Questions about two parts

[Streetwind]

I know that KSP, coming out of a sandbox game mode, rarely ever features redundant parts. They all have some sort of niche, some use case where they are ideal. That said, there are two items that have me scratching my head: the advanced inline stabilizer, and the stack separators.

The advanced inline stabilizer is a 1.25m reaction wheel part. However, there is another 1.25m reaction wheel part: the inline reaction wheel. This other part comes earlier in the tech tree, and has exactly the same stats as the advanced variant... with one difference: it is lighter. So I am asking myself: if I have two parts that are mounted in an identical fashion and perform an identical function with identical stats, except that one of the two is lighter and researched earlier than the other - why do I need the other part at all? What does the inline advanced stabilizer do that the inline reaction wheel doesn't, that justifies its existance with higher weight and more difficult access?

Similarly, the stack separators. In this case, I do understand where they function differently than decouplers - they eject both sides away from them instead of just one side. That still leaves the question though, as to why I would need that? A normal decoupler is lighter, comes much earlier in the tech tree, and achieves the same end result: cutting the rocket in half. All the separator does is add another part of space debris into the mix, because it floats off on its own, and that's bad. So where is the use case for stack separators?

Thanks in advance for taking the time to enlighten me!

Edited February 1, 2014 by Streetwind

[Awaras]

The advanced inline stabiliser also adds ASAS functionality to your craft if it doesn't have a command part that has it (holds your craft's orientation when you press 'T'). *edit* My bad, that was before they changed how it works...

As for the stack separators, sometimes you want to separate two things from each other without leaving the decoupler ring stuck to one of them. Say if you make a lander that drops a rover after landing for example.

Edited February 1, 2014 by Awaras

[Geschosskopf]

You're correct about the SAS units. There's currently no reason at all to use the heavier version. It became redundant a couple versions back with the major overhaul of SAS.

As to the difference between stack separators and decouplers, the primary one is that the blue ones apply no repulsive force to either stage whereas the yellow ones push in the direction their red arrows point. Depending on the application, you might or might not want that extra kick. The difference is best seen when you fire them while the rocket is coasting. When you fire a yellow one, the 2 parts of the rocket will move apart from each other. If you fire a blue one, it looks like nothing happens at all until you yaw or pitch the rocket, at which point it just falls apart and the rotating part will kick the dropped parts away.

So, the yellow ones are great for dropping stages during ascent, because they help get the dead boosters out of the way quickly so they have less chance of damaging the rest of the rocket. But the blue ones are great when maintaining your position is critical, or the added speed from the separation would be bad. Applications include dropping probes in geosynchronous orbits, ditching an orbital maneuvering engine to expose a docking port after you've used the engine to rendezvous closely with the target, and separating a rover from a lander.

[Awaras]

...ditching an orbital maneuvering engine to expose a docking port after you've used the engine to rendezvous closely with the target.

You can just place the engine directly on the docking port and simply undock it when you don't need it anymore.

[Tex_NL]

You can just place the engine directly on the docking port and simply undock it when you don't need it anymore.

True, but what if it does NOT have a conveniently located docking port?

[Kasuha]

Earlier in the game, IAS provided more advanced functionality than SAS. After change in implementation, they provide the same function, just IAS is a bit heavier (and maybe more visually pleasing). But mostly yes, it's a redundant part now.

Stack separators are good if you want to separate two things from each other (e.g. two probes) without leaving remnant of the separation hardware on either. And if you don't care about leaving a bit of space debris behind.

[Streetwind]

Ah, I see. That makes a lot of sense, especially about the SAS thing. Thanks!

[Claw]

As to the difference between stack separators and decouplers, the primary one is that the blue ones apply no repulsive force to either stage whereas the yellow ones push in the direction their red arrows point. Depending on the application, you might or might not want that extra kick. The difference is best seen when you fire them while the rocket is coasting. When you fire a yellow one, the 2 parts of the rocket will move apart from each other. If you fire a blue one, it looks like nothing happens at all until you yaw or pitch the rocket, at which point it just falls apart and the rotating part will kick the dropped parts away.

So, the yellow ones are great for dropping stages during ascent, because they help get the dead boosters out of the way quickly so they have less chance of damaging the rest of the rocket. But the blue ones are great when maintaining your position is critical, or the added speed from the separation would be bad. Applications include dropping probes in geosynchronous orbits, ditching an orbital maneuvering engine to expose a docking port after you've used the engine to rendezvous closely with the target, and separating a rover from a lander.

I'm pretty sure the stack separators also provide some ejection force. It's just that it applies it in both directions and leaves a ring in the middle, instead of ejecting one side while staying attached to the other.

EDIT: Yeah, I just tried it. Placed to Mk1 Command Pods heat shield to heat shield with aTR-18D stack separator connecting them. Laid it on it's side, then launched it. Upon separation, both command pods were ejected equal distance away from the separator. The separator was left standing on its side, completely unmoved.

I repeated it with a lander can, just so no one can claim they moved because of the angled side. Again, they slid apart and let the ring standing in the middle.

Edited February 1, 2014 by Claw

[Taki117]

Ah, I see. That makes a lot of sense, especially about the SAS thing. Thanks!

The only reason the IAS was left in the game was for backwards compatibility, basically it allowed older craft that used the IAS to still be used after the update.

[Jimmy Jazz]

Actually, I think it is not possible to eject only one side due to the action-reaction law.

[Silverchain]

Actually, I think it is not possible to eject only one side due to the action-reaction law.

Kinda depends; you could have an ejector that simultaneously fired small fast projectiles at whatever rearwards angle would allow them to miss the ship. Or dumped water or sand or something like some rocket launchers do...

[Jimmy Jazz]

Kinda depends; you could have an ejector that simultaneously fired small fast projectiles at whatever rearwards angle would allow them to miss the ship. Or dumped water or sand or something like some rocket launchers do...

As I understood, you are suggesting use of an equivalent force with an opposing vector to

anul the effect? Yes, that would do, but it's a bit off topic here, I think.

[Claw]

Actually, I think it is not possible to eject only one side due to the action-reaction law.

By "ejecting only one side," it is meant that the decoupler stays with one portion of the rocket, while the other side is clean. "Ejecting both sides," means the ring is left as a separate, third part between the two (now separate) body pieces.

In either case, if the ejection force is the same, the two newly separated body pieces will move away from each other at the same relative velocity. Perhaps that's what you're referring to.

[Jimmy Jazz]

By "ejecting only one side," it is meant that the decoupler stays with one portion of the rocket, while the other side is clean. "Ejecting both sides," means the ring is left as a separate, third part between the two (now separate) body pieces.

In either case, if the ejection force is the same, the two newly separated body pieces will move away from each other at the same relative velocity. Perhaps that's what you're referring to.

Yes. I obviously misunderstood you there.

[Claw]

Yes. I obviously misunderstood you there.

Sorry if I was unclear!

[Rickenbacker]

I use stack separators when I want my launcher to have a docking port on top, so that I can dock with it later and use any remaining fuel for something else. That way the docking port is free after decoupling. I also use the big stack separator instead of the big decoupler, because it looks a LOT better underneath a 3-man capsule, being so much thinner. And since I only use that solution for capsules reentering the atmosphere, I don't worry about the debris.

[Supernovy]

I think stack separators are good for decoupling a lot of surface attached parts at the same time, as they separate things radially attached to them as well as stack attached. I've used them with docking ports before, but because of the docking port's "decouple node" function, they're not all that useful.

One situation they may be useful in is when you have two engines attached to each other nozzle to nozzle. A decoupler would block the thrust of one of the engines after separation, but a separator wouldn't.

[breakthrough]

I use the advanced stabilizer to balance the center of mass on aircraft.