"DT, I'm in complete agreement with you. I never said that missiles are not propelled by reaction drives or gravitic drives, simply that it cannot be those types as listed in High Guard.... but exactly what does propel missiles I don't believe we are told anywhere, so looks like GM fiat."
Actually we are. It's on core book page 111. The missiles on those ships are TL 6 with a slightly more sophisticated one at TL 8. These are not complex items, they are tech when missiles became part of an arsenal replacing dumb rockets. What come out of a ship turret is a little energy seeker or an image recognizer with thrusters for turning. Cheap, simple warhead, guidance and propellant. They are probably larger versions of vehicle tac missiles meant to pierce ship armor.
"Missiles are powered by an engine, generally either a type of rocket or jet engine. Rockets are generally of the solid fuel type for ease of maintenance and fast deployment, although some larger ballistic missiles use liquid fuel rockets. Jet engines are generally used in cruise missiles, most commonly of the turbojet type, due to its relative simplicity and low frontal area."
Cruise missiles are above the Traveller ship missile as it can travel even greater distances looking for specific target. They would be more a hunter drone. That means ship missiles are simple rocket engine packages.
The biggest factor of a ship's missile is distance and that take a lot of propellant so those missiles are mostly propellant. This also explains the cost of each. Looking at the tac missile design system in Striker... yeah, for a warhead big enough to dent ship armor and fuel for flight time, you're getting a bargain at 1.25 kcr per unit.
One last thing. Zero armor in a ship is not unarmored, it is the minimal toughness (Hull and Structure) for a ship to prevent 'normal' wear and tear from high velocity debris. Considering the damage potential, that is still tough armor! Just to be sure though does anyone know if there's a rule comparing damage between ground weapons and armor and that of starships. Can't find it between the Core book and old Mercenary.