going to Jool, couple of issues

[Wallace]

Soo, I have my ship all packed up ready to go for the <dramatic movie trailer voice> Big Jool Mission </>, but i've just got a couple of things bugging me before I start.

Problem one is, I have a disagreement between Mechjeb and Kerbal Alarm Clock on the launch window. KAC says it is 13 days away, mechjeb says 19, but with a funny twist, I've actually got a few smaller probes going with the big ship and some of them it tried to give a departure over a year away on the first try, and then when I tried again later gave them 19 days. Also, all the paths mechjeb has given to the ships go through the mun's SOI but at such high speed (exit burn is ~1800m/s) their course isn't altered it's just totally straight. So i'm not sure what to trust, should I go with the mechjeb manoeuvre or make my own at the time KAC tells me?

Second issues, well, it's not really a problem, just fishing for a bit of advice. According to the DV map i've been looking at I should need 9045 m/s to go from Kerbin orbit to Jool orbit, and I currently have enough fuel tanks in tow to give me about 9100 m/s. However, I've found on previous interplanetary missions I almost always needed more DV due to ending up on a non-optimum route, soo, how much spare Dv would be sensible to add to the ship?

Edited June 25, 2014 by Wallace

[Red Iron Crown]

Problem 1: I use alexmoon's excellent transfer calculator. Learn it, love it.

Problem 2: You can save a ton of dV by aerobraking at Jool, Laythe, and upon return to Kerbin. You should have enough to do a fair bit of puttering around the Jool system.

[Taki117]

As far as when you should perform the Maneuver, I would go with Mechjeb, even without the Mun's Gravity Assist you will need to make a course correction. Also, Departure Windows aren't that small. Sure you're going to burn a little more dV in either direction, but we're talking less than 100m/s.

[Oafman]

Yes, as Taki said, windows are open for a while. I imagine either of MJ or Alarm Clocks' suggestions would be fine (6 days is not much difference), although I guess one would be slightly more optimal than the other.

The journey to Jool from LKO shouldn't take much more than 2000m/s; maybe a bit more for corrections etc. My recent trip was a 1900m/s burn, then maybe 50m/s more at a halfway correction.

When you get there, aerobrake at Jool, so you have a capture orbit. Then have a look where Laythe is, and see if your orbit gets close to it. I only had to make a small tweak to get a laythe intercept. I then aerobraked again at Laythe for capture, then circularised.

One thing I did, which I reckon was worth it, was spend ~200m/s correcting my plane at Jool's equatorial node, just before my aerobrake. This put me on the exact plane with Laythe, meaning that all subsequent maneuvers were in 2D only, so were much more straightforward

[Red Iron Crown]

Relatively easy to get good gravity slingshots in the Jool system from Tylo and Lythe, too.

[Wallace]

Seems I counted the DV required for orbital insertion when I was looking at the DV map, and forgot I don't need that if I do aero breaking, looks like I will have plenty enough DV already, I guess I'll be able to go round and visiting moons with the spare.

[Random Tank]

I'd always go with KAC for transfer windows; mainly because I don't trust any dirty AI's. MJ will tell you to burn this way to get to Jool, but suddenly you'll find yourself flying into the sun. It's like some evil Sat-Nav.

[Wallace]

Mechjeb did propose some very dodgy manoeuvres when I went to Moho, which would have intercepted at a horrible angle and used far too much Dv for braking to get into orbit, and I had to redo it all myself, so I'm fairly suspicious of anything it tells me.

[Pulstar]

I always went with what KAC suggested and never had any problems with going to Jool or elsewhere with it.

[MarvinKitFox]

9000 d-v?

Already in LKO?

You have enough legs to go to Jool, and come back.

.

.

Twice.

[Wallace]

Well, I've dispatched all my lil craft to Jool, 3 of them went according to the Mechjeb manoeuvre nodes, and 2 or them went a bit later as the main ship rather overshot it's burn time. It turned out to be a bit more unstable than I had expected so I had to turn off most of the high thrust engines that were supposed to get it out of orbit fast so it didn't quite manage it it one burn, it hard to do a second burn with it's nuclear engines when it was passing the Mun, so the two ships that were supposed to go just after it missed their manoeuvre nodes.

It all turned out ok in the end, except the arrival times for 2 ships that set off late are a few hundred days out from the rest. :S

The main ship and one carrying extra vehicles have 6000 DV left and the three ion probes each have about 3000 DV of rocket and 12000 Dv of ion, so I should be able to visit all the muns when I get there.

I've realised what my mistake was, I made some notes of DV requirements on some post it notes but I accidentally put the DV from the surface of Kerbin to jool orbit down but labelled it wrong and then I thought it was the Dv from Kerbin orbit to Jool orbit when I came back to it, and I included fuel for breaking so I wouldn't have to aerobrake oppses!

All the spare fuel will be useful as I have 2 remote probes and 2 small ships that will attach to the main space station that will need fuel, and I'm sure it will come in useful for the follow up mission to establish the Laythe base, so it went ok in the end, and next time I'll be sure to write my lil notes a bit more carefully. If only I'd included a Tylo lander with the mission I could have done the Jool 5 challenge.

Here's a picture of the 9000DV monstrosity.

You can see the high thrust engines (16 LV-T30s) halfway down, they use all the fuel behind them to give about 2000DV starting at 0.8 TWR, the engines were actually left over from a 'upsidedown rocket' experiment that managed to bring 1 and half orange tanks of fuel into orbit, which is probably why they turned out overpowered. I switch half of them off and still only had it on half throttle and it still was close snapping in half at times. The front half has about 8000Dv and performed much better but those 5 tanks still flopped about like a Wacky Waving Inflatable Arm Flailing Tube Man despite being towed from the front. It used over 3000dv on a 1800dv burn from flopping about.

Edited June 26, 2014 by Wallace
more details, also fixed punctuation.

[MarvinKitFox]

Oh.....kay.

That look solid, yes it does.

[Wallace]

Hey, it worked... not well of course, but it worked.