From FreeSpace Wiki
A mission in which the player must defend at least one friendly target until it completes a specified goal is called an escort mission. Defending a ship until it reaches a jump node is a prototypical example of an escort mission.
Here are some hints for making escort missions:
- List the ships you need to protect in the escort list.
Set the priority level in accordance with the importance of the ship. The more critical a ship is the higher Priority value it gets. Priority values range between 1 and 100. The set-priority SEXP allows you to manipulate Priority values in-mission. Use this if the situation demands it. Setting Priority to 0 will remove a ship from the escort list.
- Use the Near Ship feature for arrival conditions.
It makes the position of all attacking waves unpredictable. Without Near Ship, a player is able to observe the exact position where enemy ships come, making the mission a lot easier than expected. Wisely-planned arrival cues also ensure that hostile spacecraft or warships don't jump in too close to/far from the ships they're supposed to destroy. While 2,500 meters may be a good range for hostile bombers, 4,000+ would slow down the action and <1000 would make the mission hard to beat since the bombers would be launching warheads upon jumping in and the time to intercept them would be very limited.
- Use the good-secondary-time SEXP.
Thanks to it, you can artificially choose what kind of warhead the AI should use and when. Use it to order the ships to use their Trebuchets when they are out of the range of heavy bombs. As soon as they get close, using Cyclops warheads is better.
- Pay special attention to mission balance.
Make the mission bearable and beatable. A too easy escort mission is a target practice at best, and boring at worst. Some players might find an overly hard challenging, while others would despise it.
- Hostile bomber escorts need to be powerful.
Otherwise, your wingmen will take them down without a problem. Force the player to attack the fighter escorts otherwise he will lose many of his wingmen, which, in the long run, will cause trouble.
- Setting reinforcements.
If you think your mission might become too difficult, it is a good idea to make reinforcements available. Reinforcements must not break balance and, if necessary, they could be available only if the skill level is equal to, greater than or lower to a certain level.
- Bombers and their escorts must be together for best effect.
The Near Ship trick has one serious setback compared to just using the Hyperspace arrival location: bombers and their fighter escorts do not arrive at the same time from the same position. This way, only the first wave of bombers and fighters will arrive simultaneously. To remedy this, using multiple one-wave wings should be considered. You can control each wing's arrival conditions in a way that will make the bombers and their escorts arrive at the same time. To ensure they arrive from the same position, use the "Near Ship - <Bomber 1>" formula for the fighter escort wing. Obviously, the escort wing must arrive at the same time or shortly after the bombers in order for the Near Ship condition to work. An arrival delay of 0 seconds isn't that believable, so setting it to 1-2 seconds is usually the best option.