Using RCS only for translation

[Joshua A.C. Newman]

I'm building a dramatically pared-down vessel* on which I want to carry an absolute minimum of mass. Because it's very small, there's no issue at all with rotational torque; I use a tiny reaction wheel, and that's fine. But I need to translate in order to line up my docking*, and I need RCS for that. The issue is that RCS fires when I'm trying to rotate to "help", which not only applies more torque than I need, but also wastes an extremely limited resource.

You know how, when you set up control surfaces on wings, you can set them up to only generate pitch/yaw/roll? Can you do something similar with RCS?

*^{_{"Vessel" is an overstatement. It's a missile, designed to use the Klaw to attach an engine to dangerous debris so that I can deorbit it.}}

Edited January 2, 2016 by Joshua A.C. Newman

[Rocket In My Pocket]

Idea 1: Push "R" before and after you push "Q" or "E".

Idea 2: Leave the Reaction Wheel at home and bring more RCS fuel.

Idea 3: Do all your rotating before you do the translating.

Idea 4: There may be different keybinds for you could utilize to achieve this, or perhaps some custom staging/action groups

Idea 5: Place the RCS thrusters in such a way that they can't assist with rolling maneuvers.

Idea 6: Forget the whole idea and just delete the debris from the tracking center/turn debris allowed down to zero.

That's all I got...best of luck!

Edited January 2, 2016 by Rocket In My Pocket

[Phil deCube]

30 minutes ago, Joshua A.C. Newman said:

You know how, when you set up control surfaces on wings, you can set them up to only generate pitch/yaw/roll? Can you do something similar with RCS?

You could use some of these. If you put 4 around your craft, and a couple pointing up and down, you would be able to go left, right, up, down, forward and backward, but not rotate.

Edited January 2, 2016 by Phil deCube

[Joshua A.C. Newman]

 

45 minutes ago, Phil deCube said:

You could use some of these. If you put 4 around your craft, and a couple pointing up and down, you would be able to go left, right, up, down, forward and backward, but not rotate.

I can't believe I didn't think of that! Thank you!

Edited January 2, 2016 by Joshua A.C. Newman
Forgot to quote

[Snark]

8 minutes ago, Joshua A.C. Newman said:

I can't believe I didn't think of that! Thank you!

Just be careful to make sure that if you try this, you really have zero rotation authority and not a tiny-but-not-quite-zero amount.  Otherwise, it can go berzerk trying to use massive amounts of RCS propellant to try to rotate by very small amounts.

My own solution to this is generally to toggle RCS off when I want to rotate, then toggle it back on when I'm facing the way I want.

[Joshua A.C. Newman]

Well, toggling it is what I'm trying to avoid. It's another thing to screw up at precisely the wrong moment. But I don't think I can guarantee zero, since that would require me to know where the center of mass is after I've burned a variable amount of fuel.

Huh. I could use TAC Fuel Balancer, actually, split the tank in half, and design it so the CM is at the joint between the tanks.

[KingMonkeyNuts]

14 minutes ago, Joshua A.C. Newman said:

Well, toggling it is what I'm trying to avoid. It's another thing to screw up at precisely the wrong moment. But I don't think I can guarantee zero, since that would require me to know where the center of mass is after I've burned a variable amount of fuel.

Huh. I could use TAC Fuel Balancer, actually, split the tank in half, and design it so the CM is at the joint between the tanks.

Also, this mod (RCS build aid) might help too, if you're not using already http://forum.kerbalspaceprogram.com/index.php?/topic/33124-105-rcs-build-aid-v076/

[gliese-aerospace]

I'm surprised no one has mentioned MechJeb — there's a module called "smart RCS" which disables RCS for rotation and enables it on the fly when you need translation authority. Try that, maybe?

[Joshua A.C. Newman]

Huh. Thanks! I avoid MechJeb (and intend to continue to, up to the point where there's enough going on and we're good enough at it that it reduces tedium), but if it works around this ridiculous problem, I can install it for the one feature. Thanks!

I use RCS Build Aid all the time! It was such a relief when I discovered it. 

[Snark]

7 hours ago, Joshua A.C. Newman said:

I use RCS Build Aid all the time! It was such a relief when I discovered it. 

RCS Build Aid is an undeniably nifty tool.  However, it's worth remembering that there's a very cool stock feature of the game that lots of players don't know about, which makes RCS Build Aid less critically necessary.  The TL;DR:  just hit caps lock to turn on fine control mode when you're docking, it magically makes your RCS smarter and compensates if your RCS thruster distribution isn't centered on your CoM.  It not only makes docking and ship design easier, it saves scads of monopropellant.

More details here, if you're interested.

Edited January 2, 2016 by Snark

[Snark]

8 hours ago, Joshua A.C. Newman said:

Well, toggling it is what I'm trying to avoid. It's another thing to screw up at precisely the wrong moment.

That's fine, whatever works for you. A few points to bear in mind about toggling:

• It isn't something you have to do a lot.   Typical docking sequence looks like:  1. Rendezvous and kill orbital velocity. 2. Rotate craft to correct orientation. 3. Toggle on RCS and dock.  Step 3 is the only one that needs any RCS thrust, and you only have to turn on RCS once.  Sometimes there's a step 4, "realize your orientation isn't quite correct, toggle RCS off momentarily to adjust", but that's not always, and it's generally just once.
• If you forget once in a while, it's not like the end of the world-- RCS maneuvers are slow slow slow, and gentle, and you haven't lost much for a momentary goof.
• The alternative, which is trying to aim for zero torque authority, is difficult to do right and can fail catastrophically if you're wrong (see below).

 

8 hours ago, Joshua A.C. Newman said:

But I don't think I can guarantee zero, since that would require me to know where the center of mass is after I've burned a variable amount of fuel.

That's not what I meant.  I'm not saying that your RCS thrusters have to be perfectly balanced around the CoM.  In fact, they don't have to be!  Just turn on caps lock and they magically balance themselves (a lot of folks don't know this); it's one of the best-kept secrets of the stock game.

No, what I'm saying is that you have to have zero torque authority, which is a completely different thing.  I'm saying that to use @Phil deCube's suggestion, you need to be very sure that you've designed your RCS so that it's unable to induce rotation.  Put another way:  It's okay if your RCS has perfectly zero torque authority, as Phil suggests.  It's okay if it has plenty of torque authority, as in a conventional RCS setup.  What you need to watch out for is a setup that has nearly but not quite zero torque authority, which is what you're likely to end up with if you try for zero and don't quite make it.

Here's an illustration of what can go wrong:

Let's say you're building a ship and try the "can't rotate" approach we're talking about.  You put one linear port on each side.  But you can't do that on the back, because the darn engine is in the way.  "Oh, that's not a problem," you say.  "I'll just put two ports, one on each side of the engine.  Nice, symmetric, and no rotation."  Right?

Well, the problem here is that they can rotate.  For example, if the RCS port on the left fires but the one on the right doesn't, then the ship will rotate to the right.  It would be a really stupid way to try to rotate the ship, but it would do it.  And there's the problem:  SAS is really stupid.  If it decides "I want to rotate the ship this way", it will use whatever means necessary, even if those means are incredibly wasteful and counterproductive.  Let's say you have the above ship, and SAS decides that it needs to rotate the ship slightly to the right.  None of the other RCS ports anywhere on the ship can do it, since they're all perfectly aligned on the CoM, so SAS decides it'll accomplish that rightwards rotation by firing just that one port on the left.  But that's insane, because firing that port induces only a tiny bit of rotation, but a lot of translation.  Since it induces only tiny rotation, it means you have to really fire it a lot to accomplish a small amount of rotation, and that's exactly what SAS does.  it goes berzerk.  Suddenly your ship is taking off like a steamkettle and you have no idea why.

The moral of the story is that it's dangerous to have an RCS setup that can do only a little rotation, because SAS cares only about the rotation it needs, not the rotation power it has. If you reduce the torque authority of your RCS to try to save fuel, it accomplishes the exact opposite of what you want:  it makes it so that SAS has to spend a lot more monopropellant to accomplish the same rotation, and it cheerfully does so.  And the closer your torque authority goes to zero, the more nuts your SAS will go.

So basically, you have two realistic options:

1. Give your RCS lots of torque authority, by placing them as far from the axis of rotation as possible.  This won't stop them from using fuel, but at least they will use less fuel when SAS fires them, because less monopropellant will generate the same torque and therefore SAS will use less of it.
2. Make SAS use genuinely zero monopropellant for rotation.  And the only reliable way to do this is to turn off RCS completely when you're rotating, unless you have some mod that can do what you want.

[Plusck]

The most important thing to do, IMHO, is never use rotation while you're on a docking approach.

You shouldn't need to rotate at all, as long as the other docking port is pointing at you and isn't itself rotating.

Just make sure you use RCS translation to push the target prograde marker to the other side of the target as you approach. Rather than you needing to rotate to aim at the target, the target will slide back over your crosshairs.

[Joshua A.C. Newman]

Thanks, Snark! Good point. It might make sense to put extendable arms out to make the torque seriously nonzero, since this is a really tiny object.

Plusck, part of the issue here is that I'm not approaching a docking port (see the image at the top). I'm using the Klaw, often in the dark, and have to be able to hold it on for the half second (or whatever it is — why doesn't anyone know the actual specifiation for the Klaw?) as I'm bouncing by a 5m/s.

[Snark]

11 minutes ago, Joshua A.C. Newman said:

Plusck, part of the issue here is that I'm not approaching a docking port (see the image at the top). I'm using the Klaw, often in the dark, and have to be able to hold it on for the half second (or whatever it is — why doesn't anyone know the actual specifiation for the Klaw?) as I'm bouncing by a 5m/s.

The same principle applies.  A few things to bear in mind:

• Dark, shmark.  That's what lights are for.  Mount a couple of lights (the broad-beam short-range ones, not the narrow spotlights) on your Klaw ship, facing forward.  Saves a lot of hassle.
• Why are you bouncing by at 5 m/s?  That's like 20 times too fast.  Attaching with the Klaw is just like docking:  you set up an orbital rendezvous, fine-tune your approach as you get near, come to a dead halt relative to the target when you're a few meters away.  None of those operations require any RCS at all, and they work just fine even in total dead darkness-- all you need is the target marker and target-relative-velocity-retrograde marker on the navball.  After you come to a dead halt a few meters away, then you approach at centimeters per second, not 5 m/s.
• Note that you don't have to have your Klaw lined up perfectly on the CoM of the target as you approach.  Pretty close is good, and sure, perfect is great if you can manage it.  But it's okay if you're a little bit off, because after you grab hold, you can unlock the Claw joint and rotate your ship (while grabbed) until you're perfectly aligned on the CoM, then re-lock the joint.

Which raises the larger question.  Why do you need any RCS at all in this situation?  Docking with docking ports requires RCS, because you need to latch on to a very specific object in a specific location at a specific orientation, so you really need the ability to adjust your sideways position.  With the Klaw, on the other hand,  it's much less critical; you can grab on pretty much anywhere in any orientation.  So all you need to do is approach to within a few meters, come to a dead halt, rotate to point at the CoM, and thrust forward by a few centimeters per second to complete the connection.  None of that requires any RCS at all.

Edited January 3, 2016 by Snark

[Plusck]

3 hours ago, Joshua A.C. Newman said:

Plusck, part of the issue here is that I'm not approaching a docking port (see the image at the top). I'm using the Klaw, often in the dark, and have to be able to hold it on for the half second (or whatever it is — why doesn't anyone know the actual specifiation for the Klaw?) as I'm bouncing by a 5m/s.

Ah ok - I don't know why but I totally missed the klaw info. The Klaw is a bit of a pain, tbh. I don't know if it's me (most likely) or a common issue, but controlling from the Klaw always seems to end up being "off" to a degree that the docking port isn't, so I end up sliding over the target rather than hitting it dead on.

[Waxing_Kibbous]

* I just want to second that I'd like rotation to be non-RCS, or at least as a  toggle-able option- it seems like when RCS is helping with rotating my approach speed/angle get buggered as well.

Edited January 3, 2016 by Waxing_Kibbous

[Snark]

1 hour ago, Waxing_Kibbous said:

it seems like when RCS is helping with rotating my approach speed/angle get buggered as well.

Not if you turn on fine control mode. 

But yes, it would be nice to be able to turn off rotation.

Edited January 3, 2016 by Snark

[Joshua A.C. Newman]

Ah! Everything I was reading said you needed to hit with the Klaw with force! That makes things much easier!

Edited January 4, 2016 by Joshua A.C. Newman