Why rapier engines shut off on their own?

[ChromeDome]

When I get higher in the atmosphere the raper engines just die. I heard they are supposed to switch to a closed cycle when they run out of air but they just won't do it

[James Kerman]

Welcome to the forum @ChromeDome, your question has been moved from KSP Discussion to Gameplay Questions. 

[Streetwind]

Have you brought oxidizer?

Plane engines typically run on liquid fuel alone, and this includes the RAPIER engine in open cycle mode. That is because they harvest oxygen from the atmosphere to act as oxidizer.

To switch the RAPIERs into closed cycle mode, they need a new source of oxidizer that isn't harvested from any atmosphere. Meaning, you need to bring some in a tank that they can draw from. Did you?

[JadeOfMaar]

10 hours ago, Streetwind said:

Have you brought oxidizer?

In addition to this there's another condition that will cause the RAPIER to simply die: If you toggle the engine mode while in the ship editor (when you measure your vac dV), their other toggle (switching mode) changes from Automatic (default) to Manual. They wait on you to trigger the engine mode switch or you have to change the switching mode back to Automatic.

Edited January 29, 2023 by JadeOfMaar

[ChromeDome]

Is there an option to have dedicated oxidizer tanks? I know there is one for liquid fuel that is the liquid fuselage.

[JadeOfMaar]

15 hours ago, ChromeDome said:

Is there an option to have dedicated oxidizer tanks? I know there is one for liquid fuel that is the liquid fuselage.

Nope. For most use cases where propulsion is concerned, the oxidizer by itself is pointless or an extreme hazard. No one wants to be hauling that around for the bulk of a mission.

If you really want an option for a dedicated Oxidizer tank you can either get a fuel switching mod or dedicate an LFO tank by either draining the LF side and locking it so it can only be filled on its OX side, or having that be the only actual LFO tank on a ship where every other tank is LF alone.

 

[ChromeDome]

Aw. That sucks. I was planning to make a spaceplane that could have oxidizer on drop tanks so that after deorbiting, I could detach them so I wouldn’t haul around tonnes of dead weight.

[king of nowhere]

7 hours ago, ChromeDome said:

Aw. That sucks. I was planning to make a spaceplane that could have oxidizer on drop tanks so that after deorbiting, I could detach them so I wouldn’t haul around tonnes of dead weight.

you could have a mixed lf/ox tank and drop it when it's empty of both. in any case, you don't need that much oxydizer, so it wouldn't save that much weight. in fact, I'm failing to see when it would ever be relevant; if you deorbit on kerbin, you are likely just going to land and recover the plane, and if you deorbit on laythe, you will want oxidizer to orbit again.

 

anyway, there are mods that let you make oxidizer-only tanks. but as I said, they rarely have uses.