Gigantors for a distant ion-powered flight

[Sharpy]

I (still) have a rescue mission on outskirts of the Kerbol system on a retrograde orbit outside Jool. I said "screw it, I won't bring your MK3 cockpit back" but the kerbal should be rescued.

I recently unlocked ion engines but I still don't have the RTG batteries. A lengthy flight in a command seat on a lightweight vehicle should do the trick, but the ion engines require a lot of power. And the mission will require some 20,000m/s of delta-V so I think nukes "there", ion "back" will allow me to do it. (just bring it into Kerbin orbit; a separate lander will pick her from there.)

If I give the rescue craft enough gigantors, will that suffice, or will the mass ruin the delta-V? What power input from them can I expect on an orbit halfway between Jool and Eloo?

And what would be their optimal placement considering the path to get there (most burns pro/retrograde in circular Kerbol orbit)?

Edited August 4, 2015 by Sharpy

[X-SR71]

I might be mistaken, but with the amount of gigantors for continuous power of an Ion engine up there in Jool's orbit you'd just be better with a fuel tank and an LV-1 Ant..

RTGs wouldn't be a good idea either, the wiki http://wiki.kerbalspaceprogram.com/wiki/IX-6315_%22Dawn%22_Electric_Propulsion_System lists a required 12 RTGs to power the Ion. That would make your rescue craft super costly too (in career).

[Sharpy]

I'm seriously starting to consider taking a lot of batteries and dumping them a'la depleted fuel tanks... 12 medium batteries per 1 medium xenon tank, for a 1.32t of weight.

I guess fuel cells could be a viable alternative. One fuel cell bank could power two ion engines...

Edited July 30, 2015 by Sharpy

[Yukon0009]

Try the fuel cells, a single FLT-200 and a couple of fuel cells apparently can provide a lot of electricity for little weight.

[Mystique]

What is wrong with solar panels in Jool's orbit? IIRC, stock KSP doesn't calculate amount of electric charge based on distance from the Sun.. Is it?

[Yukon0009]

Nope, they changed it.

[X-SR71]

What is wrong with solar panels in Jool's orbit? IIRC, stock KSP doesn't calculate amount of electric charge based on distance from the Sun.. Is it?

Eh.. You haven't been in Jool lately, have you? Like since version 0.90 or so..

[KerikBalm]

I'd say ions there, Nukes back...

Or Nukes both ways

But for just a command seat, that far out, an ant engine and a bit of fuel and a heat shield for aerobraking should work, no?

BTW, solar power always decreased with distance, but it didn't follow a proper incerse square law, but rather some arbitrary curve.

It now properly follows the inverse square law... and as Jool is about 5x as far away from the Sun as Kerbin, solar panels produce about 1/25th as much power out there...

[Sharpy]

I'd say ions there, Nukes back...

Or Nukes both ways

Nukes are heavy, moving them with ions would take forever. I checked my options for nukes both ways but to get the needed delta-V with the return stage I'd need obscenely heavy first stages, something like 3 LF Fuselages Long. For a time I seriously considered picking an asteroid for a fuel tank.

Ant's advantage is that it's very light. You can get some 2-3000 dV in a very tiny package. Unfortunately with its poor ISp that doesn't scale up well, a craft with 10,000 dV on Ant will probably be bigger than one based on Nuke.

as Jool is about 5x as far away from the Sun as Kerbin, solar panels produce about 1/25th as much power out there...

Wiki says "The amount of electricity needed to keep one ion engine running at full thrust is roughly equivalent to half the output of one Gigantor XL Solar Array ". 25 gigantors at 0.3t each, 7.5t, not a total disaster but to accelerate some 10t of the craft with one ion by 10,000m/s... nope, just nope.

And no, no heat shield or parachutes. I'll just bring her into Kerbin orbit and send a separate lander to pick her up.

[Geschosskopf]

If I give the rescue craft enough gigantors, will that suffice, or will the mass ruin the delta-V? What power input from them can I expect on an orbit halfway between Jool and Eloo?

And what would be their optimal placement considering the path to get there (most burns pro/retrograde in circular Kerbol orbit)?

What is wrong with solar panels in Jool's orbit? IIRC, stock KSP doesn't calculate amount of electric charge based on distance from the Sun.. Is it?

As of 1.x, a Gigantor with 100% sun exposure in the Jool system gives you only about 0.9 EC/sec. This makes it hardly better than an RTG. Smaller solar panels hardly register as working at all. The bottom line is that if you are out beyond Duna, solar panels by themselves are incapable of providing high volume EC/sec, such as to run an ion engine at full power. Thus, you need another approach. Some options:

* Run the ion engine at VERY low thrust, just consuming whatever pitiful amount of EC/sec the solar panels provide out there. This will result in burns of epic length, which might make a delicate rendezvous rather difficult.

* Have enough battery to store all the power you need for the longest burn you have to do at full power and use a couple of Gigantors to SLOWLY refill the batteries at a bit less than 2 EC/sec. Then you have to hope they can do this in time for the next burn.

* Use fuel cells to run ion engines. IIRC, this provides about 1600s Isp, considering both the xenon and the LFO consumption. Not too shabby.

* Not use ion engines but just go with nukes.

[Sharpy]

For a while I considered running the nuke with a very heavy thrust limit, utilizing its alternator, but it's really very inferior to fuel cells in efficiency.

[Streetwind]

I'm in the "fuel cell to power twin ions" camp.

Since you're in a retrograde orbit, you can't do much with gravity assists to lower your orbit... I mean sure, you can try, but it would be much easier in a prograde orbit.

So you're going to need a lot of dV. And unfortunately, some engines (like the Ant) are not going to give you that dV. I mean that literally - the engine is incapable of passing 7000 m/s even with the entire observable universe in fuel. But the Isp of the Dawn is so high that you don't have to worry about such things.

Alternatively, you can have a multistage rescue vessel. But the resulting vessel will be heavier than the ion craft, and thus more difficult to transport to the Kerbal that needs to be rescued... remember, you need to reverse the orbit direction to initially rendezvous with him too!

Note: if you pack some solar panels as well, you'll need to carry less liquid fuel for the fuel cell. After reversing your orbit you can leave your periapsis close to the sun, coast down there, and then lower your apoapsis to Kerbin's orbit on solar power. If you get inside Moho's orbit, four OX-4L should do (but arrange them so they have a way to align themselves).

Edited July 30, 2015 by Streetwind

[LordFerret]

I (still) have a rescue mission on outskirts of the Kerbol system on a retrograde orbit outside Jool. I said "screw it, I won't bring your MK3 cockpit back" but the kerbal should be rescued.

I recently unlocked ion engines but I still don't have the RTG batteries. A lengthy flight in a command seat on a lightweight vehicle should do the trick, but the ion engines require a lot of power. And the mission will require some 20,000m/s of delta-V so I think nukes "there", ion "back" will allow me to do it. (just bring it into Kerbin orbit; a separate lander will pick her from there.)

If I give the rescue craft enough gigantors, will that suffice, or will the mass ruin the delta-V? What power input from them can I expect on an orbit halfway between Jool and Eloo?

And what would be their optimal placement considering the path to get there (most burns pro/retrograde in circular Kerbol orbit)?

From a PSA posted back in May:

forum.kerbalspaceprogram.com/threads/118819-PSA-Solar-panels-now-useless-past-Dres-(as-it-should-be)

[tomf]

The ion engine and Fuel cell aray combination has an effective ISP of 2897s 1292s.

if you have a 1 tonne payload (a lander can, parachutes etc) then a ship with a fuel cell array, a FL-400, a pair of ion engines and 15 tanks will have enough electricity to give you a delta-v of 11,000m/s which is a big chunk of what you need.

Edited July 31, 2015 by tomf
Forgot that the wiki has the fuel cell in units not kgs

[Nich]

Rather then a full array why not go with a large battery (2000-3000), 2 6 panel solar cells, rtg and single fuel cell and round 8. It should buy you 2-3 minutes of burn time which should suffice for most burns. when you need a longer burn you can turn on the fuel cell to increase your burn time and you have an RTG to refill the battery in the Jool system. All the systems really synergize well

[cantab]

If you can deal with the long burns it will be fairly straightforward I think. Things like hold prograde / hold manoeuvre make it not so bad, I just set the burn going then watch some TV or play on my 3DS until it finishes. (And if you use an autopilot you don't even need to pay the game any attention at all).

At a rough estimate, 5 tons for the ship excluding the fuel tanks, then ten of the largest xenon tanks should get you the delta-V you need. That gives a ship that's 15 tons overall. You'd be looking at roughly 8 hours of burns at 4x timewarp, which is in my view doable, and you could add extra ion engines to use when closer to the Sun with more available power. If you set it to drop the empty xenon tanks that helps too.

[tomf]

Rather then a full array why not go with a large battery (2000-3000), 2 6 panel solar cells, rtg and single fuel cell and round 8. It should buy you 2-3 minutes of burn time which should suffice for most burns. when you need a longer burn you can turn on the fuel cell to increase your burn time and you have an RTG to refill the battery in the Jool system. All the systems really synergize well

Because to sustain the main long burn you need on the order of 2.5 tonnes of battery. You are masively better of taking a 0.24 tonne fuel cell array and .2 tonnes of fuel to power it.

My theoretical ship comes in at a total of 2.9 tonnes fully loaded

[Sharpy]

One fuel cell and low thrust or impulse burns could work. The problem here is the balance between mass and power production between single fuel cell and an array.

Array: 0.24 t, 18 âš¡/s

Cell: 0.05 t, 1.5 âš¡/s

6 cells: 0.3t, 9 âš¡/s

Meaning I'm getting half the electricity for 25% more mass using single cells, although one cell certainly weighs less than a whole array. Luckily the fuel usage is the same.

[DancesWithSquirrels]

Fuel cells powering ion engines have an Isp of 1293 (math and relevant thread here), which might work reasonably well for you. Alternatively, as proposed by Nich and I believe discussed elsewhere (can't find the thread at the moment) if you have battery power for about 300-500 m/s delta-V and more incoming solar power than your probe requires you can make required orbital maneuvers (pumping orbits at periapse) and making interplanetary transfers at whatever rate your incoming electricity can manage.

There's a neat demo of this method

(~600 m/s for a full jetpack) with refills at pods in the appropriate elliptical orbits.

Note: You might use a Jool gravity assist to reverse your orbital direction on the way out.

[X-SR71]

The ion engine and Fuel cell aray combination has an effective ISP of 2897s.

if you have a 1 tonne payload (a lander can, parachutes etc) then a ship with a fuel cell array, a round 8, a pair of ion engines and 9 tanks will have enough electricity to give you a delta-v of 10,000m/s which is a big chunk of what you need.

Wait, I thought the ISP of ion + fuel cell array was just under 1300: http://forum.kerbalspaceprogram.com/threads/116611-New-ion-craft-using-fuel-cell?p=1911963&viewfull=1#post1911963

[DancesWithSquirrels]

Yep, the Isp of a fuel cell/ion engine combo is 1293, so for 20,000 m/s velocity change the ship has to be 80% fuel, 30% of which is Xenon and 70% of which is fuel/oxidizer by mass.

e ^ (20,000 / (1293 * 9.81) ) = mass ratio of 4.8392, or 20.7% of the ship as empty weight.

So a ship where ~24% of the shop mass is Xenon (which is a lot of xenon), 56% of the ship mass is fuel/oxidizer, will give you 20 km/s delta-V. That's going to be expensive - your contract probably won't cover it.

Staging tanks will, of course, improve that ratio, although I'm not gonna bother running the math right now.

Note: LV-N's with an Isp of 800 would require a ship that's 92.2% fuel for 20 km/s single staged, although again improving with staging of tanks/engines.

[Laie]

I've had great sucess using a few RTGs and a sizeable battery as buffer.

Can't really remember, but I think 2*RTGs and 2000k of battery were good for 5 minutes of ion thrust (and required 40 minutes to recharge). On my vessel, that equalled perhaps 400m/s in a single burn.

I do remember that I could complete every maneuver necessary to scan all Joolian moons with a single probe.

[DancesWithSquirrels]

Sharpy, the originator of the thread, hasn't unlocked RTGs yet, so that's not an option.

[Laie]

Aargh, should have read the OP first. I though I knew what this thread was about after reading a few replies.

Going all-nukes is totally doable and probably a lot easier on the part count than a nuke/ion combo. My Milkrun vessel had 25km/s, 2/3rds of that from nukes (skip forward a few pics to see the nukes alone). Due to the nature of that challenge, my vessel had to have a pretty high TWR -- if you're content with less, a 20km/s nuclear vessel should be doable in less than 50 parts. Try to make it simple enough so you can run it at high physics warp, for your own sanity.

I'd suggest to launch it half-empty and refuel at any old asteroid before going on the main mission.

[cantab]

Yet another thought: If my maths is right the alternator on one nuclear engine generates enough electricity to run 10 ion engines as well, and the combination has a specific impulse of around 1600 s, better than what's being quoted above for fuel-cell-ion propulsion. That might work well, especially as you might have the option of using pure ion power at times.

Scrap that, I mixed up the xenon and Ec consumption!

Edited July 30, 2015 by cantab