Asteroid Tug Engine Flare clearence?

[Stickyhammy]

So I tried and failed to push a 3500 ton E class asteroid into Kerbin orbit but that was fine as I learned a lot. Things now are much more dire. A unknown tonnage E class Asteroid is on its way to Kerbin and this time I need to stop it from crashing into the planet.

To that end I built a tug to pull the asteroid rather than push it. My question is do the engines need a clearance for the exhaust and if so how much? I have 3 main engines and their rocket flare at full thrust will be burning holes in the asteroid, if that was possible. I don't want to make the chain with the claw any longer to preserve its stability. If I have to I guess I can add another huge fuel can to the middle part of the ship and that should give the room I need. But before I bother with that I would like to know how close is too close in terms of engines flares on a tug.

Edited June 21, 2014 by Stickyhammy

[cantab]

Yes, if you have something too close behind the engines you'll get no thrust. It's the same whatever the "something" is, so you can do your testing on the launchpad, make a plate out of structural panels.

[Tery215]

Yes, if you have something too close behind the engines you'll get no thrust. It's the same whatever the "something" is, so you can do your testing on the launchpad, make a plate out of structural panels.

Because the jet of spent rocket fuel is on collision with a part attached to the engine..?

I don't know how far away it needs to be either. I recommend just not making the spent rocket fuel ray overlap with the asteroid.

[Padishar]

As cantab said, you should definitely test it on the launch pad first by attaching your puller to a large flat plate and see if you get any thrust. You don't want to have all the hassle of flying out to the asteroid only to discover you get no thrust once you latch on (or worse, unbalanced thrust, because the asteroid is uneven and blocks engines only on one side). This is one of my class E pullers that I tested by removing the claw, replacing it with a large flat plate and taking most of the fuel out to get the TWR above 1. The three long trusses were enough for it to lift off but two weren't...

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The trusses (and the probe core/RCS tank/size adapter they connect to) gave too much wobble when trying to turn the asteroid with RCS so I've braced them with more trusses and struts in later versions. Also, it makes sense to set up an action group to toggle the gimbals on the engines as, when you are flying to the asteroid the gimbals should work as the engines should still be behind the centre of mass but when you grab hold of any decent sized asteroid the center of mass will move behind the engines and SAS will use the gimbals wrong and cause you serious stability issues while towing...

[RidingTheFlow]

Because the jet of spent rocket fuel is on collision with a part attached to the engine..?

I don't know how far away it needs to be either. I recommend just not making the spent rocket fuel ray overlap with the asteroid.

Newton's third law... If your rocket exhaust impacts your ship (or anything attached to it), you will get net zero thrust, because all engine exhaust will slow down your ship (on impacting) exactly cancelling out acceleration (on leaving nozzle).

This is actually what will happen in reality. Otherwise it would be very easy to create "reactionless drive" which allows to fly without actually expelling any exhaust - note that with external heating (like nuclear thermal rocket) this would've allowed to produce thrust without actually spending any reaction fluid (since you could've just capture exhaust, re-heat it and repeat). But newton third law won't allow for that - if you want to accelerate, you need to accelerate something in opposite direction (and leave it untouched afterwards).

There are some mods that allow thrust balancing, to make summary thrust vector to go exactly through centre of mass. So you can attach several engines to the asteroid, just roughly on same side and in same direction - and plugin will set individual thrusts so asteroid won't rotate.

Edited June 20, 2014 by RidingTheFlow

[FleshJeb]

You could angle your engines outward by 15 degrees or so and still get cos(15) = 96.6% of your thrust to the rear.

[RidingTheFlow]

I think with class E it will require very long boom/truss though, because asteroid will have to be placed far behind enough for exhaust to clear the sides and E class is so big.

[Claw]

I recall a rough guide on exhaust clearance is something like 2x or 2.5x the length of the plume, starting from the engine nozzle.

You can also turn them slightly outward (as suggested). If done right, it looks kinda neat (IMHO).

[Stickyhammy]

Okay thanks that's what I needed to know. I will have to make it longer. Got plenty of time before the rock hits Kerbin.

Visual aid of test. It looks like the flare is hitting the empty gas cans but I am going up so I guess its going to be good enough. The real test will be the asteroid.

Edited June 21, 2014 by Stickyhammy