Brachistrone trajectory tips?

[MaverickSawyer]

So, I've decided to, for once, turn ON the infinite fuel option in the debug menu so I can have some ludicrous (speed) fun. Basically, I want to make a torch ship out of a habitat module, a Round-8 tank, and at least one Vector, and just go sightsee around the system. Now, this kind of setup would no doubt be insanely impractical to make without the infinite fuel, but since when has that been a requirement? 

 

Anyways, I'm looking for tips on how to pull off these extremely high impulse maneuvers, or how to even do a true Brachistrone trajectory and not shut down the engines until I've arrived at my target. Anyone have some advice?

[cantab]

Point at destination. Burn. You may need to slightly "lead" your target, but remember your trajectory will be a straight line pretty much.

The Mammoth offers slightly greater TWR than the Vector, but your acceleration limit is still "only" 266 m/s². Both are blown away by a couple of the SRBs, with the best being the Launch Escape System, capable of 833 m/s². Even that I think won't let you burn all the way on an interplanetary trip with a reasonable playing time.

[Snark]

I think you mean brachistochrone.

First, the bad news:  abandon all thoughts of using maneuver nodes or projected trajectory in the map view, if you had any.  The game gets horribly confused when your velocity is very high (and by "very high", I mean significant multiple of solar escape velocity, and you'll be going a whole lot faster than that).  Specifically, the intersect markers get really screwed up, basically becoming unusable.

Next, the good news:  with the kind of speeds you'll be dealing with, you don't really need to care about maneuver nodes.  Just point your nose straight at the target and burn.  When you're halfway there, about-face and burn straight away from the target.

Judging "halfway" is gonna be the hard part, since you'll have to just eyeball it and won't have any help from maneuver nodes or the like.  I'd say, if you settle for only accelerating a third of the way there, then leave yourself a dead spot in the middle so you can fiddle with deceleration.

<boring_anecdote>

In case you're interested in how I know all this:  I've never played around with the scenario you're planning.  However, back in 0.90 before re-entry heat and aerodynamics was a thing, I had an Eve land-and-return mission, and it had nearly arrived at Eve when I had a facepalm moment: I realized that I didn't send my kerbalnaut any way to get home.  That is, I had this big mean ol' lander that would do the job to land on Eve and then return to low Eve orbit, but then didn't have any more fuel to get home.  I had originally been planning to send a separate interplanetary crew-return vehicle, but then stupidly forgot about it.

I only had eight days (that's eight Kerbin days, i.e. 48 hours) until it would arrive.  I decided to make it a point of honor that the kerbalnaut would have his return vehicle waiting for him when he arrived at Eve.  So I built a ship with the most insane dV I've ever done in KSP, basically an ion engine with oodles and oodles of xenon drop tanks.  Pre-1.0, reentry wasn't dangerous, so I figured just burn for Eve hell-for-leather and arrive super-fast, then aerobrake to orbit.

So I ended up going from Kerbin to Eve in 48 hours.  Was going several dozen kilometers per second when I hit Eve's atmosphere.  It was... dramatic.

</boring_anecdote>

[Temstar]

13 minutes ago, Snark said:

So I ended up going from Kerbin to Eve in 48 hours.  Was going several dozen kilometers per second when I hit Eve's atmosphere

[MaverickSawyer]

23 minutes ago, cantab said:

Point at destination. Burn. You may need to slightly "lead" your target, but remember your trajectory will be a straight line pretty much.

The Mammoth offers slightly greater TWR than the Vector, but your acceleration limit is still "only" 266 m/s². Both are blown away by a couple of the SRBs, with the best being the Launch Escape System, capable of 833 m/s². Even that I think won't let you burn all the way on an interplanetary trip with a reasonable playing time.

SRBs can't be shut down, though.

[Wcmille]

If it turns the travel time to a very short period, like an hour, that's probably fast enough that you just point at where you are going, and fire. If the travel time is much longer than that, like days, are you really going to not use time warp?

[MaverickSawyer]

Well, currently scooting along in my prototype at 126 km/s and climbing fast (22G acceleration), and I'm under a month to Jool. We'll see how much more I can wring out of it before I chicken out and coast.

 

EDIT: Cut the throttle at ~150 km/s. 22 days and change to Jool encounter.

 

This is fun. Highly impractical, but fun.

Edited December 23, 2015 by MaverickSawyer

[Snark]

Incidentally, the thing that makes continuous-burn trajectories so insanely fast is the fact that they're continuous; they actually don't need particularly high acceleration.  That Kerbin-to-Eve run in 48 hours I mentioned had a ridiculously low TWR... but it kept it up for several hours of game time.

At a measly 1G of continuous acceleration, you can get from Earth to Pluto in something like 18 Earth days.  That's accelerating to midpoint, then turning around and decelerating the rest of the way.  And that's a whole lot bigger than the Kerbol system.

That's not to say that ludicrous accelerations can't be fun.    Certainly, staring at the screen for a couple of hours of 4x physics warp was an experience I'm not particularly eager to repeat.

If you enjoy that sort of high-speed hijinks-- and also want to feel a little less "cheaty"-- I'd suggest giving RoverDude's Karbonite Plus mod a try.  It adds some sci-fi super-engines with ridiculous thrust levels and an Isp of 125,000, so you can do burns pretty much like what you're describing.  The catch is that to fuel those engines, you have to mine a resource that's very hard to obtain (it's present only in very hard-to-get-to locations like the surface of the sun, or on Eve).  Makes for a fun challenge mining the stuff, and when you do, boy howdy what a payoff.  And it really looks great 'coz, well, it's RoverDude.