Tank Types
Fuel tanks in RO are fully procedural, meaning that their height, length, and material are fully customizable. These are provided through either Procedural Parts or RO Tanks; their functionality is slightly different in terms of shape and aesthetic customization, but they both feature identical material selections that are similar to real life tanks. There are four main tank types, explained below. Additionally, as you unlock better technologies, you will gain access to lighter and better materials (Steel -> Al -> Al-Cu -> Al-Li -> composites).
Conventional (previously called separate) tanks, also known as stringer or fuselage tanks, are available from the start and are the most basic tank type. Historically used by early Soviet launchers, these tanks have no common bulkheads (i.e. the main outer structure houses a completely separate internal fuel tank), making them cheap and quick to manufacture. However, this comes at the cost of high dry mass and low utilization (and therefore low mass ratios and lower delta v). High pressure tanks used for pressure fed engines will suffer from an even higher dry mass required to withstand the pressure. They should therefore only be used on lower stages where the payload penalty for extra dry mass is not so severe.
In the early career (sounding rocket era to around first orbit), you will have access to Sep-Steel tanks, and then later Sep-Al and Sep-Al2 tanks, which will bring your dry mass down and your utilization from the upper 80 percents to the low 90s. Upgrading your tank types is a quick and easy way to improve your delta v as you move onto larger and more ambitious rockets.
Integral tanks unlock later in the tech tree, but are much lighter than separate tanks. Their external load-bearing walls also serve as the walls for the internal fuel tanks, which results in higher utilization. First used on early American launchers, these tanks are common in the rocket industry and are still used by many launch vehicles to this day. Their slightly more complex manufacturing techniques means that they will be more expensive and take longer to build than separate tanks, but the delta v increases and mass saving are more than worth it. Larger diameters will also become more expensive to tool, so you may want to think about using diameters that you will continue to use on future rockets in order to save on tooling costs later on.
As your space program progresses, you will find that integral tanks will become your go-to, middle of the road tanks, striking a good balance between mass and cost. They are also the lightest tank type that can be highly pressurized, making them useful for pressure fed engines.
Balloon tanks are the most advanced type of tank. While they offer the lightest mass of any tank type and feature a utilization from 99-100%, balloon tanks are exorbitantly expensive to tool and manufacture (especially with larger diameters) due to their extremely thin walls, which can be less than one millimetre thick. They also need to be constantly pressurized in order to maintain their structural integrity (with potential catastrophic results should they lose pressure); in RO this means that they cannot be used for HP fuels.
You will find that the need for balloon tanks will come up later in your career as you progress beyond LEO. These tanks should only be used on upper stages with high energy fuels like LOX/LH2 or LF2/LH2 (and corresponding engines such as the RL-10 or the XLR81-LF2) where every m/s of delta v per unit mass is crucial. Tanks larger than 4m in diameter will be on the order of hundreds of thousands of funds to tool and therefore should not be used for hydrolox engines that require large quantities of fuel such as the LR87-LH2 or J-2.
Service module tanks are used to house special payloads such as sounding rocket and satellite payload, life support resources, electric charge, small amounts of fuel for fuel cells, and small amounts of RCS fuel. They are heavy, somewhat expensive, and start out with 50% utilization (progressing only to 75% for the most advanced tech levels), and should only be used for "special" payloads which can only be stored in these tanks. Regular fuels should not be stored in these tanks. These should only be used to store consumables and life support, and if you try to use them as regular fuel tanks you will have a very bad time.