Killing relative velocity

[gilflo]

When performing a Rendez vous, assuming that you have done all the maneuvers to align orbit and approach the target at less than 500m, how do you kill relative velocity without increasing this small distance of separation ?

Even when the retrograde vector (yellow one) is nearly align with the retrograde vector of the target (pink one), every burn to decrease my relative velocity increase the render vous distance between me and the target.

I tried burning on the pink target retrograde, burning my retrograde, I burnt to align the yellow retrograde my retrograde vector on the target retrograde (pink), it always make relative speed decrease but it also always make the rendez vous distance increase toom ugh.

[corvus2606]

If you reduce relative speed to 0, then you are as close as you are going to get. The standard process I follow is to zero my relative velocity, then burn towards the target, reduce my distance by half, then repeat

[iEvermore]

I guess i would need to know what your velocity is at compared to the target. What you want to do is make sure both prograde markers match up and leave the speed alone until your closer. At 500m I would move to docking with RCS and use time warp for more control and less spinning your craft around.

Again if u tell me your at 500m and relative velocy is 100m/s then yes slow down on retrograde with engines. You always want to be moving towards the prograde of the target. You sometimes need to adjust many times but at 500m from target you'll have this down pat in no time.

[iEvermore]

Ps u know u MUST have SOME velocity prograde to get closer right? I just thought about ur "increasing distance" comment. If u retrograde too much u are now moving away from the target. The two prograde must be lined up and have some sort of velocity.

[DMagic]

You're not quite doing it right. The 2 pink icons do not indicate the target's velocity vectors, they indicate the actual, current, position of the target. Pointing at the pink circle means you are pointed directly at the target. You need to switch the navball to target mode, click on the the little window above the navball until it says target. This is how you can see the target's relative velocity. If you point at the yellow x in target mode you can then reduce your relative velocity to 0. Then you can use the pink icons to move toward the target.

Edited May 15, 2013 by DMagic

[Vanamonde]

Once you're within the last 2kms or so and have matched velocities, there's no need to be subtle or fancy anymore. Just point at the pink dot-in-circle marker, as DMagic notes that's the current position of the target, and burn straight for it at about 20-30m/s. You may have to do some correcting along the way, but nothing too difficult.

[gilflo]

DMagic thanks for explanations.

iEvermore, I mean, I target a maneuvers that make my closest approach point at 500m from target, then I burn and after 2 or 3 maneuvers I get this closest approach point at 500m from my target, but I am still 20mn or more from this point and my relative speed to target is around 75 to 125m/s, I warp and even if the speed is lightly decreasing, I need to burn to be at the 0,5 to 2m/s when I will be at my closest approach point. The fact is that when I burn in order to do this, my closest approach point increase dramatically so that I need to perform others node maneuvers to decrease this closest approach point. I thought that if I succeed to align perfectly my retrograde with one of the pink icon, I could burn without loosing this point or without increasing it dramatically.

[RoboRay]

If you're having trouble with the distances, wait until you're at or near the closest point of approach distance to kill relative velocity. For example, if your CPA is 500m don't start trying to match velocities while 2km or more away, as that's not going to work. Wait until when you hit 600m, point at your yellow retrograde marker (in Target mode) and start burning to zero.

Edited May 15, 2013 by RoboRay

[dr_jt]

Consider Kerbals

Edited June 6, 2013 by dr_jt

[DMagic]

I see what you mean about aligning the icons. But if you are passing by the target at 100 m/s that means your orbits are very different. That's why it's a good idea to have fairly similar orbits before trying to get a close approach. Even if you can't do that, it's still possible to get in close, you just need to burn as close to approach point as possible, obviously this is easier if you have lots of thrust. The distance might increase from 500m to maybe 5km, but that's not really that bad. Even if you are 5 km away, if your relative velocity is 0 it should be easy to get closer, not as easy as if you were starting from 500m away, but not too tough either.

[iEvermore]

DMagic thanks for explanations.

iEvermore, I mean, I target a maneuvers that make my closest approach point at 500m from target, then I burn and after 2 or 3 maneuvers I get this closest approach point at 500m from my target, but I am still 20mn or more from this point and my relative speed to target is around 75 to 125m/s, I warp and even if the speed is lightly decreasing, I need to burn to be at the 0,5 to 2m/s when I will be at my closest approach point. The fact is that when I burn in order to do this, my closest approach point increase dramatically so that I need to perform others node maneuvers to decrease this closest approach point. I thought that if I succeed to align perfectly my retrograde with one of the pink icon, I could burn without loosing this point or without increasing it dramatically.

I used to have the same problems. You can see some of my earlier posts asking for help..

- assuming u have the same inclination as target...

- Basically, you need to watch your speed and where your "Prograde" is. What I do, is this. From the beginning I get myself into orbit, I mark my target, and I set up a lower orbit if I need to catch up, or a higher orbit if I need to slow down. (This depends whether or not the target is in front or behind me in orbit.) I do this the second I detach my ascent rockets.

- Then I time warp until I'm as close as I can get, sometimes you need to tweak it with maneuver nodes, but for the most part if your no more than 5-15km different in orbital height it will be close. Note it will take a few full orbits to get closer, but you will see each time around the planet you get closer and closer.

- Once I'm within let's say 20km away from target, I switch from map view to ship view and switch my camera to "Chase"

- I slow down by burning Retrograde. You watch your target speed above your Nav ball to see this.

- Once I get it to about 0.1 - 0.5m/s. I then turn the ship towards the pink prograde of the target and burn until I get the Yellow prograde to line up. Now this will need constant adjustment as you move closer. This is because of how orbital mechanics work and you constantly need to adjust your self. Once it gets too far off like your yellow prograde is on the edge of the Nav ball, I go find my Yellow Retro grade and zero out the speed again. Then once is slows to 0.1 - 0.5 I burn toward the pink prograde again. You may need to rinse and repeat this a few times, but you will get there. I had a hard time with higher speeds as I got closer so I choose to keep it at a comfortable 10m/s for anything under 2km. Some people are different, but I find this gives me a lot of time to make adjustments. Time warp is your friend.

- Once you get closer I switch to docking mode. This turns off your engines and moves you to only RCS control. You toggle between Vertical/Horizontal/Speed adjustments to pitch adjustment. You hit space bar to move between the two. (Now keep in mind, by the time you switch to docking mode, you need to have your speed down to not much more that 1-1.5m/s depending on the size of your craft. You will spend too much Mono trying to slow down if your going too fast. )

- Now that your in Docking mode, you keep adjusting your ships trajectory (Prograde marker) with the WSAD keys to keep your yellow prograde aligned with the Pink target Prograde. (Note: in docking mode, the direction your nose is pointing does not matter, only where the yellow prograde is in relation to the pink one. This changes when you actually need to make contact, but I worry about pointing the nose only at the last 10m. Just don't let it go crazy) WSAD adjusts horizontal and speed and Left Shift and Left Control changes vertical. Keep in mind some times making any type of movement will change your speed to target, you need to keep it under control. Should your Prograde marker disappear you are at 0.0m/s and just need to hit W to speed up toward target.

Here is a video that helped me understand it a bit. You should subscribe to Scotts Youtube channel. He has a lot of helpful tools.

http://youtu.be/AHkY3FusJIQ

He doesn't use the Docking mode, but you can chose what you like.

Let me know if you need more info. I've done a lot of research before getting the hang of this.

Edited May 15, 2013 by iEvermore

[Specialist290]

In short, what you're really trying to do in the big picture is match orbits with the other craft. It's much easier to do this if they're already quite similar (particularly if they're in the same orbital plane; the map screen gives you the angle of relative inclination to your target at the ascending and descending nodes, which are the points where the planes of two orbits intersect) and if you're target is already in a circular or near-circular orbit.

Another thing I'd advise (if you haven't already done this): Put some RCS fuel and thrusters on the craft you're piloting. Once you get within "eyeball range" of your target, RCS thrusters are much more versatile than your main engines. They allow you to translate (i.e. change your direction of movement without changing the way your nose is pointing), which is quite useful indeed for rendezvous maneuvers.

EDIT: Crossposted with Evermore. That's sound advice there.

Edited May 15, 2013 by Specialist290

[magnemoe]

If you're having trouble with the distances, wait until you're at or near the closest point of approach distance to kill relative velocity. For example, if your CPA is 500m don't start trying to match velocities while 2km or more away, as that's not going to work. Wait until when you hit 600m, point at your yellow retrograde marker (in Target mode) and start burning to zero.

And yes wait for the marker where you orbits cross, reducing speed to 0 here will put you in the same orbit.

Its pretty easy to end up circling the target, Very easy if you have to high twr. Regarding TWR, it might be an idea to have some tiny engines like two orange radials for orbital work, an mainsail on an almost empty orange tank is hard to control.

[gilflo]

Well ok, thanks for those replies. With the vid it is quite easier to understand how to decrease relative speed.

The fact is that I am using a rockomax tank to Rendez vous on my space station to fill its big KSP tank and even if rockomax is well balance it is not so easier to maneuver than a small ship.....

[gilflo]

This morning I used the Scott Manley Method to approach my SS1, I was afraid to kill a differential velocity of 150m/S with my 4 LVT45 mounted on my Rocko Tank but it was quite easy to do it just 30s before minimum closest approach point which was 200m.

So I stopped at 600m and let the target approach at 2,5m/s then at 350m I used ORDA automatic docking. ORDA has improved as

it is not using so much Mono to dock. I filled completely my KSP 3,5m Tank and I kept one of my Rocko tank with its four engines

to undock and try some manual docking. Hope I won't crash in my SS1

Here it is

[iEvermore]

Sweet!! Nicely done!

Have fun.

Edited May 16, 2013 by iEvermore

[loiske]

When you have the basics of killing relative velocity, you can try to mess around with that. Lets say you have closest approach of 2000m. When you start your burn, watch where the "away from target" -marker is relative to the retrograde marker and aim your burn slightly to the opposide side. This will bring you closer to your target at the same time it kills the relativäe velocity. I have just begun to explore this myself. And it helps to have mechjeb etc. To tell you the closest approach and time to it.

Edit: this works with higher relative velocities. When you actually get closer, its best to keep rvel to ~10m/s and use timewarp to travel the distance

Edited May 16, 2013 by loiske

[Colonel_Panic]

DMagic thanks for explanations.

iEvermore, I mean, I target a maneuvers that make my closest approach point at 500m from target, then I burn and after 2 or 3 maneuvers I get this closest approach point at 500m from my target, but I am still 20mn or more from this point and my relative speed to target is around 75 to 125m/s, I warp and even if the speed is lightly decreasing, I need to burn to be at the 0,5 to 2m/s when I will be at my closest approach point. The fact is that when I burn in order to do this, my closest approach point increase dramatically so that I need to perform others node maneuvers to decrease this closest approach point. I thought that if I succeed to align perfectly my retrograde with one of the pink icon, I could burn without loosing this point or without increasing it dramatically.

Okay, this is simple once you have the hang of it, but kinda difficult to explain...

First things first, get your orbit as close to the target as possible. I presume you know how to do this, and get within 1km or so. If not, and this is where you're stuck, let me know and we can back up a bit.

once at the target's intersect in your orbit within 1-2km or so, you want to switch the navball to target mode if it isn't already (it should be switched automatically if you're close enough) and drop the map screen. The first thing you want to do is kill your relative velocity as much as possible. To do that, with the navball in target mode, point to the retrograde indicator (the green (X) ) and burn, keeping a close eye on your velocity indicator. You should see it approaching 0. As it gets closer to 0, kill your burn, and make finer adjustments with smaller, softer pulses of power until you're showing <1 m/s.

Next, get a feel for your distance to target and how long it will take to reach it. Point toward your target vector indicator (the pink (o) ) and make a small burn to bring your speed up and close on the target. You want to adjust your vector until the prograde indicator (yellow O ) is over the target vector indicator, by overshooting the target vector indicator by a little bit, 'tugging' the prograde vector toward it. Once close enough, burn toward the target until your relative velocity is high enough to reach it in a minute or so.

Once you've done that, turn around to face your retrograde indicator again, which should now be near the target retrograde indicator (pink Y ). As you get close to your target, start to slow down by burning toward your retrograde, but at the same time you want to be overshooting your retrograde a bit on the opposite side from the target retrograde indicator, 'pushing' the retrograde indicator toward the target retrograde indicator. This will fine tune your approach to intercept as you continue to decelerate and get closer, until you are right on top of it, at which point burn directly toward your retrograde indicator until your relative velocity reads about 0.2m/s or less. Congratulations, you're ready to switch to docking mode.