Eject Asteroid from Solar System

[AHHans]

I saw this contract, and immediately accepted it:

And now I wonder how to best be about it. I don't want it to take too long in in-game time so I don't want to wait long for gravity assists.

My first idea was to send an automated but mining capable probe and aim for a single gravity assist on Jool. The second idea was to just convert as much of the asteroid into radioactive vapor (by passing it through a few LV-N NERV engines) as it takes to boost it out of the solar system in a set of "stupid" prograde burns in solar orbit. (Who cares about efficiency!) My current plan is to use my existing, crewed(!) asteroid tug to do these kind of burns, and then convert even more of the asteroid into radioactive vapor to get back to Kerbin. (Or Jool if I don't make it back to Kerbin.)

Any other suggestions? Do you have comments?

[Streetwind]

You definitely want to mine the asteroid ASAP. Ideally before you do any burns at all. As you do so, the asteroid loses mass, becoming easier to push. Now obviously, with a D-class rock you're unlikely to get it all the way to empty with just a single craft attached to it... but you can still make sure that craft arrives as empty as possible. Waste some fuel doing space acrobatics around the rock before grabbing on if you must!

Another thing to watch out for: if you are lucky, this can get a lot easier. Lucky in this case being that you spot a D-class asteroid that you can capture into Kerbin orbit in such a way that the apoapsis points roughly prograde with respect to Kerbin's solar orbit. Suddenly, you don't even need to go into solar orbit yourself to fetch a rock, which is faster to start with; and then, when you do have it captured, you can just let yourself fall back to periapsis and do a big, Oberth-assisted burn that'll see you to solar escape velocity before you have even passed the orbit of the Mun. Which makes it extremely easy to decouple and return a crewed tug. But, as mentioned before, you gotta be lucky to get such a rock.

[AHHans]

8 minutes ago, Streetwind said:

Lucky in this case being that you spot a D-class asteroid that you can capture into Kerbin orbit in such a way that the apoapsis points roughly prograde with respect to Kerbin's solar orbit.

Well, I decided that Kerbin looks much better if dressed with a nice, tidy asteroid belt. So asteroids that cross into Kerbin's SOI are currently fresh out of stock.

But on that topic: does anyone know hoe "new" asteroids are generated? Can I increase the chance of finding an asteroid that comes into Kerbin's SOI by not having sentinels look for asteroids around other planets? (Like Eve, Duna, Jool.)

[swjr-swis]

2 hours ago, Streetwind said:

Now obviously, with a D-class rock you're unlikely to get it all the way to empty with just a single craft attached to it...

If you find the need to do this, remember that you can 'Jettison' full tanks of ore! Just bring a sizeable ore tank, and keep dumping as you mine what you no longer need for the ejection burns.

[Capt. Hunt]

4 hours ago, AHHans said:

My current plan is to use my existing, crewed(!) asteroid tug to do these kind of burns, and then convert even more of the asteroid into radioactive vapor to get back to Kerbin. (Or Jool if I don't make it back to Kerbin.)

Any other suggestions? Do you have comments?

I wouldn't use a craft that you want back.  Remember, anything you send to Kerbolar escape velocity is going to need almost that much delta-v to get back to kerbin.

[AHHans]

21 minutes ago, Capt. Hunt said:

I wouldn't use a craft that you want back.  Remember, anything you send to Kerbolar escape velocity is going to need almost that much delta-v to get back to kerbin.

I know. But I also don't want to loose an asteroid once I captured it. (See my comment about the asteroid belt.) And if I brake back to Kerbin anyhow, then I can also use a crewed vessel. The big question is if burning up all the mineable ore in the asteroid will give me enough dV to get onto an escape trajectory and then back to Kerbin again.

For the time being I've sent my big asteroid tug onto an intercept with an ejection candidate. Once I'm there I'll do some math and try to figure out home much dV the ore in it gives me. If that doesn't work out then I'll make further plans.
Probably build a robotic asteroid tug, that con go on a one-way trip.
Hmmmm. ... maybe I should send a probe out into the deep dark yonder? Just to let the further universe know that the Kerbals exist.

[paul23]

1 hour ago, Capt. Hunt said:

I wouldn't use a craft that you want back.  Remember, anything you send to Kerbolar escape velocity is going to need almost that much delta-v to get back to kerbin.

You need more, much more for "there and back again".

https://what-if.xkcd.com/38/

[AHHans]

11 hours ago, paul23 said:

You need more, much more for "there and back again".

https://what-if.xkcd.com/38/

Well, but that's fuel, not delta-v. You do only need as much dV to get back as you needed to get there. But to have that much dV you need exponentially more fuel.

I'm planning to use the asteroid for fuel anyhow so the question is not "How much fuel do I bring?", but "Will it be enough to go there? And back again?".

[swjr-swis]

4 hours ago, AHHans said:

Well, but that's fuel, not delta-v. You do only need as much dV to get back as you needed to get there. But to have that much dV you need exponentially more fuel.

I'm planning to use the asteroid for fuel anyhow so the question is not "How much fuel do I bring?", but "Will it be enough to go there? And back again?".

Keep in mind that contracts of that type are already fulfilled when the orbit just barely becomes an escape trajectory. All you really need at that point is to expend a tiny puff of fuel in retrograde to close the orbit again, and you can spend the remainder of fuel to creatively lower Ap. Perhaps lowering Pe a bit to aerobrake and lower Ap 'for free'?

Edited October 16, 2019 by swjr-swis

[AHHans]

6 hours ago, swjr-swis said:

Keep in mind that contracts of that type are already fulfilled when the orbit just barely becomes an escape trajectory.

Ah, yes, I'm aware of that. It's not like I haven't gotten "into orbit" by slowing down just enough to have the Ap inside the SOI, and then accelerating again 20 seconds later. (20 seconds because I first want to check that the contract is indeed fulfilled and then need to re-orient the spacecraft). At least for this contract it makes sense because actually leaving the SOI that is literally the whole universe may take some time.

7 hours ago, swjr-swis said:

Perhaps lowering Pe a bit to aerobrake and lower Ap 'for free'?

I don't think I'm going to aerobrake on the sun anytime soon.

(And for other bodies: if I accelerate e.g. close to Kerbin then I may get onto a trajectory with enough energy to take me out of the Sun's gravitational well, but I'm sure that until I left Kerbin's SOI and am in the Sun's SOI the contract will not be fulfilled.

[swjr-swis]

2 minutes ago, AHHans said:

but I'm sure that until I left Kerbin's SOI and am in the Sun's SOI the contract will not be fulfilled.

Yes that is true.

[Laie]

On 10/15/2019 at 4:20 PM, AHHans said:

The second idea was to just convert as much of the asteroid into radioactive vapor (by passing it through a few LV-N NERV engines)

I second this approach.

A quick sanity check suggests that it takes about 4000m/s to eject from a Kerbin-like orbit. That doesn't require any fancy tricks. With nukes, about 40% of the gross mass (rock + tug) has to pass through the nuke(s) in order to eject the asteroid. On Poodle power, that becomes ~70%... so yes, stick with nukes.

I'd say the fastest way is to have a mining rig that doesn't need much babysitting: set SAS to prograde, light the engines, and go make a sandwich. Browse the forums. Whatever. Preferably while the game runs at physics warp. You'll probably have to pay attention every so often in order to mine and refine more fuel, but as long as that's all... it will be over and done with in one afternoon.

Even if it takes several Kerbin days to mine and boost and mine and boost, you'll still be pretty close to PE by the time you reach ejection velocity. The tug can then just turn around and double back, I'd guess it will still be under 5km/s to make it to a gentle Kerbin encounter. Your unburdened tug is good for 6+km/s, I presume?

 

[AHHans]

12 hours ago, Laie said:

A quick sanity check suggests that it takes about 4000m/s to eject from a Kerbin-like orbit.

Well, the asteroid I'm currently aiming for is in a lower orbit, between Eve and Kerbin. My one asteroid-watch satellite - which is in a similar orbit - tells me that it would need about 4400 m/s.

 So for 9000 m/s I need a wet/dry mass ration of about pi. That's not too unrealistic for a large asteroid. (SCNR )

12 hours ago, Laie said:

[...] and go make a sandwich.

Ah, no! I'm already overweight as I am, no need to contribute to that. My plan is to answer questions on the KSP forum instead.

12 hours ago, Laie said:

Your unburdened tug is good for 6+km/s, I presume?

Fully fueled with Lf and ore it's more than 9000 m/s. But it will need two (well, 1.5) conversion stops in between. But as you discussed, that shouldn't be a problem.

[AHHans]

An update on this endeavor: after a version 1.8 induced delay of the currently also ongoing Eve mission(*), I finally caught me a suitable asteroid, and it is definitely one that I want to get into Kerbin orbit for closer study:

According to my calculation I have 12km/s dV available if I use up all the mine-able ore in that asteroid, and the maneuver node says that I need about 4 km/s at the PE to eject it from the sun. (The current orbit is between Eve and Kerbin.) So I should have enough ooomph to get the contract done, and then slow down into an intercept with Kerbin.

But there is some more tricky maneuvering to be done on and around Eve and another asteroid to catch, before this craft gets to the PE. So it might take a few days before that happens.

(*)I believe I already complained enough in other threads...

P.S. Yes, that's my first "magic" asteroid.
P.P.S. In the meantime I accepted a contract to rescue Asler from a low orbit around the sun. (I guess I never learn...)

[Update 16. Nov.:] After the activity on and around Eve had coalesced into one big group-hug on Gilly-Station, I finally got around to maneuver the asteroid. One humongous cloud of slightly radioactive vapor later I had not only fulfilled the contract but also have the asteroid on a course to intercept Kerbin.

Edited November 16, 2019 by AHHans