Is electrical generation via alternators free or at a cost of fuel/thrust?

[THX1138]

Is electrical generation via engine alternators free or at a cost of fuel/thrust?

[cantab]

When the engine is running it will generate some electricity proportional to its fuel consumption. Fuel consumption is unchanged whether or not there is any storage for that electricity, and you can't turn the alternator off. So it's "free" in the sense that everything is the same whether you use that electricity or not. But it's not "free" in the sense of needing no resources - when the engine is switched off and not burning fuel, the alternator does not work.

[THX1138]

Thanks.

[Streetwind]

There is also no difference in fuel flow or Isp or thrust output between an engine that has an alternator and the same engine with the alternator commented out in the config, for the record.

[VikingStormtrooper]

In real life, production of electricity from a thermal engine requires a part of the energy (or enthalpy, if you prefer!) otherwise used to expand the exhaust and provide thrust to be moved to the alternator. Considering the unavoidable losses that characterize this process, the result is that you would obtain electricity at the price of lower engine performances, and a lower burning time as well. Something similar happens with the dynamo of bicycles, where a percentage of the power provided by the cyclist is diverted to the dynamo, with the result of making it a bit harder for the cyclist to cover the same distance in the same time.

They probably did not model this physical phenomenon in KSP for simplicity, but IMHO it would not be a difficult one.

[Palaceviking]

Electrical generation on rl rockets affects the output in an infinitesimally small manner as to be almost unmeasurable so I think it would be a waste of time to model it.

[ForScience6686]

10 minutes ago, Palaceviking said:

Electrical generation on rl rockets affects the output in an infinitesimally small manner as to be almost unmeasurable so I think it would be a waste of time to model it.

Exactly, the thrust produced by these engines is quite high when you think about it, so the small draw of the alternator is quite puny in comparison.  

[bewing]

IRL, most rocket engines produce a lot of waste heat that builds up in the nozzle. Some (or even most) of this waste heat can be converted for free into electricity with a simple heat engine. Alternately, some engines produce plasma, and that plasma must be kept away from the engine walls. The easiest way of doing that uses magnetic fields, and happens to produce a heck of a lot of electricity as a by-product.

I pretend that KSP uses the first method -- running waste heat from the refractory parts through a heat engine. The recovered EC amounts are rather pathetic, but it's useful on jets to keep small batteries charged.

 

[VikingStormtrooper]

 

10 hours ago, VikingStormtrooper said:

In real life, production of electricity from a thermal engine requires ...

Indeed, I used the term "thermal engine", that is very general - even your car is based on one of them, at least if you have a normal car. I agree with you, the SSME releases a huge amount of thermal power, and it is reasonable to think that the alternator removes an incredibly small fraction of this power in order to get electricity. But what happens if you move to smaller engines, and not necessarily rocket engines? I simply gave a theoretical explanation, and I suggest that, since KSP covers a large variety of engines, and since it allows users to make mods able to further expand it, the function to remove a small amount of thermal power and divert it to the alternator would be a nice one. If the developers are not interested in this, someone may create a plugin or something different, I don't know. Everyone is free to choose what to model and why. Of course, if you use only the F-1 engine for Saturn V rockets all this stuff is completely useless, but we are open-minded, and unfortunately our cars rely on smaller engines 

[Aegolius13]

Fun fact: you can buy a camp stove called Biolite that burns twigs, and then uses the thermoelectric effect to generate electricity from that heat.  The electricity, in turn, runs a fan to intensify the fire.  It even generates a couple of surplus watts to charge a phone battery.  

I imagine the Kerbal engines work on vaguely similar technology - especially the Poodle, which (according to the in-game description) doubles as a barbeque at low power.  Too bad we can't select twigs as a power source, but maybe that's what the "solid fuel" is, anyway.