How far in front of an asteroid must the tug be?

[Justin_Kerbal]

Finally caught up with this thing just to find out that I could fire at full thrust and make no change in orbit.  I assume these nukes need more room to operate.  Does this seem too close?

[Aegolius13]

Yeah, that does seem too close.  I think it takes around 2 orange tanks' length between the back of your engines.  But there's a pretty easy way to test.  Sub out the Klaw on your ship for something wide enough to get behind the exhaust plumes, like some 3.75m tanks.  Then launch it.  You can see if those parts heat up, or you can warp it to space and see if the ship generates thrust.  If so, it should work just as well with the Klaw grabbing an asteroid. 

[Aaron Also]

Why, I think your rockets are pointed the wrong way. 

I always bring an extra reaction wheel or two and leave them turned off until I have captured the rock. Then turn them on and orient for burn. 

KAS struts are very helpful in this situation to rigidly secure the vessel to the rock. 

Tweakscale is very helpful too. A 2.5m claw is much better than the stock 1.25m one. 

[Superfluous J]

Your rocket is doing exactly what you built it to do. If anything the rocket/asteroid conglomeration should move backwards because of the rocket exhaust bouncing off of the asteroid back forward.

You should either do what @Aaron Also said, flare your engines out so the exhaust doesn't hit the asteroid, space the engines way out from the body so they still point backwards but go around the asteroid, or... sure you can also move them really far forward and trick the physics engine.

One tip if you opt to switch to a push instead of a pull method: Autostruts can be great friends, as I showed in the video below. I start talking about the autostruts a little after 7:00 in, and you really start to see it at 22:50.

Spoiler

 

 

[tomf]

From memory the distance needs to be about 3 of the XL girder segments. I measured it by creating a test craft with an engine pointed directly at one of the structural panels held at various distances below the engine. 2XL girders away and he craft went nowhere from the pad, 3 or so and it was able to take off and fly crazily around KSC.

[Justin_Kerbal]

Thanks everyone!  I'll watch the video after work.  I normally push the smaller asteroids and this is the first time I've tried capturing a Size E asteroid, and I read it was better to pull that to push it.  I don't currently use the mods for tweakscale or KAS struts, but might need to check them out. 

Point taken about cheating the physics engine.  I'd rather not cheat doing it the right way. 

[Sharpy]

Yup. One way to test engine range: make a rig of engine and a lot of smallest, cheapest fins along the way of the flame. They get blown out as far as the engine reaches.

Generally, while engines differ, I found none exceeds the length of five modular girder segments XL ( ) from nozzle end to thrust end (note in some cases the flame may extend farther visually, but it's harmless - and in others, it may seem to end way early but still be blocked.)

Your overall design is fine, but you do need a longer arm, and nukes, due to low thrust, make really lousy asteroid tug engines. This one runs on Rhinos, and moves Class E asteroids with scary ease.