How do I correctly use an ion probe?

[Mihara]

I've spent quite a while trying to make a probe running stock ion engines actually, you know, fly, but the success so far has been minimal. Sure, ion drives as implemented are for tiny, under-one-ton probes. Well, it's those that I am trying to make work. In particular the kind that start from Kerbin orbit with their ion engines and then go on to, say, Jool, and spin around a few of it's moons.

The biggest problem I've been running into is that a stock ion engine probe is constantly running out of power well before it can possibly complete it's maneuver, and, having limited amounts of solar panels and/or RTGs forces me to cut the throttle, which makes the maneuver even longer, which generally screws everything over. Throwing in enough batteries to last through a maneuver kills the probe's expected delta-v. Throwing in more xenon to compensate drops max acceleration even further and makes maneuvers take yet longer, as if that wasn't enough of a problem. Eventually that spirals out of control and results in a probe that is no longer tiny by any means and still doesn't reliably maneuver.

An unexpected discovery that the large Gigantor solar panels actually are less weight-efficient than a large number of OX-4B, which needs to be arranged in a pattern where they don't cover each other, somewhat helped, but it's still not enough -- while I'm sure it's possible to get the probe to do what I want, in theory, using some orbital trickery, doing maneuvers in many small chunks, etc, etc, it's simply not fun to pilot it, and not worth it, when a comparable -- or even lighter -- quantity of regular rocket fuel and a tiny engine gets the job done better simply because, due to it's high TWR, maneuvers are completed without energy losses.

I tried a probe carrier to get a bunch of probes to Jool on LV-N, but the only halfway workable ion probes I did manage to get result in so many individual parts (I had to mount solar panels on cubic struts to reduce mass and squeeze enough in) that a bunch of them makes the ship completely uncontrollable due to lag.

No matter how I twist it, it appears to me that stock ion engines consume way too much electric charge for the thrust they do generate.

Can anyone tell me they have succeeded in sending an ion probe to visit Jool's moons? More than one moon with the same probe, multiple polar orbits? Stock parts only? How did you fly it?

[RoboRay]

I sent a small ion probe to Moho, which is harder to reach than Jool. Here are some pics of the craft: http://imgur.com/a/K1YnZ#0

From your description of your craft, you seem to be on the right path. You need more solar panels if you can't maintain full throttle. And remember that insolation decreases as you get further away from the sun. A probe to Jool is going to need many more panels than one which stays in the inner solar system.

Batteries are just dead weight. Leave them at home. Keep one RTG on board if you want to ensure the probe is always under control, but if you have no sunlight you can't thrust anyway, so what does it matter?

I would probably plan to aerocapture into Julian orbit, saving you a lot of time and some xenon.

But a bigger probe using more conventional propulsion would probably make more sense.

Edited May 12, 2013 by RoboRay

[HoY]

Your second attempt, using lvn's for the interplanetary burn is the right approach. A single lvn with one flt-800 I think has enough DV to get a 1t probe to jool and capture, BUT. The jool system is so far from the sun that even with 2 gigantor solar arrays you will only be able to run the ion engine at half throttle. If personally not send ions in hat direction, just to the inner planets. This is the probe setup in sending to jool in my exchange thread

http://forum.kerbalspaceprogram.com/showthread.php/28431-I-Present-the-Firebat

The thread is really about the launcher. But it carries 4 probes at 0.7 tonnea each to the jool system and delivers them to a pair of other nuclear ssto's I have there already, which will then in turn place one probe in a polar orbit around each of the four moons and return to laythe.

Design is simple. Docking port, battery, scanners, RCs, rtg, and probe core. The RCS is used to adjust their orbits after the larger crafts get them close

In closing, ion probes don't work well the farther away from Kerbol you get

[Francesco]

yup, solar panels generate half of their power when you are near Jool than when you are near Kerbin.

but I second your question about using ion engines for travelling to outer planets: since you do the escape burn on the dark side of Kerbin, how do you guys manage to do this? batteries, batteries and more batteries?

if my math is correct, a 4 minutes periapsis kick consumes 2880 units of electricity at full throttle.

Edited May 12, 2013 by Francesco

[Scotius]

I tried the same as you Mihara, and ran into the same kind of problem. Not enough power to do one continuous burn-->more solar panels-->more xenon-->TWR drops drastically as probe become larger and heavier. Finally i gave up after my last probe ran out of xenon half-way through capture burn into Duna orbit, and ended in solar orbit without a drop of fuel. It might be possible, but surely not efficient - and probe won't be small and light. Only way around it are mods. There is some adding ion/electric engines with more thrust, or bigger xenon tanks, solar panels etc. Stock? Not really worth the hassle - LFE driven probe would be simpler and more efficient.

[Mihara]

Yours doesn't have a kethane detector on it, which lets you have more solar panels. I've been looking around the mods for solar panels which would be in scale with stock panels in terms of mass and energy output, and somewhere inbetween OX-4 and Gigantor, but with no success so far -- everything is either unrealistically too light or far out of scale in terms of energy output. And I'm banking on aerobraking already, otherwise the probe would simply take too long to get captured.

The one other trick I figured out is to incline the Kerbin orbit you start from. This makes no significant difference to interplanetary transfer, but ensures you don't start your maneuver from Kerbin's shadow, which can make it impossible to complete in time.

[HoY]

Mine actually have both ISA mapsat ANd Kethane scanners on them, tested and working even tho they are clipped ontop of each other. It was the only way i could think of to keep it perfectly balanced while still having a docking port which was also perfectly balanced

[magnemoe]

You need lots of solar panel area, one of the huge ones should be enough most of the time. and yes using the small struts and the panel who point straight out is an good idea 6-10 of them.

You don't need much batteries 2-3 xenon tanks is usual enough. You require less solar panels towards the sun.

This should give you an twr on around 0.25-0.2, remember asas to keep this pointed the right way, total weight should be less than a ton.

Ion probes has an slot between the Oscar and ant who give you 1000-1500 m/s for an small probe and a 4 ton nuclear ones. 8000m/s for less than a ton is impressive

[RoboRay]

Yours doesn't have a kethane detector on it, which lets you have more solar panels.

Lol, what? That makes no difference. The design would have just been slightly different if I had taken one.

As to making your transfer injection over the night side, this is a niche case where you may want to consider launching into a retrograde orbit. Orbiting to the west would put your injection over the day side, and a small ion probe will not require a much larger launch vehicle to deliver the extra 350m/sec launching on heading 270 requires.

[Mihara]

The thread is really about the launcher. But it carries 4 probes at 0.7 tonnea each to the jool system and delivers them to a pair of other nuclear ssto's I have there already, which will then in turn place one probe in a polar orbit around each of the four moons and return to laythe.

I'm nowhere even near sending SSTOs to Laythe. Like I said, I tried to make a probe carrier for six ion powered, halfway workable ion probes, each at 0.8 for 4600 delta v or so.

The thing was way too laggy to fly. I gave up and made a carrier with 8 liquid fuel probes each at 3300 delta v. Much lower part counts on probes, much less pain actually getting them into polar orbit after the carrier drops them as it bounces off the moons, and two spares in case I do mess up.

You need lots of solar panel area, one of the huge ones should be enough most of the time. and yes using the small struts and the panel who point straight out is an good idea 6-10 of them.

Huge ones are definitely worse than lots of small ones, they are much heavier for the same amount of charge.

Lol, what? That makes no difference. The design would have just been slightly different if I had taken one.

Hey, every kilogram counts. Not to mention the unwieldy shapes this ends up making.

As to making your transfer injection over the night side, this is a niche case where you may want to consider launching into a retrograde orbit. Orbiting to the west would put your injection over the day side, and a small ion probe will not require a much larger launch vehicle to deliver the extra 350m/sec launching on heading 270 requires.

Good idea, actually, gotta be better than just inclined.

[HoY]

I was more referencing my method of delivery instead of ion engines. Also, if each of your probes has 3300m/s on its own you can drop some of that fuel, that's loads too much since your delivering them one at a time at each moon with a carrier type dealie.

[DerekL1963]

I tried the same as you Mihara, and ran into the same kind of problem. Not enough power to do one continuous burn-->more solar panels-->more xenon-->TWR drops drastically as probe become larger and heavier.

I've sent a probe to Jool with two Gigantor, two RTG's, five fuel tanks, and eight batteries and have been able to do long burns quite easily so long as I'm not in Jool's shadow.

The tricks to ion engines are: Patience. Roll your vehicle so your Gigantors get full sun. Monitor your throttle and resources to minimize your electrical depletion rate. Patience. They'll get the job done even out at Eeloo, but it's not "firewall the throttle and done in a minute or two". Let the game run in the background while you browse the forums.

[nyrath]

Even in real life, NASA engineers say things like "The problem with ion drives is that we do not have a long enough extension cord"

Solar cell arrays are subject to the inverse-square law, and RTGs or nuclear reactors have too much mass.

In real life, some theorists talk about putting titanic kilometer-wide solar power stations in orbit, and having the stations power the ion-drive craft with beamed power.

http://www.projectrho.com/public_html/rocket/infrastructure.php#id--Solar_Power_Stations

[HoY]

thats a nice concept, and may be an alternative to Ion drives, but it uses H2 rather than Xe

[foamyesque]

Leave from a higher Kerbin orbit. It'll give you more time to complete your burn, and you'll be in shadow for a much shorter interval.

[RoboRay]

"Leave from a higher orbit" is simply saying "use something other than ion engines for the first part of the trip."

[DerekL1963]

Even in real life, NASA engineers say things like "The problem with ion drives is that we do not have a long enough extension cord"

And that goes triple for VASIMR - it's a power hog, not the miracle drive it's sometimes believed to be.

[Read have Read]

Sounds like Periapsis Oberth Kicks would be the best way to escape kerbin with Ions.

Personally I'll be retrofitting Ion drives to my KeoStationary Remotetech relays along with larger dishes *because* they are weaker than the Rockomax 24-77 orange radials to perform orbit correction burns. With two 24-77s I have to tell the flight computer to do a 1% burn for 0.5 seconds and I still overshoot the target.

Edited May 12, 2013 by Read have Read

[magnemoe]

"Leave from a higher orbit" is simply saying "use something other than ion engines for the first part of the trip."

No problem raising orbit from 100 to 500km with ions, before the transfer window. Anyway, an twr of 0.2 is not much worse than an underpowered nuclear ship where 0.4-5 is common.

Look like the point is to avoid the huge solar arrays, with them you get terrible twr.

[T3E]

Hello everyone,

I dare to abuse this Thread quickly for my own sake:

Quite new and after i managed to send stuff in the orbit I find this Idea with the probes quite interesting.

May I ask where to find the Ion engine?

Also I would like to know if theres some necessary "science" or "utility" equipment I need?

So if possible, hit me up with the MUST HAVE Parts, no need for big explanations, I like to figure it out myself mostly (just a kick start necessary )

Cheers

[magnemoe]

Ion engine and tank are under utility. Just one of each.

[Francesco]

PB-Ion Electric Propulsion System a.k.a. ion engine

there are no "must have" utility parts: a parachute is useful on Kerbin, but totally useless on the Mun.

the only must have parts that I know of are boosters