This page describes some common knowledge about space travel. Spacers would all know and understand these things. Passengers on the Zheng He might not know it as well, but many are probably familiar with these basic facts too, either from explicit training or prior space experience, or having picked it up from the crew.
Kinds of Spacecraft
Spaceships come in lots of varieties - there's as many kinds of spaceship as there are airplanes or boats. They can be categorized in several ways.
Atmospheric ("shuttles") spacecraft: Most big ships are built in space and would be destroyed if they tried to land on a planet, by some combination of the heat of atmospheric entry or the forces experienced during it. Atmospheric shuttles turn the atmosphere to their advantage by having wings - basically, they're a combination spaceship / airplane. (The NASA space shuttle is a familiar real-life example, though modern shuttles are single-stage-to-orbit craft more like the proposed REL Skylon.)
Landers: The atmospheric spacecraft's counterpart for landing on planets with no significant atmosphere. Big ships often lack the acceleration to make a soft landing even on fairly low-gravity worlds, so these specialized craft with appropriate high-acceleration engines are used to land on, and take off from, airless bodies. They may also double as orbital tugboats or space-to-space shuttles, though their efficiency is lower than purpose-built craft.
Orbital Craft: Orbital craft move about generally within a planetary system, and may also be atmospheric spacecraft or landers. They are usually fairly small. They accelerate and manoeuvre well, being very good for rendezvous, docking, and supporting activities. Usually, they do not have comfortable long term accommodations for their crew and conduct short missions of a few days maximum. They are often based at space stations where the crew live. Many orbital craft have no crew and are actually drones, controlled by crew in a control room on a planet or station. If they break down in orbit, another vehicle is usually available to rescue them, so reliability, while still important, is not their dominating design consideration - self-sufficient maintenance is not expected.
Interplanetary Ships: The backbone of space commerce. As important as interstellar commerce is, most cargo and passengers that go to space do so to travel to another planet, space station, or moon in the same star system. Economical interplanetary shipping relies heavily on economies of scale, so that large, efficient engines can be used. Some interplanetary ships are uncrewed and use highly efficient, low-acceleration engines, taking years to get between planets. They are maintained by teleoperated onboard drones. (This approach is not as effective as having humans onboard or at least nearby, where they can operate the drones efficiently without long speed-of-light delays, but it is very cheap.)
Laws concerning the rights and training of interplanetary ship crews are not determined by the Allied Worlds in most systems, but by system-level or lower organizations. In most cases, they are less favorable to the personnel than the equivalent AW policies either for the Allied Worlds Space Force or for interstellar merchant vessels. An area of current controversy is hibernation pay. Interstellar ships, regulated by the AW, are required to pay crew at least 75% of their normal pay for time spent in hibernation, and the Space Force pays 100% as an inducement. However, this regulation does not apply to interplanetary ships that do not use the wormhole gate system.
Interstellar Ships: Interstellar ships are those that go through wormhole gates linking star systems. They necessarily have the capabilities of a high-performance interplanetary ship as well in most cases, to travel between wormhole gates within a system. Interstellar ships are regulated by the Allied Worlds Gate System Commission. Interstellar ships tend to be very long in relation to their width and height because the gate stations have a fixed diameter aperture that limits the size of ships. (Many interstellar ships use very long fusion drives as well.)
Interstellar ships are a tremendous capital outlay and are owned by governments, megacorporations, huge public trusts, investment banks, etc.
Things that do not exist: (1) There are no "fighter" spacecraft in common use - the closest thing would be small armed atmospheric spacecraft intended to secure air supremacy ahead of landing craft during military operations. These have limited applications and are not especially common. (2) There is no equivalent of a "space submarine" because technologies for achieving stealth in space are not available as far as the Allied Worlds' know, and in fact are believed to be exceedingly difficult to develop. (3) In most parts of space, a "tramp freighter" would be uneconomical to operate. Exceptions might be within separated asteroid settlements and the complex moon systems of giant planets. (4) The Allied Worlds Space Force has mostly controlled piracy, which is not at present much of a problem; in any case pirates tend to use ruses, extortion, drones, and inexpensive orbital craft, hiding amongst civilian populations rather than politely placing themselves on a centralized pirate ship for convenient disposal by the authorities.
Spacecraft Systems
Engines: Spacecraft need some kind of engine to propel them through the vacuum of space. All engines work by flinging some kind of material - the propellant - opposite the desired direction of travel. Most spaceships have to carry a very large amount of propellant and accordingly it's desirable to use the propellant as efficiently as possible. This is achieved by flinging out the propellant as fast as possible, which requires a lot of energy. In the simplest rocket engines, chemicals are burned to produce hot gas, which moves briskly out the engine - the chemicals serve as the propellant as well as the energy source. Chemical rockets are very limited in their performance, and while still used, are not considered suitable for the needs of interplanetary travel.
The modern interplanetary or interstellar ship has a fusion powerplant, which generates relatively low thrust but expels the fusion products from the ship at several percent of the speed of light, making very efficient use of it. Military ships and expensive fast transports use fusion torches that can achieve high acceleration at the cost of some efficiency by injecting supplemental propellant (usually water, which is abundant and has convenient storage and radiation shielding properties), tuning their thrust to mission requirements.
Most fusion spacecraft use deuterium/deuterium fusion, because deuterium is cheap and plentiful. The highest performance ships use deuterium/helium-3 fusion instead, which is more efficient in several ways and requires less shielding to protect life support areas from neutron radiation. Unfortunately, helium-3 is much scarcer than deuterium, and most of the Allied World's supply is stockpiled for future military use. The supply of it on the Zheng He and Marco Polo represents something like 10% of the Allied Worlds total helium-3 stockpile.
Long-Range ships like the Zheng He are equipped with "in situ resource utilization" equipment to collect and filter propellant from comets, icy asteroids or small moons, etc. Civilized destinations have orbital propellant depots to save ships the trouble of having to carry ISRU equipment.
Gravity: Human health is adversely affected by long-term exposure to microgravity, even in hibernation. Crewed interplanetary and interstellar ships use large centrifuges to simulate gravity. A small centrifuge must rotate very fast to provide sufficient gravity. This tends to make people nauseous, though they can, to some extent, habituate to it. Since hibernating people do not get nauseous (and are lying down, which would further reduce the effect), they can be packed into comparatively small centrifuges. Large ships and space stations with big centrifuges are much more comfortable for long journeys. There is no magic "artificial gravity."
Life Support: All short-duration and most long-duration ships rely mostly on food stores, but water and air are usually recycled, and aeroponic gardens are fairly common on large long-duration ships, their positive effect on crew morale being widely appreciated.
Force fields/shields: Not something that has been invented, except for the magnetic deflectors that reduce impacts with charged cosmic rays, etc. These would do nothing against a laser or missile attack.
Teleporters: There is no useful macroscopic teleportation technology known to the Allied Worlds, unless you count wormholes, which they can't build (but can use.)
Reaction control thrusters: Tiny engines, usually resistojets on modern ships, that apply off-axis forces to rotate the ship or move it slowly in any direction - necessary for docking and for pointing the ship in the desired direction. They also don't emit harmful radiation, meaning that they are the only propulsion option available in some contexts (departing a space station, for example; space stations don't like being irradiated by the main engine...)
Drones: Modern ships, crewed or not, are full of drones that repair and maintain them, clean them, etc. Drones usually have some autonomous intelligence and can be taught new tasks, but lack flexibility and executive reasoning ability. Usually they can be controlled remotely by a human at a console somewhere. If there are no humans onboard (or at a nearby station), drones might be controlled from very distant bases. The speed-of-light delay, which could be up to hours long, makes such control operations tedious and inefficient. In cases where the stakes are low and the operations predictable, as with large interplanetary freighters, this may be an acceptable compromise for not having to carry humans onboard at all - a big savings.
Heat: Spacecraft systems, especially drives and power plants produce staggering amounts of waste heat. A fair amount of engineering effort must be spent to get rid of it, because in space, heat can only leave by radiation. Ships have huge radiators to get rid of waste heat. Ships operating in deep space primarily use fluid droplet radiators, where a spray of hot coolant is passed through empty space - where it cools - and is caught in a sort of channel for reuse. These are efficient, but operationally they inevitably lose a certain amount of coolant during orbital manoeuvres. Ships that do a lot of orbital maneuvering, especially in crowded orbits where space junk - just as lost coolant droplets - would be a navigational hazard, use ordinary radiators instead.
Spacecraft Operations
Weak engines have to use efficient but exceedingly slow transfer orbits to get between planets, making short engine burns and then coasting for days, months, or years. Modern crewed spacecraft use brachistochrone trajectories instead, which are the fastest for a given acceleration. In a practical brachistochrone, the ship accelerates continuously for about half of the journey, then turns around and accelerates in the opposite direction (i.e. slows down) for the rest of the journey, so as to arrive at its destination at a safe speed in the least possible time. If the ship doesn't have enough propellant to make such a trip, it can decrease its acceleration - using less propellant but at a higher efficiency.
Accelerations are usually described in terms of earth gravity - 9.8 meters per second, per second is "one g." A ship accelerating at one g is gaining speed at the same rate it would if it were falling straight down under Earth gravity - with nothing to hit, that would quickly become very fast indeed. After a day, it would be going over eight hundred kilometres per second or 1.8 million miles per hour. Few ships are capable of such acceleration. The Zheng He has a maximum acceleration of one-tenth g, but that is at a prodigious compromise in propellant efficiency and the service life of some drive components. Its normal acceleration is about 0.01 to 0.05 g, colloquially a "1% brach" to a "5% brach."