The forcefield drive, as a torch level propulsion system, has an effect on the design of the spaceships used in the same 'Verse. As a pure fusion drive it needs only a small amount of fuel/remass compared to a fission thermal craft, although still more remass is needed than in a electric drive craft. NASA's Discovery II is a pure fusion design, but a forcefield drive is likely to have performance closer to a design utilising Zubrin's Nuclear Salt Water Rocket, or even higher. Exhaust velocity for a physical nozzle is governed by temperature and pressure, two things that can be increased almost indefinitely if forcefields are assumed, making even relativistic Ve possible.
It may be noted that in a hard SF setting a torch ship can have acceleration as low as 0.1g, meaning that structural design is probably similar to any other spaceship. In classical space opera, however, torch ships are capable of constant - or at least extended - 1g burns, changing the structural requirements considerably. At this acceleration a trip to saturn from Earth takes seventeen days, as opposed to the years needed by a conventional drive. This means of course that requirements for food and air are greatly reduced; where a sub-torch ship would need hydroponics and closed cycle environmental systems, the torch ship can make do with canned food and air. It also greatly reduces the physical and mental impact of the trip, meaning that passenger or casual trips are plausible.
The acceleration also provides artificial gravity, removing the need for a spin-hab that would be necessary on a slower vessels to stave off the physiological impacts of prolonged weightlessness. If considerable time is to be spent in orbit with crew onboard a spin-hab might be included, but it would have to be of the folding type, as shown.
Due to the high speed the torch ship reaches it would need much heavier whipple shielding than a slower craft, probably taking the form of a large disk protecting the bow, and in several layers; although that assumes that forcefield generators are to expensive to use except for engines. It could become common, especially among military crews, to adorn the shield with the ship's emblem, or that of its owning country of company. Also due to acceleration the structure will be more compact and solid than the structure of many sub-torch drive spacecraft. Another factor is that as the forcefield used in the engine provides radiation shielding, there is no need for the extended central truss that is used in many designs to reduced irradiation of the crew. Depending on the efficiency and operation of the forcefield the ship's radiators may also be smaller than a more realistic ship design, greatly changing the overall look of the ship. The ship will doubtless have plenty of auxiliary electrical power, and so in a warship weapons will likely lean toward lasers and various electromagnetic projectile devices, which may affect the design somewhat.
It is also important to note that the power of such drives enables truly massive payloads to be lofted into orbit, so there is no needed for space elevators, or other non-rocket space launch systems. It does not mean that interplanetary or interstellar ships will be capable of landing on a planet; there parameters for space flight and surface to orbit are such that one vessel capable of both is in most cases impractical in the extreme.
The vastly shorter trip times afforded by torch drive spacecraft has a significant impact on the commerce of a SF universe. As can be seen from various tables on this page of Atomic Rockets the times are reduced as much as years for trips into the outer solar system, even with only 0.1g acceleration at the lower end of torch ship performance. The most obvious implication is that with travel so mush easier far more people will be travelling. As a result there will be regular dedicated passenger services, instead of the occasional spare berth on a mining or freight ship. Secondly, any colonies in the outer system will be much larger and more diverse; the reduced trip time and difficulty allowing for more people to be thrown at a job, or even to immigrate. Instead of a few sparsely manned mining bases there may spring up 'boom towns' filled with the kinds of characters far more likely to make for an interesting story than the staid types employed by the mining enterprise. This large number of people will generate a secondary economy of luxury or possibly illicit items/services, offering large profits to the daring entrepreneur. Another result of these torch ships is that although they can be made bigger than is possible with real technology, they will only have small crews, even a sole person. On trips of months it would be psychologically dangerous to have one man aboard, and as I postulated in my first blogpost, it is unlikely that ships will be unmanned; with trips of a few weeks that is no problem at all.
The nature of the cargo being transported also changes with the introduction of torch level drives. If you are willing to travel a little slower a 0.1g torch ship can carry a lot more cargo at lower acceleration, for practically the same cost. When mass budget is limited the cargo must be high return - rare earth metals, machinery and electronics, medicine, fissionables, etc. - with the ability to move more mass other commodities become more important - iron, copper, aluminium, etc. I would think that the faster ships with larger mass budgets would also tend to favour a few large ships in the merchant fleet, rather than many smaller vessels; but it is a question beyond my pay grade.
In the early stages of the colonisation of the solar system Earth will undoubtedly remain the political centre of attention. On Earth will reside those who own the mining interests, those who buy the few valuables that space has to offer, and from Earth come ships bearing supplies without which any effort at industry or colonisation is doomed. This is likely to be the same in any system with an inhabited planet; what the case would be in a newly colonised system is quite different, and the topic of another planned blogpost. This, of course, enables all sorts of interesting shenanigans when a habitat or colony grows independent enough to want independence; a device that has been used to advance the plot of many SF works, among them Robert A. Heinlein's The Moon is a Harsh Mistress. However, the inclusion of torch drives makes this situation unlikely in the extreme. It seems to be the case that our solar system will function as a hydraulic empire, in which the water is replaced by technology that can only be manufactured on Earth. As trip times are short, it is relatively easy to keep a few spares on hand and order more when needed, so any impetus to develop technological independence is removed from the colonists. Purely social reasons for a colony to separate from Earth government may arise, but these are unlikely to lead to a armed conflict if the colony depends on Earth for technology. There is also the fact that it will only take the Earth military, with its much vaster manpower, a short time to respond to any military action.
In a setting or 'Verse of almost any kind there are bound to be small armed military vessels. These will have the duties of providing patrols to anywhere not immediately visible from their home base, carrying diplomats and politicians, intelligence gathering, providing COIN and anti-terrorist mission support, boarding and inspections in orbital space. They may be armed, but lightly; a few missiles, a chain gun, possibly a HV kinetic cannon or laser. In the universe of forcefield enabled torch drives, however, large fleet ships, or any kind of direct warfare vessels are unlikely due to a ferocious device known as the torch missile.
The torch missile is a simple device; a large independently targeted missile equipped with a miniaturised torch drive unit. Most torch drives that have been theorised in the real world are incapable of too much miniaturisation, or are prohibitively expensive to be used as disposable weapons. A forcefield drive, however, is only limited by the size and cost of the field generator; a variable which the author is free to adjust to suit the 'Verse. Unless the forcefield used in the setting are so strong as to make any spaceship using them invulnerable to any amount of kinetic energy torch missiles will be able to destroy any ship. All you have to do is launch enough of them at a high enough speed and some will get through the targets point defence. Only one hit will be needed; no-one walks away from a impact with a ten tonne missile travelling at several hundred kilometres per second relative velocity. And they have the same range as a ship unitising the same drive system, meaning that an attack can be launched from the surface of a planet or a orbital station as easily as from a battlestar - not to mention accelerations in the hundreds of g.
The result of this is that conventional spacecraft are useless in a all-out war between proper and equal space militaries. Battles will be between defensive systems and swarms or missiles - or between two opposing swarms. If major warcraft exist, they may well fill in the role of mobile AAA batteries, tasked with the protection of a vital asset. The military in space is thus unlikely to be as large as Army, Navy, or Air Force, composed mainly of orbital assets, and most of its personnel geeky technicians rather than glamorous pilots.
A mine of information regarding torch ships is available on the Atomic Rockets website, along with various interesting tables that help explain their performance benefits; and Rocketpunk Manifesto has an excellent article on torch missiles, (somewhere).