Ion engines thrust to weight

[Captain Katawa]

Is it possible to make Ion craft with any reasonable TWR so i wouldn't have to wait forever for burns or will the diminishing returns of engine and solar panels mass ruin the whole thing?

Is it a reasonable idea to use multiple ion engines on one craft?

[Drewskii]

Are you planning on using them for a spacecraft or probe?

[Captain Katawa]

Originally posted by Captain Woodchuck ll:

Are you planning on using them for a spacecraft or probe?

I was thinking about reasearch probes loaded with mistery goo and maybe even small manned spacecraft.

[Jupiter3927]

1 engine is always best for getting the most dV out of your fuel.
You can always do a physical timewarp using [alt+.] to speed things up like you're flying on Kerbin for up to 4x timewarp.

Walking away to do something else or play something on your phone is an option too.

I had an emergeny stage on one of my rockets with 4 ion engines and 4 big xenon tanks.
It was never meant to be used but it had 4 for symmetry and had a ton of dV with extra long burn times.

[andylaugel]

"Reasonable" is subjective. Your vessel's acceleration in m/s/s is going to be equal to it's total thrust divided by its mass. But if you put a basic XL solar panel, modest fuel tank, and an ion engine together, you get 2.05 m/s/s or 0.2g worth of acceleration. So the best case scenerio without much deadweight isn't that great in terms of TWR and burn times.

That said, ion engines do come in handy sometimes. Duzer got himself stranded well below the orbit of Moho, and the 9 km/s dV "Nerv" powered craft I sent to rescue him from Moho got stranded after picking him up. So I built this mighty dV ship to collect him:
http://steamcommunity.com/sharedfiles/filedetails/?id=970797943
0.44 to 1 m/s/s acceleration (4.4% to 10% g) isn't great. But the Poodle got it to my Gilly refueling base, and Duzer is now on his way back with enough dV to orbit Kerbin again.
http://steamcommunity.com/sharedfiles/filedetails/?id=969869071

[andylaugel]

I'm not sure if you want to see this, but here is another example of Dawn ion engines being put to good use.

I like having a survey scanner in polar orbit over every world. So I stuck 4 of them on a single launch vehicle with enough dV to get them into Kerbin orbit and out of Kerbin's shadow. Burns take a while, but the large dV means they can enter polar orbit around any world.
http://steamcommunity.com/sharedfiles/filedetails/?id=759966245
http://steamcommunity.com/sharedfiles/filedetails/?id=755395716

[Captain Katawa]

Originally posted by andylaugel:

"Reasonable" is subjective.

Well right now I have had an absolutely abhorrently boring mission to Jool and back on Ion engines and the worst part about it was not the 40 minutes burn times but the fact i had to pause them and recharge (solar panels arent so great away from the sun) It was way too much real life time spent even with physics warp.

Is there a way to configure ion engines so that they would be more like nuclear engines (less dV but somme more TWR)?

Is it a reasonable configuration to power them by fuel cells or a lot of isotope generators?

[dnrob7]

I did a mussion similar to andylaungel's^
Stranded a kerbal on Moho and had to get drastic with the dV
This is what I came up with in an attempt to have a bit more twr when needed.
http://steamcommunity.com/sharedfiles/filedetails/?id=757072659

Ultimately it wasn't great. I think my save file got nuked somehow before I even got him back from Moho but basically, that 20,000 + dV only applies if you are not using it to fight gravity.

As for actually changing how the dawn engines work, yeh, as I understand it, it's easy. It's just a text file from what I've gathered. Never done it myself but I'm sure google can tell you all about it in a flash.

[andylaugel]

Originally posted by Captain Katawa:

Well right now I have had an absolutely abhorrently boring mission to Jool and back on Ion engines and the worst part about it was not the 40 minutes burn times but the fact i had to pause them and recharge (solar panels arent so great away from the sun) It was way too much real life time spent even with physics warp.

Tip: Click on the engine and limit the thrust to whatever power your solar panels provide. You can see the consumption rate in that fuel window on the upper right side of the screen. It'll still be a long burn, but it won't need as much of your attention.

Originally posted by Captain Katawa:

Is there a way to configure ion engines so that they would be more like nuclear engines (less dV but somme more TWR)?

You can edit part files. This is how mods begin.

Originally posted by Captain Katawa:

Is it a reasonable configuration to power them by fuel cells or a lot of isotope generators?

Fuel cells--no maybe (see below). Fuel cells really come into their own as a means of powering drills and/or an ISRU on the ground. Drills draw ore, ISRU turns it into fuel, and the fuel cells use a fraction of that generated fuel to power the drills and ISRU. If you're packing fuel like that, just install a conventional engine to use it directly.

RTGs (isotope generators) are handy, but consider how many you'd need. Each one produces 0.75 power per second. One ion engine needs 8.74 power per second. You'ld need a dozen to power an ion engine at full. I've done it and it works, but it's pretty pricy ($276,000).

[andylaugel]

Originally posted by Captain Katawa:

Is it a reasonable configuration to power them by fuel cells or a lot of isotope generators?

I ran some numbers and built a test ship here. I'll walk through what I found.

KER tells me an ion engine with a large xenon tank can burn for 3 hours 11 seconds. Working manually gives us a consistent answer. 5250 xenon / 0.486 xenon per second = 10802 seconds. Convert into hours/minutes/seconds and we get 3 hours 2 seconds.

An ion engine and a RC-001S Remote Guidance Unit draw 8.79 electricity per second. A fuel cell can produce 1.5 electricity per second. 8.79/1.5=5.86 fuel cells needed, rounded up to 6.

A fuel cell consumes 6.08 liquid fuel/hour. This times 6 cells times 3 hours = 109.4 liquid fuel. It also consumes a proportionate amount of oxidizer.

Building this ship gives us 7,711 m/s dV, with an acceleration of 0.65 m/s/s, or 0.663g. Or at least that is what KER tells me. And it probably isn't accounting for Liq/Ox fuel consumed by the fuel cells, so the dV is higher.

If we remove the xenon and dawn engine, and replace it with a Terrier, we get 3,298 m/s dV and a 2.93g acceleration.

So, I guess it is possible to do. I just wouldn't recommend it. The probe core will require some power anyways, meaning a solar panel or RTG is still a good thing to have.

[andylaugel]

Originally posted by RoofCatA:

Also ion is not about solar power. It's about batteries giving you enough juice for even the longest burns (15min+) recharging slowly over time in between. You can use small panels for that going down to the sun or 2 RTG going to Dres and beyond. Batteries are much lighter than large solar panels. Also solar panels have the shadow disadvantages - both shadow from themselves as shadow from all imaginable bodies at the worst time possible. You have to fly maneuvers in particular directions you can't change much.

The battery equivalent of 2 large solar panels give a single ion engine 22 minutes, 53 seconds of burn time. (All batteries store 20,000 electriciy/ton.) If our acceleration is 1 m/s/s, that's 1,373 m/s dV per burn. If you're recharging from a single RTG, you're waiting 4h 26m 40s to fully recharge for your next burn.

The big problem with the primarily battery route then is that your vessel effectively has an upper cap on how much dV they can burn in a single maneuver.

Out of curiousity, I ran a few numbers based on my Duzer rescue mission, picutred above. His transfer burn from low solar orbit to Kerbin is 6,100 m/s with ~0.5 m/s/s acceleration for a 12,200 second (3h 23m 20s) burn. It currently uses 2 XL solar panels that weigh 0.6t. But if we relied on battery with the same 8.79 electriciy/second usage rate, we'd need 5.36 tons of batteries for the same burn. Naturally the extra mass would have reduced dV and acceleration, meaning we'd have needed even more batteries.

[Gob]

If you don't mind a TWR and dV hit then you can use a small fuel tank and a fuel cell array and forget solar panels altogether.

[andylaugel]

So many options, so many less than optimal choices.

So it looks like there are four main power options then when using ion engines.

• Solar. Performance is based on distance to the sun per the inverse square law. See http://wiki.kerbalspaceprogram.com/wiki/Electric_charge#Solar_panels.
  
• Fuel cells. You need to pack fuel cells, fuel tanks, and fuel. This gets a little complicated design-wise as you're accounting for 4 different resources used during a burn. How many cells do you need? How long will you ever burn for? How much fuel does that consume? How much dV does it really have, when you factor in Liq/Ox use? This can be the cheapest fund-wise, but it's not mass (TWR/dV) friendly.
  
• Batteries. Each burn is limited to a fraction of your overall dV. You may need to plan out every burn your craft needs to make--or be prepared to make multiple burns spaced apart to complete a single maneuver.
  
• RTGs. Simple, but expensive. A dozen RTGs ($279,600 funds) can fully power one ion engine with power to spare.

That said, you'll need some RTG or solar power anyways to maintain control via a control pod. Batteries or fuel cells alone means the probe will run out of power at some point.

To complicate things further, you can reduce thrust so the engine consumes less power.

Example: While you need 9+ XL solar panels to fully power an ion engine around Jool, you can get by with only 2 XL panels for 21% of the thrust. Burns may take x5 longer, but decreased mass means your dV is higher, the spacecraft requires fewer funds to construct, and acceleration is--well, you lose some in throttle reduction, but gain some in having less dead weight.

[=)]

Never once seen the point of them because they take so long to accelerate, you lose out on the efficiency of burning from periapsis (because after about a minute you are now burning at a much higher altitude and nolonger prograde either). Nukes better.

[Chibbity]

Originally posted by =):

Never once seen the point of them because they take so long to accelerate, you lose out on the efficiency of burning from periapsis (because after about a minute you are now burning at a much higher altitude and nolonger prograde either). Nukes better.

For any longer duration burn (More than a min or so) you should be splitting the burn, Ie. doing half of it before the maneuver node, and half of it after. For example you have a 10 min burn, so you start burning 5 mins before your node. Using this method, you won't experience that problem.

[Captain Katawa]

Originally posted by =):

Never once seen the point of them

Well they have IMMENSE efficiency you can go anywhere and back on a tiny rocket.
My problem is that such efficiency might even be too much but in the same time burn times are just getting crazy