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Practical interplanetary rendezvous guide


duckunlimited2
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Hi guys. I took some time off of my last days of summer vacation to create a guide on interplanetary rendezvous. Due to the limitation of photos we can upload here and my lack of video recording software, I have opted to format it in PowerPoint. Let me know what you think about it. Tell me if you have any problems viewing it or if you have trouble understanding it and I will best see how to remedy the situation.

Also, it is possible to test out the rendezvous methods yourself out in kerbol orbit. Just grab the lightest command module, put the stock ASAS module on it (it protects you from the space kraken) the smallest fuel tank and the lightest extrasolar engine on. Config the fuel in the tank and give yourself ridiculous amounts of fuel to give you enough leeway for practice. Strap it on a rocket to help get it out of Kerbin's surface and atmosphere and head off for solar orbit. With planets about to come out we need to prepare ourselves. So start practicing.

Here's the link to the ppt: https://dl.dropbox.com/u/97825123/THE%20PRACTICAL%20WAY.pptx

Have fun and good luck

Edited by duckunlimited2
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Fiddling with your orbit after you reach the target planet's altitude is a bad time to do it.

For example:

If going from a low to high orbit and setting up so that you'll arrive at the rendezvous point in front of the planet, you could have burned slightly longer at the burn point to cause an intercept.

Otherwise, it's a simple method, but costs a lot more fuel and time.

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Having digested the method fully now, I'll have to agree with Kosmo-Not on this one. Going to an intermediate parking orbit to make angle-measurement easier can pose significant extra fuel costs in Kerbol orbit, depending on how far away your parking orbit is.

Still, the idea of using the navball as a protractor gives the option of using trigonometry to measure the current phase angle on a ship in interplanetary space using the angle between the nadir pole on the navball and the KSC marker, and the ship's and Kerbin's semi-major axes..

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Still, the idea of using the navball as a protractor gives the option of using trigonometry to measure the current phase angle on a ship in interplanetary space using the angle between the nadir pole on the navball and the KSC marker, and the ship's and Kerbin's semi-major axes..

Now we only need someone with enough mathematical abilities to come up with a nice formula that solves this problem...

Someone out there? :)

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With that navball, i actually dont use the ksc marker at all. The orbital periods of both methods are both in a ratio of 2:1 (smaller orbit always being the one). When the navball spins, im not just measuring how far my craft goes around the orbit but also how far the planet goes as well in its orbit. I can do that because i know the orbital periods ratio. If the smaller orbit completes 1 orbit, the bigger completes a half. Therefore, because one can track both orbits in time, one can work out when the angle is right for a rendezvous burn.

In regards to the intermediary orbit being a wasteful energy effort i disagree to an extent. When we r captured by a soi via an H-transfer, ourpejected path is always an escape trajectory. There fore we must use delta v to prevent it. The question on how much do we use. It depends how fast we were when we entered the soi. Now if we imagine that we burn a transfer from kerbin to another planet. U would need alot of delta v to prevent the escape trajectory cause u r slow at the top. Now if we did some effort to first establish a intermediary orbit then transfer, the amount of delta v needed to prevent escape trajectory is less then the straight transfer. So much so, that it makes up the effort we put in to our intermediary orbit. Im no expert in delta v but the way imagine it makes sense.

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Uh. Nope.

All you have to do is use some basic math to find out a launch window. Then launch into it and perform the intercept at the right phase angle and burn point with the proper Delta V that you'll be sent on a course towards the planet and get pulled in by its gravity. There's no need to screw around with orbits with a proper ratio, navball angles and all that other nonsense. The math for this stuff is not very hard. You're going to end up wasting a huge amount of Delta-V, fuel and your own time doing this.

Kosmo-Not's guide has the math written up. It's got the equations and what each letter means. If you want to go to another planet, do it the efficient way and simply take some time to learn the math. It's easy.

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I did the calculations, and came up with the following results:

This calculation involves getting from Kerbin to another planet orbiting at 30Gm (roughly twice the orbit of Kerbin). Ship is assumed to be starting from a 100km ASL orbit around Kerbin. Effects due to the target planet's gravity are ignored for the purpose of simplicity.

Method 1: direct Hohmann transfer to the target planet

Method 2: the method described in this guide

[TABLE=width: 500]

<tbody>[TR]

[TD]Method 1[/TD]

[TD][/TD]

[TD][/TD]

[TD]Method 2[/TD]

[TD][/TD]

[/TR]

[TR]

[TD]Description[/TD]

[TD]delta-v[/TD]

[TD][/TD]

[TD]Description[/TD]

[TD]delta-v[/TD]

[/TR]

[TR]

[TD]1st burn[/TD]

[TD]1300[/TD]

[TD][/TD]

[TD]1st burn[/TD]

[TD]999[/TD]

[/TR]

[TR]

[TD]2nd burn[/TD]

[TD]1311[/TD]

[TD][/TD]

[TD]2nd burn[/TD]

[TD]669[/TD]

[/TR]

[TR]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD]3rd burn[/TD]

[TD]847[/TD]

[/TR]

[TR]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD]4th burn[/TD]

[TD]754[/TD]

[/TR]

[TR]

[TD]Total[/TD]

[TD]2611[/TD]

[TD][/TD]

[TD]Total[/TD]

[TD]3269[/TD]

[/TR]

</tbody>[/TABLE]

As you can see, the second method requires 658 more delta-v.

If the target planet has an atmosphere, we can ignore the final burn in each method due to the atmosphere capturing the spaceship for landing.

Method 1: 1300 delta-v

Method 2: 2515 delta-v

Method 2 requires 1215 more delta-v than method 1 (almost twice as much).

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Kosmo-not, can you please label what each burn is accomplishing so I can.... well see what each burn is accomplishing.

And a further inquiry: Did any of those burns simulate a burn to park the craft in orbit with the target planet? Remember my argument is that I agree it takes more delta v to set up the intermediary orbit and transfer, but it makes up for it by reducing the necessary delta v needed for it to park in orbit.

edit: Oh If I am proven wrong, can you tell me why my method's wastes delta v. I would want to know where my thinkings gone wrong.

Edit: Oh, just looked at your post again. I see now that you did do a parking orbit. scratch that second part. I also believe that these calculations take in consideration the Oberth effect right. I can see how that works against me in the realm of your calculations. But now that I think about it, I'm not that entirely sure that your calculations truly reflect the game mechanics. More specifically I honestly don't think the Oberth effect is reflected in the game. As far as I can see, the community just accepts the fact that the Oberth effect IS reflected in game. However, no one has proven it does or it doesn't. So I think the next logical step is to determine if the Oberth effect is reflected or if it isn't. That should be the deciding the factor that either disproves or supports my rendezvous method. (Unless their is another factor working against me)

I'll repeat what your calculations say their doing in game (except I won't start in kerbin's SOI; I'll start outside it). That should directly prove or disprove the delta v problem and also prove or disprove the Oberth effects existence.

Edited by duckunlimited2
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Yes, the Oberth Effect is in the game. If the Oberth Effect doesn't work, then the orbital equations don't work, because you're calculating kinetic energy wrong.

The parking orbit is the problem. You generally spend a significant amount of fuel circularizing at the intermediate parking orbit, and then more fuel burning out of it.

For what it's worth, here's what I got.

Planet V at 9.79 Gm semimajor axis. (Venus analog)

Planet M at 20.7 Gm semimajor axis. (Mars analog)

Planet J at 70.7 Gm. (Jupiter analog)

For the direct Hohmann transfer, assuming you aim it to dump you directly in the destination SOI..

Total Delta-V from 100km Kerbin orbit:

Planet V: 1476 m/s. Arrival relative velocity: 856 m/s.

Planet M: 1058 m/s. Arrival relative velocity: 821 m/s

Planet J: 1947.9 M/s. Arrival relative velocity: 1758 m/s.

For the method posted above

Planet V. Intermediate Orbit SMA 15.6 Gm (yes, this works out to be higher than Kerbin, though planet V is lower)

From 100km Kerbin orbit to Intermediate orbit: 788.6 m/s.

Circularizing at Intermediate Orbit: 297.5 m/s.

Transfer from Intermediate Orbit to Planet V: 1050.6 m/s.

Total Delta-V: 2293.0 m/s. Relative velocity at arrival 1,180.4 m/s.

Planet M. Intermediate Orbit SMA 13.0 GM (yes, this works out to be lower than Kerbin, though Planet M is higher)

Transfer from 100km Kerbin Orbit to Intermediate Orbit: 931.4 m/s.

Circularizing at Intermediate Orbit: 94.4 m/s.

Transfer from Intermediate Orbit to Planet M: 1012.7 m/s.

Total Delta-V: 2038.5 m/s. Relative velocity at arrival 902.3 m/s.

Planet J. Intermediate orbit SMA 44.5 Gm.

Transfer from 100km Kerbin Orbit to Intermediate Orbit: 1620.9 m/s.

Circularizing at Intermediate Orbit: 1620.0 m/s.

Transfer from Intermediate Orbit to Planet J 554.2 m/s.

Total Delta-V:3785.2 m/s. Relative Velocity, 493.2 m/s.

So for Planet V and Planet M, Direct Hohmann results in lower delta-V (817 m/s less for V, 980 m/s less for M), and lower arrival velocity (324 m/s less for V, 81 m/s less for M) over the method in the powerpoint.

For Planet J, the trip still takes less delta-V on the direct hohmann. ( 1837 m/s less) The arrival velocity is higher than for the powerpoint method( 1264.8 m/s higher.), but not enough to offset the delta-V settings....if you were to come to a dead stop directly at the SOI edge.

At the surface of the planet, thanks to the Oberth Effect, the difference between the two planet-relative velocities will be smaller. Much smaller, if J is a gas giant. And you get the best efficiency by braking at the last minute, rather than coming to a dead stop at the SOI edge, if your destination is the planet. In addition, if planet J is a gas giant you can aerobrake to shed velocity, as Kosmo-not mentions, instead of burning fuel.

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Yep. Ive been humbled. Upon my own testing in game, i have found out that i am wrong. Sending a craft that weighs 13.6kmu from kerbin altitude to a hypothetical planet located 60gm with an intermediary orbit located 30gm wastes about 380 fuel units more than just sending your craft straight up to the planet.

As a guide meant for ease of understanding, practicality, and versatility (like a one size fits all type of thing) it accomplishes its goal quite respectively. However, its not a custom tailored suit. So its not the best option in most cases.

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Yep. Ive been humbled. Upon my own testing in game, i have found out that i am wrong. Sending a craft that weighs 13.6kmu from kerbin altitude to a hypothetical planet located 60gm with an intermediary orbit located 30gm wastes about 380 fuel units more than just sending your craft straight up to the planet.

As a guide meant for ease of understanding, practicality, and versatility (like a one size fits all type of thing) it accomplishes its goal quite respectively. However, its not a custom tailored suit. So its not the best option in most cases.

It is less efficient fuel wise but it is easy to throw a couple more tanks on. It is a lot harder to figure out and perform the launch angle out of kerbin orbit.

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