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[Tutorial] Interplanetary How-To Guide


Kosmo-not
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I feel like I'm too dumb for this. Can someone explain further? Let's say I want to get to Duna from Kerbin at a 100km parking orbit. The calculator that Olex made is helpful, if I knew how to use it. Also, I am using MechJeb.

What is a Phase Angle, and how do I get there? 44.36°

What is an ejection angle, and how do I get there? 150.91°

What is an ejection velocity and how to I get there? 3289.2 m/s

If just someone could explain further and much clearer, then I can figure this out, but these to me are just random numbers.

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Patq911,

The phase angle is the angle between Kerbin and Duna with the vertex at the sun. You get there by time-warping before you launch until the planets are properly aligned. This is just like how in real-life the NASA scientists wait for certain launch windows before sending stuff to Mars.

The ejection angle is the angle formed by a line extending along Kerbin's prograde direction of travel along its orbit and a line through the current position of your ship, with the vertex at the center of Kerbin. You get there by time-warping once you're in a circular equatorial parking orbit. This will be a much shorter time warp than the one to align the phase angle and won't screw up your phase angle.

Your ejection velocity is the orbital velocity you need to accelerate to. You get there by firing up your engines while your nav-ball is pointed prograde until the display reads the desired velocity.

To find these angles within KSP, you can use this plugin: http://kerbalspaceprogram.com/forum/showthread.php/21111-PLUGIN-0-17-AdamKSP-Show-Phase-and-Ejection-Angle-and-closest-approach-v0-5

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Patq911,

The phase angle is the angle between Kerbin and Duna with the vertex at the sun. You get there by time-warping before you launch until the planets are properly aligned. This is just like how in real-life the NASA scientists wait for certain launch windows before sending stuff to Mars.

The ejection angle is the angle formed by a line extending along Kerbin's prograde direction of travel along its orbit and a line through the current position of your ship, with the vertex at the center of Kerbin. You get there by time-warping once you're in a circular equatorial parking orbit. This will be a much shorter time warp than the one to align the phase angle and won't screw up your phase angle.

Your ejection velocity is the orbital velocity you need to accelerate to. You get there by firing up your engines while your nav-ball is pointed prograde until the display reads the desired velocity.

To find these angles within KSP, you can use this plugin: http://kerbalspaceprogram.com/forum/showthread.php/21111-PLUGIN-0-17-AdamKSP-Show-Phase-and-Ejection-Angle-and-closest-approach-v0-5

This is helpful, but in general, how big is your window for each of the 3?

Also, is the ejection velocity how fast you have to go, or that much faster than what you're already going?

Edited by Patq911
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"it depends"

For example I was able to get a duna encounter just by eye-balling both angles, but I've heard that Jool takes more precision (haven't been there myself yet - been busy working on a different project)

The ejection velocity is how fast you have to go, as it reads out on the nav ball. In reality this ejection velocity isn't important. It would be the exact speed you'd have to reach if you could apply an instantaneous "burst" of thrust up to that speed, but since rockets accelerate smoothly, the actual speed you're going when you see the encounter pop will be somewhat different.

Another tip is to start your burn somewhat before the ejection angle. Since, once again, the rocket doesn't immediately get up to speed you need to give yourself some time to accelerate.

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  • 2 weeks later...
  • 7 months later...

The picture in the original post that contains the formula to calculate the ejection angle seems to be malfunctioning(bad link). . . Does anyone know what this formula is? Perhaps the original post could be amended to re-include it?

Thanks in advance

EDIT: Found the linked PDF halfway through that has the original equations. Original post should probably be updated to include the missing info, or link the PDF.

P.S. This guide is brilliant, very informative, perfectly laid out. Doing things in a more manual fashion just makes landing on another planet more exciting:) Thanks to all involved in making this happen!

Edited by Uberick
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  • 2 weeks later...

Read all 9 pages, couldn't find this answered... When I start my ejection burn, should I

1. keep my ship pointed at prograde during the burn (it obviously changes) or should I

2. point it at prograde at the start, and maintain that angle through the whole burn despite the prograde drift?

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  • 1 month later...

For some reason the images on the calculator aren't showing up at all; I have both java and flash fully updated and am using Chrome, don't know what the problem is.

Since I don't have a visual representation and am a bit dense-when, precisely, should I be starting my burn? I'm using Kerbal Engineer, trying to get to Eve, and, if I've understood it correctly, I want to burn when the "angle to prograde" value in K.E equals the Ejection Angle, right?

Sorry if I've completely misunderstood this, I'm rather inexperienced with anything more complicated than Mun-crashes, and am not exactly the smartest person alive :huh:

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  • 4 weeks later...

Was looking at Interactive illustrated interplanetary guide and calculator for KSP

by olex, based on Kosmo-not's interplanetary how-to guide

http://ksp.olex.biz/

http://forum.kerbalspaceprogram.com/showthread.php/16511-Interplanetary-How-To-Guide

How does one calculate ones ejection angle for a hyperbolic transition?

Ideally I am looking for an answer based on mass and velocity, not geometry as KSP does not provide me with eccentricity or formulas.

eg eccentricity = sqrt(a*a +b*b)/a

Thanks

Kessel_Run

PS very good guide can be found at

http://www.braeunig.us/space/interpl.htm#transfer

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Here's a formula I just derived which should give you the ejection angle:

cLw6FMc.png

Burn when you are behind the planet's prograde / retrograde direction by that angle.

v is the velocity of the ship after the ejection maneuver (current orbital velocity + transfer dv)

μ is the gravitational parameter of the planet (grab it from the wiki.)

r is your orbital height above the planet's center (orbital altitude + planet radius from wiki.)

Haven't tested it yet; hopefully I haven't made any mistakes :P.

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  • 2 weeks later...
  • 7 months later...

This is the first time I've ever looked at a math problem and thought, "Wow I can't wait to get into that!" I saw the calculator page too and thought that was well done, but I kind of want to do the math for myself for some reason even though I'm pretty rusty.

If math was taught to me in terms of tantalizing rockets blasting and gravity wells and just generally trippy stuff instead of how many rocks are left over I might have taken more of an interest in math's potential.

I love the Kerbal Space Program! I can't even really understand why except that I love how gravity becomes your best friend and your worst enemy at the same time and the energy required to have "free will" is enormous. After growing up with Star Trek I really appreciate the stark reality of the 'real' problems of space travel rather than dealing with subspace interference messing up magic crystals.

I think there should be more emphasis on telling stories of the huge seemingly impossible mountain that must be climbed for travel within the solar system to become practical rather than on aliens with tight asses because you're only horny sometimes but gravity is forever. Also one story is easier to tell than the other, but the other is more spiritually rewarding in the end. :confused:

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  • 1 month later...
Read all 9 pages, couldn't find this answered... When I start my ejection burn, should I

1. keep my ship pointed at prograde during the burn (it obviously changes) or should I

2. point it at prograde at the start, and maintain that angle through the whole burn despite the prograde drift?

Ideally, like time, you'd split the difference. Thought experiment, you are orbiting a body. We've stolen the third dimension so can say you are orbiting clockwise such that your ship is oriented to 3 o'clock when your position is at 12 o'clock. I.e., You're ship points to the right. You wish to give an impulse at 12 o'clock in your prograde direction but your thrust is so low you calculate you'll have to start at 9 o'clock and end at 3 o'clock. As such you should start the burn at 9 o'clock when you are radial -, burn through the intended 12 o'clock (now halfway through your burn and prograde), and finish at 3 o'clock when you are radial +. The vector change will be the same as if you instantaneously burned prograde at 12 o'clock.

In KSP just set up a maneuver node and burn on it half the time (plus a touch for acceleration) before the node encounter time and half after.

PS - Hmm. Thinking on it the total vector would be off but not sure how you'd do better with the stock navball.

Edited by LannyRipple
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  • 1 month later...
  • 1 month later...

I've been trying to work out the delta-v for a trip to Duna all night, so I can compare my work against the Olex calculator to know if I'm doing it right. I'm certainly not doing it right. And I'm about to bang my head into the desk.

I'm sure I'm getting confused somewhere with either mismatched units, or I'm getting the variables from the firsts equation confused with the ones from the second.

To start with... what I THINK I understand... is that the first velocity equation computes the deltav required to get from kerbin's orbit to duna's... so R1 and R2 are Kerbin and Duna's orbits, in km, respectively(, and u is kerbol's gravitational parameter... 1.167922e9 km3/s2. Is that all correct? I'm coming up with 1.92389...

And the second equation is calculating the delta-v to escape kerbin orbit... so for a 100 km parking orbit, r1 would be 700km, r2 would be 82,000 km (the limit of kerbin's SOI), v2 is 3431.03 ( kerbin's escape velocity) and u is kerbin's gravitational parameter, 3530.461 km3/s2.

I'm coming up with 4.6661434...

Those numbers don't make any sense as answers to the question....

Am I starting with the right numbers? If so... then it's just a dumb math error... but it's apparently a dumb math error I'm making over and over again. :( I'm attaching a pic 'showing my work' in the hopes that this will make it easy to point out my stupidity.

maths.png

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Straight off the bat, it looks like

A) You didn't do the second square root in the first equation,

B) You're mixing units, using meters per second for the value of v2 where you're using kilometers as your length unit everywhere else, and

C) Kosmo-not's equations mean that the second equation's v2 = ÃŽâ€v1 from the first equation, rather than any particular altitudes escape velocity.

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