Direct Time Travel
Direct time travel describes traveling through the timestream of a single timeline without leaving that timeline during any point of the movement. This method is most commonly used by novice time travelers or those moving only backward in the timestream, as it leaves little risk of degradation or damage to the vehicle in question. Direct time travel is less efficient when attempting to travel forward through the timestream because the branching points of the timeline can cause the time traveling vessel to sustain quick-turn damage and go down unfavorable paths if not carefully navigated prior to traveling.
Tethers are a useful tool to employ direct time travel when returning from a trip backwards in time. An electronic device programmed to emit an energy frequency can be placed at the point of origin and used as a relative waypoint for the time traveling vessel to make a catalog of the branch points used in its journey backwards in time. In this manner, a tether can allow a ship to safely return home using the simpler method of direct time travel.
Indirect time travel
Indirect time travel is a riskier but more precise method of traveling between points on a time stream. Much more energy is required to use this method, and the potential damages are much more severe if improperly prepared for. The general sense of indirect time travel is to create a bridge between one's initial and final points on the timestream in either direction and hop from the former to the latter via an external medium. The most common versions of this method are the bridge technique and the shell technique.
Using a vast amount of energy and a temporal map, a time traveler can plot a course from their current time point to another, distant time point in either the past or future. Once the course is plotted, a computer with an advanced set of AI is commanded to use micro-adjustments or rapid-fire decisions to create a "bridge" timeline between the two points, micro-adjustments for a past destination and rapid-fire decisions for a future one.
Rapid-fire decisions occur as the computer uses a minute amount of energy to create enough quantum decisions to create or isolate a new timeline with parameters to directly link the current point in time to the desired ending goal. The computer travels ahead of the ship in time by an infinitesimal amount, just enough to create the bridge in front of the ship and allow it to safely travel.
Micro-adjustments use a similar principle in the opposite direction. The computer goes back in time a quantum amount and builds a bridge backwards through time with the goal being to match up the constructed bridge timeline backwards to the starting point. Essentially, by knowing where its pilot wants to go, the computer uses smaller time travel intervals to make that time a reality.
Naturally, the bridge method is a costly, energy-intensive, and incredibly dangerous method of time travel. A wrong calculation or a poor algorithm could leave a pilot stranded in the Abyss for eternity as his/her ship spun out of control building a bridge to nowhen. Likewise, failing to fuel up properly or landing in a destination without sufficient fuel could also leave a novice crew up time creek without a temporal paddle. The trade-off is that this method is extremely accurate, and that the bridge does provide a permanent two-way street to travel in case of repeated trips.
The shell technique uses a much more brute-force attempt at indirect time travel. The vessel is wrapped in a tight cocoon of temporal energy, the titular shell, and the course is plotted from the assigned starting point to the proposed ending point. The computer calculates a temporal trajectory through a different dimension, typically empty space or an undisclosed n-th spacial dimension which provides a straight line path to the target destination. Once the route is established, the shelled vessel is hurled along the route at a high velocity, using the temporal shell as a cushion against the harsh consistency of the travel medium and the high speeds of travel. Stopping usually occurs via the destruction of the temporal shell in the direction of travel; the energy from the reaction then counteracts the momentum of the ship.
Naturally, this can have the unintended side effect of collateral damage if a route is not navigated within proper parameters. The unleashing of a temporal shell is not to be trifled with, and the destruction caused by a careless stop could very well endanger the pilot and crew as well as their surroundings. Also, this method tends to be slightly less accurate than the bridge or direct methods, but is quite fast and requires less overall energy than a bridge.