Ykhjuy

I'm writing a story in which a civilization previously capable of (firmly technobabble-based) FTL travel has become stranded, orbiting an isolated star with no rocky planets. (There was a planet, but it was destroyed by their crash-landing in the star system.) The civilization is comfortably capable of forging an existence in artificial habitats orbiting their new star, but for the plot to function, it must be prohibitively difficult for them to leave the star system (to return home, or colonize elsewhere).

Their best chance would be to get their hands on an already-constructed FTL travel device, but I'm having trouble constructing a convincing (within-universe) reason why they couldn't simply build a new one from stuff they have at hand.

The simplest way is probably to leave them with a shortage of unobtainium. A substance named for obvious reasons, that happens to be required for the production of FTL drives.

Perhaps the limited quantity they had in their existing drive was destroyed (causing the accident) or dispersed in the accident. Either way, they're now limited to space habitats and STL drive until such time as they can come into possession of more unobtainium.

It costs a lot of money and they lack the political will to use all those resources for that scope.

See what happened with the Apollo program. As long as USSR was leading the space race the US Congress was scared by the enemy supremacy and had no troubles providing funding to the program.

Once the Moon was reached more than once and public attention lowered, together with the manifest incapacity of the Soviet space program to replicate the result, suddenly the budget was limited and even scheduled launches were canceled.

Their ship and her payload were designed as a colony expedition. They had not just asteroid mining craft, and seeds for hydroponics, and modular habitat sections, they also had the blueprints and machine tools to replicate them. And they had the engineers to read the blueprints and use the tools.

For the stardrive, they had just the operators' maintenance handbook, and pilots and engineers trained to operate it.

Consider: You are obviously able to use a computer, like billions of people worldwide. And there are many millions who can program a simple website. But how many can design a microchip, or manufacture one? A programmer would know about silicon wafers, and doping it with other elements, but how many can explain the physics behind that?

So the colony would have to replicate a R&D project. They get a head start because they know it is possible, and also the principles how it works. This is balanced by the need to build the colony as planned. They know that they have a fleet of sublight ships, and that these sublight ships will wear out, and that they should start assembling the shipyard to build new sublight ships.

They were never able to construct FTL drives - they got them from a forerunner factory that was still intact, which they had discovered during their own early steps into space.

(Or they purchased the FTL drive from extant aliens, or FTL tech is a particular company's closely guarded secret, etc...)

Perhaps they have tried to create a new FTL drive from remains of their old one, and it backfired so much - risked so many lives, nearly tore through the ship, messed up with their resources, etc - and leaves barely anything of the attempt that the rest simply say 'it's not worth it' and stop any further pooling of resources into making one.

When they crash-landed in the new star system, the scientists and technicians who were knowledgeable and part of producing FTL died, the computers and backups containing information on how to build/rebuild the equipment were also destroyed.

The remaining scientists and technicians have little knowledge of physics involved to produce FTL engine, and they were not property trained.

It was unfortunate for the scientists and technicians to die. There was a malfunction in the ship they were in when exiting FTL, sending the ship crashing down to the planet.