How would I handle a very low TWR but very high delta-V transfer?

[OutInSpace]

I've built an ion-powered ship that drops spent Xenon tanks to achieve insane delta-V, on the order of 50kms. My intent with this is to basically jump Hohmann in an alley with a large blunt object, slowly accelerating to the point where I basically have a straight line trajectory to my encounter, coasting, then slowly decelerating into orbit.

My question is: what's the best way to execute this? I can use a porkchop plotter to pick a suitable trajectory, but I'm dealing with a pathetic TWR, only made passable through Better Time Warp. Doing a burn like that all at once with a ship like this all at once would be insane.

[bewing]

I've done something like this myself in the past, and it was fun. I'd suggest making a little rocket ship, turning on "infinite fuel", and then just trying it a few times for practice.

The basic deal is that you look first at where you want to go. Even at a high speed, your target will still move a bit in its orbit before you get there. So you have to lead your target a bit. Then you look at your current location and velocity. If you have blackjacked Hohmann, then (on average), all of your current momentum is wasted, so you need to finish killing it -- or at least kill all the part that you don't want. (Your current momentum can vary from being 100% in the correct direction, to exactly 100% in the wrong direction, so on average it's 0%.) Then you point your nose where you actually want to go, and start accelerating (make sure you get your plane inclination correct).

As you burn, your projected orbit will stretch until it reaches your target CB's orbit, then the intersection point will move along that orbit, then your close approach markers will converge on your target point, and then you will get an SOI intercept shown. Then cut your engines and start looking at how you need to fine-tune your approach. A little burn north, a little burn east or west, a little burn prograde, and you can get a nice approach. You will already be moving so fast at that point that you do not want to continue burning. Adding more deltaVs won't save you that much time, and it totally messes up the process of getting a close approach.

Doing a constant acceleration, followed by a flip ship, followed by a constant deceleration and ending up at exactly the right point in space is an exercise for a computer, not a KSP player.

 

Edited October 28, 2020 by bewing

[swjr-swis]

2 hours ago, OutInSpace said:

My intent with this is to basically jump Hohmann in an alley with a large blunt object, slowly accelerating to the point where I basically have a straight line trajectory to my encounter, coasting, then slowly decelerating into orbit.

You do realize that what you're suggesting to do ends up being a curve anyway, right? Except now instead of aiming for a known and (relatively) easily pre-calculated target, you're trying to aim for a target that keeps moving at an increasing then decreasing speed, right up to the moment you arrive.

 

2 hours ago, OutInSpace said:

Doing a burn like that all at once with a ship like this all at once would be insane.

Insane, perhaps. Also the only possible way to get even close to an actual straight line trajectory (as straight lines go in a gravity-controlled lightspeed-limited universe).

[Fierce Wolf]

Is your craft named Rocinante?

[OutInSpace]

9 minutes ago, Fierce Wolf said:

Is your craft named Rocinante?

Unfortunately no . And I think the Roci has 0.03c of delta-v, or something insane like that. I just have 50kms and a crap TWR.

13 hours ago, bewing said:

Snip

 

Awesome! I tried it with the infinite fuel cheat like you suggested and I got pretty close. I tested it with the ion ship (using Pe kicking to get out of Kerbin) and I got similar results, although some precision was lost due to the insane physics warp I was pulling to get through the multi-day burns.

I do wonder if it would be more effective to eject from Kerbin using a chemical booster before firing up the ions to shift the course onto the intercept. Shouldn't be too hard to test, and it negates the risk of a Mun encounter when periapsis kicking.

Edited October 28, 2020 by OutInSpace