I think I'll just re-iterate and expand on the opinion I had previously:
1. Fuel should totally be a thing;
2. Engines should be ridiculously efficient.
3. Engines are all essentially the same - they have magnetic nozzle that expels stuff.
4. Running out of fuel shouldn't be a death sentence, but rather a challenge to overcome.
I am uncertain whether the power plant should be decoupled from the engine or they should be one entity. Having separate reactors and engines opens up a lot of possibilities. I think they should be separate; it adds to management but if fuel is cheap it shouldn't be a problem to enable automatic refuelling on every station you visit.
So, if we have separate power plant and engine, we can have:
Fission, fusion and anti-matter reactors.
- pretty straightforward, fission is the least and anti-matter is the most powerful and efficient.
- they may require power for ignition or fuel storage, but once they are on they don't consume energy. They can be of fusion
type (cheap fuel; extremely efficient: quite big, quite heavy and quite expensive engines), fission
(more expensive and less efficient fuel, but cheaper, smaller, more light-weight engines that require almost no energy for ignition), and anti-matter
(insane fuel prices; insanely efficient; require some power for anti-matter containment but no energy for ignition). The most important thing about them is the absence of the cruise mode - you just accelerate and accelerate till you're capped to your maximum speed, with weapons remaining operational.
- they use external power to run themselves. Their characteristics depend heavily on the power input.
Can be of ion/plasma/VASIMIR type
- that is, they can use anything you can ionise and expel to produce thrust. Have some water in your cargo hold? Feed it into the engine. Have some mined ore? Feed it into the engine. Have an annoying sibling? You got the gist
That way, if you're low on fuel you can always mine something. Efficiency should vary depending on what you feed into the engine; specialised fuel mixtures give you 100% efficiency, but with something like glass or unrefined ore you'll get much lower efficiency -> you'd go through your fuel reserves faster and spend more energy. And finally, the most advanced is a quantum vacuum plasma thruster
- you don't need fuel for it as it extracts it from vacuum fluctuations, but it needs a lot of energy to run. Those engines can enter the cruise mode when weapons are off and all your energy goes into the engine, Freelancer-style.
That way, explorer's choice would clearly be a powerful fusion reactor with quantum vacuum plasma thruster (Q-thruster), and mining equipment to get some hydrogen or helium-3.
Let's talk a bit about what efficiency and other things actually mean.
Engine efficiency = specific impulse = how fast it eats through the fuel, if it has it. Q-thrusters are the most efficient since they don't need fuel.
Engine power = thrust = how fast you can accelerate to your maximum speed both in normal flight and in cruise mode. Antimatter, fusion and fission drives are the best and are an obvious choice for really big, massive capital ships; fighters, cruisers and transports will live perfectly fine with plasma or Q-thrusters.
I think combining several engine types on one ship would be fun.
And finally, every ship should have some sort of back-up power plant, for example a solar panel and/or a small fission reactor, that will kick in if your main reactor went out of fuel. This should be accompanied by an annoying 'low on reactor fuel' message, but should allow player enough room to get to the fuel depot. Solar panel+Q-thruster allows for indefinite flight at the cost of no power to weapons, low shields etc., and you're screwed if you're in the system with no star. For extreme cases, there should be an option for distress signal.
The most obvious thing for explorers (like me!) should be:
1. Get yourself a fusion reactor;
2. Get Q-thrusters or fusion engine or both;
3. Get some mining equipment;
4. Get a portable refinery to extract hydrogen from what you've mined;
5. Get both a back-up solar panel and a back-up fission reactor just in case.
That way, you'll be able to fly indefinitely with some mining/extraction on your way. Use of local resources to continue your journey is an integral part of exploration, folks!
Survivor of the Josh Parnell Blackout of 2015.