How to purposefully collide 2 spacecraft

[capran]

So I was bored, and I figured I'd like to see what happens when 2 spacecraft collide going in opposite directions. I built a simple stock rocket capable of ~100 km orbit in 2 stages (with Mechjeb attached). I launched one going east and a second going west. I tried my best to get them on perfectly circular orbits at that same apoapsis and periapsis.

I tried for over an hour an half to get them to collide! I couldn't do it. Even if the Rendezvous Planner said the next closest approach was within single meters, it would always miss! Also, occasionally I'd notice my orbit would suddenly change by a few hundred meters, even if my engines were off (no RCS on this ship.)

I'm wondering now if the game's engine simply can't or won't allow a head on collision? Perhaps the relative velocities and momentums are too high so Squad programmed it so they just don't happen?

Edited November 25, 2013 by capran
use hyperedit or edit persistence file

[DrMonte]

1. You must definitely drive one of those ships for physics to take effect.

2. Throw away MechJeb.

3. Try for over a day and a half

[leopardenthusiast]

There are a couple of problems here:

One, tiny changes of a few metres are constantly happening, and there's nothing you can do about it in normal time.

Two, craft do not interact when in timewarp. Even if they do collide, nothing will happen unless you're in normal time, and if you are, the changes will probably just prevent the collision.

The best way to get similar effects would probably just be to do a normal rendezvous, but have one craft have a mainsail, and when within a kilometre, point at the other craft (you can use MechJeb if you want, but I'd do it manually) and fire the mainsail to massively increase speed. It's not quite the ~4km/s relative velocity you'd get from opposite orbits, but neither spacecraft will survive.

[capran]

1. You must definitely drive one of those ships for physics to take effect.

2. Throw away MechJeb.

3. Try for over a day and a half

1. I was piloting one at a time.

2. Nope. The Smart ASS alone is incredibly valuable. And all the information windows make the game much better. Yes, Engineer has those too, but I like the arrangement of Mechjeb better. Plus Smart ASS and auto maneuvers. Without Mechjeb, you have to go back and forth from the ship screen to the map screen just to see the orbit info and ship controls.

3. I'm not THAT bored.

There are a couple of problems here:

One, tiny changes of a few metres are constantly happening, and there's nothing you can do about it in normal time.

Two, craft do not interact when in timewarp. Even if they do collide, nothing will happen unless you're in normal time, and if you are, the changes will probably just prevent the collision.

The best way to get similar effects would probably just be to do a normal rendezvous, but have one craft have a mainsail, and when within a kilometre, point at the other craft (you can use MechJeb if you want, but I'd do it manually) and fire the mainsail to massively increase speed. It's not quite the ~4km/s relative velocity you'd get from opposite orbits, but neither spacecraft will survive.

Why do these changes occur? The game only uses simple Newtonian physics, right? No quantum or relativity, and aren't all sources of gravity treated as point sources?

I only time warped to get the ships fairly close together (say about 100 km) then go to 1x.

I've done similar things to your last point before, not always intentionally!

[SofusRud]

I believe the changes he's referring to are either floating point errors or center of mass moving. also remember that loading distance is 2.5 km, and when travelling in opposite directions that literally gives the game less than a second to make the calculations, which can result in the craft glitching through each other. To compensate, I recommend that you do the collision at a much higher altitude, giving you lower orbital speed, perhaps between the Mun and Minmus

[Wallace]

Try it round the mun, the velocities will be lower and the orbits smaller, so should be easier.

[Kasuha]

Colliding things in orbit is tricky because arithmetic errors in thousandts of milimeter on one side of the orbit add up to tens of meters on the other side. Your best bet is probably editing your persistence file, putting the two ships on exactly the same orbit, then time warping them to sufficiently small distance (about 100 km) and watching from there.

[DrMonte]

I believe the changes he's referring to are either floating point errors or center of mass moving. also remember that loading distance is 2.5 km, and when travelling in opposite directions that literally gives the game less than a second to make the calculations, which can result in the craft glitching through each other. To compensate, I recommend that you do the collision at a much higher altitude, giving you lower orbital speed, perhaps between the Mun and Minmus

So no fun in collision

I would try quicksave/load just before encounter course correction.

For example 3km - just before physics load or 2km - just after physics load. If you bypass just tens of meters you should be able to correct the course.

[Motokid600]

Honestly a slower collision would be more spectacular. With opposite orbits all your gonna see is just a quick POOF! Ships gone...

[Starwhip]

Well, if the orbital velocity is approx. 2000 m/s, and they are in counter orbits, then they are coming at each other at 4000 m/s. Divide that by about 10 fps (I know, accounting for lag...), and you get:

4000 / 10

400 m/s. That's PLENTY of time for them to just *whoosh* by one another. No wonder they don't collide.

[Nuke]

ive tried to knock things out of orbit with surface launched kill vehicles (just a small rocket built with half meter parts which was incapable of making orbit). i had to use the lazor system to keep a beam on the target which assisted navigation greatly. pretty much as soon as the object crossed the horizion you launched, and you had to throttle very carefully to arrive at the right place at the right time. i got damn close but never actually hit the thing (a big cluster of large tanks). i even tried firing missiles at it (also unsuccessfully). you could probibly do it without mods with a little bit of patience and a whole lot of luck.

[Cranium]

Install HyperEdit. Alter the crafts' orbits so that they are exactly the same, but with opposite rotations. Wait for the boom.

[Deadweasel]

Well, if the orbital velocity is approx. 2000 m/s, and they are in counter orbits, then they are coming at each other at 4000 m/s. Divide that by about 10 fps (I know, accounting for lag...), and you get:

4000 / 10

400 m/s. That's PLENTY of time for them to just *whoosh* by one another. No wonder they don't collide.

erm... wouldn't that mean 400 meters per frame? Either way, agreed, you could easily miss seeing what went wrong at those (relative) velocities, and course corrections to get a collision are extremely slow and arduous. Try getting properly aligned with a target going the SAME direction, but with a 200 meter per second difference between the two. RCS takes an age to make a difference in alignment.

[Starwhip]

Exactly. 400 meters a frame. Which means that if the craft are on the exact same orbit, they could still go "through" one another, because it is a game and they are not really moving.

[Deadweasel]

Exactly. 400 meters a frame. Which means that if the craft are on the exact same orbit, they could still go "through" one another, because it is a game and they are not really moving.

Regardless of the render rate, physics is still active for both ships in range at regular time.

The only case where you'll get them passing through one another is if you're time warping at that moment, at which point both ships are "on rails" and exempt from physics calculations, including maneuvering and collisions.

Here, this video demonstrates the point more clearly:

Only time warp can result in two ships "missing" one another when they're otherwise on a direct collision course with one another.

[Markus Reese]

It is possible, I have had it happen by accident with space junk

The main thing is never to time warp after you get everything set up, when you bring it out of timewarp, the numerics can be slightly different resulting in your 100m spacing It is a bit of trial and tweak. Mostly getting the orbital planes exact, then focus on just tweaking elevations each time they go past eachother. if you miss. Myself, I find it tough to get the within 1m on a perfectly circular orbit simply due to the fact that the orbital indicator Ao and Po start to freak out as you perfectly circularize an orbit.

[Smidge204]

+1 for using HyperEdit. That will get you to within a few hundred meters easily, and you can use RCS to tweak things. Keep your prograde and target markers lined up.

Record video of it.

=Smidge=

[Deadweasel]

+1 for using HyperEdit. That will get you to within a few hundred meters easily, and you can use RCS to tweak things. Keep your prograde and target markers lined up.

Record video of it.

=Smidge=

*plays Barenaked Ladies - It's All Been Done*

Woo hoo HOOOO!

[Vanamonde]

The root of the problem: space is big, ships are small.

[capran]

Install HyperEdit. Alter the crafts' orbits so that they are exactly the same, but with opposite rotations. Wait for the boom.

Yeah, that may be the only easy way. I don't know enough about the config files to alter the persistence config.