Achieving Interplanetary travel outside of Kerbin's SOI

[DelayedReaction]

Hi All,

haven't posted for a while but i've been having a recurring issue trying to get to Duna for a contract.

I use Mechjeb to help me out but it still doesn't seem to help.

Basically, I build a craft to get me to Duna using the D/V map to check I have enough fuel, get into LKO (~90km), then tell mechjeb to transfer. this usually gives me a node time of ~170'ish days.

the issue I have is that you are limited to a very low speed of time acceleration at a 90km orbit and accelerating for ~170 days will take me a good couple of hours, is there any way to change this?

another thing that i've been considering is whether it would be worthwhile pushing myself into Kerbol orbit and then hohmann (sp?) transferring to Duna that way, or would that just be a ridiculous waste of fuel and/or an impossibility?

thirdly, Mechjeb offers a 'Porkchop' Transfer option but I don't understand the graphic that i'm looking at, is anyone able to explain it? or point me in the right direction to find out the information? (I get that the picture looks like a porkchop).

Cheers

DR

Edited January 18, 2015 by DelayedReaction

[ghpstage]

Disclaimer, i've not used mechjeb so I don't know how it works. I do suspect the 170 days is just waiting for the window to open, which is when Duna reaches around 45 degrees ahead of Kerbin.

To time accelerate to it, you could go to the tracking station, where you can use max time acceleration as your not controlling any ships.

A Kerbol orbit would still need to wait for its own transfer window to open up in order to get there in a reasonable time frame, at a reasonable dV cost. It will also not have Oberth to help in the same way, and would suffer more gravity drag during escape due to the lower escape velocity.

It can certainly be done. I did exactly this when I started trying interplanetary transfers, but it does waste a quite lot of fuel.

Edited January 18, 2015 by ghpstage

[Kryxal]

The Launch Window Planner at http://alexmoon.github.io/ksp/ uses a porkchop plot, just plug some numbers into it and play with it to get a better feel (click on the graph for combinations of departure time and trip length).

[juanml82]

170 days means the time to the maneuver is 170 days, not that you have to burn for 170 days (which isn't possible in game because no ship can carry enough fuel). I suggest you get the mod Kerbal Alarm Clock. Once you have the maneuver node, you add an alarm and Kerbal Alarm Clock will warn you when 170 days have passed. You can either time warp at the Space Center or do other stuff in the meantime (say, send a probe to Jool, continue to explore Kerbin's moons, or anything you like)

I don't think doing a Hoffman transfer from outside Kerbin's SOI is efficient, although it can certainly be done.

Regarding MJ advanced calculator, the colors indicate how much dV the maneuver will require. Red means prohibitive, the darker the blue, the more economical the burn. You click on the spot in the colored chart where you want MJ to create the node and then you click on add the maneuver node (or whatever it's called, I don't have the game running atm).

A word of warning about MJ's calculators, though. Typically, MJ calculates how to get there, but disregards at which inclination you'll make it. If you're sending a single ship, it may not be important. And if you're sending ships to Moho, Dres or Eeloo, just reaching them without enough fuel to circularize, do something there and return might be enough. But if you're sending fleets instead, you might want to make sure all ships circularize at the same inclination once they reach their destination, so you can rendezvous the ships if needed without expending a lot of fuel matching planes. In that case, I'd suggest you use MJ to pinpoint the date and location of the maneuver node, but you then change it for one of your own which takes inclination into account. The mod Precise Node can help you with that.

Finally, while not necessarily relevant for a Duna (or Eve) mission, interplanetary burns can take a lot of time (say, 10 minutes). If you're starting from a low Pe (say 75-80 km) and you just divide the burn time by two, you might dive into the atmosphere during the burn. A way around it is to do several (usually two is enough) burns: get MJ (or by yourself) to pinpoint where and when you need to launch. Now, instead of the full interplanetary node, just burn some 800 dV prograde. That will get your Ap near the Mun orbit, meaning your next Pe (and interplanetary burn) is a few hours ahead. Now, plan your interplanetary burn at your Pe. Since you've already burned 800 dV of the burn, you'll need to burn a lot less time the second time and you won't risk going into the Atmosphere. Plus, long burns tend to be innacurate. If this sounds a bit complicated to explain, there are graphics somewhere which explain it better.

Also, as a rule of a thumb, an interplanetary transfer stage should have four nuclear engines, unless you're sending small probes. Chemical engines are too inefficient (although used ascend stages can give you a kick if they have enough leftover fuel), ions are too slow. And one or two nuclear engines will require very long, and inaccurate burns.

[DelayedReaction]

170 days means the time to the maneuver is 170 days, not that you have to burn for 170 days (which isn't possible in game because no ship can carry enough fuel). I suggest you get the mod Kerbal Alarm Clock. Once you have the maneuver node, you add an alarm and Kerbal Alarm Clock will warn you when 170 days have passed. You can either time warp at the Space Center or do other stuff in the meantime (say, send a probe to Jool, continue to explore Kerbin's moons, or anything you like)

I understood that the node was in 170 days not a 170 day burn, I have Kerbal Alarm Clock, I've just struggled to work out how to use it properly

I don't think doing a Hoffman transfer from outside Kerbin's SOI is efficient, although it can certainly be done.

I didn't think it would be efficient, just speculating on if it could be done.

Regarding MJ advanced calculator, the colors indicate how much dV the maneuver will require. Red means prohibitive, the darker the blue, the more economical the burn. You click on the spot in the colored chart where you want MJ to create the node and then you click on add the maneuver node (or whatever it's called, I don't have the game running atm).

I figured it was something along these lines, red for inefficient, blue for efficient, just didn't realise it was to do with the d/v, I assumed it was to do with how long until the window.

A word of warning about MJ's calculators, though. Typically, MJ calculates how to get there, but disregards at which inclination you'll make it. If you're sending a single ship, it may not be important. And if you're sending ships to Moho, Dres or Eeloo, just reaching them without enough fuel to circularize, do something there and return might be enough. But if you're sending fleets instead, you might want to make sure all ships circularize at the same inclination once they reach their destination, so you can rendezvous the ships if needed without expending a lot of fuel matching planes. In that case, I'd suggest you use MJ to pinpoint the date and location of the maneuver node, but you then change it for one of your own which takes inclination into account. The mod Precise Node can help you with that.

I generally only send one rocket at a time, my pc doesn't run KSP too well (even when I use my linux 64bit installation) so I try to minimize required computing power as much as I can.

Finally, while not necessarily relevant for a Duna (or Eve) mission, interplanetary burns can take a lot of time (say, 10 minutes). If you're starting from a low Pe (say 75-80 km) and you just divide the burn time by two, you might dive into the atmosphere during the burn. A way around it is to do several (usually two is enough) burns: get MJ (or by yourself) to pinpoint where and when you need to launch. Now, instead of the full interplanetary node, just burn some 800 dV prograde. That will get your Ap near the Mun orbit, meaning your next Pe (and interplanetary burn) is a few hours ahead. Now, plan your interplanetary burn at your Pe. Since you've already burned 800 dV of the burn, you'll need to burn a lot less time the second time and you won't risk going into the Atmosphere. Plus, long burns tend to be innacurate. If this sounds a bit complicated to explain, there are graphics somewhere which explain it better.

This... This makes perfect sense to me strangely.

Also, as a rule of a thumb, an interplanetary transfer stage should have four nuclear engines, unless you're sending small probes. Chemical engines are too inefficient (although used ascend stages can give you a kick if they have enough leftover fuel), ions are too slow. And one or two nuclear engines will require very long, and inaccurate burns.

haven't toyed around with nuclear engines yet, a bit unsure about how to use them but i'll have a play around later on...

The Launch Window Planner at http://alexmoon.github.io/ksp/ uses a porkchop plot, just plug some numbers into it and play with it to get a better feel (click on the graph for combinations of departure time and trip length).

this is very helpful, thanks for the link

thanks for the responses everyone, will have a play around and see if I can't get me to Duna

[Hannu]

It is possible to mod time warps, but it causes many nasty side effects. I do not recommend that. Better solution is to put an alarm on KAC and go to KSC to wait.

It can be shown, that if you select two places around central body and two moments of time, there are two possible trajectories between them. In Porkchop plot you choose places, departure time and arrival time (or travel time), put them on coordinate axes and calculate dvs of these two trajectories. Lower dv is selected and point is plotted with color depending on dv. You can see areas of lowest dvs and choose them as transfer windows.

Alexmoon's www-page is better than Mechjeb, because Mechjeb takes only departure dv into account. It tells also more information, which you can use for example to select optimal inclined LKO instead of equatorial.

[Raptor9]

@Delayed Reaction, regarding the transfer window wait, there are two ways to speed this up.

1) Park your ship in a higher orbit around Kerbin say at 250km (my preference). This gets you above one of the warp limit intervals of 240km allowing you to warp much faster. The lower you go, the more altitude intervals set limits on your time warp.

2) Go to the tracking station and sit out the wait there. The tracking station gives you the max allowable warp speed, but it'll be easier to pass the node if you don't have something like KAC set properly to kill your warp.