Class E rocks to KO

[steuben]

I'm planning on pushing some Class E rocks into Kerbin orbit.

I'm budgeting about 3k/s delta v. That's before any gravity assist and aerobraking come into effect.

The outbound engines will be Nervs, no question there. The question is reducing inbound burn times. Do I stock up on Nervs, or do I haul up a few Mammoths?

 

[Shadowmage]

Having just done this myself (with a 1200t class-E), a good point to remember is that a 3km/s dV laden craft will only have ~30m/s once it is hooked up to the asteroid (which adds >1,000t of dry mass, greatly reducing your available dV).  Of course this will vary depending on your exact craft setup/mass ratios, but the dV for the craft means little after it is attached to the rock.  The 3km/s should likely get you intercepted and rv'd, but for pushing the rock around you'll need a lot more (fuel+engine).  Just remember, those things are big/heavy.... very big and heavy.

My solution was basically a single skipper + ISRU + drills, and taking a few hours to refine fuel and slowly bring the rock into a capture orbit (which required about 300t to be refined from the rock, no idea how much fuel that actually made).  Once captured then yes, aerobrake + gravity assist to get it into actual position -- free dV is especially useful on 1000t+ monsters

 

If I were to do it again, I would certainly bring more engine (probably KR2-L, the 3.75m upper stage engine; higher efficiency than mammoth/mainsail; possibly even multiples of them) and either more fuel or more drill and refinery unit pods (they don't need to have engines necessarily, merely drills; the ISRU can be on the craft with engines and fuel tanks).  NERVAs would be great.... if you didn't need 20-30 of them to get any appreciable thrust on a 1000t+ rock.  Even with the large 3.75m engine(s) you are still going to need some pretty long burn times (and a few hundred tons of fuel).

 

[Streetwind]

I personally found that when pushing rocks, too high a thrust can be a hindrance. If you are not perfectly centered with your thrust vector, opening the throttle makes you put the rock into a spin. At low thrust, SAS can counteract that, but if you have multiple mammoths pushing you might run into issues.

Edited July 20, 2016 by Streetwind

[Laie]

23 hours ago, steuben said:

The question is reducing inbound burn times. Do I stock up on Nervs, or do I haul up a few Mammoths?

Depends on whether you play with Kerbal Joint Reinforcement or Kerbal Attachment System or similar mods, or of you want to do this with stock (wobbly) rockets. In the latter case, it doesn't take much force to bend the Claw and send things all out of whack. The common recommendation is to set your grabber up in a puller configuration, but still: bending will be problem.

What this means is that a) you can't seriously expect to apply the full thrust of a Mammoth without ill side effects, even without gimbals. For what it's worth, my personal limit was on the order of 500kN, but I guess you can get 5 different opinions from three people. And b) engine gimbals may give you a bad case of the wobblies even when pulling, and certainly when pushing. Expect to steer it with reaction wheels and RCS; it's advisable to have several control pods, each with a claw, and to distribute them around the rock.

Personally, I'm much in favor of a bundle of nukes. Besides fuel economy, you may tweak their thrust individually to make up for the (inevitable) offset between Center of Mass and Thrust.

And these days, bringing an ISRU kit and converting the Asteroid to fuel is probably the smart thing to do. If you don't want to do it that way, assume 2500t for a Class E rock, 200-300m/s to get a close Kerbin flyby, and again as much to get it into a very lopsided Kerbin orbit. If you find one that has a very Kerbin-like orbit to begin with, you can get away with half as much.

[cantab]

If you want to bring your own fuel, the setup will be a lot smaller with LV-Ns. If you plan on drilling the asteroid, consider the Rhino for more power without needing to spam parts.

Either way I recommend meeting the asteroid in solar orbit. Then it doesn't take much delta-V to correct its Kerbin periapsis to where you want - low for an efficient powered capture, even lower for an aerobrake, or you can fish for a gravity assist. Back in the day I got rocks to Eve and Ike that way (although not E classes).

[steuben]

I know that 3k/s in LKO will be less with 4 kt of rock attached. But, I figure it will take about 3k/s to move 4.5 kt of mass to KO.

It will be a puller design to address some stability issues that will arise.

I'm playing with stock parts and just the design class of mods, ex. KER

The intercepts will be in solar orbit. The rock I'm testing against looks like it will be about 1k/s and a couple of burns to get it into a kerbin intercept of any shape. The frelling thing is a beast to klaw as well, <red leader voice>it's just pecking on the surface. </red leader voice>

Thinking of gravity assists are there any tools that will help me plan them? For example if I wait 2 more münar orbits I can catch a nice one.