This has usually led into drawn out arguments in my experience, mostly due to people being grossly misled by popular science fiction, but I wanted to share a basic gameplay system for realistic space combat here. This was originally written in mind for videogames, but the basic principles remain the same for tabletop. [Albeit, any kind of 3D vector system for space combat on tabletop becomes rather unwieldy and complicated in my experience.]
RTT gameplay, combat is done on a WEGO system.
You control 1-5 ships. The biggest perhaps as large as a Nmitz carrier, with the smallest about the size of a Typhoon submarine.
Basic gameplay mechanics/axioms:
Nose has most armor and weapons [generally]
Sides and rear of spacecraft are vulnerable
Engines are weapons
Physical weapon damage based on newtonian physics, if you both accelerate into each other, your guns will hurt more
Penetration is modeled instead of hitpoints
Weapons are: Missiles [long range; Kinetic kill and nukes], Cannons [long range; gauss/railgun and conventional], and lasers [short range; ultraviolet or higher wavelength].
A combat scenario takes place with consideration for the setting. Scenarios allow for various kinds of interception orbits which determines, partially, closing velocities in the game. Both players choose their initial closing velocities and trajectories before the game starts, using up fuel to change their initial closing velocities.
Full control is given to the pitch/roll/yaw of each spacecraft. UI design should focus on automating things like keeping a nose pointed at a designated enemy so the player doesn't have to do it manually. Different spacecraft designs will have different rotational and primary engine speeds. Mass is taken into consideration with the newtonian mechanics, a heavily armored, massive, spacecraft is much harder to maneuver than a less massive, lighter spacecraft. Mass changes during gameplay as parts are destroyed and fuel is used up. A heavily armored spacecraft could go into battle with a quarter tank of fuel, and come out with similar maneuverability as a result to a 'lighter' opponent. Fuel use becomes part of the tactics/strategy.
Subsystems within spacecraft can be damaged, penetrations that destroy battery or munition sections within the ship could cause internal explosions or power outages. A 'lucky bb' could destroy or disable an internal subsystem. To help differentiate design types, some ships put a focus on additional internal component armor, but as a trade-off have lower outer armor. Penetrations might occur more often, but internals aren't raped when the penetrations do happen.
General combat will consist of trying to present the most armor and guns to your enemy. Missiles and cannons can be fired into the maneuvering arc of the opponent, forcing him to turn his nose into the hits to blunt the damage and expose his side or rear to your lasers; Or forcing him to change his direction of travel while keeping his nose pointed at you and hoping his point defense lasers can take care of any missiles that might re-maneuver to hit his side/rear armor. High speed closing velocities between two spacecraft can result in weapons that would normally be unable to penetrate instead cutting a hole straight through one side and out the other even through the thickest armor.
Heat and power management is also important. Firing weapons, especially lasers, may send your ship past its ability to manage the heat effectively, and firing all your energy intensive weapons, like railguns and lasers may drain your batteries and leave you unable to return fire or use laser point defense for several turns afterwards. [Better hope that alpha strike was worth it!] Going over heat capacity reduces the overall effectiveness of your ship.
A good resource for info on realistic rocket/spacecraft design can be found here: http://www.projectrho.com/rocket/