Why won't my asteroid budge?

[Navel lint]

Hi,

So I caught my first asteroid today, but only to discover I wont budge. When I make a waypoint and fire up my engines, the delta-V meter on the naveball doesn't move a single tenth of a m/s, even when I give my engines full throttle for some time.

It's a class C @ 30 tons. My grabbing vessel is weighing 5 times as much and moved just fine up until the point where I attached it to the rock.

I'm thinking part of it could be because my engine exhaust hits the rock making the entire system like a man blowing on the sails of his own sailboat, but even if that was at play, I should still see move movement, right...? Well I don't. Are some asteroids bolted to the firmament in an extra dimension we can't perceive? Or did I do something wrong?

Here's a screenshot of the thing:

https://photos.app.goo.gl/vD7ZYohDKMoBMdSq9

[Geonovast]

Simple: Your engines are too close to the asteroid.

Your engines are pushing you forward, but the exhaust is actually hitting the asteroid and causing it to pull you backward with the same amount of force.  So, you go nowhere.  So yes, you're powering a sailboat by blowing on the sail.  In real life you might still get some movement from this, but not in game.

When moving an asteroid, you need one of three things:

• To push it, so there's nothing behind the engine.
• To pull it, but with the engines far enough out so that the asteroid isn't in the way.
• To pull it, but with the engines far enough forward that the game no long considers the exhaust from the engines.

Edited January 8, 2019 by Geonovast

[MaverickSawyer]

I'm going to have to agree with Genovast here... I've almost always used a pusher configuration when going asteroid wrangling...

Figure 1: Solar electric asteroid tug performing a burn

 

Figure 2.1: Nuclear electric asteroid tug orienting for burn

 

Figure 2.2: Nuclear electric asteroid tug performing a burn

 

The other option, which I've personally never attempted, is to use either a long standoff truss or KAS winches and "tow" the asteroid behind your ship. I'm sure, however, that it'd be a truly epic sight to behold.

Edited January 8, 2019 by MaverickSawyer

[Kryxal]

I could see using RCS for maybe minor adjustments here if you have it ... I'd hate to do it that way, but given the engine and claw positions, what alternative is there without a new craft?

[Zhetaan]

@Navel lint:

First, welcome to the forum!

Second, the others are correct; your engines are 'way too close to the asteroid.  The way the game calculates thrust and impingement and all that makes it so that if a part interferes with the engine exhaust, then all of the thrust is cancelled, so indeed you will see zero motion.  There's no half-measure where it takes into account cross-section or anything like that.  Normally, rocket parts have such low thermal mass that they burn up and thereby get out of the way (this is explosive decoupling if done deliberately), but asteroids have a bit more tolerance.

Your design can be improved simply by keeping the drills where they are and moving the engines to the front.  Obviously, you'll need to design to avoid burning your solar panels and radiators, and you may need another fuel tank as a spacer to ensure that the exhaust plumes really don't impinge on the asteroid--it's surprising just how long the exhaust plume is.

Alternatively, as others have said, you can improve it by keeping the engines where they are and moving the drills and grabber to the front.  In that case, you'll need to avoid bumping your solar panels.

Personally, I favour puller designs over pusher designs because pullers are easier to correct if you don't aim precisely through the centre of mass of the asteroid.  Pushers tend to fall into positive feedback more easily, though that can be corrected by enough RCS nozzles or reaction wheels.  I do not, however, like designs that splay the engines because the cosine losses are incredible.  I'd do it if it were the only way, but it's not.  Straight engines on outriggers are okay if the outriggers can take the strain, but I mostly move Class E asteroids, which makes that impracticable.

[Aegolius13]

You can also angle the engines so the rocket exhaust clears the asteroid,  but you lose a bit of effective thrust (proportional to the cosine of the angle,  hence called "cosine losses"). But this doesn't work so well on really wide asteroids. 

[Kryxal]

I tend to use multi-engine pushers, so long as the CoM is framed by a box defined by the engines you can adjust thrust output individually to compensate.  That time I took a class-E to Moho, I was limited by the refinery processing rate anyway...

[Rocket In My Pocket]

I like Pullers for the added stability, you just have to get far enough from the asteroid with an "arm" or clear it's width with engine sponsons. 

[Navel lint]

Hi all,

Thank you for all the answers. It appears you are right. The rock is not fixed to the firmament with 4-dimensional bolts after all. I tried moving the asteroid with my RCS, and it does in fact move the rock. I suppose I could have tried that before posting. Doh... Sorry.

[GoatRider]

I use a pusher ship with integrated miner. As long as I make sure I am aimed right at the CoM when I grab, it stays stable when pushing. A generous number of vernier thrusters takes care of any residual torque. The only problem I've ever had was with the kraken when trying to push around an E sized asteroid. I finally figured out I can't use the trick where you cancel rotations by briefly going into time warp. When I do that, physics easing will sometimes lock the auto-struts too short, bending my ship out of shape which then explodes.

[Superfluous J]

Note this happens with ships too, just you don't tend to build this way. Asteroids aren't special in this.