how do you mechjeb correctly?

[endl]

so i have read/followed lots of advice on ship design and messed around with alot of orbital stuff but now its for me to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no kerbal has gone before, now that ive gotten that out of my system heres my current situation.....

i have designed a broad mission that consists of a universal tug and many different craft. atm im just sending them 1 craft and 1 tug at a time into orbit to dock and proceed onward, my dilemma is getting mech jeb to cooperate and not waste my fuel.

i have my tug parked at 150k orbit but mechjeb wont xfer to eeloo because the mission window is 200+ days away so to speed that up i have to move the craft to 600km orbit by doing 3 large burns in order to circularize. this eats alot of fuel.

so this comes down to 2 questions.

1. am i bringing enough fuel

2. is there a better way thats less fuel intensive

its basically an orange can with 4 nukes as the tug, the back is the lander thats a karbonite processor its about the size of half an orange can. im using near future solar/construction for the components.

Edited October 1, 2014 by endl

[Sonny_Jim]

i have my tug parked at 150k orbit but mechjeb wont xfer to eeloo because the mission window is 200+ days away so to speed that up i have to move the craft to 600km orbit

The standard trick is to switch to another vessel that's stationary on Kerbin, which will allow you to do maximum time acceleration until it's close to the mission window, then switch back to the vessel in orbit. Kerbal Alarm Clock is handy for making sure you don't miss the window.

The other option is to delay launching into orbit until the mission window.

[endl]

awesome i hadnt considered that

[John FX]

You could also consider Kerbal Alarm Clock which will set you an alarm for that node and let you get on with doing other flights.

So you would launch to orbit, get MJ to plot your interplanetary node, set an alarm for 10 minutes before the node, <do lots of other stuff>, have the alarm go off with a button that takes you to the craft 10 minutes before the node, select `execute node` on MJ.

I have about 40 flights on the go right now, 8 of them are `live` with alarms for action in times ranging from 45 minutes (LKO orbital maneuver) to 1yr 340d 3h 45m (SOI alarm for Eeloo)

EDIT : Oops, didn`t see KAC had already been suggested. Get KAC...

[Red Iron Crown]

You can also do your timewarping in the Tracking Center.

[Pecan]

...

so this comes down to 2 questions.

1. am i bringing enough fuel

2. is there a better way thats less fuel intensive

1. If you're worried about the amount of fuel it takes to move from a 150km orbit to 600km then, no, you don't have enough fuel to go to Eeloo. Such a small orbital adjustment should be inconsequential compared to interplanetary escape (from Kerbin) and injection (at destination). Same for the amount of time the burn takes.

2. Not really. One thing to remember is that with a long burn-time starting from a low orbit you may well push the ship back into the atmosphere, or even crash, while making the burn (pointing down ahead of the node when you start). Starting from 600km should solve that problem as well as allowing faster timewarp. As has been pointed out KAC is great, you can use maximum timewarp just in the spacecentre screen, without focusing on any ship.

[Hannu]

1. If you're worried about the amount of fuel it takes to move from a 150km orbit to 600km then, no, you don't have enough fuel to go to Eeloo. Such a small orbital adjustment should be inconsequential compared to interplanetary escape (from Kerbin) and injection (at destination). Same for the amount of time the burn takes.

No, it is significantly more effective to make transfer orbit injection burn at low altitude. Then you can use so called Oberth effect to get most kinetic energy out of your velocity change. If you start from 150 km orbit and go first circular 600 km orbit and then make a trans Jool injection, you probably lose several hundreds of meters per second.

It is the reason why real planet probes (Galileo, Cassini) took high risks when they went very near to the target planets during the first braking burn. It was necessary to keep needed dv's achievable.

However, there is one problem in low initial orbit. Orbital prediction of KSP and Mechjeb assumes that burns are instantaneous. But large interplanetary burns are very far from that. Vessel can rotate over large angle around Kerbin and prediction fails severely. In worst case you fall back into atmosphere and even in good case you need corrections of hundreds of meter per sec. Therefore I prefer higher orbits for interplanetary burns. If you leave from 300 km orbit and have TWR of 0.25-0.35 actual dv deviates some tens of m/s (10-30) from predicted and is relatively easy to correct by aborting burn manually or by midcourse correction.

2. is there a better way thats less fuel intensive

Yet there is. Optimal way is to start from lowest possible orbit and burn whole burn to prograde direction. It means than you rotate your craft during the burn. Mechjeb's Smart ASS can do that easily, but the problem is where and when you begin the burn. First you have to determine and achieve exact orbit (LAN and inclination). Alexmoon's porkchop plotter gives inclination straight and you can get correct LAN by using angle to prograde -information. Most difficult thing is determining of burn start time. I do not know suitable software. It would not be an extremely difficult task to program a calculator and I have considered it, but it takes several days if I want reasonable user interface and it gives quite little benefit over traditional "add moar boosters", at least in current version where money is not bottleneck resource at all. And on the other hand, it is fun to plan, build and launch monstrous rockets.

[John FX]

2. is there a better way thats less fuel intensive

You could always try a gravity assist (although if this is your first trip to Eeloo i`d not advise it)

There is a calculator for a fly by available here

The guy who makes it has made a trip to Jool in about 1050Dv (IIRC)...

[MarvinKitFox]

No, it is significantly more effective to make transfer orbit injection burn at low altitude. Then you can use so called Oberth effect to get most kinetic energy out of your velocity change. If you start from 150 km orbit and go first circular 600 km orbit and then make a trans Jool injection, you probably lose several hundreds of meters per second.

Yes, you will lose delta-v. Several hundreds??? no. More like a few dozen.

Considering that you are using ion drive, or Nukes, you need the altitude to make up for the non-instantaneous nature of your transfer boost in any case.

Your losses due to reduced Oberth at altitude will be more than offset by your reduced losses due to your burn requiring a much smaller arc of orbit.

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you *are* using a nuke for your Eeloo transfer, aren't you?

[Hannu]

Yes, you will lose delta-v. Several hundreds??? no. More like a few dozen.

Let us calculate. Example is first Jool window and target is hit to Jool. Alexmoon's calculator shows that you need 2037 m/s from 100 km orbit and 2042 m/s from 600 km orbit. It is almost the same value. Transfer from 100 km orbit to 600 km orbit needs 278 m/s + 243 m/s = 521 m/s.

Considering that you are using ion drive, or Nukes, you need the altitude to make up for the non-instantaneous nature of your transfer boost in any case.

It is true, that low TWR and low orbit is problematic combination. I prefer use of heavier rocket which launces interplanetary transfer stages directly to 300 km orbit. It takes 4900-5000 m/s. If you want to make burn in two phases it is better to burn first to high elliptical orbit with low periapsis and continue at next periapsis pass. In this way you have to pay special attention to timing of maneuvers and avoiding the SOI of Mun. I think that current game gives so easy money that I prefer easy solutions in this thing.

you *are* using a nuke for your Eeloo transfer, aren't you?

Of course, but I try to keep TWR about 0.3 except extremely heavy vessels.