Can you also elaborate how the momentum works while doing somersault in space for completeness I suppose, thanks.

... it works like momentum. There is a translational variety, this is what keeps a car rolling forward when the engine suddenly dies. Without friction from air and the road it would continue to travel at the same speed down the highway. There is also rotational momentum. This is the sort that a spinning top has, and if it weren't again for friction it would keep spinning forever. In space a ship has translational momentum (which keeps it in orbit) and rotational momentum (how it is spinning). Applying a force to spin the craft has little/no effect on the translation momentum, and vice versa.

While there is a mass penalty to having a second set of engines, there is also an advantage, that it allows you to keep your weapons pointed at the enemy while you decelerate. Ships in The Expanse almost always have their guns pointed the wrong way as they decelerate. See here: youtube.com/watch?v=gfW8hP3LZJ4&

@AdamReynolds Fair enough but a warship is very different thing to an ocean liner, or even a freighter. Find me a nuclear freighter, or an ocean liner that disappears below the waves. I believe that there is a warship in the expanse with dual and opposingly orientated engines, which allows it to strafe incoming fire. Also the smaller war frigates actual have a 360 by 360 degree weapons coverage.

@Kain0_0 not really, all obey the same laws of physics. Many freighters and ocean liners were in the past pressed into service as warships, usually transports but sometimes seaplane tenders or even fitted with weapons and used as light cruisers and commerce raiders.

@AdamReynolds in the future, someone will eventually invent turrets, I'm sure of it.

@jwenting Yes, that is historically correct, and quite likely to be correct in the future. And you are also correct in that a ship still has to deal with the same not foundering problem. That wasn't my point though. A ship of sea, air, or space still has to be a ship, and still has to contend with the same basic problems of that medium. My point was that a ship is designed for its purpose, a ship designed for conducting war will contain design features that accomodate conducting war. Such as dual opposing engines that would normally be considered extravegant for commercial designs.

Weapons that can't be aimed independently of ship direction are kind of worthless. As Starfish Prime says, turrets. Also, weapons and sensors can't usually be pointed through engines. So every set of engines gives you an extra blind spot.

@AdamReynolds Get close enough, and most engines are weapons.

@Chronocidal: …but for most engines, that's very close. As a weapon, a rocket engine is like a gun loaded with blank cartridges: sure, it can kill you if you stick it in your ear and pull the trigger, but at any longer range the exhaust soon disperses and becomes harmless. Furthermore, the dispersal is not only inevitable with gaseous exhaust, but also a desirable safety feature.

Ships in The Expanse almost always have their guns pointed the wrong way as they decelerate And when they accelerate too. In the expanse, main engines are used to decide the broad path of the ship, that path will hopefully be strategically useful. When firing they point the ship in whatever direction suits best. When dodging incoming fire they use something like KSP's RCS to make small strafing movements. In the one time I know of where there was an engagement at close range, and with cover, ALL manouevring was done with RCS.

Reaction Control System

One advantage of having only one engine and flipping the ship in order to slow down is that the same side of the ship is always "the floor". If you slowed down with an engine on the opposite side of the ship then the deceleration would push you to the ceiling

It should be noted that you can use gyroscopes for most of the spinning.

I have to agree with this answer. Additional thrusters would be used to rotate the ship and the Main Thrust would serve as a brake. This isn’t necessarily Fiction either but a realistic modern approach to problem.

@puppetsock Imagine a weapon spanning the whole ship. You can only point it by turning the ship, but when it fires, it's not worthless. Obviously, you'll need smaller turret-mounted weapons, too.

"All the ship needs is: A means to rotate the ship. Normal main engines. " Not quite. You also need at least one thruster that uses non-liquid fuel (usually compressed nitrogen). This thruster must be used for a few seconds before you can switch on the main engine, it is needed to move the liquid fuel content to the bottom of the tanks. If your main engine's fuel intakes fail to deliver liquid fuel, your main engine will usually disassemble rapidly.

@cmaster-reinstatemonica yes that is one solution to the requirement of feeding liquid engines. There are alternates ways to achieve the same outcome though. One way is to ensure that the liquid fuels tanks are sufficiently pressurised before main engine start, or to have pressurised feeder tanks that ignite the main engine. Alternately the rearward RCS might be enlarged using a hypergolic. More fancifully perhaps it is a different engine technology such as an ION drive, or fusion torch style engine.

TLDR in space, all your thrusters are "reverse" thrusters, because in space forward-backward are wherever you put them.

@Kain0_0 Pressurizing the tanks does nothing to move the liquid to the engine intakes. As long as you don't physically reduce the tank volume as to not allow any steam bubble to form (for example by placing an inflatable balloon inside the tank, or installing a movable compartment wall), you must apply some acceleration to control where the liquid is located. The only good alternative to a cold gas thruster would be to place the engine intakes at the perimeter of the tank, and then rotate the entire ship along its axis to use the centrifugal force for gas-liquid separation.

@cmaster-reinstatemonica Interesting, so I'm very wrong in how to get the liquid into the engine. I would have thought pressure differentials would have been sufficient in line with how space x is making the falcon engine reignitable by essentially pushing warmed up gas into the top of the tank.

:q