Best way to power a large amount of ion drives?

[OutInSpace]

It turns out that the large ion array on my WIP Jool5 mothership needs an obscene amount of power, at least relative to stock KSP. We're looking at 24 Dawns consuming a total of 216ec/s at full tilt. Solar obviously isn't an option because the inverse square law is a thing, and since I'm limited to stock parts I can't just slap on a NearFuture fission reactor and call it a day.

Currently, I'm considering clipping a ring of 12 RTGs to each Dawn, which should provide enough power to run them all at max throttle. The problem is that the total mass of all these RTGs will be about 24 tons - not huge relative to the rest of the ship, and the Dawns have the ISP to burn, but every ton counts when your TWR is almost non-existent.

The alternative is using fuel cell arrays. Each array can pump out enough juice to run two drives, so only about 3 tons would be added in total. But, they consume LFO, slashing the effective ISP of the Dawns from 4200 to 1300ish, and that's before the TWR impacts of all the fuel I'd have to carry with me.

So... I'd like to get y'alls opinion. Which one is the better solution for a large ship? Or is there a third option I haven't even considered?

Edited July 25, 2020 by SomewhereOutInSpace
clarification

[Cant think of a username]

NO ABORT THIS ION ENGINE PLAN NOW! 24 IONS IS WAY TOO MUCH! AND WITHOUT A WAY TO GET MORE XENON GAS, THE IONS WILL NOT LAST LONG!

ps. sorry for the caps if this is rude

Edited July 25, 2020 by Cant think of a username

[Spricigo]

The obvious solution is to drop most of those ion engines. Yes, TWR will suck but what did you expected with engines that have more weight than thrust to begin with? 

If reducing the number of engines is not an option you may try to add some batteries to the mix and see if you can make it sustain the engines "some time" so you can still do your mission dividing the longer burns in multiple maneuvers with time to recharge between consecutive maneuvers. 

 

Edited July 25, 2020 by Spricigo

[Streetwind]

Try to figure out what the longest burn you must do in one go at max throttle is.

For example, let's say you have a TWR of 0.1, and you wish to leave Kerbin on ion power. Your acceleration is roughly 1 m/s² (one tenth of Kerbin's gravity), and you need to expend 1000 m/s in order to escape. Once you have an escape trajectory, you can keep burning as long as you want, but until you have one you're still stuck in orbit, so escape is your effective limit. At this TWR it would take you 1000 seconds, or 16 minutes and 40 seconds, in order to escape. This is clearly impractical, because any burn larger than roughly 1/6th of your orbit is going to start making problems for you with imprecision. We'll assume that you are in a 36 minute orbit of Kerbin, so 1/6th of that orbit is 6 minutes. This will clearly be a periapsis kick sequence made of three burns. The first two burns will input 720 m/s, so there's no risk to encounter the Mun as you loop around for the final burn, so this is a good setup. You can fly with your spacecraft as it is.

Going futher with this example, now answer the question: on this mission, do you ever expect to have to make a burn that is longer than six minutes? One that asks you to expend more than, mass loss included, let's say 400 m/s worth of dV in one go? You're going to Jool, so an aerobrake-assisted capture burn is an option, which might well let you get away with this amount. After all, as long as you get any capture at all, even a super eccentric one, you've safely arrived. Afterwards, you can either lower this orbit through successive aerobraking/further burns, or tweak your orbit at apoapsis to target Laythe and aerocapture there.

The point of this thought exercise is this: once you have figured out your maximum expected burn length, you can design your spacecraft to the most mass-efficient specifications possible to supply full engine power for just that long and no more than that.

For example: to generate 216 Ec/s with RTGs, you need 288 of them, and that weighs 23 tons. However, if you brought only enough RTGs for 66 Ec/s, you'd only need 88, and they'd only weigh 7 tons. Then, you need to provide 6 minutes of 150 Ec/s worth of battery storage, or in other words, 54,000 Ec. Since all batteries weight the same - 1 ton for 20,000 Ec - you can calculate that that will weigh 2.7 tons. Now your final mass for your power solution is 7 + 2.7 = 9.7 tons, instead of 23 tons for straight RTGs.

[king of nowhere]

but then, if you wait 10 minutes to recharge the batteries to run 20 engines for 1 minute, you may as well have 12 engines running for 2 minutes, or 6 engines for 4 minutes, and so on. it doesn't change much, your thrust is still limited by how much electricity you can produce.

and i'm afraid there are no other solutions. getting so much electricity has a price. i'd try with a mix: instead of 20 tons of rtg, only 5 tons of rtg or so, and about 40k battery power. that's enough for 3 minutes continuous burn, with a dozen minutes to recharge. it should be enough to make small manuevers in one go, while you'll still need multiple burns for the large manuevers anyway, and it should be lighter than 300 rtg

[Laie]

On 7/25/2020 at 5:10 AM, SomewhereOutInSpace said:

Currently, I'm considering clipping a ring of 12 RTGs to each Dawn

Ouch.

Streetwind has really covered the essentials: your power consumption is bursty, so what you really need is enough storage for the worst burst. Between burns, you can fill 'er up on a trickle charge. The most mass-efficient solution will probably be something like 3 RTGs for every 4000u of battery capacity, but it really depends on the vessel and mission profile.

Once you are at Jool, I don't think you will have to do a maneuver of more than 300m/s in one go, and may get by on 200m/s.

However. leaving Kerbin for Jool on an acceleration lower than 1.5m/s² will require more effort than just a few periapsis kicks. Even 2m/s² is already pushing the envelope.

Edited July 27, 2020 by Laie

[Streetwind]

2 hours ago, Laie said:

However. leaving Kerbin on an acceleration lower than 1.5m/s² will require more effort than just a few periapsis kicks. Even 2m/s² is already pushing the envelope.

...You sure? Three easy 4-Minute burns at 1.5 m/s² is 1,080 m/s dV, more than enough to escape. Maybe you mentally moved the comma a step to the left? Because yes, that sure would complicate things

[kedrednael]

When using the engines that require that much electricity I always use a nuclear fission reactor from nearfuture. But otherwise I would use capacitors from nearfuture, with enough batteries to hold the electric charge that wouldn't be used by the engines when discharging a capacitor.
When going stock, the batteries are the best option I think, as shown by Bradley Whistance: https://www.youtube.com/watch?v=L8zhadZRedI

[Laie]

 

10 hours ago, Streetwind said:

12 hours ago, Laie said:

However. leaving Kerbin for Jool on an acceleration lower than 1.5m/s² will require more effort than just a few periapsis kicks. Even 2m/s² is already pushing the envelope.

...You sure?

Fixed my post. The problem isn't leaving Kerbin, of course, but doing a Kerbin->Jool transfer. After periapsis kicks, you still have  >1000m/s to go, which is a bit much for a single maneuver.