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michaelhester07

Asteroid grabbing tips

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I've gone through grabbing a few asteroids. I'll share how i've done it. I've grabbed a Class C so far and put it into kerbin orbit.

1. Build big... really big

This is why SQUAD gave us all those big rockets. In order to successfully grab an asteroid and bring it back home to Kerbin you have to deliver some serious firepower to it. This especially goes for anything Class C or bigger. While you're building big... those liquid rocket boosters are amazing at getting your big rocket into orbit. My second attempt to move an asteroid was on a class E that was going to hit kerbin (oh noes it's Deep Impact/Armageddon). I tried to use the Skipper rocket but it was woefully under-powered for the task.

Use of RCS is recommended but not necessary.

1b: The liquid rocket boosters will probably be the last engine you ever need to unlock

2. Plan your launch

This comes in 2 steps...

A. Track the asteroid you want to grab

B. Line up your launch with the direction it will pass kerbin

When you see an asteroid in the tracking station you can tell the station to track it. This lets you know when the asteroid will intercept Kerbin's SOI. Now on a spacecraft you have in orbit target the asteroid. You'll be able to see how it is going to pass by kerbin. This is important to let you align your ship with the orbit of the asteroid when you take off. You'll save a lot of delta-V doing this. You need all the dV you can get when you get to the asteroid. You'll be launching in directions you've never thought of before. I've had one launch going due south.

This is also a good way to intercept a spacecraft at an odd angle or to go to Minimus without doing the phase correction burn.

3. Launch

You want your interceptor to be in orbit before the asteroid enters kerbin's SOI. This is especially true for shallow passes where the asteroid is beyond the orbit of Minimus.

4. Intercepting the asteroid

Before you can intercept you need to match the phase. Plan a maneuver at the ascending or descending node. Adjust this maneuver so that the node's angle reads 0.0. If it's dancing around a lot then you've got it. Now you can plan the intercept burn. Place this burn on the exact opposite side of your orbit from the asteroid's periapsis to start. Expand the prograde marker so you get the intercept bugs to appear. Now project several orbits ahead until the target position bugs are past the Periapsis. This is where you look to intercept. Move the position and strength of your intercept burn to get one of your position and the target position bugs to line up. For shallow intercepts you can be off by a few hundred km.

5. Shallow intercept course correction burn

Half way to the intercept you may need to do a course correction burn. Stick to purple or blue direction and get your intercept within 100km. You have ample time to get the burn right.

6. Intercept time. When you're within 300 km of the asteroid switch to "target mode" on the navball by clicking the spot where your velocity reading is. It defaulted to "orbit". There's "surface" and "target". You want "target". Now find the retrograde vector (green x marker) and burn towards it till your velocity is under 200m/s. (highest this has ever been for me was 1200) There should be a pink asteroid retrograde marker. Burn on the opposite side of that marker to make your retrograde marker line up. Keep 100m/s until you're within 15km then drop it to 50m/s. Keep the markers lined up so you can use the velocity you have. Within 2 km drop the speed to 5m/s.

When you set the trust limiter on your engine you can get finer control over the throttle. That big Kerbodyne rocket can be as accurate as the 909.

7. Capture. You brought the grappling hook with you didn't you? Arm it. Right click the asteroid and select "target center of mass". You're going to dock with the asteroid now. RCS helps here but you're not docking with a docking port. Aim doesn't have to be too precise. Keep your speed under 4m/s.

8. Aligning the center of mass. Now that you've hooked onto the asteroid you have to align the center of mass so that you can drive it. Right click the hook and choose "free pivot". Turn off RCS and use reaction wheels for this: align the spacecraft such that the center of the navball marker is centered on the asteroid's center of mass purple marker. If you don't do this when you throttle up you'll induce spin on the asteroid. If you do this then you can fly straight. Once aligned right click the hook and choose "lock pivot". Now you can fly striaght. The angle of incidence for your hook doesn't matter, only that the center of mass is lined up.

9. Bring it home: Switch to orbit mode on the navball and burn retrograde until you have a periapsis. Put the Periapsis around 1 million meters or a little lower. Park your asteroid as you would a normal spacecraft.

I recommend doing this with a probe so you don't have to inadvertantly sacrifice kerbals when you find out your rocket ended up with 30 deltaV attached to the asteroid. This is also why you built big in step 1. My class C asteroid grabber has 5000 delta V before it latches on but only 900 when it's attached (depending on the size of the rock). Class A and B asteroids can be handled by smaller craft.

Edited by michaelhester07
lock the pivot

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Actually, one of the most successful methods of capture I have found is to lower/raise the periapsis to 35 Kilometers. That way you get an aerobrake capture, which you can then extend the periapsis back up to 70+ kilometers. I tried that with an E-Class (Was on a 500,000 Km periapsis, I was trying to capture it. Don't ask.) and failed miserably, as the 6-NERVA engine cluster only had about 75m/s DV once I attached it... (about 200,000 Km periapsis when I was done.)

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I didn't think to aerobrake with an asteroid. Would 50km be a safer aerobrake for the Class E rock?

For me i'd rather put it at 500-1000km and launch a second craft to grab data off it. I inadvertently turned an asteroid into a refueling station on one of the grabs with 2 of my asteroid grabbers locked onto it.

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50 Km doesn't create much drag. 35 Km doesn't either, but it gives you an apoapsis around 100-200 Km, which you then raise the periapsis to. A 50 Km aerobrake wouldn't really do much, maybe a Minmus-altitude apoapsis instead of a Kerbin escape.

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If the asteroid is already in Kerbin's SOI you may need a big rocket. However, I prefer to go out and meet them in interplanetary space, where even a small rocket can manage burns that will be magnified over time/distance to big changes in the roid's trajectory by the time it reaches Kerbin. With multiple aerobraking passes, this design

eEz4jEL.png

has put two E class asteroids in Kerbin orbit.

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