Trying to Capture a Class E Rock, Need Some Pointers...

[capi3101]

Howdy, y'all. I'm sure this one's been answered loads of times before now, but y'all know how whacked out the forum search function has been for...well, forever. It's been a while since I had a question I needed to ask anyways...

Okay, so here's the sitrep: I've got a Class E asteroid headed to a 2.3 million meter Apoapsis in about six days. It's already in Kerbin's SOI. I'd like to try to capture the thing. This would be the first time I try to capture a rock (let alone one of that size class), so I'm thinking I just want to do something simple and try to put it into a stable orbit. I've got about 2,000,000 funds. Heaviest engine I've got unlocked is the Mainsail, I've got the Claw and NERVAs.

My main question - how much delta-V should I plan for, and roughly how much thrust should I plan for the payload? What kind of mass should I expect a Class E to have anyways? For that matter, is there anything that lists out the range of masses of each type of asteroid (i.e. Class A is X tonnes to Y tonnes, Class B is W tonnes to Z tonnes, etc.).

I ask mainly because I have a Class C on a collision course in a little over a month, and I'd like to try to avoid it landing on somebody's rear bumper if at all possible...

I have done the first tutorial (and I went ahead and docked with the rock), I've got loads of experience with rendezvous and docking and I understand the mechanics involved with all the piloting, so I'm mainly asking for design help here, but any help y'all can offer would be greatly appreciated.

Edited September 4, 2014 by capi3101
changed thread status to "awesome"

[DeMatt]

Six days from periapsis? It's probably too late to capture the E-class - they measure in the thousands of tons, so you realistically have to catch them far out and tweak their approach early. The C-class is probably doable; of course, you can always just let it crash and look for an easier target.

As to general design guidelines:

• Start with a "pathfinder" probe. This is a Klaw, a probe core, and the solar panels/batteries/ion drive to get it to the asteroid. Light, fast, not expected to move the asteroid itself (although "absurdly low acceleration" multiplied by "absurdly long time" does tend to equal "acceptable movement"), but gives you a mass readout in the Tracking Station.
  
• Now that you know how heavy the asteroid is, take an info plugin like Mechjeb or KER, and the adjustable NRAP Test Weight, and start tinkering.
  
• A's can probably be moved around by a single LV-N, being only a couple dozen tons in mass. That's about the mass of a big orange tank, which makes them good as your first asteroid-moving practice.
  
• Especially with the larger asteroids, you'll want lots of RCS to turn and/or stabilize it. You can stick more than one ship onto the asteroid at a time; stud it with super-RCS pods - they'll all work together when you control the combined craft.
  
• "Puller" configurations, if you avoid aiming the engines directly at the asteroid, tend to be more stable than "pusher" configurations. But they're harder to build and control.
  

[cantab]

Plan for an E massing 3000 tons. It might be lighter, but it's very unlikely to be heavier though I have heard rumours. A D might be 500 or so, a C on the order of 100, and an A can be as light as a few tons.

The most fuel-efficient approach to capture a large asteroid is to intercept it in solar orbit, then you need only a small correction to set up an aerocapture. You'll only need a few hundred m/s with the asteroid for that, depending on how picky you are about your orbit. For your E class if you can get to it pronto you might be able to make a radial burn and put it onto an aerocapture course, but I'm not sure how much delta-V you'll need. My Macbeth 3 mission, https://flic.kr/s/aHsjXR1JAr , is my sole asteroid capture at Kerbin. A 500-ton D class I met it in solar orbit and only had about 100 m/s with it in tow, but that was enough. You can see the design in the album.

For a fully powered capture I suggest aiming for 2000 m/s of delta-V with the asteroid, plus whatever you need to get to meet it.

[capi3101]

Would y'all consider it to be excessive cheating if I were to launch a pathfinder, dock it, get the mass readout...and then revert?

3000 tonnes. Wow...that's a bit of a load now, ain't it?

Okay...if I'm doing the math right, with the periapsis around 2,300,000 meters the gravitational influence of Kerbin should be around 0.41 m/s^2...so with 3000 tonnes of mass, if I want a TWR of 1.0, I'd need 1,259 kN of thrust. That sound right?

Sounds like I want to haul a Mainsail up to do the pushing. It'd be way lighter than a cluster of 21 NERVAs...

Also sounds like steering the sucker is going to be the tricky bit.

That's something to start working with anyway. Thanks for y'all's advice so far.

[cantab]

If you don't mind minor cheating you can always search through your savefile for the name of the asteroid and its mass should be there to read off.

Generally speaking when talking about TWR for orbital and deep space craft the standard is Kerbin TWR. You don't actually need to fight gravity, it's just in some situations you don't want to be burning for too long. But with an E class asteroid long burns are going to be inevitable.

For this mission I would stick to the LV-N's. True they're heavy, but the difference in engine weight will be peanuts compared to the mass of the asteroid. That's not to say a Mainsail-powered design can't work, but it will need an awful lot more fuel.

For steering, an option with the really big asteroids is to simply unclaw, fly around the asteroid until you're facing the right direction, and reclaw. If you do try to turn it I suggest disabling SAS, then you can put a turn on and let it slowly coast.

[Oafman]

A while back I captured a class E, wit a mass of about 3000t. To give you an idea of what this might do to your available delta v, I had 2100m/s left on arrival at the roid, and once I had docked this became 42 m/s!

Luckily I had intercepted it before it entered Kerbin's SoI, so 42m/s was enough.

This is why people are talking about it maybe being too late to capture this roid. Because, now it's so close, you'll need a fair bit of DV to push it into Kerbin's atmosphere for an aerobrake, and it will be a hard design to get a ship there capable of the job.

But, it might be a lot less massive than 3000t, so if you act quickly, and try to get a ship to it with as much power and stamina as possible, you might have a chance. Keep us updated if you try...

[Padishar]

Six days from periapsis? It's probably too late to capture the E-class - they measure in the thousands of tons, so you realistically have to catch them far out and tweak their approach early.

As long as you can make a rendezvous not too far after periapsis then you may well still be able to slow it enough for a capture into a highly elliptical orbit. The first class E I grabbed only needed about 40 m/s to capture it. If you leave the orbit as elliptical as possible (and as long as the orbit ends up well clear of the Mun and Minmus SOIs) then you have all the time you want to send up another ship or even, if you still have a reasonable amount of deltaV, to wait until it gets back out to apoapsis and then slow it a little again to drop it into one or more aerobrake passes.

This is one of my large asteroid pullers for inspiration...

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[Yakky]

You have three possible strategies:

(1) Nudge it enough to allow aerobraking in Kerbin's atmosphere. Six days is not as much time as would be ideal, but it could work if you get there quickly enough with a very large rocket. Some simple math shows that it can't be traveling more than about 175 m/sec right now. A delta-v of maybe 10-20 m/sec might be enough if applied very soon in the optimal direction (use maneuver nodes to explore which direction accomplishes the most). But hurry, before it speeds up much!

(2) Just use rockets to slow it down enough to capture it into an elliptical orbit around Kerbin (i.e. so it's not on an escape trajectory anymore). This will take about 35-40 m/s of delta-v if you do the burn exactly at periapsis (to maximize Oberth Effect), if I've guessed your orbital parameters right. So if the asteroid weighs 5000 tons and you have a 50-ton rocket, the rocket needs to have about 4000 m/s of delta-v without the asteroid attached.

(3) Do a combo of (1) and (2). Get something out to the asteroid right now to lower the periapsis as much as possible, even if you can't do enough to aerocapture. Then, do a second push at periapsis to slow it down and capture into an elliptical orbit. By lowering the periapsis, you will greatly enhance the speed at periapsis and thereby greatly enhance your Oberth Effect benefit when you do your periapsis burn.

Hope this helps and good luck!

Edited September 3, 2014 by Yakky

[capi3101]

Thanks for your advice, y'all; I can report that I was successful in my efforts to put the rock into a stable orbit around Kerbin. An ugly orbit, but an orbit nonetheless.

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Wound up catching it slightly past its periapsis. Fortunately, it turned out to be relatively light for a light class E, a mere 1600 tonnes, and it caught Kerbin at such an angle that it really needed only a verrrrry slight nudge to put it into orbit (as the pictures show, it was in an orbit within Kerbin's SOI with only about 5 m/s of delta-V).

In short, I was lucky conditions set up the way they were. I do appreciate y'all's advice; next time I see an E coming my way, I'll try to catch it a little more ahead of time. I'll definitely be doing that with the C coming my way.

Will change this thread to "awesome".......er, "answered". Thanks again.

[cantab]

Congrats! Ulations!