Non-Newtonian chemical rocket???

[maddog59]

I built the ship/base pictured below: I want the two ships to land the base, then lift off after undocking. But when I cheated it into Mun orbit and tried to land it, the engines fire, the fuel gets used up, the parts below get hot, but nothing happens to the trajectory: the apo/periapsis don't change. The TWR is 5.77, and there's plenty of fuel (nearly 1700 m/s dV), the control point is the MK2 command pod (either one, doesn't make a difference) and full control is available - I can pitch/yaw/roll and throttle up/down 'til the cows come home.

I'd expect to see SOMETHING happening to the trajectory. I've tried restarting the game but there's no change. I even tried shutting down the engines on one ship, but there was no change - firing the engines should have sent it pinwheeling, but nothin'. To the best of my knowledge I don't have any mods installed such as MJ2 which provide assists to handling the ship. 

I tried undocking one of the ships and it behaved normally.  I've used all of these component's before on other ships and those ships behaved normally.  I've used this 'pull you' style before.

I tried splitting it in two, and orbiting (cheating) just one of the ship/base combo's with the same result. Then I added a Sr Docking Port to the top of the base, below the ship's docking port, undocked/re-docked in orbit and it still failed. Then I put a Jr. Docking Port on the bottom of the base, brought the ship around and docked it so it would be pushing the assembly, and VIOLA! it worked as normal. 

  ! 

So: any suggestions on what I can do to get this beast to behave?

Edited August 14, 2021 by maddog59
Added results of other scenarios

[HvP]

What's happening is that components below your engines are blocking the thrust from those engines. The game doesn't model exhaust flow around parts in the exhaust flow of the engines. If there is anything within a certain distance of a straight line coming directly aft of the engine then it assumes the thrust is getting blocked by the ship and you will have no net forward velocity.

That's why your craft worked when you put the engine cluster behind the main vessel. There was no longer anything blocking the engine exhaust. Even a small winglet in the path of a jet engine won't allow exhaust flow around it. There literally can be nothing placed directly in line below the path of the engine exhaust within about 20 or 30 meters of the engine (I'm unsure of the exact distance required.)

Solutions include rotating your engines so that they aim slightly away from the center-line so that the line of thrust will bypass parts below them; or mounting your engines with nothing under them or at least on arms long enough so that nothing is blocking their exhaust.

Be sure to account for engine thrust vectoring, too. Those "Thud" engines have a very large gimbal angle and could very easily aim back towards parts of your craft during hard pitch and yawing. If that happens then the line of exhaust will be blocked on one side of your vessel but not the other, leading to an uncontrollable situation.

Edited August 14, 2021 by HvP

[maddog59]

1 hour ago, HvP said:

Be sure to account for engine thrust vectoring, too. Those "Thud" engines have a very large gimbal angle and could very easily aim back towards parts of your craft during hard pitch and yawing. If that happens then the line of exhaust will be blocked on one side of your vessel but not the other, leading to an uncontrollable situation.

Brilliant! Thanks so much! And as usual, what looks good to me turns out to be a really poor design. Angling those bad boys outward gave me thrust, but anything more than a couple of clicks of the throttle and the beast turned into a very pretty pinwheel. Back to the drawing board... 

<later>

Ha! With enough careful testing I got it to work:

Unfortunately one ship didn't survive my attempt to land it nearby. 

Of course, this was all simulation by way of the cheat menu. Now I've gotta figure out how to get the d*** thing up the Kerbin well...

Edited August 15, 2021 by maddog59

[HvP]

Glad I could help!

The advantage of this game is that it gives you so many options to create. Sometimes that means having many options to create havoc. But that's fun too!