Alternators and turbo pumps

[Leafbaron]

Maybe somebody with more rocket anatomy knowledge can tell me if this is stupid or not. Are the alternators on rocket motors a kerbal thing or are they an analogue of a real world alternator on real rockets? if so i think it would be a nice feature to run the alternator at the expense of fuel ina pinch when electric charge is required, however it would be almost like a fuel dump situation where the fuel is not ignited, just forced throught he turbo pumps. this way you don't have to burn and mess up your orbit if you are in a very dire situation. 

edit: this would be for use in career mode before unlocking fuel cells or solar panels. 

Edited September 16, 2016 by Leafbaron

[Wjolcz]

Fuel cells.

[KSK]

On 16 September 2016 at 9:39 PM, Leafbaron said:

Maybe somebody with more rocket anatomy knowledge can tell me if this is stupid or not. Are the alternators on rocket motors a kerbal thing or are they an analogue of a real world alternator on real rockets? if so i think it would be a nice feature to run the alternator at the expense of fuel ina pinch when electric charge is required, however it would be almost like a fuel dump situation where the fuel is not ignited, just forced throught he turbo pumps. this way you don't have to burn and mess up your orbit if you are in a very dire situation. 

edit: this would be for use in career mode before unlocking fuel cells or solar panels. 

I'm fairly sure they're a kerbal thing to provide a gameplay workaround if you need a bit more electrical power for early spacecraft before you've unlocked batteries or solar panels. I guess you could run an alternator off the turbopumps in real life but it would make the engine a lot more complicated for negligible benefit and probably make the turbopump (and therefore the whole engine) less efficient. Interestingly, battery powered electric turbopumps are a thing, so it works perfectly well in reverse.  

 

Edit: I am not a rocket engineer, so this is just my best guess.

Edited September 17, 2016 by KSK

[Temeter]

The discussion already came up once, basically it's not done. An alternator and cables going from the bottom of the rocket up to the crew capsule would be quite the effort, and for what? You'll only have the carrier rocket engines for the first <20 minutes of a flighty usually, so why even bother?

Recharge of crewed craft is usually either done by fuel cells (Apollo), or just solar panels (Soyuz, at least early versions had no cells).

[Guest]

What about thermoelectric generators? You could slap one onto the rocket engine, use cryogenic fuel as the "cool" side and the combustion chamber as the "hot" side.

[Stoney3K]

4 hours ago, Aperture Science said:

What about thermoelectric generators? You could slap one onto the rocket engine, use cryogenic fuel as the "cool" side and the combustion chamber as the "hot" side.

Again, same "problem": The infrastructure from the rocket engine (which is going to be detached) towards the crew capsule is going to be too complex and too expensive. Remember, adding a lot of meters of copper cable means adding a lot of mass that you're going to discard when the first stage is jettisoned. Basically just adding dead weight when you really don't want it.

Crew capsules usually have batteries or fuel cells and solar arrays to power themselves. The Space Shuttle is the only spacecraft that has turbomachinery to power alternators -- it has three APUs that are basically turbines powered from RCS nozzles (aside from fuel cells). These are used to power the hydraulics and provide back-up power from launch.