interplanetary burns using ion engines

[codepoet]

Does anyone have any tips on how to go about doing an interplanetary burn using ion engines on a quite large probe. I have 5t probe powered by one ion engine which means long burns (12hrs to burn all the propellent). What is the best strategy for doing burns? I need to get far enough from kerbin to be able to a long enough burn on one side of the orbit to go interplanetary. Should I do lots of burns at prograde and then one long one at the apoapsis to leave kerbin, or go for a circular higher orbit before leaving, or just burn prograde all the way around until I am higher? Added tothis is the problem of not being able to burn in the shade of kerbin, which is where I probably want to most of all.

Any tips?

[KvickFlygarn87]

I would go for a higher orbit.

[Spica]

I only turn on my ion engines once I have reached escape velocity by using my main rocket.

[Dante80]

A question would be why do you need a 5tn probe to have a single ion engine. You either have too much of "probe", or too little of "engine" in your design I think. Why not use a stage before the probe to make the kerbin escape burn, and ditch it when you are done?

Having said that, I think the best way would be to bring your peri above 120km and burn there (3' before until 3' after) for as many orbits needed to clear kerbin SOI.

[Superfluous J]

I'll forego the customary "don't" here because you really seem committed to this.

You paired up engines that require power with a flight plan that robs your ship of power. You must overcome that obstacle before you can do anything. A higher orbit will make Kerbin smaller in your field, thereby lessening the time you are behind it. You could also tilt your orbit a bit (once you've made it higher) so you come in under or over it, keeping Sun in your sky longer or (ideally) make it not be blocked by Kerbin at all.

I hope you're using kOS or MechJeb

[sewerbird]

To echo others here, longer escape burns typically want a higher periapse around kerbin, not only because you'll avoid clipping through atmosphere whilst pointing at your maneuver node, but also because the curvature of your ellipse will be less pronounced: this means that more of your burn is 'tangent', which is a good thing. If you're open to fixing some of this problem in the VAB, I personally enjoy an 'Ejection stage' consisting of a large gray tank and a skipper engine: it'll push your payload out of LKO handily and quickly, and your ions can then do the rest of the burn along the way out of Kerbin SoI.

[Geschosskopf]

Well, 5 tons of payload is a bit much even for the more powerful ion, VASIMR, and other types of engines in the Near Future mod. Still, even doubling your tiny amount of thrust will cut your burntime in half. The problem, however, is that all these exotic engines require beaucoup electricity PER SECOND, creating which quickly adds a lot of weight, offsetting the benefits of the higher thrust.

So the real question I have is, what do you need a 5-ton payload for? I'm starting to use ion and Near Future engines a lot these days and I can usually do everything I need for about 6 tons total, including engines, fuel, and the massive electrical power planet. Science parts are very light and even other stuff doesn't weigh that much. For instance, last night I built a probe mining unit for use on Pol. It carries the small refining unit, 2 of the small drills, small tanks for every type of fuel, and flies on 2 of the Near Future ion engines powered by a mid-sized nuclear reactor. The whole thing weighs only 6.5 tons and has about 8500m/s delta-V even fully loaded with enouth TWR to land and take off from Pol.

[codepoet]

Since many of you are asking - the reason I have a large probe with a small ion engine is I am trying to put together my own baseline entry for a challenge (which I might post on the challenge thread if I can figure out how to do it myself) which is something along the lines of:

"What is the smallest lauch mass to establish a kethane plan on laythe?" So for 5t I have a kethane tank, drill, coverter, empty fuel tank, ion propulsion, solar, and Entry-Decent-Landing system. The design challenge boils down to the fact that using a small kethane drill creates a lopsided craft, so you have to find ways of balancing it. Also landing it on laythe is a challenge without lots of fuel+rockets. There is also the piloting challenge that we are discussing here, and then the challenge of getting into LKO as efficently as possible. Once I have got to laythe I will consider putting it on the challenge board.

So back to the case in point:

Is it better to go for a high circular orbit, or a high elipse? The problem with the eplipse is that it takes much longer to raise the apoapsis, and then you can end up missing your transfer window. You also need to get the AP to be opposite to the sun at the moment of the transfer window, but it would not be when you start doing your AP raising burns. Also there is the problem of the time at PE being so small to do the AP burns.

Yes I am using mechjeb to take the boring away.

[Geschosskopf]

Hmmm. Well, an obvious solution is to put each Kethane function on a separate ship. Then the ships might be small enough for stock ion engines to be viable for the interplanetary burns, provided you leave MJ to do them while you go watch TV, read a book, or whatever. And landing on Laythe only requires about 50m/s to deorbit and then just parachutes, provided you're good at landing on small targets which is all you have there.

Before doing that, though, you might want to reconsider the underlying mission plan. Kethane on Laythe is a dodgy proposition. First off, Laythe has very little land area so there's a decent chance you might not get any Kethane on land at all. And even if you do have land-based Kethane, there's a very good chance it will be at an inconveniently high latitude, thus entailing a significant plane change both going and coming. That's luck of the draw and you won't know until you get there and scan.

But more importantly, why in the world would you want your Kethane rig to ever take off from Laythe? Taking off from Laythe requires about 3/4 the delta-V of launching the same payload from Kerbin so it's not economically viable to use Laythe as a Kethane refueling base for spaceships. Kethane on Laythe should stay on Laythe refueling airplanes and the odd lander or spaceplane used for crew rotations. To refuel spaceships in the Jool system, at a convenient distance from Laythe, use Vall.

[codepoet]

Hmmm. Well, an obvious solution is to put each Kethane function on a separate ship. Then the ships might be small enough for stock ion engines to be viable for the interplanetary burns, provided you leave MJ to do them while you go watch TV, read a book, or whatever. And landing on Laythe only requires about 50m/s to deorbit and then just parachutes, provided you're good at landing on small targets which is all you have there.

Before doing that, though, you might want to reconsider the underlying mission plan. Kethane on Laythe is a dodgy proposition. First off, Laythe has very little land area so there's a decent chance you might not get any Kethane on land at all. And even if you do have land-based Kethane, there's a very good chance it will be at an inconveniently high latitude, thus entailing a significant plane change both going and coming. That's luck of the draw and you won't know until you get there and scan.

But more importantly, why in the world would you want your Kethane rig to ever take off from Laythe? Taking off from Laythe requires about 3/4 the delta-V of launching the same payload from Kerbin so it's not economically viable to use Laythe as a Kethane refueling base for spaceships. Kethane on Laythe should stay on Laythe refueling airplanes and the odd lander or spaceplane used for crew rotations. To refuel spaceships in the Jool system, at a convenient distance from Laythe, use Vall.

Thanks for your thoughts. You are right - it is probably not a good idea to set up Laythe as a base of kethane operations. However I do have a backstory to make the challenge make sense. I hope to provide a game file where there is kethane in an appropriate place on laythe - the challenge is to get it with the smallest possible launch (in fact it will probably be to refuel (and so rescue) a crew that has been stranded on laythe by a fuel leak).

So back to the OP... I am thinking the best option is to burn prograde for a couple of orbits, until I have a roughly circular orbit at about 1000-1500km, than then to a series of big burns on the sun side to raise the AP to near the edge of kerbin SOI on the night side. Once a the highest AP then do the interplanetary burn. I think the problem is that I will need about 5-6 days lead time to get to the high AP, so really setting off 2 hours after the transfer window is not the best start!

[Sirrobert]

If you really want to do it with that 1 engine, you are going to have to do it in multiple orbits. A low periaps to use the orberth effect, and than just burn at periaps until you are to high, orbit around the planet again, burn again, until you reach escape velocity.

Or just land and get something with real engines, cause you are going to have the same problem trying to slow down.

[Designer225]

Ion engines have very low thrusts and uses a lot of power, so I don't recommend you to use them to escape. Instead, use LV-N's or Poodles for escape. The ion engine is best for interplanetary/interstellar (not possible) travel, but not escape-to-interplanetary stuff.

[numerobis]

Burn in daylight to get your apoapsis up high radially out from the sun. Then wait 3 months and your apoapsis is up high retrograde (so you're well on your way to an exit burn to Eve); another 6 months and it will be high prograde (for an exit burn to Jool). It can be tricky to time things precisely like this, but most launch windows are several days long, so you can afford to wait one whole orbit. When the time comes, burn starting close to periapsis, and keep burning; you won't be in darkness very long on your way out.

[numerobis]

Ion engines have very low thrusts and uses a lot of power, so I don't recommend you to use them to escape. Instead, use LV-N's or Poodles for escape. The ion engine is best for interplanetary/interstellar (not possible) travel, but not escape-to-interplanetary stuff.

This is almost equivalent to saying "don't use ion engines ever"; you will be burning only a few tens of meters per second in interplanetary space, so why bother with ion engines for the adjustments? Use your main engine instead. If you need to be able to make tiny adjustments in the cm/s range, you can use a single LV-1 or an LV-1R mounted close to the center of gravity (but not necessarily right on it; gyros can easily compensate for its tiny thrust), for just 0.03t extra mass, use the same fuel, and get twice the thrust of an ion engine -- you can even splurge and use a symmetric pair.

The only way to make the roughly 0.5t mass of the ion engine subsystem worthwhile, masswise, is to use it for a long burn.

[Superfluous J]

This is almost equivalent to saying "don't use ion engines ever".

You pretty much nailed my opinion.

[Geschosskopf]

Well, if you want a circular Kerbin orbit, you'll have to raise your Pe, too. If you wait until you've got your Ap all the way up, you'll face an equally long burntime raising the Pe, or just as many orbits as used to raise the Ap.

So it might be better to "walk up" in small steps on both Ap and Pe. on each side of your orbit, burn for a minute or 2 to raise the other side above where you are now, repeat until you're where you want to be. That way, you're busy all the time, not doing 1 long, boring burn. Plus, after a while, you can start using really high warp speeds to get back around quickly.

But you know, being that far from Kerbin makes launch windows kinda meaningless. How long does it take Minmus to orbit Kerbin? A week? A month? And you'll be going even slower. So it's very probable that you'll be days or weeks off the best launch window by the time your ship circles around to proper ejection angle for the transfer burn. But one of the beauties of ion engines is that this really doesn't matter because you've got delta-V out the yinyang. So why bother waiting for the proper window at all? Once you get your orbit where you want it, plunk down a transfer node and see what you get. If it's within your budget, go for it.

[Kulebron]

I managed to send a some-tonne probe to Mün encounter by doing several burns in periapsis. But then I needed a fast insertion burn, which was impossible, instead Mün gave it a slingshot into Kerbin surface. So you need either a very good planinng of slingshots (which you can't plan exactly for a long period, because of floating point calculations imprecision), or a backup liquid fuel engine.

[Burninate]

I just finished up a 12 ton no-docking Moho heavy lander and return mission. 6x (5x xenon, goo tank, gel pod, 2 batteries, 2 6x1 panels, 1 passive panel, 3 legs, 200-size tank, 30kn rocket engine, parachute), plus a central pod with 1x 400-size tank, a one-man capsule, two more 6x1 panels, two passive panels, and an ion engine:

No parts were left on Moho. This used some Rockomaxes to get it into orbit, a 3200+atomic transfer stage to loft it through a tight Eve gravity assist, and the rest of the ship made it all the way from the entry to the Moho SOI back to Kerbin. The ions are of course useless for achieving an escape trajectory, necessitating the mixed approach. A long inclination burn and then a high-elliptical apoapsis burn on the way back (after many orbits of waiting and adding maneuver nodes for predictions) let me aerocapture my way (@ 30km)into Kerbin orbit and finally plan my descent directly to KSC HQ with almost empty xenon and liquidfuel tanks.

This was around 25 hours gametime of burning, plus many more for orbital maneuvers. No mods, no Mechjeb, no Protractor, no Alarm Clock (though I could have used the last two).

Things that were vital:

* Just enough solar panels, in a wide enough variety of angles, to assure that you're almost always juiced enough for 100% throttle. A few batteries for if you're forced to make a multi-phase transition on the dark side of a planet (this was a 20-phase final approach to Kerbin for me)

* alt + > 4x fast forward mode

* Reserving the liquid fuel for landing or taking off, and in my case for a very good gravity assist that took a few times to get right.

* Multi-orbit transfer forecasting. Place half a dozen maneuver markers and then modify the first one, and the game will tell you if any of them result in valid transfer trajectories. My last transfer burn to get to an across-system Kerbin encounter was only 1.7m/s and happened five orbits out from the actual encounter, and right after the SOI transition I was moving at 3300m/s, so I had to plan the encounter with Kerbin's atmosphere *long* before the SOI by looking at the post-SOI colored predictions, and make several adjustment burns.

*Have a sense of the resonance effect - where coming close to a multiple or the orbital period will end up having the ship moving a little bit ahead or back in relation to the target each orbit

* Live, manual transfer correction. Because the forecasted maneuvers are for point accelerations and only go so far, while the ingame map will tell you your current forecasted path very well.

* Having something else to multitask on during burns. 25 hours of burning gametime converts to 6.25 hours at 4x speed, but add another 2 hours retrying landing burns and 1 hour on achieving the perfect Eve assist and 1 hour mostly waiting and forecasting for the return trip, and you end up with 10 hours spread over an entire weekend, much of it alt-tabbed to a browser window.

* Quicksave. If you're operating close to the line of possibility, you need to use it strategically.

Edited November 18, 2013 by Burninate

[numerobis]

Impressive patience!