RCS As Shown In Fiction VS Realism....

[Spacescifi]

I have given much thought to the look of RCS on space vessels and the reasons behind it. Specifically for large classic scifi SSTO vessels.

At first I wondered why often RCS is shown as nozzles dotted into the hull itself spaced apart, or sometimes in rows, but never clustered into some circular port.

Why? Two reasons come to mind.

1. RCS may draw it's propellant from a main tank that feeds all of them... even if they have  a few smaller auxilliary tanks nearby.

2. For large SSTOs, you may want to cover the nozzles on the roof of your hull to protect against hail, snow, and debris while landed on a planet. This is easier to do with metal doors if nozzles take up less space such as with linear ports or simply spacing out nozzles embedded into the hull. But a circular port with a cluster of nozzles would by extension require a broader door to cover it from rain/hail/dust on the rood while landed.

IRL I never see a cluster of nozzles used for RCS, only main engines, but if you are using multiple nozzles anyway for RCS, why not cluster them in their respective areas? Probably for aesthetics? I dunno? Looks ugly?

Any functional reason?

Like if you have 4 thrusters each for yaw, roll, and pitch, is there any advantage/disadvantage of clustering them in their areas VS spreading out their thruster locations more?

Maybe it has to do with maneuverability I dunno and COM.

 

Edited July 12, 2023 by Spacescifi

[razark]

3 hours ago, Spacescifi said:

IRL I never see a cluster of nozzles used for RCS, only main engines, but if you are ysing multiple nozzles anyway for RCS, why not cluster them in their respective areas?

I'd call that a cluster (or several clusters).  The aft RCS system was clustered on the OMS pods.

Spoiler

 

 

Or perhaps I misunderstood what you are saying.

3 hours ago, Spacescifi said:

Like if you have 4 thrusters each for yaw, roll, and pitch, is there any advantage/disadvantage of clustering them in their areas VS spreading out their thruster locations more?

Are you talking about having all the thrusters clustered together in one location on the vehicle?  As in, not like the Shuttle above, but having all the RCS in one big group?

If so, having them out at the ends of the ship, rather than closer to the center allows for increased leverage.  There's also the benefit of being able to keep the exhaust away from more portions of the ship, as having it hit the vehicle would likely be bad.  The same goes for anything the ship may dock to during its mission.  You wouldn't want to damage your station by badly located RCS engines.

[sevenperforce]

Fictional spaceships depicted in film typically don't show any RCS thrusters at all -- the ships turn magically with engines running at full blast but without any forward impulse.

Part of this was the combination of a lack of knowledge (film directors don't know how spaceships work) and rule of cool (it felt more "advanced" to have spaceships that turn without any visible means of rotation). The other part was the cost of CGI. CGI used to be incredibly expensive, and so any added animations drove up the budget. It is only in recent years that the cost of high-quality animation has dropped to the point that animating things like RCS pods is no longer a big-budget item.

The decrease in CGI cost has combined with an additional desire to show space as grittier, with more realism and dynamic elements. The CGI ships in The Mandalorian and the rest of the Disney expansion of Star Wars are a good example of this. Prior to the Disney expansion, CGI ships in Star Wars basically just floated around without any visible propulsion or mechanisms, but now you see animations of stuff like RCS and vertical thrusters. For example, vertical thrusters on the Razor Crest here:

Vertical thrusters on the N1 Starfighter here:

And the use of active gimbal on the main engines, to control the ramming action of the Hammerhead Corvette:

An astute observer will of course note that the engine plumes from the corvette appear to have Mach diamonds showing overexpansion, which is de facto impossible in the vacuum of space. Perhaps these aren't actually Mach diamonds but are instead some sort of magnetohydrodynamic vortices that just look like Mach diamonds? /s

One possible in-universe reason we don't often see RCS in science fiction is that the inertial control mechanisms are just massively more powerful, or they have used some sort of magnetic field coupling as the primary orientation control. In real life, magnetic field coupling can be used to desaturate reaction wheels (the Starlink satellites use an electromagnetic rod to push against Earth's magnetic field for wheel desaturation to since they don't have any attitude thrusters), but you'd need immensely more power to do that for your primary mechanism.

Science fiction, of course, is famous for huge power sources. The TIE fighters in Star Wars are said to be extraordinarily maneuverable, and the X-Wing pilots are always saying "Lock S-foils in attack position." Why? The X-Wings have their weapons mounted on the wingtips, so opening the wings would increase their field of fire...but that might not be the only reason.

The X-Wings do have sings that look vaguely like they could provide aerodynamic lift, but the TIE fighter's wings certainly do not:

What are those wings useful for, other than looking cool? They could be solar panels (at least, that's one in-universe answer offered in some of the extended universe stuff) but their utility would be meager given the enormous power requirements of Star Wars vehicles. They could also be thermal radiators of some kind, although we don't see them glow like radiators. My head-canon is that any flat, thin surfaces on Star Wars vehicles are high-energy magnetically-coupled reaction control planes, capable of pushing against any ambient planetary or stellar magnetic field in order to rapidly change orientation. That's why the X-Wings appear to maneuver in space just like ordinary aircraft: their "wings" are acting like real control surfaces. That's also why the TIE fighters are so maneuverable: they are basically just two giant RCS surfaces with a pilot and engine and blasters in a pod at the center. Even the Millennium Falcon's long, flat external surfaces are acting this way.

This allows some aspects of basic physics to still apply. The moment of torque around the center of mass of a ship is going to depend on the physical orientation of those reaction planes. X-Wings and Naboo Starfighters have good roll and pitch authority ("let's try spinning, that's a good trick") but poor yaw authority and will probably rely on differential thrust for yaw. The TIE fighters have ridiculously good yaw and roll authority but very little pitch authority other than what phased gimbaling of their ion thruster can provide; this explains the strange, otherworldly way in which they seem to zip around through the sky. The Falcon has some yaw and roll control but FANTASTIC pitch control, which allows it to simply pitch its nose up and use its sublight thrusters to blast its way into space like a KSP spaceplane loaded down with oversized RCS wheels.

To summarize, Star Wars ships basically use the following engines and maneuvering systems:

• Repulsorlifts: a "gravity brake" which "locks" to a particular point in the gravitational gradient and allows a vehicle to hover with little or no power consumption; similar technology is used for inertial dampening to keep the crew from getting smeared
• Reaction control planes: a magnetically-coupled "wing" which pushes against planetary or stellar magnetic fields to provide roll, pitch, and yaw authority as necessary
• Vertical turbine-based thrusters: a turbine bypass which sucks in atmospheric gases, diverts them through the main engine system, and then expels them downward at high velocity in order to provide vertical translation for liftoff when you need to ascend or descend vertically
• Sublight thrusters: extremely powerful reaction engines with ridiculously high specific impulse which somehow do not torch everything behind them...perhaps using a exhaust made of something which decays into non-interacting particles shortly after leaving the nozzle
• Hyperdrives: whatevermajiggy that lets you go to hyperspace 

It's a good head-canon anyway.

[darthgently]

6 hours ago, sevenperforce said:

The other part was the cost of CGI. CGI used to be incredibly expensive, and so any added animations drove up the budge

Well, there was the respectable RCS done in the Babylon 5 series on the modest Amiga computer.  Respectable in the sense that the RCS firing vectors and duration corresponded with craft attitude changes

6 hours ago, sevenperforce said:

In real life, magnetic field coupling can be used to desaturate reaction wheels (the Starlink satellites use an electromagnetic rod to push against Earth's magnetic field for wheel desaturation to since they don't have any attitude thrusters)

Did not know this.  Very elegant

[Nuke]

i always figured it was easier to turn small thrusters on and off than throttling them. they are all gonna be plumbed off of the pressure fed system through a solenoid valve. i suppose lining them up simplifies the plumbing greatly.  also if you have multiple thrusters you have some redundancy, and you can turn on any number of thrusters if you need more or a specific amount torque. sometimes you see small and large thrusters and the smaller ones are for fine control. or with patterning of larger and smaller thrusters, achieve a lot of different thrust levels. 

pretty sure thrusters were just bang bang in the apollo era, with two settings, on and off with most of the control done with timing instead (turn on, wait a specific time, turn off). the rcs contol stick used had more in common with an arcade stick than my hotas. it even seemed that way on the dragon docking simulator space-x put out. the shuttle may have been different, idk.

[Nuke]

1 hour ago, darthgently said:

Well, there was the respectable RCS done in the Babylon 5 series on the modest Amiga computer.  Respectable in the sense that the RCS firing vectors and duration corresponded with craft attitude changes

Did not know this.  Very elegant

in the middle of a b5 binge right now, so im actually impressed about a lot of stuff like this. b5 even has radiators (though they look like solar panels, but the station has a fusion reactor, so you would need radiators more than panels, probibly an artistic choice, they would look kind of drab in the grayish coating you see on radiators actual). 

Edited July 12, 2023 by Nuke