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How is Energy generated on a Spacecraft?


MalfunctionM1Ke
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Hi Guys,

I am really not the Guys who is nagging about something that I got stuck with for almost a full year but right now I am watching a Documentary called "Moon Machines" which is pretty detailed.

Anyways, what came to my mind is the following:

In the Apollo Program, Energy was generated continuesly by Fuelcells in the Service Module, which are using the onboard Fuels (Liquid Hydrogen and Liquid Oxygen) to produce Water and Electricity.

While playing KSP, Energy will be generated by RTG's (like Voyager, Curiosity, etc.), Solar Panels (Satelites, Calculators) and Rocket-Engines.

My concern is about the Engines.

Is there any real example for a powerproviding Rocket-Engine?

In which way would that Energy be produced? There is no drivetrain that could power some kind of Generator.

Also the LV-N's or NERVA Engine is something ... well... worrying to me.

In a Nerva Engine, the Fuel is passed by the Reactor to heat it up. So the heated Substance that could be converted into Energy is released with "high" Thrust out of the System. So actually they should not produce any Energy at all, but they do.

Also there is no Energy-Output Meter on them when you right-click them in Flight.

I dont want Squad to turn the whole System round and let the Fuel-Tanks produce the Electricity on the Spacecraft.

Actually I only want some help, Brainstorming this.

It also could turn out that I am completely wrong here :)

Thank you for your kind Support.

Mike

Edited by MalfunctionM1Ke
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I thinks SQUAD's main concern was game balance and functionality, not all out realism.

BTW, here is an power producing engine: http://en.wikipedia.org/wiki/Gas-generator_cycle_(rocket)

Hi Tank,

isnt the Gas Generator just a preburner that drives the Turbopumps by a Drivetrain?

I dont think that this is generating Power.

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Yes, there is. Granted, they tend to be used in ICBMs and other forms of long-range missile, but they could conceivably be used in rocketry as well.

Oh wow. I didnt know about that.

I am not worried about the Gameplay or Realism, it is just something that came to my mind while watching some Space-Documentaries :)

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Oh wow. I didnt know about that.

Actually, I didn't know about them either until you brought it up. A quick Google search on rocket alternators turned up information on flux-switching alternators, and there you have it. Isn't the internet wonderful?

But yes, classically space missions have used fuel cells for power generation purposes. It's a much simpler solution than developing an alternator that can work with a high-exhaust-heat rocket.

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Actually, I didn't know about them either until you brought it up. A quick Google search on rocket alternators turned up information on flux-switching alternators, and there you have it. Isn't the internet wonderful?

But yes, classically space missions have used fuel cells for power generation purposes. It's a much simpler solution than developing an alternator that can work with a high-exhaust-heat rocket.

Google is wonderfull, but I wouldnt know what to search for because my Skills in english arent that perfect when it comes to technical Terms.

So I figured out to ask the "other Guys and Girls" :)

I really would like to know what would add more weight to a rocket, the alternator or the fuelcell.

But I think that is too much to ask for.

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I don't think we can realistically say how much an alternator set up to work on a space-capable rocket engine would weigh. Until we have a clear example of one, at least. But it would probably be lighter than a fuel cell array overall. Of course, it would necessarily also rob some of the capacity of the rocket engine to convert some of the mechanical energy from the rocket exhaust into electricity.

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I don't think we can realistically say how much an alternator set up to work on a space-capable rocket engine would weigh. Until we have a clear example of one, at least. But it would probably be lighter than a fuel cell array overall. Of course, it would necessarily also rob some of the capacity of the rocket engine to convert some of the mechanical energy from the rocket exhaust into electricity.

Yeah you are right.

In Addition it would also only generate Power when the Engine is firing, like the ones in KSP.

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what about a thermo-electric generator? (like the type on RTGs) also, on your nerva question,its going through the exact same process as chemical rockets, but instead of relying on a chemical reaction in the propellant, it used a nuclear reaction outside of the propellant. the result is still the same: a mass of rapidly expanding gas, which is vented out of a nozzle that directs the gas all in one direction.

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A rocket engine needs auxillary components to let it run, such as pumps for fuel and whatnot, and some of these pumps can require horsepower in the tens-of-thousands range. This is why I'd guess that most rocket engines would need some means of extracting shaft power from their fuel. If that’s the case, slapping an alternator on the shaft should be no big deal. I’ll surmise that the reason for using fuel cells is so you don’t have to run a whole rocket engine in order to power a hand warmer, and where RTGs would be inappropriate (due to mission length/ power requirements?/ weight?).

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I don't think we can realistically say how much an alternator set up to work on a space-capable rocket engine would weigh. Until we have a clear example of one, at least. But it would probably be lighter than a fuel cell array overall. Of course, it would necessarily also rob some of the capacity of the rocket engine to convert some of the mechanical energy from the rocket exhaust into electricity.

The amount of power robbed is tiny. The Gas generator on a Saturn V produced something like 20,000lbs of thrust. The main engine itself, was what? 700,00lbs of thrust? Each? To generate, say, 10kw of electricity, it would likely utilize something like 200-400lbs of thrust. A very, very, very tiny portion of the energy produced.

The reason why such a thing isn't a good idea for anything other than ICBMs, is that those spend a lot of their time under thrust, so the battery requirements are minimal and the amount of time spent under generator is a large portion of the overall "mission" length.

On something like the Apollo mission, the total amount of time spent under thrust is an infintesmal portion of the overall mission. So you'd need MASSIVE batteries to store all the charge you needed. Really gas generator powered alternators are a poor option. For something where you'd be under thrust for a large portion of a mission they are great, and significantly better for power to weight ratio compared to any other solution, which is why ICBMs use them.

For space missions, not so much. They might make sense on something like a solar powered probe mission, just for the launching vehicle, since in all likelihood the launcher is going to require significantly more power than the probe itself will, between initial guidance, control surface operation, rocket motor gimbals, etc. It might be on the order of dozens of kw or more, compared to the probe which might use dozens of watts to hundreds of watts. A 50-200lb alternator might make more sense powering the thing (even one per stage) than hundreds of pounds of batteries for the 10-20 minutes that the launcher is likely going to be under thrust.

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Yeah a thermoelectric generator would be fairly simple I think... Hot side, part of exhaust, cold side, the liquid hydrogen fuel tank.

Quite a big temperature difference and no moving parts required.

NASA PDF: Knock yourself out.

Can't use the fuel tank as a heat sink. You NEED the liquid hydrogen to stay super cooled there. Heating is a BAD idea. You could conceivably run the liquid hydrogen/Oxygen fuel over a thermoelectric generator heat sink, before it gets dumped in to the combustion chamber, or before it passes in to the engine bell, depending on rocket design, and in to the combustion chamber.

Issues is TEG are very low efficiency, no matter the temperature difference you can get, they also tend to be a little sensitive, so you might not be able to construct one that can handle several thousand degrees.

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My first though was Peltier-Seebeck (TEG) devices, you would need a lot of them though as they are in single digits efficiency wise though arrays of them are used to generate upwards of 5kw, maybe Kerbals with their very dense planet have ready access to cheaper metallics and can build better versions of these than we can (for a reasonable cost).

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There is a related question that's bugging me about Apollo. Why NASA did not supplement their power generation with a solar panel or two? They were space-worthy by then - didn't they? Weight constrains?

The energy density available was still much smaller than contemporary fuel cells or even RTGs, so they generally weren't worth using on spacecraft that required relatively high power levels. NASA was launching earth-orbiting sats with RTGs as late as the 70s for this reason.

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This topic makes me wonder whether you can use the turbopump configurations available in most engines for electric power generation. Seeing as Apollo's F-1 pumps already had a power output of 30.000 hp, I guess it is well possible. Maybe not the most efficient, but hey.

Edit: that will be the alternator discussed here :D

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That's what I was saying. I'm pretty sure almost every rocket engine has some sort of turbine to draw power from the fuel. How else can you supply the stupid amount of horsepower needed to drive the pumps? Once you have such a configuration, attaching an alternator to it should be no biggie. But again, you're not going to carry with you and burn a 10000000000000000000000 lbf rocket engine just to run a few hundred watt dynamo.

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I don't think that kerbal space program should be entirely realistic, because if it was then it would probably take alot of system power when it is completed, but if you really wanted to make an extremely realistic game then you could probably install a lot of mods! In actuality, I am sure if NASA really wanted to make a power providing option for engines they could, but doing so would be kind of expensive for a lot of power, for only about 3 minutes maximum of burn time. (for example i know that some engines burn for longer) See the world is all about finances, and creating an energy harvester for engines is pointless since we have solar panels that we can use, to power batteries for absent-lighting travel. But like i said i'm sure we could definitely do it! And for the technicalities to answer your question on how they would do it, scientists have came up with many different ways to harvest almost any type of energy whether it be radioactive energy, vibration atoms, heat, or even solar power. So i am sure that they could develop a way to take the radiation from the NERVA engine and harvest it's energy if you really wanted to. But like I said, you will only be burning for a few seconds at a time, and very rarely in inter-planetary travel so you would still have to have a backup power system such as solar panels, which makes it kind of pointless to have another energy input because once you leave earth the sun is always shining. I hope this helps you out a bit i enjoyed writing this as i do all of my posts! Hope you have a great day!

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A bimodal nuclear thermal rocket would, instead of flushing the reactor with hydrogen to keep it cool, use a brayton generator to produce power. I guess it would also work while the NTR is firing.

At the end of the day, for the inner solar system, solar is probably the best option. For the outter solar system, a small nuclear reactor would be nice.

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In any liquid rocket engine you have to figure out a way to get the fuel into the combustion chamber. In a large design that means you need to force a vast amount of fuel (the Space Shuttle burned 535,000 gallons of fuel - about an Olympic swimming pool full - in 8 minutes) against an incredibly high pressure (200 atmospheres for the Space Shuttle). That requires some enormously powerful pumps.

In fact these turbopumps are generally the most important and expensive component of any design - sometimes launch vehicles get called "a turbopump with a rocket attached". Basically they work a bit like a jet engine, only instead of burning fuel to accelerate air, they burn fuel to accelerate more fuel. This generates some serious power.

The most powerful rocket engine ever built is the Russian RD-170, originally built for the Soviet shuttle programme and now used in the Ukrainian Zenit rocket. This design has 4 separate combustion chambers gathered around a common turbopump, which produces 190 MW - enough to power Salt Lake City.

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