SAS / ASAS vs Manned/Unmanned

[Kaiz0r]

A few weeks ago I started fooling around with KSP and managed to fly around Kerbin, get rockets in orbit, visit the moon and do some docking. But now I found something to which I can't find or figure out an answer.

I have scanned through the first 20 pages in the General section and the How To... section

I was sending a Mun base that needed refueling up in a 3000km orbit around Kerbin. At the time the unit came in orbit it had a total weight of 90t, with the Mk2 lander-can as command pod and 3 SAS units spread acros the height of the craft (1 at the top of the lander, one at the bottom of the lander (so in this case it's in the middle) and one at the bottom of the transit rocket). This unit doesn't use any RCS to "steer" it prograde/retrograde.

But then I send my refuel unit up after it. This is just a unmanned fueltank with some RCS and uses the same base rocket as the MunBase. It is just a bunch of small fuel tanks (2160 liquid cap.) with a small engine for emergency's, RCS for docking, 4 SAS units (two at the bottom and two at the top), 1 ASAS unit and the Probodobodyne OKTO command pod. And this unit doesn't move at all with RCS turned off. And this unit only has a weight of around 35t when docking.

And now the funny part starts. The only difference that I could think of is that one is manned and the other is unmanned, the manned and unmanned command pod have the same torque value, the refueler should have more torque (if more SAS units mean more torque) and the refueler is a lot lighter. So I put a manned command pod on the refuel unit. And all of a sudden it can also turn without RCS.

So I'm trying to figure out how SAS works and how it affects the craft. I cannot find satisfactory answers on the internet or here on the forum. I did learn how gyroscopes can be used to orient stuff in space. And how tilting them creates torque that turns stuff when in space. But the way I read it that is what the ASAS unit does, but the fact that the SAS unit has a torque value suggests that the SAS unit does that, even though they say that SAS is used to stop spinning and nothing more

I would be forever gratefull if anybody could explain how to use SAS/ASAS and why the torque factor seems to be working very well with a clumsy 90t object and doesn't work at all on a more streamlined 35t object.

The units have a Mechjeb 2.0.7 unit and a Romfarer Lazor unit. When testing the manoeuvrability of the craft I used both mods (only 1 at a time of course) and also hand steering and the original ASAS lock position button.

[OUScooby]

I've notice this too. My unmanned space craft seem to react completely differently to my manned one. Even with the RCS on they are so slow to rotate around in orbit and so slow to react when trying to stop it moving. I often overshoot my target and waste RCS fuel trying to slow it and get it back pointed at the target. It's frustrating because I'm trying to construct a large ship in orbit by sending up its pieces as unmanned probes, but docking them is so difficult because they are so slow to react when trying to maneuver it. If I put a manned capsule on any of these ships suddenly with out changing anything else, I'm able to move it around just fine even without using RCS.

[Shrike42]

SAS only stops rotation when it's active, ASAS locks the craft onto the current heading/bearing as best it can. Sadly, you can't have SAS and ASAS active independent of each other, making it hard to use the torque of the SAS module.

Manned modules have far more torque than probes, and it's always on. For unmanned probes, you can sorta get around this by starting with a probe and adding a manned module (which will be empty) to it. For example, the new 1-man lander can will give you a lot of torque even with the ASAS off.

And you can also consider the lander can an Emergency rescue vehicle, with room for one Kerbal. Or, they could simply be the gas station attendant, who gets out and wipes down the windows with a greasy rag while the Kerbals on the active mission listen to the radio and eat snacks.

Does that help?

[Astrobug]

Alright. These parts lack much documentation, so what follows is just my own understanding and YMMV.

Command pods, both manned and unmanned, generate their own torque, though the manned capsules generate a great deal more and are easier to control.

SAS modules only function to kill a vessels rotation, and will not add to the torque used to steer the craft.

ASAS modules maintain a craft's heading at the time it is activated, and will use gimbaling/command pod torque/fins/RCS to do so. To be honest, though, I never seem to have much luck with these, and don't put them on my rockets much. It's probably just the fact I see this as a challenge to my flying prowess.

EDIT: It may or may not have taken me more than 12 minutes to get this posted, why do you ask?

[rhoark]

Looking at the .cfg files, the mk 1-2 pod has 40 times the rotational power of any probe cores (20 vs 0.5)

[stupid_chris]

SAS are sadly not torque wheels, which I really don't understand. I just made the proposition in the development page to add torque wheels since they are really needed.

[Eric S]

Looking at the .cfg files, the mk 1-2 pod has 40 times the rotational power of any probe cores (20 vs 0.5)

This. They all report the same torque values in the VAB, but that value is lying. It's the value of rotPower in the part.cfg file that matters, and for that, all of the manned pods have at least 10 times as much rotPower as any of the probe cores.

[OUScooby]

So is there any way to make an unmanned probe more maneuverable other than adding an empty manned capsule to it?

If there is no other way could I make a duplicate of the probe parts and edit their core values in the config file, would that make them more maneuverable?

Also why did they design the parts this was, is there some real world physics they are trying to emulate? Is this a problem NASA also has to overcome with their unmanned missions, or is this just an oversight in the game design?

[whatisthisidonteven]

So is there any way to make an unmanned probe more maneuverable other than adding an empty manned capsule to it?

If there is no other way could I make a duplicate of the probe parts and edit their core values in the config file, would that make them more maneuverable?

Also why did they design the parts this was, is there some real world physics they are trying to emulate? Is this a problem NASA also has to overcome with their unmanned missions, or is this just an oversight in the game design?

You can use RCS, but yeah, other than that it's incredibly hard to maneuever big unmanned craft.

SAS should provide torque, but it doesn't for some reason.

[AmpsterMan]

I also remmeber reading somewhere that the amount of torque is based on the mass of the command pod/ unmanned probe.

[Designer225]

Yup, the heavier the mass the larger the torque. The torque-to-mass ratio for command pods and probes should be the same.

[OUScooby]

I'm going to try and add an empty manned capsule to unmanned ships that I can just jettison as soon as i no longer need it.

[stibbons]

The torque-to-mass ratio for command pods and probes should be the same.

Out of curiosity I just took masses and rotational power from the wiki and plugged them in to this spreadsheet. There's one or two outliers, but the ratio is similar for most modules.

Problem is that the unmanned probes are just so light. I'd say there's plenty of room for a set of parts that do nothing but add mass and rotPower. Would be handy for large unmanned payloads like mining rigs or habitation modules and the like.

[OUScooby]

I'm still wondering if any knows of a reason the game was designed this way. If I take two ships the first with a manned capsule, the second with an unmanned probe but with enough extra mass that the two ships are equal, the manned ship will be able to maneuver much more easily than the unmanned one.

If this is just an over site in the design of the game I'm prepared to edit the config files to make the unmanned probes easier to fly for the purpose of making casual game play more enjoyable, but if this is an intentional design in the game to make the player engineer a solution around it, then I would feel like editing the file would simply be using a cheat.

[Temstar]

In versions 0.17 and earlier, SAS was indeed a reaction wheel as well as a PID controller (but without the I, so it won't lock you into a heading just like how it works now). When SAS is engaged it's controller will dampen rotation using P and D and the onboard reaction wheel. Back then in the staging list each SAS will actually show up with a "torque" bar underneath to show how much force each SAS's reaction wheel is exerting at any moment, like wise the manned pods also have a torque bar. When you steer you can see all of those torque bars fill to maximum as all the wheels engage. When ASAS is engaged on a ship the ASAS's PID controller overrides any SAS + command pod PD controller and issues commands to all the reaction wheels to keep the ship's heading. When it does this you can see the torque bar fluctuate as ASAS issues turning commands this way and that.

Fast forward to 0.18 and Squad decided to decoupler the SAS reaction wheel control from manual control. Thus nowadays SAS's reaction wheel will only respond to commands issues by SAS or ASAS, it will no longer contribute its torque to manual control input.

Why was it done this way? If I were to guess I reckon Squad thinks that SAS and its powerful (relative to real life) reaction wheel makes it too easy to steer automated crafts if you could just slap some on and use it for manual steering. Since in real life most altitude control are done via RCS systems they wanted us to follow suit and mimic that. The comprise is that for the larger manned crafts one would assume there's enough room to install big reaction wheels (ala ISS) so they didn't touch command pod's reaction wheel and still allow players to steer using those.

Keep in mind almost all spacecraft every flown in real life used RCS system for altitude control. The only exception I can think of is ISS with it's control moment gyroscopes. Even then the ISS CMGs imparts very little torque to the ISS and is only used for holding the space station at a fixed attitude relative to the surface of the Earth. The CMGs also become saturated with stored momentum and must be dumped regularly with the help of RCS system. Thus when you think about spacecrafts in KSP you should really think "why are manned spacecraft with their magical reaction wheels so easy to steer?" rather than "why are unmanned crafts so hard to steer?"

Edited May 30, 2013 by Temstar

[Shrike42]

It is completely intentional. Probes don't need to be big, heavy systems with life support, seats, and snacks. They're designed for small, light craft (typically), but we make them into workhorses. Probes also don't get dizzy or run the risk of death from high G, so having stability systems built in isn't a major consideration. The amount of torque produced, as mentioned, is related to the weight of the command module. I'll agree it'd be nice if we could add extra torque sources (seems like it should be completely possible to mod an SAS unit to provide torque) to help with controlling larger craft, but RCS will also get you turned around more quickly than torque can.

[stupid_chris]

Keep in mind almost all spacecraft every flown in real life used RCS system for altitude control. The only exception I can think of is ISS with it's control moment gyroscopes. Even then the ISS CMGs imparts very little torque to the ISS and is only used for holding the space station at a fixed attitude relative to the surface of the Earth. The CMGs also become saturated with stored momentum and must be dumped regularly with the help of RCS system. Thus when you think about spacecrafts in KSP you should really think "why are manned spacecraft with their magical reaction wheels so easy to steer?" rather than "why are unmanned crafts so hard to steer?"

This is true to the exception of interplanetary crafts. They all use torque wheels. All the satellites sent around other planet or on solar orbits use torque wheels. Or Hubble, for example, uses torque wheels. Basically anything that is supposed to stay in space for extended periods of time.

[Francesco]

<snip>

The CMGs also become saturated with stored momentum and must be dumped regularly with the help of RCS system.

I'm curious about this, what happens when they do that, exactly? it's like they turn off the wheels and have to use RCS to prevent the ISS from spinning due to the release of energy?

Also, I would like to know why Squad has listed all command pods as having max torque = 5.

we know they actually don't, but this can cause confusion to a novice player.

[Temstar]

I'm curious about this, what happens when they do that, exactly? it's like they turn off the wheels and have to use RCS to prevent the ISS from spinning due to the release of energy?

They spin down the wheel. Of course spinning down cause the craft to rotate in the direction of the spin. The RCS system is then used to counter this spin.

[m4nti]

Also, I would like to know why Squad has listed all command pods as having max torque = 5.

we know they actually don't, but this can cause confusion to a novice player.

Because they do have that torque. It's just that torque is related to mass and the smaller pods have smaller mass leading to lower resulting torque.

[CookiezFort]

I have this problem too, I do not wnat to send anybody up for the second part of my space station but with the remote control 'capsules' it does not want to rotate/turn mid air it is very annoying! I guess i will have to send someone else up there

[Commander Zoom]

They don't just "turn off" the wheels, which are of course still spinning (at close to their max rated RPM, that's the problem); instead, they gradually bleed off the speed (energy) by letting them push against the RCS and vice versa. (Action and reaction, good old Newton.) The effect is to trade flywheel velocity and potential/stored/mechanical energy for monopropellant, requiring that the latter be replenished in the next supply run.

EDIT: And I should have refreshed this window since I began my latest play session, 'cause of course Temstar beat me to it.

[Francesco]

I have this problem too, I do not wnat to send anybody up for the second part of my space station but with the remote control 'capsules' it does not want to rotate/turn mid air it is very annoying! I guess i will have to send someone else up there

the solution is to have a command pod on your ship anyway, and either exit the Kerbal(s) inside or use the crew manifest mod, so that you can send it up "empty" but with the benefits of a command pod torque.

Because they do have that torque. It's just that torque is related to mass and the smaller pods have smaller mass leading to lower resulting torque.

if you read the post by stibbons in this very thread, you would see that it's not true.

[Rune]

As things stand now, SAS modules are dead weight. But try stacking small probes in groups of five around your ship: there you go, reaction wheels with actual electrical consumption.

[rkman]

And use RCS to change attitude, just remember to turn it off when done changing attitude so that no RCS fuel is wasted on maintaining attitude.