high delta-v transfers

[psyper]

I want to be able to do high dv transfers for instance from lko to minmus. This is so I can get to a base or station in an emergency (I have tac life support installed). Fuel saving requirements aren't important so I want a heavy burn to leave Kebal and then a heavy burn to get in to orbit when I get to the moon. However as soon as my exit trajectory leaves kerbins soi I lose my close approach markers so I can't work out if I'm getting close to an intercept.

Has anyone else pulled off this manoeuvre successfully and how best do I do it?

Edited January 9, 2015 by psyper

[eddiew]

So basically, Kerbin to Minmus in as straight a line as possible, rather than a Hohman transfer?

Interesting question... sadly I don't have an answer, but as another option, have you considered leaving a supply-cache in Minmus orbit? Probe core, hitch hiker can, plenty of TAC supplies - any problems on the ground, land it nearby and move the kerbals into it as an emergency shelter

Alternatively if you use KAS, you can just pop a pipe end point on the cache-ship and forget the command pod. Wire it up to whatever vessel or station needs supplies and transfer them over like you would with fuel.

[soulsource]

If I understand your question correctly, it'd help you if you could follow the patched conics for more SOI changes, right? Luckily, there's a game setting for this.

Edit: I posted a wrong link. Now it should be correct.

Edited January 8, 2015 by soulsource
Ooops, that was the wrong link.

[Yasmy]

In addition to high delta-v, for direct intercept, you probably also need a high TWR.

Given enough thrust, all (non-singular) gravity wells are shallow.

Perhaps then you won't need the intercept markers.

What happens if you target Minmus and just burn until the prograde marker and target indicator are aligned?

With high thrust, high delta-v, I say wing it.

[merendel]

What happens if you target Minmus and just burn until the prograde marker and target indicator are aligned?

I wouldn't recommend this approach. Minmus is still traveling at around 270m/s around kerbin. If your doing a really high energy transfer your going almost strait up so thats 1km every 4 seconds tangental to your vector. If you spend enough dV to make the trip in 2 hours and align your vector dead center on where minus is now you'll most likely still miss the SOI by the time you get there.

From my own experience I generaly find the departure time for a high energy burn to minmus is prety close to the time you'd leave for a hohman if you've got a good TWR. The more energy you put into the transfer the more your vector deflects closer to minmus's current position. Its not quite 1:1 so you'll slide off target the faster you go but it will still get you prety close. delay the burn by a minute or so and give it a normal componant so your vector still crosses minus's orbit.

The other trick is to look at your SOI departure from kerbin time. Minmus has about a 300 hour orbit give or take. Its orbit is also about 2/3 of the way to the SOI limit so you'll only be crossing its orbit a few hours before hiting the boundry if your going fast enough. for every hour to the SOI limit you need to aim for your vector crossing minmus about a degree or two ahead of it. Drop a node, give it as much dV as you wana devote to the outbound leg, aim your guestimate number of degrees ahead of minmus and give the node enough of a normal componant that your projected vector crosses minmus's orbital track and you should get an encounter. Play a bit with draging the node if need be and it should pop up eventualy.

as a test case 3km/s burn from LKO well away from the AN/DN (so part of that is a normal componat) gets me about 6hours to kerbins SOI crossing minmus's orbit a about the 3 hour mark. Aiming 5-10 degrees ahead of minmus and I start geting encounters with minus. Departure time is a couple minutes after when a homan to minmus would burn.

[Jouni]

You don't get closest approach markers on an escape trajectory, but you do get encounter markers.

If you want to go to Mun or Minmus quickly, first launch the craft to roughly the same orbital plane as the target moon. Once in orbit, create a maneuver node with the desired amount of prograde delta-v. You can create the node anywhere in the orbit. Then drag the node marker around the orbit, until you get a close encounter with the moon.

[Laie]

However as soon as my exit trajectory leaves kerbins soi I lose my close approach markers so I can't work out if I'm getting close to an intercept.

Approach 1: just blindly toy around with a maneuver node and try to get an encounter. You may not have the closest approach thingy, but you will still see when you're projected to pass Minmus' SOI. If your initial burn went off well (high TWR helps a lot), that maneuver to regain an encounter shouldn't take more than a few m/s -- so there's only a limited number of possibilities to explore.

Approach 1a: Try to match planes with Minmus as close as possible before the initial burn. When trying to regain an encounter, you'll only have to explore two dimensions rather than three.

Approach 2: just trust that your initial burn wasn't too far off and pull the brakes as you approach Minmus. Once your projected trajectory no longer leaves Kerbin, you'll be somewhere near Minmus and have your closest approach markers back. Proceed as usual. Should cost you perhaps one or two hours more, and an extra 500-1000m/s.

Approach 2a: Put a maneuver node to where you will cross Minmus' path. That node doesn't need to do anything, it is just for timing purposes. At perhaps one hour before reaching that node (crossing the line), start thrusting sideways so that direction to target and target-relative prograde line up. Repeat in five-minute intervals. Again, this assumes that your initial burn wasn't too far off.

[psyper]

Hi guys, many thanks for the feedback - you're all pretty much saying the same thing - drop a maneuver node down pull it out past minmus orbit until the part that intersects is the right time (using a 2nd maneuver node it should say how long before it takes to get to that point and then remove it) and then pull the node around kerbin until I get an encounter. I think this should work well for mun but for minmus I might have to launch my rescue vehicle in to an orbit which matches minmus' inclination first.

As for putting in an emergency hab system in orbit around minmus - that doesn't sound very kerbal but might implement that with excursions to duna or the jool system - having a spare habitation module ready and close by would be a good idea.

In scifi films and books when going to another star they burn the engines constantly until they are halfway to the star system then turn around and then burn to start reducing their velocity - high dv, low duration flight. Would be interesting to see if I can get a minmus encounter which would take twice as long to get there (give or take a minute to turn around) as the burn takes. Then burn out would be pretty simple but then turning around and burning retrograde might change the orbit a great deal - perhaps I could use that burn to also pull my trajectory in close to the moon so I could hug the orbit in close to the ground and land - now that would be an epic mission! - a suicide burn starting halfway between minmus and kerbin!

[tomf]

A proper torch ship (continuous burns) requires an etremely high ISP. It is still going to several hours to make the trip, which means your engine is going to burn for several hours, which means a huge amount of fuel. It is probably possible to build an Ion engine powered craft that would burn for long enough.

[MarvinKitFox]

No need to go full torchship.

By just burning 200m/s extra at lko (and about 450-500 extra on arrival), you can cut your transit time from Kerbin to Minmus by factor 8-12

**totally** worth it if your limiting constraint is Life Support, or Battery lifetime.

(Learnt this in BTSM, where getting delta-v ability is Much easier than lugging enough potatoes batteries)