Launch pellets (of fuel or propellant) or pallets (of other ship's stores) using an accelerator of some sort (e.g. coilgun, railgun, starwisp, light sail). Make sure the fuel pellets are the proper isotope mix ratio for you fusion power plant and carry enough "other stuff" for propellant. You should also charge the pellets so that your electrico-magnetic scoops can "catch" them.
A preseeded trajectory is one in which the fuel and/or propellant for a Bussard Ramjet like craft is launched prior to spacecraft launch. The velocity and launch time for each pellet is carefully calculated so that the spacecraft which follows it encounters it under ideal conditions.
The problem with a Bussard Ramjet is that the reaction cross-section of a $4 H^1 \rightarrow He^4$ reaction is very small. This means it takes a long time for the reaction to occur, which means that if the reaction occurs at all, it likely occurs after the fuel left the engine of your spacecraft. Using a fuel with a larger cross-section can greatly improve the gain possible from the reaction which might make the Bussard Ramjet possible.
Your pellets should contain the proper isotope mixture of fusion fuel (e.g. tritium + deuterium). Bear in mind that you need not only consider Hydrogen isotopes. $H^1$ fuses with $B^{11}$ for a primarily aneutronic reaction ( $H^1 + B^{11} \rightarrow 3He^4$ ).
In this case your pellets would just provide the $B^{11}$ portion of the fuel. You would scoop the $H^1$ component for this reaction from the interstellar medium. You could also get all of your reaction mass requirements from the interstellar medium's $H^1$. Such a system would provide a "clean burning" (aneutronic), "fast burning" (higher reaction cross-section) fusion reaction with higher thrust (extra mass provided by the interstellar medium) than a pure fusion reaction.
This technique eliminates the worst problems of a Bussard Ramjet and enables an interstellar craft to make the trip without accelerating and decelerating huge masses of fuel and propellant.
You must accelerate the pellets and time their launch very carefully and the spacecraft using this trajectory must not deviate from it by very much or it'll lose all benefit.
Although most of the benefits possible to achieve by such a system are realize if just the fuel and propellant are supplied this way, it should also be possible to seed the trajectory with other supplies. Some mechanism from grappling those pallets of other supplies must be included and the ship using the seeded trajectory must know it is coming or normal operations of the ship would destroy the goods.
This method has a few big benefit from the perspective of the crew:
- All resupply shots are fired before committing to the launch of the
spacecraft (your resupply is laid down before launch).
- The prelaunched fuel would provide some visibility into the
interstellar medium – thus alerting the trailing spacecraft of unseen
hazards (e.g. brown dwarfs).
A post-seeded trajectory
A similar mechanism by which pellets of fuel and propellant and/or pallets of other stores are shot at the receding spacecraft. This technique has the added benefit of providing positive momentum transfer to the spacecraft (thus accelerating it), however, each shot must be carefully timed and calibrated to keep from hitting the spacecraft like a hypervelocity weapon instead of a gently captured resupply pallet.
Once again, the biggest benefit comes from resupplying the fuel and propellant. Other ship's stores would be nice to add but not nearly as critical to overall mission mass.
If something happens to your civilization after spacecraft launch but before all the necessary supplies are launched, then your crew will die. There's simply nothing they can do about it.
Important Note
It's important to note that in either system, the group launching the supplies must be very careful about the supply's trajectory.
- Shots not in line with the ship's path provide no benefit.
- Shots fired are the wrong velocity either fail to reach the craft or
hit it so fast that they would likely destroy it.
Also the spacecraft can only stray from the designated trajectory by a little or it'll fail to get the vital supplies it requires.