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There are three fuels for each of the three base engines, each has its own "efficiency" in terms of km achieved per kg used:
The efficiency of a fuel is important since its a linear relationship between kg of fuel used and distance traveled, however the weight penalty is non-linear (credit to neoazureus):Pentality = Max[Shipweight, (Shipweight/300)^3.2]
The key take away is that every kg you add to your ship costs more than the last in terms of achievable distance. This is a nice mechanic that prevent mega rockets ever making sense as the additional weight costs too much distance.
Importantly, engines (except steam) require an oxidizer now in a 1-to-1 ratio with the fuel in order to burn. Oxylite has a burn efficiency of 100%, Liquid oxygen has an efficiency of 133%, this would give for example Petroleum the power of 40km/kg and 53.2km/kg respectively.
Storage containers weigh very little. Interestingly, the oxidizer storage carriers exactly enough to burn 3 full fuel storage containers of fuel. So either you build a ship with 3 fuel tanks per oxidizer tank, or measure/limit the amount of oxidizer you pump in on smaller ships.
Seeing the difference between petroleum and liquid hydrogen we can see that 3 tanks of petroleum will provide a base distance (before weight penalty) of 108,000km. (40km/kg * 2700kg). This is the same distance provided by only 2 full tanks of hydrogen (60km/kg * 1800kg). This distance would be boosted to 143,640 km if you use liquid oxygen instead of oxylite as the oxydizer.
This is significant since the mass of these two rockets would be totally different:
So despite having the same base achievable distance, the net distance after weight penalty is 10,000km more for the hydrogen example.
Whilst all the base rocket parts like fuel storage, engines and command capsule are all relatively light. All the other rocket components are relatively heavy and will balloon your weight penalty rapidly:
For example, even adding on just one of these cargo bays to the petroleum rocket above drives the weight penalty up to -39,575km (23k more) and a second takes it to -79,567km (40k more), so it will barely get off the ground.
The solid fuel thruster is an interesting item, whilst it add 12,000 base distance. It weighs 200kg and requires another 800kg in wet mass (400kg fuel, 400kg oxydizer) to be powered. The solid booster is not always a good choice, you can see its effects with the calculator. Generally if your ship already weighs 7000kg wet (including fuel) adding a solid booster begins to reduce the net achievable distance. It's generally most useful as a small bump to steam engines to bring back materials from early planets.
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