Nuclear engine not working in KSP

[TeleTub]

I assembled a large and relatively modular ship in orbit recently for the purpose of interplanetary travel. I wanted to attach a nuclear engine to the bottom of the ship for more efficient long-range thrust however it seems that It isn't drawing fuel properly. It is attached using a few sets of decouplers and a few service bays and a few hitchhiker cans lie between the engine and the tanks. I make this configuration on the ground and it works just fine but no matter what I try in space it won't fire or even recognizes the fuel being attached to the engine. The only real difference between the ground config and the space one is that the space one was assembled in space and the ground one being assembled in the VAB. Here is a picture, any ideas?

Edited February 25, 2019 by Gargamel
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[Gargamel]

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[Aegolius13]

Have you doubled-checked the nuke has been enabled?  I sometimes get weird results like that when I separate and combine craft independent from their normal staging order.  Can do this from either the right-click menu, or an action group button if you assigned one. 

Beyond that, could definitely be a fuel flow issue. I'd check every docking port or similar part between the fuel source and the engine, and make sure that crossfeed is set to enabled.    Possibly when you do dual docking ports like that, the game only recognizes one as the official connection? 

There are also a few parts that don't allow crosfeed at all, but I think that's just stuff like the heat shields (though that would not explain why it worked in testing but not here).  What's the part immediately in front of the nuke?

 

[TeleTub]

1 hour ago, Aegolius13 said:

Have you doubled-checked the nuke has been enabled?  I sometimes get weird results like that when I separate and combine craft independent from their normal staging order.  Can do this from either the right-click menu, or an action group button if you assigned one. 

Beyond that, could definitely be a fuel flow issue. I'd check every docking port or similar part between the fuel source and the engine, and make sure that crossfeed is set to enabled.    Possibly when you do dual docking ports like that, the game only recognizes one as the official connection? 

There are also a few parts that don't allow crosfeed at all, but I think that's just stuff like the heat shields (though that would not explain why it worked in testing but not here).  What's the part immediately in front of the nuke?

 

 

The part order goes as follows: nuke, coupler, coupler, service bay, service bay, battery, battery, coupler, coupler, sci junior, reaction wheel, hitchhiker can, and another set of couplers on either side attaching the engine/ tanks assembly. And I have made sure the engine is on. I've played around with about every setting and made sure that all the couplers displayed "disable crossfeed" (this disable and enable toggle thing is kinda confusing) on both sides of the pair.

[bewing]

I'm not sure I have time tonight, but I think there's a possibility that you need to have at least one teeny fuel tank in that center stack for the fuel to flow into, before it goes to the nuke.

Below the cupola and engines, I think you have another tank and another set of docking ports? Do those docking ports have crossfeed enabled?

Edited February 25, 2019 by bewing

[TeleTub]

14 hours ago, bewing said:

I'm not sure I have time tonight, but I think there's a possibility that you need to have at least one teeny fuel tank in that center stack for the fuel to flow into, before it goes to the nuke.

Below the cupola and engines, I think you have another tank and another set of docking ports? Do those docking ports have crossfeed enabled?

Ya, I was kinda thinking of doing something like this as a last resort. I'll give it a try but I'm afraid that it will just drain this tiny tank super fast and I will have to have fuel transfer constantly running to keep it going. And yes I checked crossfeed.

[TeleTub]

Update: I sent up a new module with a small tank on it and it does run while there is fuel in that small tank but it requires me to be constantly transferring fuel into it for it to work. Even with that, it drains faster than I can transfer fuel. I don't really know what else to do. I might just send up new engine/tank modules altogether. However, if I do that I want to know what is the best thing to do. Should I go full nuclear and maybe even reduce my fuel capacity (I won't need any oxidizer). Should I still keep some kind of liquid engines around?

[Kryxal]

You might want to use the Mk1 LF-only tank ... I believe fuel transfer rate is proportional to the destination tank's capacity, by the way.

[Capt. Hunt]

On 2/25/2019 at 3:54 AM, bewing said:

I'm not sure I have time tonight, but I think there's a possibility that you need to have at least one teeny fuel tank in that center stack for the fuel to flow into, before it goes to the nuke.

Below the cupola and engines, I think you have another tank and another set of docking ports? Do those docking ports have crossfeed enabled?

nope, you don't actually need a tank on that stage, as long as the fuel can flow to the engine it will fire.  If I had to guess, I'd say that the fuel crossfeed is turned off somewhere, probably one of the docking ports.  If you have KIS/KAS you can bypass the crossfeed by installing a fuel line on EVA.
 

You may run into other issues with those docking ports though.  From here it looks like everything is connected with Clamp-o-tron juniors.  That thing is going to shimmy like a belly dancer when under thrust for more then a few seconds, because of how flexible the .625m joints are.  I'd recommend a minimum of standard clamp-o-trons to connect modules on large interplanetary ships.

[TeleTub]

8 hours ago, Capt. Hunt said:

KAS

 

8 hours ago, Capt. Hunt said:

nope, you don't actually need a tank on that stage, as long as the fuel can flow to the engine it will fire.  If I had to guess, I'd say that the fuel crossfeed is turned off somewhere, probably one of the docking ports.  If you have KIS/KAS you can bypass the crossfeed by installing a fuel line on EVA.
 

You may run into other issues with those docking ports though.  From here it looks like everything is connected with Clamp-o-tron juniors.  That thing is going to shimmy like a belly dancer when under thrust for more then a few seconds, because of how flexible the .625m joints are.  I'd recommend a minimum of standard clamp-o-trons to connect modules on large interplanetary ships.

3

Thanks for the advice, I've decided to actually send up new tank modules with only nuke engines on them. This will save weight (no more oxidizer) and I won't have to worry about any shimmy (I have actually been using the normally sized docking ports, not the jr.s but it still had some wobble). I will also have another large docking port center mass freed up which I can use for larger landing modules. 

[Capt. Hunt]

19 hours ago, TeleTub said:

 

Thanks for the advice, I've decided to actually send up new tank modules with only nuke engines on them. This will save weight (no more oxidizer) and I won't have to worry about any shimmy (I have actually been using the normally sized docking ports, not the jr.s but it still had some wobble). I will also have another large docking port center mass freed up which I can use for larger landing modules. 

ok, it was hard to tell from your pic what size those were.  Seniors work the best, in fact, they were added to the game because people were complaining about how wobbly the small and medium docking ports are.