Aligning prograde with target?

[Orum]

I'm trying to do some high speed impacts on one of my space stations to test my faux whipple shielding, and I came across a problem--trying to hit another object in orbit at high velocity is actually fairly difficult. Trying to do these by hand has proven to be a complete disaster, and when I looked to MechJeb, specifically Smart A.S.S., I realized there's no way that I can see of accomplishing aligning the two. It doesn't seem to have any context that considers both your own orbital velocity as well as the target's position (yes there's RVEL, but RVEL+ aligns you with your current relative velocity and does not attempt to maximize it by aligning prograde with the target).

I've also tried a counter rotating orbit (same orbital params but going around the other way), but the rounding errors make this nearly impossible with the sort of precision I need. I'd prefer some way to do this while under constant thrust--from lift off to eventual impact, or stopping just a second or so before impact. I also looked at some of the "warfare" mods (namely BDArmory), but my concern here is I think most of the missiles in that also contain some sort of "explosive" payload (like a fuel tank blowing up), and I want a pure kinetic impact. I know what you're saying, "But if you crash your own rocket into it under thrust, you'd have some fuel left in your tank too!" While this is true, I was hoping I could dump the fuel just before impact with TAC Fuel Balancer and avoid that problem.

So, what's the best way to go about something like this? Is there a mod I should be looking at, or is there some way to accomplish this with MechJeb that I've overlooked?

Edited October 5, 2014 by Orum

[Taki117]

Mechjeb does have a Target+ option which will align you towards the target, regardless of your actual velocity vector. You will have to do some manual adjustments as you get closer, but you should hit whatever it is you are aiming for assuming it is decidedly large.

[Sirrobert]

If you want to hit, you need to get the close encounter distance down to 0.

There is no Mechjeb default window for close encounter distance, but it is an option when you make a custom window. Try that

[StrandedonEarth]

Kerbal Alarm Clock can also give you a 'closest approach' distance under the rendezvous tab, which I was once able to tweak down to 0.0m. Silly me, I never thought to quicksave it and have fun with the encounter.

[Skorpychan]

Use mechjeb to get a rendezvous. Set the distance far away. When it stops you dead, accelerate towards the target.

[Orum]

I previously used the TGT+, but I would keep missing it as that doesn't consider your velocity relative to the target. The "closest approach" is in MJ as well, but the issue was always rounding errors that sometimes were induced while warping or even just rotating or waiting.

After some more experimenting, I found that the best thing was just to establish a higher orbit where the pull of gravity was less, and that made hitting things easier. Still not ideal as I miss every now and then, but it's enough that I can test things.

Thanks.

[Sirrobert]

Well you have to remember that it will always be difficult to hit an object in space. Space is BIG, and relative to the empty space around it, your target is basicly a needle in a haystack

[tomf]

If all you want to do is test the shielding your best might be to build something very light and maneuverable with overpowered engin, empty it of fuel, enable the infinite fuel cheat and then keep burning straight at your target. Carry powerful RCS thrusters. Set up a good intercept and then when you get close burn straight at the target and use RCS to keep your prograde pointed straight at the target.

[Wanderfound]

It's a matter of burning or RCS translating until your target and target prograde markers coincide and holding them there until impact. I usually find it easiest to get up to speed and roughly aligned on the main engines, then point the nose at the target and use RCS translation for fine tuning. It's just high-speed docking, and the techniques are the same.

The closer you get to the target, the bigger the angles involved, so unless you are 100% perfectly on target it will look like the target prograde marker is moving away from the target marker. The target isn't moving and your course is straight (ish, orbiting); the navball effect is just trigonometry. Translate or burn to push the markers back together.

The later you apply corrective control inputs, the more vigorous they'll need to be. Using powerful manoeuvring thrusters (Vernors or spammed RCS) may help.

And quicksave on approach so you can practice repeated shots until you've got it.

Also: if you're just testing the station, why not use Whack-a-Kerbal?