Wcmille

Ion Engine Cogeneration

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Anybody tried pairing Dawns with Nervs? How'd the go? Can't tell if the boost (Nervs  have alternators) is worth it in practice.

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It's an interesting concept, but it would take the alternator output of two NERVs to supply enough power for just one Dawn engine. That only raises your thrust output from 120 kilo-newtons to 122 kilo-newtons. Also, 2 Nervs weigh a total of 6 tonnes with just the engines alone. 12 RTGs on the other hand, enough to power the same number of Dawn engines, only weighs a little less than 1 ton (0.96 tonnes).  I personally don't see it as being practical. But if you want to make something awesome using that combo, more power to you! I can't wait to see it!

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Aren't those engines on the opposite ends of the usefulness chart? Ion engines for small light stuff, nerv for big stuff. 

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So, an ion engine which depends on liquid fuel?  That's a bit of a waste.  With enough solar panels, it could keep going long after the NERVA's are exhausted.

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1 hour ago, sardia said:

Aren't those engines on the opposite ends of the usefulness chart? Ion engines for small light stuff, nerv for big stuff. 

Why are ion engines bad for very large things?

1 hour ago, Corona688 said:

So, an ion engine which depends on liquid fuel?  That's a bit of a waste.  With enough solar panels, it could keep going long after the NERVA's are exhausted.

I only had stock parts in mind. The idea would be a craft that consumes both fuels.

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24 minutes ago, Wcmille said:

Why are ion engines bad for very large things?

Because they deliver so very little thrust.  The bigger the craft, the less difference thrust makes.  On a large craft, an ion engine would take years to decades to make a serious impact.  Sometimes that's tolerable, especially since they're so efficient, but your craft doesn't have years, it only has until the NERVA's run out of fuel.

24 minutes ago, Wcmille said:

I only had stock parts in mind. The idea would be a craft that consumes both fuels.

Solar panels are stock.

Off-hand I don't know any other in-game way to turn electric power into thrust than ion engines.  It'd be nice if the game had more in-game uses for power.

Edited by Corona688

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There's also the question of what kind of TWR your craft has.  Admittedly LV-Ns aren't exactly what you'd call muscular, but they're hot rods compared with the Dawn.  The Dawn's likely to be wasting a lot of its thrust just accelerating its own mass to try to keep up with the LV-N, which means its effective efficiency will be a lot lower.

That's assuming that you plan on running them concurrently.  If you have a ship that has ion engines for doing burns when you can tolerate really low TWR (and you have incredible patience), and then you switch to the LV-N when you need a bit more voom, then that could work, I suppose.

But in general, I wouldn't think they'd make a particularly good combination.

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2 hours ago, Waxing_Kibbous said:

I keep posting videos, yes it's a sickness

 

How many Dawns are in this craft?

It looks like your ejection burn from Kerbin was perfectly timed to run out of battery at the conclusion of the burn. Very nice engineering.

How are the Dawns and panels attached?

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2 hours ago, Waxing_Kibbous said:

I keep posting videos, yes it's a sickness:
[SNIP]

 

That music is great.

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It would be better to combine wirh fuel cells.

ive done lvn ion ships before... For reusable moho craft. The lv-ns do mostof the work, but i turn on the ions too for the captureburn ( some of which is when moho blocks the sun, so the lvn power is nice)

mostly though, i use the ions after the liquidfuel is gone, for thefinal part of the mission (it needs a significant retroburn when returning to kerbin, as it burns up otherwise)

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19 hours ago, Wcmille said:

How many Dawns are in this craft?

It looks like your ejection burn from Kerbin was perfectly timed to run out of battery at the conclusion of the burn. Very nice engineering.

How are the Dawns and panels attached?

I do not take credit for that video! it's just one of my favorite channels  :) I am not even 1/10 of Turbopumped in the KSP skills dept. Many times he will post the craft files in the youtube description

*I meant posting videos here from YT, not actually posting original content, sorry... :blush:

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21 hours ago, Snark said:

The Dawn's likely to be wasting a lot of its thrust just accelerating its own mass to try to keep up with the LV-N, which means its effective efficiency will be a lot lower.

This sounds misleading.

The inefficiency isn't the low thrust of the Ion causing the ship to accelerate slower than if the ion engine wasn't installed.

The inefficiency is in running the high isp ion engine while there is still liquid fuel as payload instead of burning off the "low" isp LV-N fuel first.

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2 hours ago, suicidejunkie said:

This sounds misleading.

The inefficiency isn't the low thrust of the Ion causing the ship to accelerate slower than if the ion engine wasn't installed.

The inefficiency is in running the high isp ion engine while there is still liquid fuel as payload instead of burning off the "low" isp LV-N fuel first.

Except that the low thrust of the ion engine does have an effect on efficiency (or at least, it may, depending on how the numbers work out).

You have an ion engine on the ship.  It generates 2 kN of thrust.  It also has a certain amount of mass-- not just the engine itself, but the mass of any support equipment needed, such as xenon tank empty mass, xenon gas, perhaps some extra battery storage and/or solar panels.

Take the acceleration of just the "ion stuff" -- i.e. the 2 kN thrust, divided by the total mass of ion-specific paraphernalia.

Then take the acceleration of the "rest of the ship" -- i.e. the thrust from your LV-N, divided by the total mass of the ship excluding the "ion stuff".

If the former is less than the latter... then adding the ion stuff to your ship isn't helping-- it's actually hurting you by acting as dead weight that your LV-N's are having to accelerate.  You'd be better off just leaving the ion accoutrements off the ship.

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There are 3 and 1/2 ways currently to run an ion craft reasonably. You trade convenience for efficiency. 

The most efficient is solar cells, usually gigantors. These are the lowest mass way to power ions. Their downside is that they have to be in sunlight and you might have to keep re-orienting your craft. Also, they don't work so good for the outer planets. 

Then there is using fuel cells. You have to haul the mass of the fuel cell array and the LF+O but it's still a high dV solution. There is the risk of using up all your LF+O if you leave your fuel cell on while warping. 

Finally, using RTGs. They are the heaviest solution if you take enough to burn continuously but the craft requires no special attention. Still a high dV solution compared to any other engine. 

The 1/2 solution is to combine any of the above with big batteries, especially with solar cells. This lets you reduce the energy generation requirement but it does limit the length of your burns. 

 

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9 hours ago, Foxster said:

There are 3 and 1/2 ways currently to run an ion craft reasonably. You trade convenience for efficiency. 

The most efficient is solar cells, usually gigantors. These are the lowest mass way to power ions. Their downside is that they have to be in sunlight and you might have to keep re-orienting your craft. Also, they don't work so good for the outer planets. 

Then there is using fuel cells. You have to haul the mass of the fuel cell array and the LF+O but it's still a high dV solution. There is the risk of using up all your LF+O if you leave your fuel cell on while warping. 

Finally, using RTGs. They are the heaviest solution if you take enough to burn continuously but the craft requires no special attention. Still a high dV solution compared to any other engine. 

The 1/2 solution is to combine any of the above with big batteries, especially with solar cells. This lets you reduce the energy generation requirement but it does limit the length of your burns. 

 

I'm suggesting "cogeneration" as the 4th way. You power the ions, whole or in part, by the alternators of your other engines. The Kerbin ejection burn from the video above illustrates what I was hoping to do.

As @Snark says, the composite engine with reduce your TWR, but increase your Isp. 

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17 hours ago, Snark said:

Except that the low thrust of the ion engine does have an effect on efficiency (or at least, it may, depending on how the numbers work out).

You have an ion engine on the ship.  It generates 2 kN of thrust.  It also has a certain amount of mass-- not just the engine itself, but the mass of any support equipment needed, such as xenon tank empty mass, xenon gas, perhaps some extra battery storage and/or solar panels.

Take the acceleration of just the "ion stuff" -- i.e. the 2 kN thrust, divided by the total mass of ion-specific paraphernalia.

Then take the acceleration of the "rest of the ship" -- i.e. the thrust from your LV-N, divided by the total mass of the ship excluding the "ion stuff".

If the former is less than the latter... then adding the ion stuff to your ship isn't helping-- it's actually hurting you by acting as dead weight that your LV-N's are having to accelerate.  You'd be better off just leaving the ion accoutrements off the ship.

I'm not saying it doesn't accelerate slower.  But anyone installing ion engines on a ship cannot be very concerned about low acceleration.

The ion is basically an upper stage that gives you tons of dv.  You can't just leave it off the design.

What I'm saying is you could get more dv by leaving it as inert mass and saving it until your ship is lighter instead of doing the cogeneration thing and wasting Xenon pushing liquid fuel.  Except in those situations where every notch of acceleration is critical, like landing and takeoff.

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Yes, that goes for any craft: use the low Isp fuel first, and the high Isp last... at least when there are no staging events.

If you have monoprop thrusters on your pod, and a decoupler, you'd get more dV by saving the monoprop until everything else is used up, and then decoupling the pod and using the monoprop.

I think the point is that if your ions only work when the LV-Ns are thrusting, the Ions aren't going to increase your dV because they slow the LV-Ns down more rather than helping them... except for very low accelerations.... ie very high dry masses where the addition of an ion propulsion system won't matter much

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I actually think it's a somewhat interesting idea.  If it's a heavy ion craft meant for operation further out than Duna, the nukes can provide boost thrust and keep the batteries from emptying as fast as they might otherwise.  My own excessive dv designs like to use a few Puffs and some extra mono to provide a boost mode for use in gravity wells and at burn midpoints, but using a nuke instead is an interesting idea.  Using such a system would involve some interesting modifications to the fuel pod design, though.

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Not sure if you're into mods, but the Atomic Age mod (made by Porkjet himself) has the Candle, which is basically a 0.625m Nerv with an alternator. That might work better with ions

EDIT: Also it has a built in generator

Edited by Brownhair2
Wrong radius

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9 hours ago, Wcmille said:

@SnarkI'm suggesting "cogeneration" as the 4th way. You power the ions, whole or in part, by the alternators of your other engines. The Kerbin ejection burn from the video above illustrates what I was hoping to do.

Once again, it's possible, but the benefit of doing this looks to me like a lot less than the benefit you'd get from bringing along one gigantor array.  Powering the ion engine for a 2 minute burn, vs powering the ion engine for a 200 minute burn, is 100 times the benefit.

As for fuel cells, a fuel cell converts 1 unit of fuel into 400 units of electricity, while a "swivel" engine converts one unit of fuel into 15,680 N of thrust.

(Aside:  Assuming they're roughly equally efficient, and as a very rough metric, I think this means 1 unit of electricity is worth about 40 joules of energy.  Finally!  Electricity in KSP has a unit!)

The Dawn engine takes 3.66 electricity per second to produce 2,000 N of thrust.  Fed from fuel cells, (400 / 3.66) * 2000 = 218,579 N from one unit of fuel.  So, wow, yeah, you can get more thrust out of fuel cells and ion engines than you could by just burning fuel.

I still don't think cogeneration is a viable strategy.  Not because it wouldn't work, but because so little of the ion engine's potential would be used.  For just 0.3 tons more weight, you could keep running the ion engine long after the rocket fuel runs out.

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Running with fuel cells makes more sense. An effective Isp of 1293s is nothing to sneeze at. 

Adding two ions to a nuke only yields 842s, and the nuke's alternator has no chance of keeping up as it's only supplying about 1/4 the necessary EC/s. If you go with the sustainable two nukes for each ion it's only 810s, hardly worth the added complexity.

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On 7/1/2016 at 10:51 AM, Wcmille said:

 the composite engine with reduce your TWR, but increase your Isp. 

While this is true, it will also, more importantly, reduce your delta-V as well. A high Isp engine won't necessarily mean high delta-V. This is where the mass of the nuke engines and associated liquid fuel will hurt the Ion engine's performance. The 'Dawn' will still move a large & heavy ship really efficiently, but it will move it very little.

7 hours ago, Red Iron Crown said:

Running with fuel cells makes more sense.

That depends on the fuel cell output rate as well .  You would need 6 Fuel Cells to continuously power a single Dawn engine at full throttle, or a single Fuel Cell Array to power 2 Dawns under the same conditions.  While a Fuel Cell Array's Elec Charge rate per ton is only 92% that of the Gigantor XL solar arrays, that doesn't include the mass of the fuel tanks and LF+O needed to power the Fuel Cell Array.  A single Gigantor XL powering 2 Dawn's IMO would be way less mass for the same effect, and wouldn't have a finite fuel source.  If you were trying to go to the outer planets, than solar energy drop-off would be an issue of course...

Generally, my rule of thumb is Ion engines are good for small payloads in the inner planets (maybe even Dres if you have enough additional solar arrays), and nukes are for large payloads and/or outer planets (due to solar energy drop-off).

Not saying everyone should adhere to this, but it's how I manage all the variables of interplanetary craft design (mass, delta-V, solar energy, power consumption) between nukes/ions.  Play however it's most fun for you.  Why put nukes and ions on the same ship? Because you can. :)

Edited by Raptor9

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