Head on intercepts

[NFunky]

Hey all,

Been messing around with a class-E ARM. Took me a while to figure out how to cram 7k dV into a Klaw ship (with KIDS "real-life" setting), but I managed it and a launcher that could orbit it.

So here I am, in an 80k LKO, in the same plane as the incoming rock. The rock is on an impact trajectory, so I need to do a stright line, head on intercept. Now I've set up a maneuver node and make sure its apoapsis is on the line of target's course. I run the prograde out until the "Target position at intersect 1" and the apoapsis are as close as I can get them.

Here's a screenshot

https://www.dropbox.com/s/yqb35r8vxi2efwd/ARM_node.GIF?dl=0

I think this is the right way to go about it, but I can't seem to get my intercept precise enough to come within rendezvous range. Never at any time during my node setup or burn do I get an intercept marker with an actual seperation distance. I'm thinking I could maybe use the Kerbal Alarm Clock's closest approach option to roughly get my burn right, but I'd love to be able to get it right with the node beforehand. What do you guys usually do for these sorts of ARMs?

Edited April 1, 2015 by NFunky
Paragraph spacing was wrong

[Randazzo]

Target position at intersect is only half the equation, the other important bit is -your- position at intersect, which I can see from your picture is on the far side of Kerbin close to your periapsis. That's why you aren't getting a rendezvous node. You can try right clicking on your maneuver node and adding an orbit to it (which will add a full revolution to the T- time) and see if your nodes get closer. If they do, you can keep adding orbits until you're close enough, then adjust your flight path as necessary. If they don't, you'll need to take another approach.

Also, heading in the opposite direction as the asteroid is going to make the capture harder, since you have to consider your relative speed to your target object. Since you will be heading towards each other, that relative speed is going to be much higher and you'll have to cancel it almost completely out to safely make contact with the object. If it's only a redirect mission, you'd probably be better off approaching from behind in the same heading, and maneuvering to latch on in a radial position, pushing the asteroid away from Kerbin without trying to slow it down.

You can take the latter bit with a grain of salt, I've only ever done one asteroid capture mission.

Edited April 1, 2015 by Randazzo

[TheXRuler]

ARM missions can be hard...

As Randazzo says, the best way is get your relative velocity at closest approach to a minimum.

Eg you could get a kerbin escape, then adjust your solar orbit to intersect the asteroids orbit before it enters Kerbins SOI. This should significantly reduce the dV costs, as only a small velocity change is required to alter the ateroids course when you are further away from Kerbin.

If however the asteroid is inside kerbins SOI your best option might be to launch to an orbit wich is going the same way as the ateroid will when it passes Kerbin, taking account of the inclination, as launching to an inclined orbit is cheaper than making an inclination change burn while in LKO. Once in LKO you could raise your ap and then try getting a close approach to the asteroid as far from Kerbin as possible.

Edited April 1, 2015 by TheXRuler

[radonek]

…You can try right clicking on your maneuver node and adding an orbit to it (which will add a full revolution to the T- time) and see if your nodes get closer. If they do, you can keep adding orbits until you're close enough…

I don't think this is good approach in this situation. All the time you spend down in well waiting for best launch window, asteroid will be falling down gathering speed and will be that much more difficult to both intercept and move. I'd say its better to shoot off asap, even at cost of wasting a lot of dv. Hohmann is efficient, but slow – you spend a lot of time up near AP.

I'd say: plot a fast track close to target orbit. Could be even escape trajectory if fuel permits. Only when close to target do a transfer burn. If both orbits are close enough, it will amount to full reverse. Yes, it will take a lot of dv, but you'll need WAY less fuel to move that rock.

[Randazzo]

I don't think this is good approach in this situation. All the time you spend down in well waiting for best launch window, asteroid will be falling down gathering speed and will be that much more difficult to both intercept and move. I'd say its better to shoot off asap, even at cost of wasting a lot of dv. Hohmann is efficient, but slow – you spend a lot of time up near AP.

I'd say: plot a fast track close to target orbit. Could be even escape trajectory if fuel permits. Only when close to target do a transfer burn. If both orbits are close enough, it will amount to full reverse. Yes, it will take a lot of dv, but you'll need WAY less fuel to move that rock.

I was specifically addressing this part of the original post.

I think this is the right way to go about it, but I can't seem to get my intercept precise enough to come within rendezvous range. Never at any time during my node setup or burn do I get an intercept marker with an actual seperation distance. I'm thinking I could maybe use the Kerbal Alarm Clock's closest approach option to roughly get my burn right, but I'd love to be able to get it right with the node beforehand.

Edited April 1, 2015 by Randazzo

[Mister Dilsby]

I've done it both ways in a successful Class E asteroid capture: the first, intercepting the rock in solar SOI and steering it, the second making an intercept in Kerbin SOI to refuel my tugship and finish the capture. As others have remarked what you're trying to do is hard, but I've done it and it's not impossible.

Head-on intercept is easy, but most call that "impact". You need to match the asteroid's orbit, in whatever SOI the asteroid is in at the time you intercept it. If it is in Kerbin SOI and on a collision course with the planet, the intercepting ship also needs to be on a collision course. Another way to think of it: pretend the asteroid isn't there at all, and you are fulfilling a contract to match its orbit.

I think the intercept you're showing is about as good as you're going to get given the velocity involved. I would fly the ship out on that elliptical track and then just thrust straight at the asteroid as it approaches. You do this by putting the navball in 'target' mode and thrusting like crazy to "pull" your prograde marker onto the marker showing direction to target. Then as you approach you need to match its velocity. You do this by thrusting even crazier to "push" your retrogade marker onto target direction--ideally slowing your velocity relative to target to zero as your distance to target goes to zero.

You may find yourself running out of delta V as you do this. I would be surprised if you have enough left, even after intercepting, to shift a class E rock out of a terminal trajectory. My big tugship has several thousand m/s dV on its own, but only about 150 when attached to 3000 tons of asteroid... that's why I needed to run a refueling intercept in Kerbin SOI, but by that point the rock was already half captured and I just needed to prevent its escape. Your kilometrage may vary, of course