First Asteroid Capture

[JarvisTbone5150]

I'm currently working on capturing my very first asteroid. It's a class c, any tips? My plan is to bring it to orbit with one craft and then send another craft to bring it to Kerbin. Wish me luck! Feel free to ask me anything!

[Anquietas314]

Use the atmosphere to slow it down, or better yet just deorbit it in one shot via the atmosphere if you're going to land it anyway. Unless of course you have a contract that wants it to be put into orbit first.

This part's very important though: line up your thrust with the asteroid's center of mass! You can target it by right clicking when you're close enough. That should help to minimize the spinning, although it's pretty much impossible to stop that completely. Unless you are mechjeb and can compute corrections on the fly (I don't think mechjeb the mod can actually do that though )

[Norpo]

Tips: Use a large craft. Bring more fuel than you think you'll need. Because once you capture that asteroid, you're going to be hauling not just the ships mass, but also the asteroids mass. If it's big, it can lower your total delta-V (with it) by a lot compared to without it. I had around 1800 m/s delta-V once, when I latched onto the asteroid, hauling it's dry mass lowered the things delta-V to 350 m/s or so. Thankfully, that was barely enough to slow it down enough.

And quicksave OFTEN. If you can't (e.g hard mode career), then you may want to practice in a sandbox environment first. Trial and error was how I did it, and that was hard enough.

I've only captured one asteroid (a very heavy D class, as I recall), but I hope these tips help!

(also, bring RCS systems, even if it's only a small amount. They *really* help for maneuvering once you're about to capture the asteroid.)

[Reactordrone]

The big reaction wheel is always a good idea. I forgot one on my last asteroid mission and chewed through 96% of my RCS aligning for burns with a class B.

[Anquietas314]

Bring more fuel than you think you'll need

For an idea of how much deltaV you'll actually have with the asteroid (assuming you have something like KER/VOID/MJ to tell you), swap the claw for a docking port (in the VAB! don't launch it!), then stick 3-5 full rockomax orange tanks on top of that, and lock the tanks (right click them and click the little green play button). That should give you a pretty good analog of the asteroid; those tanks weigh 36 tonnes each, some of the bigger asteroids are around 150 tonnes.

[JarvisTbone5150]

Thanks everyone! I practiced in sandbox first.

line up your thrust with the asteroid's center of mass!

I ran into this as a problem and I can see what you were talking about. I'll post pictures of the mission soon!

[FlyingPete]

This part's very important though: line up your thrust with the asteroid's center of mass! You can target it by right clicking when you're close enough.

Do this to get 'close enough' to the center of mass, then use the 'free pivot' function on the claw to allow you to swing the attached ship into line.

For bringing it into orbit, aerobraking is your friend. I landed one on Minmus after the resulting trajectory got me fairly close, with no fuel used.

[Anquietas314]

For bringing it into orbit, aerobraking is your friend. I landed one on Minmus after the resulting trajectory got me fairly close, with no fuel used.

No fuel used? o.O Minmus has no atmosphere and a typical encounter arrives at 200m/s (well beyond crash tollerance)?

[bdito]

For an idea of how much deltaV you'll actually have with the asteroid (assuming you have something like KER/VOID/MJ to tell you), swap the claw for a docking port (in the VAB! don't launch it!), then stick 3-5 full rockomax orange tanks on top of that, and lock the tanks (right click them and click the little green play button). That should give you a pretty good analog of the asteroid; those tanks weigh 36 tonnes each, some of the bigger asteroids are around 150 tonnes.

Good tip. Never thought about doing that. Thx.

[JarvisTbone5150]

By the way, what's the main difference between using the LV-909 engine and the nuclear engine? Don't they both have about the same fuel consumption rate? Is the ISP in vacuum the only real big difference?

[RexKramer]

Here's a tip for moving large asteroids: Don't rely on reaction wheels or RCS to rotate the asteroid when you need to change direction.

Instead, it's sometimes faster to 'un-claw', reposition your tug, and reattach to the asteroid.

As others mentioned, thrust towards the asteroids COM. Even doing that, you will eventually induce some rotation. If the asteroid starts to rotate and you have trouble stopping the rotation, briefly time warp at 5X, that stops the asteroid rotation, just like it stops your ship rotation.

With a bunch of reaction wheels and/or RCS, you can probably keep a class C asteroid under control. Anything larger, it's definitely easier to just reposition your tug instead of wrestling with a giant rock.

[bdito]

It's generally easier to pull an asteroid than to push it.

[Anquietas314]

It's generally easier to pull an asteroid than to push it.

This is actually generally true of any big object . Stuff won't flex as much if you're pulling it. I've never tried it with an asteroid though, but I think you'd need to be quite careful about the design. In particular, you don't want the engines close to the asteroid, or they'll be less effective (the engine plasma'll just push the asteroid in the opposite direction, cancelling some of your thrust...)

[worir4]

A tip to stop the spinning. Use four engines that are on structural pylons in symmetry, then use the thrust limiter on one of them to compensate for the spin.

[LABHOUSE]

Or place a few rockets on the craft and set them to action groups. Use it as rcs.

[technion]

By the way, what's the main difference between using the LV-909 engine and the nuclear engine? Don't they both have about the same fuel consumption rate? Is the ISP in vacuum the only real big difference?

ISP is a HUGE difference, and it's precisely why you generally use them in these scenarios.

The fuel consumption rate doesn't mean a lot because it doesn't factor in the thrust you get for that consumption (like the ISP does).

[JarvisTbone5150]

Thanks! That actually clears up a lot.

[FlyingPete]

No fuel used? o.O Minmus has no atmosphere and a typical encounter arrives at 200m/s (well beyond crash tollerance)?

As in no fuel needed for the Minmus encounter. For landing on Minmus, I used the asteroid tug to slow its descent for landing. Had to climb slightly so I could scoot the tug out from under the asteroid. The asteroid rolled around a bit on the flats and I landed the tug on its engines near where it stopped.