Here are some justifications:
- The Rocket fuel item is rather end game and giving the player access earlier will entice the player to construct their building pipeline to include it.
- Figuring out the proper Crude oil production ratios, specially for beginners such as me, to not have an overabundance takes quite a bit of time; my reaction is to store the excess into Solid fuel; it would thus give players something else to do with their excess Solid fuel that is stacking up.
- It would give the player another enticement to research Nuclear power to get Nuclear fuel before reaching end game conditions.
- Force Rocket silo research to require Rocket fuel research (obviously; though I realize this would increase end-game research reaching time; though this could be mitigated by dispatching the used research packs between the Rocket fuel research branch and the Reactor silo research).
- Put the Rocket fuel research perhaps after Rocket speed 3, thus requiring Military pack, ensuring that the player doesn't get it too quickly.
- Make Explosive rocketry research require it (Not sure that's an amazing idea, but it's interesting; you could also force Explosive rockets to require Rocket fuel, though I don't like that idea much either...)
- Alternatively, we could insert Rocket fuel research in the Rocket speed research; another idea I don't like a whole lot.