So you’ve got yourself a spiffy rocketship and you’re heading out into space. What do you load it up with?

Which things you put on your ship will depend on where you’re going and what you intend to do. There are three basic cases:

1. Short trip: You’re on local patrol and will return shortly to base, whether that be a planet, moon, space station, or carrier ship.
2. Civilization: You’re going somewhere relatively well-established, in which case you’ll be able to pick up supplies along the way.
3. Self-sufficiency: You’re headed off to the frontier where you’ll need to bring all your own supplies for a long duration.

In general, an item that is useful for case (1) is also useful for (2) and (3). Similarly, an item useful for case (2) is likely useful in case (3). However, the reverse is not necessarily true. In this thread items will be listed with the lowest numbered case in which they are useful, and will be assumed useful in higher-numbered cases as well. This means, for instance, that type-1 items are useful for nearly every ship, and type-3 items are only really useful for ships going “off the grid” to rely solely on what’s aboard for extended periods.

For example, a fighter or defense ship only needs a few type-1 items. A cargo freighter plying the trade routes may spend days or weeks in space, and thus will have some type-2 items as well. A prospector ship looking for new mining or colonization opportunities will probably be gone for months, so it carries far more supplies including type-3 items.

Furthermore, items can be separated into various categories based on what functions they serve. Here are a number of items, split into 7 categories and sorted by use case. For the sake of completeness, this includes some things that are likely to be built into the spaceship. It also includes some sci-fi staples that may not be physically possible, or may not appear in any given sci-fi setting. It does not list third-party cargo being carried, as that could be anything.

Spoiler

Fundamentals: Essential components of a working spaceship.

• Communication link
  Flight controls
  Generators
  Long-range sensors
  Maneuvering thrusters
  Warp drive

• Redundant backup systems

Life support: Necessary or useful for protecting the soft fleshy things aboard.

• Heat sinks
  Oxygen recycling system
  Shields and armor

• Airlocks
  Escape pods
  Radiative cooling
  Water recycling system

• Hydroponic farm
  Medical devices

Convenience: Items one could live without, but wouldn’t want to.

• Acceleration compensator
  Climate control

• Artificial gravity source
  Bathrooms
  Crew berths
  Dishes
  Entertainment items
  Exercise equipment
  Extra clothing
  Furniture
  Interior doors
  Kitchen facilities
  Personal items
  Trash compactor

Utility: For getting things done.

• Cloaking device
  Computers
  Data storage
  Weapons

• Repair tools
  Small arms
  Spacesuits

• External manipulator arms
  Fuel synthesizer or scoop
  Mining tools
  Scientific instruments

Planetary: For various interactions with planets and moons.

• Atmospheric flight wings

• Landing shuttle
  Teleportation device

• Aerial vessels
  Nautical vessels
  Terrestrial vehicles

Cargo capacity: For transporting various materials safely.

• Chemical tanks
  Cryogenic freezer
  Hermetically sealed chamber
  Passenger berths
  Refrigerated cargo space

Consumable: Things that get used up over time.

• Air
  Ammunition
  Fuel
  Water

• Food and drink
  Goods for trade
  Medicine
  Recreational drugs
  Spare parts
  Special purpose chemicals
  Toiletries

• Building materials

What else would you put on a spaceship, and in what circumstances?

Snacks. We're going to need snacks. And lots of NPR podcasts.

Under "Utility" #3 I'd put some sort of replicators - be they star-trek style rematerializes, robotic assembly part factories, or machine shops. This is different from general repair tools in that 'repair tools' would be for minor breaks (parts and the tools to replace them), while part factories would contain raw materials and the machines to make new parts.

Those factories also might be able to make new ammunition, vehicles, or other necessities; so the mass-cost of such a facility would be balanced by being able to carry less of several specific items that the facility could produce (ammunition, fighters, and spare parts can all be the same mass, and get used as necessary rather than be stored as ammunition when repairs are needed).

I would suggest adding a medical bay under Life Support for #3, as a dedicated medical ward would make it easier for you to keep all your wounded/injured/sick people under doctor's care instead of keeping them confined to their separate crew beds, and would also make it easier to do surgeries if somebody urgently needs one instead of trying to clean out an area of the ship to do an emergency appendectomy on the fly. Having medical devices such as AEDs is all good, but surgeries may become necessary for longer journeys into the unknown. (Additionally, devices like AEDs may make sense for a type 2 vessel, maybe even a type 1 vessel, so I'd suggest downgrading "Medical Devices.")