Space Simulator
1.0.9
space flight simulator

Spacecraft , except the most basic satellies have at least one way to change its velocity. Ignoring new exotic way of propulsion , most known spacecrafts achieve propulsion by the use of a rocket engine. The basics of the rocket engine are very simple known since the antiquity . Any mass expelled from an object at a certain speed ,generates an opposite force to the object called "reaction force" . It is importantn to understand that the force arises from the fact of expelling mass at a speed ,and has nothing to do with the medium (air,water, space ) onto this mass is expelled to. That is, a reactive rocket engine is the only way to accelerate an object in the vacuum of space.
A rocket engine can not generally use the oxigen in the atmosephre for combustion , hence it needs to carry its own oxygen , usually liquified and very cold to minimize tankage requirementes 
Esentially , a typical rocket engine burns oxidizer (generally oxygen ) and fuel ( generally Kerosene ) into a closed combustion chamber with just one exit , the pressure of the aforementioned combustion expells all exhaust gases outside at tremendous pressure that due to the gremetrical design of the nozzle, it is transformed into brutal velocities of several times the speed of sound. The masss expelled the mass of the exhaust a those velocities , generates the opposite force (reaction force ) that pushes the rocket forward.
This ,however simple , requires the highest grade of engineering and materials to achieve, due to some reasons
On the other hand, rocket engines are merely designed to last for a few minutes (hours at most ) , and so they dont have any longterm engineering challenges (rust , treatments, oil quality, etc) .
Rocket engines ( and airplane turbojet engines for that matter ) dont' produce work , in the traditional way. They just generate a reactive FORCE .In contraposition , a Otto/Diesel engine generate WORK (that is force over distance ). Power ,normally measured on HP is the ability to create work (force ALONG distance ) over time. For example an engine of 1 Kilowat , can Lift 1 Kilo one meter up , in one second. If it were to lift 1Kilo over to meters, it would require desmultiplication/gearing , so it takes 2 seconds. This is the reason cars have gearboxes. The further along they need to 'spread' the power , minimizes the possible force applied. But the ammount of workperhour a standard car can create is the same (a lot of force , over very small distance when in first gear , or the opposite , very little force over a large distance ,in topgear).
On the other hand ,a rocket engine doesnt generate force based on distance. It generates just force. Allways the same force for as long as there is oxidizer and fuel left. The same force , is also generataed regardless the rocket is inching up in the first second after launch ,or blasting at 10km/s at the end of a Lunar Injetcion. The force is the same, no matter how fast or slow the rocket is moving.
So... then what is the power generated? Deppends on how fast is moving. Consider this respect to a motor vehicle, in which generates at most same power at any range of speed. Indeed, at rocket launches, with all the drama , concert of light , smoke, fire, and shakes, all these.. and the rocket is generating ZERO work.Total output: ZERO Horsepower.But it generates forces. Forces that will keep accelerating the rocket constantly as opposed as the car, that needs to spread the power into increasingly larger distances, and hence needs to reduce power 
So the rocket (assuming the fuel is constant ) will accelerate constantly , 1m/s >2 m/s , 1000m/s > 1001 m/s , 10000 m/s >10001 m/s . Suddenly this apparent drawback of rockets vs motorcars converts into a huge advantage.
This effect also applies to jet engines, and is the reason behing jet engines while mounstruos inneficient become even more ecconomically than propeller planes at certain speeds. Take the mythical Aeroespatiale Concorde. With all the huge four turbofan spitting fire and fuel , it was most ecconomically to fly at its unrivalled speed of Mach 2.1.It is said that the Concorde spent more than 1/5th of its total fuel taxing , due to the gross ineficiencies of a reaction engine running at low speeds.