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Torch Ship Navigation


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Please forgive me if this is a repeat question. In my limited understanding of things interplanetary I thought it possible if you had enough delta-v you could accelerate half way there, then decelerate for the second half. And, or, with sufficient delta-v you could just go any time and not worry about launch windows. Am I B: correct in my assumptions, and B: if I am correct how would I do that in the game?

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You can decelerate for the second half - that is what you have to do for bodies without atmospheres. And yes, with enough delta-v, you could go anywhere anytime - but longer burn times will also mean they will be innacurate unless you use a ship with an insane TWR. To achieve that, open the debug menu (alt+F12) and you'll find a cheat tab. Choose unlimited fuel.

That, or build a ship with a huge amount of delta-v. If you're carrying little payload and are willing to put up with long burn times, it's possible.

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Yes, but in the game it's not really feasible because you can't time warp when accelerating, and it would take too much real-time to do it. This is why Ion Engines have way, way more thrust in the game than in real life.

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Taking these one at a time:

Could you accelerate half way then decelerate for the other half?

This is a complex question. Delta-v is not constant time wise, meaning that you could spend 1000 m/s of delta-v in 10 minutes or 10 seconds depending on how powerful your engine is. So in theory you could fit a very weak engine and burn it at minimal throttle the whole way (turning around at the half way point) but it would be far less practical and no faster then just using a proper engine and burning for a few minutes in Kerbin orbit at a much higher thrust, coasting, and then braking at Duna or whatever planet you are flying to.

If you are trying to get to another planet faster then burning the whole way is still not practical in KSP - any practical burn will easily be done while still inside Kerbin's SOI. Burning a realistic engine longer then that would fall over into the realm of using cheats or exploits to gain infinite fuel or abuse massless parts and at that point it's still more practical to use those cheats or exploits to make a burn inside Kerbin's SOI and then coast for the few seconds it takes to reach your destination. This does not even get into the fact that you'll likely break the game (Kraken) because of how the floating point numbers work (if you move too fast the numerical precision isn't high enough to keep up accurately)

I'm guessing your question comes from several futurist plans where a nuclear powered probe would fly to another star by accelerating to a significant margin of the speed of light in the first half and braking on the second half of the journey. This does not apply to visiting other planets in KSP, the distances are just so small (and you are still heavily influenced by the gravity of the sun) and the attributes of the engines aren't right for that kind of journey to have any benefit over a 'normal' transfer.

Could just go any time and not worry about launch windows

This is easier to answer. Yes you can go at any time (though in a few isolated cases it may be a shorter trip to wait a short while because a planet is blocking the ideal path and a trajectory around it would take longer then just waiting for the path to clear). The only consequence of going at a different time is the delta-v cost. In fact even leaving at the same time you have a choice in how much delta-v you might trade off to reach your destination faster. You can explore this by looking at the launch window planner. Once you enter a destination and click Plot It you'll get a colored graph showing from lowest cost (blue) to highest (red) departure times and desired travel times. If you move your mouse over the graph you will see how much delta-v that departure time will cost.

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I could see this being a real possibility ... but not with the KSP engine or anything near stock. (lack of) Time acceleration would just kill you.

It may be the OP is looking to make ships that burn nearly continuously to simulate gravity, and I'm fairly sure the tools could be designed to figure out how to get where you're going, but there's been little enough point in KSP as it is.

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B: You are correct, and

B: Use physics-less parts.

Stock trip to Jool in 2 hours, and back to Kerbin in a second or so by Mr. Manley:

If that's your thing, go for it. I prefer time warp. (Though max time warp to Eeloo does take quite a long time.)

Edited by Yasmy
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Please forgive me if this is a repeat question. In my limited understanding of things interplanetary I thought it possible if you had enough delta-v you could accelerate half way there, then decelerate for the second half. And, or, with sufficient delta-v you could just go any time and not worry about launch windows. Am I B: correct in my assumptions, and B: if I am correct how would I do that in the game?

For a classic sci-fi torchship with a very high ISP (on the order of say..... 70,000) and a reasonable acceleration (lets say at least 0.1g, preferably 1 g), you do a "brachistichone" trajectory, which differs a lot from the hohmann impulse trajectories we do in KSP.

Given you are under constant acceleration, orbital mechanics becomes less important, you just thrust in the direction you want to go, and you attain speeds well over escape velocity, your trajectories are often hyperbolic excepts near the beginning and end...

A slightly different case is a very high delta-V capable ship, that travels along impulse trajectories... lets say a ~10,000 m/s ejection burn in excess of 3 g's, and then a ~10,000 m/s insertion burn - in this case you can ignore launch windows and take much more direct routes. It wouldn't be a torchship in that your ship would only be burning for a short time, but it still gets you where you want to go much faster.

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These are some great answers, thanks. I was originally thinking about an Arthur C. Clarke novel, I think it was 2064?, where they accelerated 1g, turned, decelerated 1g to Juipter. It never occurred to me that the program might not be able to keep up. Thanks for the responses.

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they accelerated 1g, turned, decelerated 1g

Many sci-fi novels use the same sort of flight profile, Heinlein's Have Spacesuit, Will Travel comes to mind. It would be a great way to simulate gravity if you could generate thrust for that long without running out of propellant or energy. Even in Clarke's Rendezvous With Rama the alien Rama craft, with a reactionless (propellantless) drive, didn't accelerate constantly.

As for KSP, I'd hate to sit through a days-long burn, even at 4x physics warp, even if kerbal days are only 6 hours

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