Asteroid Rendezvous Outside of Kerbin's SOI

[Claw]

Asteroid Rendezvous Outside of Kerbin's SOI

Purpose: I've seen a few videos and walkthroughs for rendezvousing with an asteroid in Kerbin's SOI, but I wanted to meet up with asteroids and do it outside of Kerbin's SOI. Incidentally, this takes less math and timing.

This is a stock tutorial. Although I have MechJeb and Kerbal Alarm in the pictures, neither is required for this walkthrough. I mostly left MechJeb attached to make sharing vehicle stats easier. MechJeb didn't fly any of this, it was all by eyeball.

(Or maybe everyone already knows how to do this and that's why there isn't a tutorial?)

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The demos/tutorials I have seen that meet up with an asteroid in Kerbin's SOI generally require about 2000 dV. Turns out, meeting up with an asteroid outside of Kerbin's SOI also ends up needing about 2000 dV. (Actual amount depends on how fast you want to get there, and how far away the rock is)

So if your gut feeling is "I don't want to meet up outside the SOI because my craft will be crazy big" I encourage you to give it a try.

Using this method, you can meet up with an asteroid in about 1/3 to 1/4 of the time versus waiting till it reaches Kerbin's SOI. If that didn't make sense, imagine that an asteroid is roughly 150 days away from a Kerbin encounter. With this method, you can rendezvous with the asteroid in about 50 days from launch. That means you are at the asteroid 100 days prior to the Kerbin encounter. The exact timing will depend on a lot, but the point is that you can meet it much further out.

Even better, if that same asteroid is on a Kerbin impact trajectory, it's going to take a lot less dV to deflect it when it's still 100 days from impact.

 

Step 0: Select your asteroid and build your craft to meet your goal.

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The Asteroid

This method is for joining up on most Near Kerbin Asteroids on any inclination, but selecting one that is in a fairly flat orbit compared to Kerbin will make your first attempt easier. Once you figure out how to join up on an asteroid, getting to any inclination becomes pretty easy (even if it's straight polar).

The Demo Craft

The only purpose of my craft in this demo is to rejoin on the asteroid. As such, it's pretty minimal and consists of the claw, a probe core, a PB-Nuk for power, FL-T800 fuel tank, and a 909 engine.

This craft has roughly 5500 dV. After rendezvous with an asteroid, it has about enough dV to capture a Class A & B asteroid into orbit around Kerbin.

At a minimum, you will need roughly 2000 dV to join up with most Near Kerbin Asteroids, pack whatever extra dV you need for your own mission.

 

Step 1: Launch to match the inclination with the craft orbit opposite of the inbound asteroid.

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Yes, I said opposite! I'm going to assume you already know how to launch, but I'll walk through how to match the inbound inclination and direction by eye. If you are familiar with meeting up with inclined space stations, this method will be similar. I generally launch into a roughly 80x80 orbit.

1a. Target your asteroid:

To determine the asteroid's inbound path, put your craft out on the launch pad. Click the launch button, but don't actually start to fly it. Go to MAP mode and target your selected asteroid. Targeting the asteroid plots the asteroid's hyperbolic orbit around Kerbin.

1b. Find the asteroid's inbound hyperbolic orbit leg:

Focus on Kerbin (double click on the planet or press TAB until Kerbin is in focus). Follow the asteroid's orbital path and find the end with a circle. This is where the asteroid LEAVES Kerbin's SOI. You'll want to pay attention to the other leg of the orbit, where the asteroid ENTERS Kerbin's SOI.

1c. Line up the asteroid's hyperbolic orbit with Kerbin's Center of Mass:

Adjust the view so that the asteroid's inbound leg is in the foreground, and the outbound leg is behind Kerbin. Make the orbit lines overlap with eachother and Kerbin's center of mass circle.

1d. Line up your launch point and the asteroid's hyperbolic orbit:

Time accelerate until KSC (and your craft) lines up with the orbit lines and CoM circle, then launch. When you do your gravity turn, go to map mode and steer your orbit line so it matches up with the orbit lines. You can usually eyeball it, but also you can also use the AN/DN node that pops up to fine tune to 180 degrees inclination.

1e. Get into orbit like you always do:

You can further fine tune your craft's inclination (using the AN flag) if you want to. It's not required, but it can make setting up your transfer node a lot easier.

 

Step 2: Execute the asteroid transfer burn, and fine tune (< 1,000 km intercept).

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2a. Locating the first asteroid transfer burn:

The asteroid's hyperbolic path will have a periapsis over Kerbin (PE). Place your first maneuver node on the craft's orbit at a point that is directly underneath the asteroid's PE.

2b. Set the first transfer burn so the intercept flags and the AN/DN flags are clustered together:

This is the money maker step. There's lots of info here, but work through it slowly. No need to rush, and take an extra orbit if you need to.

** The key goal is to get the "intercept flags" as close as possible with the AN/DN flag overlaid. The closer all these flags are, the better. However, your burn will be very sloppy and correction will be required, so don't kill yourself matching nodes. Just get all three flags clustered together as close as possible. It will help this step if you zoom in on Kerbin with the maneuver node in the foreground, and the asteroid orbit intersection in the background.

The best order I have found:

- Pull the Green prograde marker to 1000-1100 dV. This will cause the intercept flags to show up.

- Adjust the maneuver node location backward or forward slightly. Find a spot that minimizes the intercept distance.

- Adjust the magenta axis to minimize the intercept distance further.

- Adjust the blue axis to get the AN/DN flag back to the intercept distance flags.

- Adjust the magenta axis a little more to minimize the intercept distance.

Try adjusting each axis a little at a time and check how the distance changed. If the intercept distance gets higher, adjust that axis back to what it was and proceed to the next axis. Adjust each axis several times. The goal is to get down to 1,000 km, but don't kill yourself to get there.

2c. Execute the burn:

For my simple probe craft, the initial transfer burn takes about 2 minutes. Be as precise as you can, but don't chase the marker all over the place. When you end this step, your intercept distance will likely be higher than what you planned with the maneuver node.

2d. Setup and execute a correction burn:

** Keep the intercept flags and the AN/DN nodes clustered together!

I like to do an early correction burn to fix the orbit now, rather than wait till later in the flight. The correction burn is usually around 100 dV. Set the node a few minutes in front of your craft (but not too far) and adjust. Do essentially the same as in step 2b with smaller changes in each axis. After you've set the node, you can slide it back closer to your craft for execution.

Repeat as many times as you want, but once your burns are down around the 0.5 m/s area, you'll simply waste time and effort.

Sometimes you won't be able to execute a correction that gets the intercept distance any closer (depending on inclination and ejection angle). Also, the intercept distance flags can be pretty unstable here. Don't worry, you can fix it later.

- After you get some experience with the initial transfer burn, you can shave off some intercept time by making the transfer burn a little longer prograde, but you'll have to play with the other axes more to keep the AN/DN and Intercept flags clustered.

- You may want to reduce the thrust output of the engine using tweakables to really fine tune the correction burns.

- After you have executed most of the correction burn (down to 10 dV or so left in the burn), delete the maneuver node. Mouse over the intercept flags and burn slowly while watching the intercept distance decrease. If the intercept distance starts to increase, shut down the engine.

 

Step 3: Update the trajectory upon leaving Kerbin's SOI (< 100 km intercept).

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I like to check the trajectory after the craft leaves Kerbin's SOI for any errors. Sometimes there isn't much change and sometimes the intercept distance grows a lot. The orbit outside the SOI is also a bit more stable and you can fine tune more if you need.

The idea is just like before. Adjust the magenta, then the blue markers. Keep the intercept distance flags and AN/DN node clustered together.

 

Step 4: Update the trajectory approaching the asteroid. (< 10km intercept)

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This will be your last real correction. The timing on this will be dependent on your how far away your intercept distance currently is, but about 1 Kerbin Day (roughly 1 Earth Day) prior is a good place to start. If your intercept distance is smaller, you can wait a bit longer to do the correction.

This is where you want to try and get your intercept distance down below 20 km. Getting down to less than 1km is possible, but be wary of getting run over by the asteroid.

 

Step 5: Final burn and rendezvous.

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In my opinion, the final burn is just as easy to execute as rejoining on a craft in orbit around Kerbin. That's because the orbits will be fairly closely aligned already, so small mistakes won't send you racing off away from the asteroid.

The speed difference will be in the range of 700-1700 m/s, and that's how much total dV you'll need for the burn.

5a. Calculate the orbital speed difference:

This step is pretty easy, but unfortunately requires some math (but the only part that does!).

Mouse over both the asteroid and your craft and write down their orbital speeds. You'll use the difference to set up the maneuver node.

In the pictures, the asteroid is 8957.2 m/s and the craft is 9877.3 m/s. So the dV required to match orbital speed is 8957.2 - 9877.3 = -920.1 m/s. The negative means we need to slow the craft down to meet the asteroid, so we'll have to set up our maneuver node by pulling on the green retrograde marker. If the speed is positive, then pull the prograde marker.

5b. Right at the intercept flags, set up a maneuver node.

This step give you a sense of timing and burn rate when approaching the asteroid at high speed. It takes less than two minutes for an asteroid to go from out of visual range (>99 km) to past your craft. Having a maneuver node can help keep from getting run over.

Create a node right at the intercept and AN/DN flag cluster. Green, Magenta, then Blue Markers

- Set up the orbital speed by pulling the green prograde marker until the dV matches the number you calculated in the above step (-920.1 m/s for this example).

- Pull the magenta marker to change the craft's orbital inclination until it matches the asteroid's orbital inclination. You can do this by eyeball, or pull until the AN/DN shows zero (0.0 degrees). (Ended up being about 3 dV for inclination change in the example.)

- Pull the blue marker to change the angle of the orbit. You will need to pull on this marker until the total dV for the maneuver node equals your target closure speed (1769.9 m/s in the example).

Look at the Navball. You should now have maneuver steering overlaid on the retrograde marker overlaid on the target marker. All three should line up.

5c. Burning Using Target Mode

You can execute this step whether you set up a maneuver node or not. The node gives you timing, but you can eyeball it if you want.

If you set up a node, the node steering, target marker, and retrograde marker should all be lined up on the Navball. If the node steering is a bit off, focus your steering on the target and retrograde marker.

- Dock like you normally do. Work in slowly and claw on, as perpendicular to the surface as possible, and pointed at the CoM of the asteroid.

5d. HELP! I messed up and my closest approach is still 20 km away from the asteroid.

No problem. Simply point your craft at the circular pink target marker and burn for whatever closure you want. Bear in mind that whatever dV you put in for closure you'll have to take out to join on the asteroid. Take your time. Even if you're 20 km apart, traveling at 50 m/s relative to the target will get you there in less than 7 minutes. (Time warp with caution)

Why can you get away with this? Unlike rendezvous in Low Kerbin Orbit, you don't have to hurry and get there out of fear of orbital mechanics causing the asteroid to run away. The orbits are huge, so closely aligned, and the body you're curving around (the Sun) is really far away.

 

[cantab]

Excellent tutorial. I've tended to launch to equatorial, burn prograde for a reasonable intercept, then change my inclination when I'm about halfway between the Mun and Minmus to get a closer intercept, but launching directly to the inclination should use less fuel, I'll have to give it a go.

[Seaview123]

This is an outstanding tutorial, very well laid out and your pictures are invaluable. I'm planning on using this method to try and rescue a botched Moho mission (ship full of Kerbals on a long elliptical orbit in Kerbol's SOI and not enough delta V to make it anywhere). If you've got any advice for meeting up with a lost space ship in solar orbit, I'd love to hear it. Thanks!

[Claw]

If your kerbals are trapped in an elliptical orbit inside Kerbin's SOI, then you might be better served checking out one of the tutorials for asteroid rendezvous within Kerbin's SOI (although your scenario isn't as complex, timing wise).

Yours would be more of a matter of timing similar to trying to hit the Mun. Get into orbit on the same inclination as your lost kerbals. I would recommend having your rescue AP/PE very close to the lost kerbal's PE altitude. You should be able to target it and warp forward a bit. Once the intercept flags start getting close together, place a maneuver node on the rescue orbit where it intersects with the lost kerbal's PE. Then you can set up a prograde maneuver that will raise your AP and make your rescue orbit a little longer. With some tweaks, you can lengthen the rescue orbit time just long enough to match up a rendezvous with your lost kerbal's orbit right at the PE intersection.

Once you are approaching with the rescue vehicle, you'll have to have enough fuel for the rescue vehicle to speed up and match the elliptical orbit so that you can complete the rescue.

Good luck,

~Claw

[moronwrocket]

Update for 5d:

(Instead of burning directly for the Target marker) Burn on the opposite side of the marker from the prograde marker (MAKE SURE YOU ARE IN TARGET NAVBALL MODE). Like this:

Prograde     Target     Where to burn
    O           X            *

[Claw]

Update for 5d:

Yes, this is definitely more technically accurate (and more efficient). If the orbit is really wide, either works. But you are indeed correct.

Cheers,

~Claw

[Madeiner]

Update for 5d:

(Instead of burning directly for the Target marker) Burn on the opposite side of the marker from the prograde marker (MAKE SURE YOU ARE IN TARGET NAVBALL MODE). Like this:

Prograde     Target     Where to burn
    O           X            *

Is there any mod that can automatically calculate the "where to burn" and put it on the nav ball? Or at least show an entire navball, even the "sides"?

This issue comes around often for me. With the navball being "round", i usually have to go back and forth to check where my prograde marker is relative to the target.

[cipherpunks]

Maybe one can use "steering error" or "angle of attack" among MechJeb statistic fields for that?...