Class E asteroid orbit deviation

[Joe Kerbal]

I'm trying to stop a passing by asteroid of class E (that's what basically my contract ask)

I have no problem targeting, reaching and grabbing the asteroid. I've some problems properly orienting my thrust towards retrograde but I attempted the whole thing without RC thrusters. I know how to do it next time... not a big deal.

What really puzzled me is the mass of the asteroid. Once I grab it, with about 5000 dv available from nuclear engine it reduces to about 5 m per second. No way I can seriously change it's orbit. What's the trick here? Class E is really big but the contract assumes I can do it. is there something I'm missing? maybe something I didn't reach yet in my research tree?

[bewing]

Mine it, to reduce the mass of the asteroid. Especially if you then convert the ore into more fuel. Many asteroids are more than 80% ore.

 

[Joe Kerbal]

36 minutes ago, bewing said:

Mine it,... 

Sounds like the only logic answer. Unfortunately I've not yet the technology but I guess I'll get it and try again. 

[Streetwind]

Well, your alternative is "more boosters". Literally.

Your dV drops to near zero because you're sending a small ship with a small fuel tank but a very efficient engine. However, the rocket equation is all about the mass fraction - the ratio between the things that are fuel, and the things that are not fuel. As soon as you dock to the asteroid, "things that are fuel" goes from "a noticeable part of the ship's mass" to "basically nonexistant", because the other part - the things that are not fuel - suddenly is thousands of times larger.

If your ship had a much larger fuel tank, it would retain more dV after docking to the asteroid, because it would be a larger fraction of the resultant total mass of ship + asteroid.

So you could try a much larger pusher craft.

Edited July 11, 2020 by Streetwind

[Laie]

2 hours ago, Joe Kerbal said:

[mining] Sounds like the only logic answer. Unfortunately I've not yet the technology but I guess I'll get it and try again. 

Well, Class E starts at ~800t and I don't know where it ends. The heaviest I've personally seen was 4500t.

Capture into any kind of orbit around Kerbin can often be done on 300m/s or so... back-of-the-envelope I guess that this would require something like 200t of propellant for the heavier rocks. Once captured, you can send refueling missions until it is in the target orbit. Totally doable, though perhaps a bit tedious (and expensive).

Another way to make it easier for you: the mass isn't set until you see it for the first time (with default settings, that happens when you first get to within 2.5km of it). So you can save the game shortly before the encounter and reload the game until you meet a rock that suits you.

 

[magnemoe]

On 7/11/2020 at 8:27 PM, Laie said:

Well, Class E starts at ~800t and I don't know where it ends. The heaviest I've personally seen was 4500t.

Capture into any kind of orbit around Kerbin can often be done on 300m/s or so... back-of-the-envelope I guess that this would require something like 200t of propellant for the heavier rocks. Once captured, you can send refueling missions until it is in the target orbit. Totally doable, though perhaps a bit tedious (and expensive).

Another way to make it easier for you: the mass isn't set until you see it for the first time (with default settings, that happens when you first get to within 2.5km of it). So you can save the game shortly before the encounter and reload the game until you meet a rock that suits you.

 

Ty did not know that.  As for OP with 5 m/s and no mining it will not work anyway. 

One nice way to make an asteroid tug is to make an puller ship as they are much more stable. 
For this you want the engines on top pointing down, also put solar panels here. now make an long structure downward, fuel tanks some radiators, ore tank, the large ISRU converter batteries, rcs fuel. add some struts so you don't damage the plates you use to test this with, add some more struts and the claw set this as root. docking port in other end so you can use this to refuel ships. 
Launch this with the claw up inside fairing, once you separate you need to change control to docking port. 

E-asteroids are pretty hard to control. 
 

[Kryxal]

I made a quad-nuke pusher with dual miners for a trip to Moho once with a class E ... it worked too, but I had to fine-tune the engines to avoid torque, and couldn't go full throttle anyway.

[OHara]

On 7/11/2020 at 8:32 AM, Joe Kerbal said:

Unfortunately I've not yet the technology but I guess I'll get it and try again. 

When you do mine an asteroid to reduce its weight, do it in version 1.9.1 or after they fix a bug in 1.10 (link) where asteroids fail to get any lighter when you mine them.