how to steer an asteroid?

[Laie]

As of last night, one of my vessels is firmly attached to an asteroid.

Turns out that this was the easy part.

The asteroid is heavy, as is the vessel: more than 3000t combined. A bit much for that poor claw, as it seems: the vessel is swaying like a tree in the wind. Not too bad, really, like perhaps by one degree or at worst two. It takes like a minute to complete one back-and-forth motion. So in, which makes it tedious to figure out the average orientation. It doesn't ever settle down, even the tried-and-true method of entering/leaving time warp doesn't do the trick. So in terms of flex and wobble, this is almost a non-issue -- but for purposes of aligning my vessel behind the asteroid's center of mass, it's already way too much. Eventually I settled on "good enough". Which isn't good enough. When I try to apply thrust, things get worse quickly. Much, much worse. I can't really tell whether it starts with my vessel swaying wider or whether the imperfect alignment leads to torque, but pretty soon I have plenty of both.

I can move this asteroid by carefully babysitting it, carefully opening the throttle only when, and for as long as, things are pretty well aligned. Needless to say, the average acceleration is horrible.

Can I do better than that? How?

And, more specifically: I have a few RCS/SAS pods spread around the rock, the main thruster is attached in a pushing configuration. I can't really bend my mind around how the SAS / RCS / gimbals are acting upon the entire vessel, but experiments conducted with pushing matchsticks across a table indicate that all controls on the pushing rocket are actually working against me. Is that right?

Edited November 20, 2014 by Laie

[pellinor]

My experience is that gimbal on a pushing engine helps, but reaction wheels attached to the engine will work against you (by flexing the engine part instead of turning the whole vessel, acting like a gimbal in the wrong direction). If the wheels/RCS are attached to the asteroid separately, they should work fine.

As long as the engine is running, all you need is a gimbal angle larger than your alignment error. To stop the engine from wobbling, it helps to have it close to the claw, with no torque acting on the engine assembly.

Edited November 19, 2014 by pellinor

[KerikBalm]

The largest asteroids are a pain to move.

I found that releasing the pivot on the claw, and "inverting" my control inputs was the best way to keep moving in more or less the direction I want.

If I want to "pitch up", I pitch my ship "down", which actually makes the roid rotate "up".

First I set up the maneuver node, detach, move so that when I face the maneuver node, my ship is pointing at the asteroid, then reattach.

Trying to rotate the whole thing is too difficult, IMO.

The claw connection is too weak for the amount of thrust you want to make course changes. I often find that my gimbaed engines cause the joint to flex so much that I get the opposite of the desired effect.

So, I gimbal my whole craft by allowing it to pivot about the claw.

[RainDreamer]

What I did for my first asteroid, as I was naive and didn't know better to equip more RCS and so on, was to abandon trying to wrangle the asteroid by my little rocket. Instead, I just plot out a node beforehand, release my claw, orient the rocket to where it needed to be pointed at, then switch to docking mode so I won't lose the direction as I maneuver around the asteroid and grab it again, this time with the engine toward the correct end of space. Seems to be much faster and more stable for me. But that was because I didn't have anything else to assist the asteroid turn like you do with other SAS and RCS module attached on the asteroid.

If the asteroid already spin like hell, well, that is a whole another problem.

[Belphegor]

maybe you can stabilize your current situation by strutting the asteroid to the vessel with KAS or that other mod, and correct the imperfect center of mass/thrust alignment with ALT+(direction).

i'm just not sure if you can place struts on an asteroid

[Zylark]

maybe you can stabilize your current situation by strutting the asteroid to the vessel with KAS or that other mod, and correct the imperfect center of mass/thrust alignment with ALT+(direction).

i'm just not sure if you can place struts on an asteroid

You can with KAS, but not directly. First you'll need to plonk down some ground pylons and then attach the KAS strut end points to those. Like so:

First of course get the gimbal on the Claw as close to center of mass of the Asteroid as possible, as when it is strutted up, that is where it'll stay with no sway. And even if pretty much dead on center, it is likely that it actually isn't all that perfectly balanced as one would like it to be. So whatever used to move the 'roid better have some torque power in excess.

And even that may not be enough, so one will have to adjust power output of the engines to achieve a nice stable push. Like this:

Naturally, then you'll also need an Asteroid pusher with an engine in each of the cardinal corners around the center of attachment to the Asteroid.

Edited November 19, 2014 by Zylark

[EdFred]

There's a reason trailers don't attach to the front of cars.

[KerbMav]

There's a reason trailers don't attach to the front of cars.

He means to say: pulling is bettern than pushing - but even that is only half true if you are not "pulling through" the center of mass, I think.

It may be to difficult to aim 100% at the COM of the asteroid, but if your ship has any length at all, I would try using RCS thrusters at its aft, furthest away from the combined COM of asteroid and ship, to have enough leverage.

[cantab]

And, more specifically: I have a few RCS/SAS pods spread around the rock, the main thruster is attached in a pushing configuration. I can't really bend my mind around how the SAS / RCS / gimbals are acting upon the entire vessel, but experiments conducted with pushing matchsticks across a table indicate that all controls on the pushing rocket are actually working against me. Is that right?

This is the approach I used, but then the largest I moved with it was 500 tons.

To start with you need to get the thrust aligned with the centre of mass. If you think you clawed badly off to start with I would declaw, back up a bit, and go in again. Assuming you've clawed as precisely as you can, then if you have an engine cluster you can use thrust limiting to fine-tune it. Turn SAS off, do an engine burn and watch which way the thing pulls, then reduce thrust on the "outside" of the path.

You might also find it helps to turn off SAS and RCS on the engine section. Let the separate pods turn the asteroid and the asteroid turn the engines.

RainDreamer's suggestion, of simply flying the engines round the asteroid into position, is a sound alternative.

On a final note, there's always the chance you've hit a game bug. Multiple claws on a vessel are notorious for this.

[Laie]

To start with you need to get the thrust aligned with the centre of mass. If you think you clawed badly off to start with I would declaw, back up a bit, and go in again.

[...]

On a final note, there's always the chance you've hit a game bug. Multiple claws on a vessel are notorious for this.

That's what I ended up doing. Connecting well-aligned was easy; using the pivot afterwards was nightmarish, even though (or maybe just because) I had to correct only for a tiny amount.

I don't think I ran into a bug as such. The claw just isn't up to the task of holding everything in it's place as neatly as it should. That EdFred thinks of it as a tow hitch speaks volumes.

[NeMeSiS]

He means to say: pulling is bettern than pushing - but even that is only half true if you are not "pulling through" the center of mass, I think.

Using some KAS ropes may help with that. But then you do end up with a large asteroid swaying behind your rocket on a rope, so that may not be the best solution. Its Kerbal though.

[cantab]

In the skyhook thread people have mentioned KAS cables are stretchy. Plenty of people have done stock asteroid pullers. The natural flex in the parts can soak up a little bit of imprecision. On the minus side, if you have engines forward of the centre of mass you need to disable their gimbals to avoid confusing SAS, so you lose a method of control.

[Laie]

In the skyhook thread people have mentioned KAS cables are stretchy. Plenty of people have done stock asteroid pullers. The natural flex in the parts can soak up a little bit of imprecision. On the minus side, if you have engines forward of the centre of mass you need to disable their gimbals to avoid confusing SAS, so you lose a method of control.

If my hunch (and your advice from your previous post) is correct, one should deactivate gimbals either way. In a pusher configuration, gimbals will make the tail swish in the same direction as RCS and SAS. If the latter two are bad, gimbals can't be good, either. I'll report back on that after further testing.

Good trick to steady your vessel: go to Space Center, return to vessel via Tracking Station. This works for me, while timewarp doesn't.

When using the pivot, deactivate SAS on all attached vessels except the one you want to re-align. Not a problem if you only have one vessel and one claw. However, I have a few torque pods spread across the asteroid. They all react to steering inputs, of course -- there would be no point in attaching them if it was otherwise. But when trying to pivot one particular claw, the torque from the pods is getting in the way / creates wobble / leads to unexpected behaviour. Life became a whole lot easier when I shut down their reaction wheels.

The above allowed me to get a much better alignment; this alone already allows for a tenfold increase in thrust I may bring to bear. That purple dot in the target marker has a diameter of perhaps 1-2 degrees, which is *a lot*. One really wants to hit it dead center. After much squinting, I remembered the magnifier. Advanced navball or similar might also help, I don't know if they do.

Edited November 19, 2014 by Laie

[FunnyBunny14]

I think you forgot to lock thr pivot. It would explain what you are experiencing.

[Laie]

I think you forgot to lock thr pivot. It would explain what you are experiencing.

When locked, it provides resistance to bending and will (absent any other forces) return to it's set position; but the claw is the weakest link and will act as pivot even when locked. This becomes ever more apparent the bigger the rock and vessel is (not a great insight: stronger forces make it bend more, who'd have thunk?).

In order to give you and idea of the kind of forces I'm dealing with, see this picture. That's a lot of thrust and a long lever -- of course I'm having problems.

You might also find it helps to turn off SAS and RCS on the engine section. Let the separate pods turn the asteroid and the asteroid turn the engines.

This helped a lot, and here's why:

It's not necessary to turn off all SAS on the pusher: leaving some of it active still makes the whole thing more maneuverable. But the rock should have more torque than the pusher.

Important hint: If your asteroid redirect mission as flexible as mine, and the whole assembly is yawing left, you should turn off some engines on the right. This will swivel the pusher relative to the CoM, so that the whole pusher acts as a gimbal and brings the rock around in the direction you want it to go.

---

Edit to add (I'm just tossing in everything I notice while doing this, for the sake of anyone who finds this thread):

As of this writing, the claw is still a powerful magnet for bugs and glitches. I've encountered a few during my mission. Save early, save often, make use of named savegames. The more friendly bugs will destroy your craft then and there; there's also more insidious one where the orbit of your vessel is no longer updated properly. When your orbit changes without your input, you caught one of these. Most of the time, the problem will go away when you quit and restart the game, but I had a variant that was peristent between savegames.

Edited November 24, 2014 by Laie

[RexKramer]

What I did for my first asteroid, as I was naive and didn't know better to equip more RCS and so on, was to abandon trying to wrangle the asteroid by my little rocket. Instead, I just plot out a node beforehand, release my claw, orient the rocket to where it needed to be pointed at, then switch to docking mode so I won't lose the direction as I maneuver around the asteroid and grab it again, this time with the engine toward the correct end of space. Seems to be much faster and more stable for me. But that was because I didn't have anything else to assist the asteroid turn like you do with other SAS and RCS module attached on the asteroid.

If the asteroid already spin like hell, well, that is a whole another problem.

This is exactly how I maneuver large asteroids. Even very large asteroids are fairly easy to manage this way. Create a node, maneuver around the asteroid so the maneuver node and asteroid COM are aligned, attach, and thrust as required.

Here's a suggestion to get rid of unwanted rotation you induce as you thrust. Reduce thrust to zero, turn off RCS and SAS, and time warp briefly. Time warp stops rotation magically. It helps to stop rotation before you detach to maneuver around the asteroid, trying to reattach to asteroids which are rotating can be a challenge.