in and of itself, it makes the perfect airship hull. Possibilities include vacuum airships, high temperature thermal, airships that change shape to attain supersonic speed, etc. Nor would an airship built in this way suffer from the fragility of conventional designs, making it much more practical than any physical airship.
- Re-entry Shielding: for a large or fast spaceship a re-entry shield is much to heavy, despite the advantage in fuel reduction that the Leonov demonstrated. A forcefield, especially if already fitted for military defence, is a perfect substitute, and if power fails, your probably screwed anyway. By allowing aerobraking the available delta Vee for a mission could be doubled, or the fuel load halved; a significant improvement. Also, as it could be much larger than the actual ship, and of a more aerodynamic shape, g-load could be altered to be less taxing.
- Cassions & Civil Engineering: a cassion is a temporary dry-dock of sorts, built to enable the construction of underwater structures. A forcefield by its virtue of easy deployment - on/off switch - is ideal for rapid or emergency construction.
- Tools: this is seldom seen, the only example I have personally come across being in Asimov's extensive 'Verse, where forcefield tools are far more capable than mechanical counterparts. The advantages of such theoretical tools are many, and are discussed on Atomic Rockets.
Update (14 JAN 2015)
As can be seen in the comments for this post, reader Yoel pointed out a two applications I had completely overlooked, so I'm adding them in.
- Ramscoop: the greatest challenge of constructing a Bussard Ramjet, one of the most advanced and powerful starship designs, is the construction of a magnetic field capable of collecting the interstellar hydrogen needed by the design. A forcefield could be just the solution, especially if it is massless and frictionless.
- Solar Sail: the solar sail is an interesting design that cannot ever come to its full potential due to material constraints. In essence the solar sail uses the momentum of reflected sunlight or solar wind, having theoretically infinite delta Vee. However, the mass of the sail, combined with the inability of find a material that will withstand solar heat at the closer, and thus more effective, distances means it is not likely ever to be widely used. A forcefield, however, might be massless and perfectly reflective, making it the ideal method of producing a solar sail, and thus providing a reliable method of interstellar transport. In fact, this is such an effective interstellar design that I have decided to do a post on Interstellar Transport, featuring such a design and explaining its implications.