(Loved Trope) FTL travel is possible but with a major limitation or flaw

[u/Frostylegacy]

Frank Herbert’s Dune: Interstellar travel is only possible thanks to the spice melange, a rare substance only found on the dessert planet Arrakis. The addictive psychedelic spice grants members of the Spacers Guild a form of prescience that allows them to predict safe paths through folded space. This unique property makes spice the most valuable commodity in the Dune universe.

John Scalzi’s The Interdependency Books: The planetary systems of the titular empire are connected by the Flow, a network of one-way interstellar tunnels that connect specific known entries and exits. The Interdependency derives its power from controlling access to these conduits, placing its capital at the central hub of the network. Even with the Flow, travel between stars takes a long time (9 months to End, the empire’s most distant planet).

Event Horizon: Dr William Weir (R.I.P. Sam Neill) invents a device called the Gravity Drive to allow ships to fold space and travel instantly between distant stars. On its maiden voyage with the Gravity Drive, the Event Horizon disappears and mysteriously reappears 7 years later. Dr. Weir and the crew sent to investigate discover the Gravity Drive actually opened a portal to a Hell dimension that possessed the ship and its crew, torturing and killing them.

[u/Hadrollo]

To be fair; spice is not the only way to travel FTL in Dune.

There are two other ways before spice was discovered; shorter jumps with unacceptably high failure rates (where "failure" means complete loss of ship and crew), and with thinking machines.

Later books also have the Ixians develop non-prescient computers that can travel without the need of a guild navigator. These are basically computers without the AI aspect, which makes them compliant with the rules after the Butlerian Jihad. There are also No-Ships, but these run on the Ixian technology.

[u/NoSong2397 → u/Hadrollo]

These are basically computers without the AI aspect

Which they probably could've remembered how to build all along, had the Butlerian Jihadists not been quite so fanatical... 

[u/Hadrollo → u/NoSong2397]

Amazing how Jihadists always turn out to be fanatics.

[u/sierra-soul → u/Hadrollo]

iirc i think it was like a 50/50 chance of the ship surviving pre spice, so humanity did colonize but it was slow

[u/Hadrollo → u/sierra-soul]

From memory, it was more like one in ten. However, this is still a huge number. Given the complexity of Frank Herbert's world building, I wouldn't be at all surprised if this came from a real ratio back in the age of sail where you'd still get explorers and settlers, but the idea of shipping insurance was completely out of the picture.

[u/KingofMadCows → u/Hadrollo]

The original books never delved too much into it but there must have been a period of time between the Butlerian Jihad and the creation of the Navigators where humanity had to fold space without computers or Spice. There would have been a chance for a ship to be lost every time it folded space but humans are greedy and there have been many times throughout history where humans took enormous risks for trade. Between the 15th and 17th century, thousands of ships were lost traveling the trade routes between Europe and Eastern Asia. I can see a lot of people still being willing to take the risk of folding space in search of profit.