Questions About RCS For Scifi...

[Spacescifi]

Why is it that in reality RCS thrusters seem to usually be just compressed gas (like mere puffs of white smoke) as opposed to the flaming exhaust we see coming out main engines?

I would think what you want most out of RCS is enough thrust to change your vessel's roll, pitch, or yaw attitude in mere seconds.

I suppose you do not need to push out the propellant as fast as possible so that is why compressed gas thrusters are used as opposed to combustion rocketry for RCS thrusters.

Also possibly that is easier to pulse fire gas thrusters than bi-propellants with oxidizer.

What you want for RCS is either a monopropellant with the oxidizer already mixed in, or compressed gas thrusters to avoid the whole issue of that.

 

So maybe that is why we don't see many flaming plumes out of RCS in real life.

 

In media science fiction we probably see them more because show/movie directos think it looks cool without giving regard to the mechanics of it all.

Thoughts? Am I right? Close? Or way off?

 

[Vanamonde]

Since RCS requires less thrust they can get away with arrangements which are less complex and less massive, so they often just release compressed gas or spray a substance onto a catalyst. It's one fuel instead of 2 and there's less waste heat to deal with. 

[Spacescifi]

1 hour ago, Vanamonde said:

Since RCS requires less thrust they can get away with arrangements which are less complex and less massive, so they often just release compressed gas or spray a substance onto a catalyst. It's one fuel instead of 2 and there's less waste heat to deal with. 

My guess is if you are using a catalyst to ignite your propellant... it's probably more thrusty than compressed gas pound for pound.

[farmerben]

Hypergolics work good.  That's what was mainly used on the Apollo lander.

[AckSed]

There is a whole forest of ways to power a small RCS thruster. Vast, though, is looking to use ethane and nitrous oxide for its space station RCS, which I think is clever: nitrous oxide is reasonably safe, storable, and decomposes like peroxide when passed over a catalyst bed to create hot oxygen and nitrogen.

[DDE]

4 hours ago, Spacescifi said:

as opposed to the flaming exhaust we see coming out main engines?

Do we? In vacuum, they produce mostly-transparent plumes miles in size, e.g. the Apollo 8 TLI.

Whereas in an atmosphere, RCS of comparable design looks like a rocket engine, e.g. syntin-oxygen.

http://buran.ru/images/gif/rsu.gif

So, basically, no, there is no qualitative difference between RCS and chemical main engines. This video, I believe, gets plume effects on the ship accurate, less so for the missiles.

 

Edited December 23, 2023 by DDE

[KSK]

I did a quick search for 'Shuttle RCS firing', since the Shuttle is one of the heavier spacecraft launched to date, so I figured that its RCS would be correspondingly more powerful. I found this image which isn't exactly plumes of fire territory but it's a bit more than a puff of white vapour. Also its from NASASpaceFlight, so should be legit.

Unless you absolutely need to make rapid attitude changes (say, for a Moon lander or something), I don't think there's any particular advantage in being able to do so, especially for heavy spacecraft. Faster attitude changes require more powerful RCS which typically means carrying more propellant for that RCS, particularly since you need propellant to both start and stop your maneuver. 

[kerbiloid]

The RCS burn rarely lasts for a second, and its thrust (thus, its fuel consumption and exhaust production) is many times lower.

So, it's just starting to burn, but immediately gets extinguished, and the burn turns into smoke..

[Spacescifi]

8 hours ago, KSK said:

I did a quick search for 'Shuttle RCS firing', since the Shuttle is one of the heavier spacecraft launched to date, so I figured that its RCS would be correspondingly more powerful. I found this image which isn't exactly plumes of fire territory but it's a bit more than a puff of white vapour. Also its from NASASpaceFlight, so should be legit.

Unless you absolutely need to make rapid attitude changes (say, for a Moon lander or something), I don't think there's any particular advantage in being able to do so, especially for heavy spacecraft. Faster attitude changes require more powerful RCS which typically means carrying more propellant for that RCS, particularly since you need propellant to both start and stop your maneuver. 

For a large SSTO freighter you actually do need to make fairly quick attitude changes if you wanna land right.

[DDE]

1 hour ago, kerbiloid said:

The RCS burn rarely lasts for a second, and its thrust (thus, its fuel consumption and exhaust production) is many times lower.

Satellite RCS tends to be built exclusively with pulsed thrust capability, it can't sustain a continuous burn. Instead it's rated for a million solenoid valve activations.

[KSK]

2 hours ago, Spacescifi said:

For a large SSTO freighter you actually do need to make fairly quick attitude changes if you wanna land right.

I wondered when those were going to turn up. And yes, you probably would, which is exactly why I mentioned needing rapid attitude changes, say for a Moon lander.

[Scotius]

Probably no one is keen of dealing with multiple small combustion chambers placed around the ship. Each coming with its own risk of a RUD happening while burning. Cold gas is usually safer, methinks.

[Spacescifi]

2 hours ago, Scotius said:

Probably no one is keen of dealing with multiple small combustion chambers placed around the ship. Each coming with its own risk of a RUD happening while burning. Cold gas is usually safer, methinks.

 

Very true. Starship uses cold gas thrusters mainly for fine attitude control in space and reentry.

In atmosphere it actually uses main engines to flip it's attitude rapidly, since cold gas thrusters are not really designed for as rapid a response. The plan I read is to switch to hot gas thrusters using methane once Mars is industrialized (mars has methane and oxygen but little or no nitrogen).

For a scifi spaceship that travels all over, what you want is versatility, but in so doing you sacrifice the power of specialization.

 

What that means is that an explorer space SSSTO would probably rely on hot or cold gas thrusters for space maneuvering, and be slower at pitch, roll, and yaw than an orbital low orbit planetary defense fleet that regularly refuels from the planet below.

In other words, an orbital defense fleet could afford to use more thrusty RCS thrusters because they always will have a place nearby to refuel from (the planet below). Which means they could dodge weapons fire better than a vessel that was designed with versatility and space wildnerness refueling in mind.

Edited December 24, 2023 by Spacescifi

[kerbiloid]

8 hours ago, Scotius said:

Probably no one is keen of dealing with multiple small combustion chambers placed around the ship. Each coming with its own risk of a RUD happening while burning.

https://howthingsfly.si.edu/media/shuttle-reaction-control-system#:~:text=A Reaction Control System (RCS,direction or combination of directions.

Spoiler

 

[JoeSchmuckatelli]

@kerbiloid Notice the use of effective countershading.  The hunter, looking up from below will only see the blackness of space whereas the hunter from above will see the craft blending in with the bright albedo of the planet. 

The small combustion chambers are hidden in the black sections to disguise their existence 

[Scotius]

Considering that allegedly Shuttle's RCS activation could be detected from as far as asteroid belt, this wouldn't help much.

There is no stealth in space.

[kerbiloid]

22 hours ago, JoeSchmuckatelli said:

The small combustion chambers are hidden in the black sections to disguise their existence 

They were just too lazy to wash it every time from soot, so they made it out of soot.

Also, this pipe organ should loudly whistle in the incoming air flow, so any acoustic station would notice its presence once it's in air.

19 hours ago, Scotius said:

There is no stealth in space.

The stealth wants you to think so. It's a stealth, how do you know if it is or isn't.

Edited December 26, 2023 by kerbiloid

[Scotius]

4 hours ago, kerbiloid said:

The stealth wants you to think so. It's a stealth, how do you know if it is or isn't.

Only way to be sure? Wide spectrum cameras covering full sphere around the craft. And laser\maser emitters firing beams randomly in all possible directions as fast as possible. Unless potential enemy is 100% invisible to all wavelengths of EM spectrum, eventually it will be hit and it will be registered by cameras.

But then, you can't be sure that whatever you looking for isn't partially shifted to another dimension - which would mean laser beams would go through it without interacting.

[kerbiloid]

11 hours ago, Scotius said:

Unless potential enemy is 100% invisible to all wavelengths of EM spectrum, eventually it will be hit and it will be registered by cameras.

Like this, but Vorlon.
 

Spoiler