Rescue in LKO

[MPDerksen]

My last thread was all over the place and I got LOTS of great help, but my questions were unfocused.  Let me be specific on the current mission(s).

I have a craft in LKO (Ap of 88.4K, and Pe of 78.5K).  My target craft is in an orbit of Ap 85K and Pe of 74.5K.  So basically a little below me, and will be catching me after many orbits.  My An and Dn are at 0.7 degrees, so my next step is to get that to 0.0, right?

Current separation at the Intersect 1 is 672km, but that should get smaller each revolution.  

My current question concerns that my Ap and Pe is not exactly at the same spot as my target.  maybe 20km apart each?  When I watch the Scott Manley videos, he shows that, once you get "close", you simply "match orbits" with your target to get your relative velocity the same.  Several tries so far, and I get within a few km for a short time, then have to thrust all over the place to stay close, and then things go to xxxx.  
Let's start there with my questions?

[James Kerman]

For a rendezvous I will usually get my inclination to 0.1° for a 'medium gravity' body like the Mun.  This will help you later on and is good practice for when you begin docking.

 I make either Pe or Ap the same as my target and wait for the target to ensure a close encounter - in your example you will be at least 3.4km away from an intercept without adjustment.  Once I'm within 50 - 80km of my target I set a maneuver node at the point I matched either the Ap or Pe and play around until I get the intersect within 1km (if you have matched planes you will only need to burn either prograde or retrograde to fine tune the encounter).  With a fairly close orbit like yours the speed of the encounter should be around 20-30m/s but even if its higher, remember that a Kerbal EVA pack has about 500m/s Dv (quite a lot for orbital maneuvers).

Once you are within 2.2km of your target, switch to it using the [ key and EVA your stranded Kerbal.  At this point I look for the rescue vehicle and 'select as target'.  From here on it's all about the nav ball so switch it into target mode if it isn't already.  You should see a target or anti-target  symbol in addition to the standard vector symbols.  All you need to do is match up the target symbol  with your prograde symbol  and you are now headed towards the rescue vessel at the speed indicated on the top of the navball.  You will need to make some small adjustments and then slow your Kerbal down when you get really close.

[bewing]

1 hour ago, MPDerksen said:

so my next step is to get that to 0.0, right?

Right.

1 hour ago, MPDerksen said:

My current question concerns that my Ap and Pe is not exactly at the same spot as my target.  maybe 20km apart each?

Not a problem.

Let's say your separation is decreasing at 20km per orbit. You wait until your separation is less than 20km, and then you know that during your next orbit, you are going to pass each other.

Now you burn retrograde (or create a retrograde maneuver node) so that your orbit crosses your target orbit -- at approximately the point where the separation goes to zero.

You get to that crossing point -- then the trick is to put your navball in TARGET mode, and burn retrograde until your speed is zero. This automatically puts you in the same orbit as your target, and you are pretty close.

Then you point your nose at your target, and burn just a little bit (10 m/s is a good number). When you get really close, you turn around retrograde (still in target mode) and burn to 0.0 m/s again. Then repeat.

[Espatie]

As @bewing mentions, making sure your Navball is in target mode is key.

 

You don't have to have precisely the same inclination or the same longitude/argument of Periapsis; reasonably close is good enough, because the following instructions will - by definition - match the orbits.

 

Once you are at closest approach, put your Navball into target mode (you will need to have made the other ship a target via the Map window).

 

In target mode the Yellow Prograde and Retrograde icons on the Navball indicate your direction of travel relative to your target. The Pink target icons show the absolute direction to the target. The Velocity readout shows the relative velocity between the two craft.

 

Unless you are incredibly lucky, the Yellow Prograde icon will be nowhere near the Pink target icon.

 

The first thing to do is to kill the crosswise relative velocity. Point your ship to Retrograde and run the engine until Velocity = 0. If your pilot is level 1 then your SAS can lock to the Retrograde icon. At a Velocity of less than 1 m/s the lock will switch to just hold, but that's fine.

 

Once you are stationary relative to the target, unlock your SAS, turn to point directly at the target (pilot level 3 to automatically lock to that direction, but easily done manually) and burn a bit. Your relative direction is now directly closing the distance. How much velocity you carry here depends on the distance between the two craft and your patience.

 

Once you are moving at an acceptable velocity, shut down your engine and turn to point Retrograde again. Don't get too close too fast! Make small burns to manage your velocity appropriately until you are within 100 metres. Your target will "wander" slightly off the direction of your movement (because the two craft are in different orbits) but that's not a problem.

 

Once you are within 100 metres of the target you can kill all the remaining relative velocity; you are now stationary with respect to the target and in (almost) exactly the same orbit. You can either switch to the other craft and EVA the occupant over to your rescue ship, or you can set up to dock using RCS thrusters.

 

One other thing is that you will now inevitably find that your rescue is taking place on the night side of Kerbin, and will wish you had put some lights on your ship.

 

Lastly I strongly recommend the mod "Easy Vessel Switch" for flipping between the ships. "Navball Docking Alignment Indicator" is very good when you get to docking ships together.

Edited January 20, 2019 by Espatie

[Daveroski]

And here's how I do it.

First I don't worry about the difference in inclination. I'll work with anything upto 5.0 degrees difference.

Then I will set my Pe to be a tiny bit less than my target's Pe. Anything less will do no matter how small so I go tiny difference. This will ensure that at some point our orbits pass each other very closely.

I will ensure that I have the Target set as "Target." but that my Navball is set for "Orbit" Not Target or surface.

I will then make a node at my pe and make my Ap roughly the same as my target's.

Before closing the node I will Right click it and using the plus symbol increase the number of orbits until I get either an encounter or in the ballpark.

Then I will right click the node again and make adjustments with radial in/out and Normal and Anti-normal and with a little work I  should be able to get an encounter at 0m or at least pretty close to it.

Once I have the node plotted I action it as carefully as possible.
For me, that means if the burn is only a few seconds, I will reduce the thrust power of my engine by as much as 90% so that I have a nice long burn at the node. I will work out from the duration of the burn when I should start my burn and begin at full throttle so that I am halfway through my burn at the time of reaching the node.
As I approach the end of the burn I will stop at around 1 second of burn left and then use SHIFT and X to get the burn to 0
Then I will close the node.

I will look at the map and see that my closest approach is pretty close to 0.

I will reset my engine to full power and use the sticky pin for my engine so that I can make it softer should I need to. I hate having to be fast on the buttons, I like to be able to take it steady.

Warp at this point is optional, but I try not to overshoot the point where my navball automatically switches to target. Because sometimes, it doesn't.

I like to end up 2-3 KM away. to give myself time to be sure I am ready for the encounter.

If it is not already, I will make my navball set for "Target" not orbit or surface.

I will aim Retrograde.

Ideally I want to come to a gentle stop as close as possible to the target.

At 1000m I will do a full power engine burst. Ready to hit X immediately if braking is too fast.
I will want to finish the braking with the shift key so after the initial burst, I will reduce the power to my engine again and use the shift key and X as required until My relative velocity is 0.

I will then set my navball back to "Orbit" not target or surface.

If I am taking someone on board via EVA I will orient my ship using just SAS not RCS

I will point the entry door towards the other ship.

For simple EVA I just remember the golden rules.

I Use space to make the kerbal face away from me.

1. Only travel in one direction at a time.

2. Only use burst of 1 second.

3 Three seconds of thrust to move need three seconds of thrust in the opposite direction to stop.

4. Go up or down then stop.

5 Go left or right then stop.

6 Go forward and prepare to stop or grab.

Repeat as required.

D.

 

[Espatie]

I generally approach with the active ship in a wider (i.e. slower) orbit and let the target catch me up, mostly habit because the game loves to give out rescue missions with the target skirting the very edge of the atmosphere of Kerbin.

 

With practice you get to just know what angle two orbits will close by.

[eddiew]

15 hours ago, MPDerksen said:

several tries so far, and I get within a few km for a short time, then have to thrust all over the place to stay close, and then things go to xxxx.  

This bit is easy(ish)  

Avoid thrusting all over the place. This isn't Elite and you don't need constant engine thrust to move.

When you do get within a few km, click the "orbit: 2xxx m/s" readout above your navball and it should change to "target: xx m/s". Now you can aim for the retrograde marker and burn gently until it reaches zero. From here on you will hang around at roughly the same distance. Point at target, burn slightly towards it, e.g. 20m/s, and wait to get closer.

Navball is everything for rendezvous and docking; ignore everything else on the screen above because,it's not relevant

[MPDerksen]

SO close!!  I got to about 600m from the Vergas craft, and m/s about 0.4.  I switch to Vergas, EVA, and start flying.  This is a whole different battle.  How do I know if I’m flying towards the craft directly?  Since the EVA pilot doesn’t have SAS, do I maneuver to keep the Prograde marker in the center of the nav ball, and my m/s reasonable?  Since I can’t pitch to face directly at my target (or can I?), I’m faced with not knowing which thrust to apply to slow myself when I get close.  I put the rescue ship directly behind Vergas, hit space, and it only orientates left/right, not pitch.  This would be a helpful binding...

[Espatie]

2 minutes ago, MPDerksen said:

SO close!!  I got to about 600m from the Vergas craft, and m/s about 0.4.  I switch to Vergas, EVA, and start flying.  This is a whole different battle.  How do I know if I’m flying towards the craft directly?  Since the EVA pilot doesn’t have SAS, do I maneuver to keep the Prograde marker in the center of the nav ball, and my m/s reasonable?  Since I can’t pitch to face directly at my target (or can I?), I’m faced with not knowing which thrust to apply to slow myself when I get close.  I put the rescue ship directly behind Vergas, hit space, and it only orientates left/right, not pitch.  This would be a helpful binding...

You can set the target for your EVA Kerbal to the rescue ship. That gives you the yellow box on screen, the pink target on the navball and directional movement markers.

 

Your Kerbal will always orient him/herself to be in-line with the North-South Axis of the planet or moon whose SOI you are in. This means that your forward and back thrust always accelerate you in one plane. Forward is always away from your Camera position in that plane, backward towards it, left and right are done with respect to the camera position.

 

Personally I remapped my EVA movement keys onto the Numpad, but it's up to you how you do it.

[MPDerksen]

2 minutes ago, Espatie said:

You can set the target for your EVA Kerbal to the rescue ship. That gives you the yellow box on screen, the pink target on the navball and directional movement markers.

 

Your Kerbal will always orient him/herself to be in-line with the North-South Axis of the planet or moon whose SOI you are in. This means that your forward and back thrust always accelerate you in one plane. Forward is always away from your Camera position in that plane, backward towards it, left and right are done with respect to the camera position.

 

Personally I remapped my EVA movement keys onto the Numpad, but it's up to you how you do it.

I did set the ship as "target", so that's how I know my relative m/s.  I'm just having a terrible time knowing what thrust to use to move in closer.  I got to 70m, but then was going too fast to slow down, and zipped right by.  It doesn't seem as simple as just pointing at it, thrust prograde, and then slow down with retrograde.  The little guy doesn't actually point AT the ship the way a ship with SAS will.

[James Kerman]

45 minutes ago, MPDerksen said:

It doesn't seem as simple as just pointing at it, thrust prograde, and then slow down with retrograde.

You're nearly there, you just need to match up your prograde vector  to your target marker  on the navball and watch your approach speed.  Relying on visuals for the approach is very difficult until you get really close.  An example (I only use the navball information until I get 20-30m from the vessel):

When you thrust to slow down it will change the prograde vector so keep adjusting to keep the two markers lined up and you will make it.

Another edit: I only backward thrust to slow down, after that, if your prograde is above the target usle ctrl to lower it, if below use shift.  Use a or d for left right adjustments.

Edited February 11, 2019 by James Kerman
added more info and image

[bewing]

1 hour ago, MPDerksen said:

SO close!!  I got to about 600m from the Vergas craft, and m/s about 0.4.  I switch to Vergas, EVA, and start flying.  This is a whole different battle.  How do I know if I’m flying towards the craft directly?

Yes, flying on EVA is different than flying a ship.

As mentioned already, the kerbal's helmet always points north. So you can't adjust that dimension with the camera. If your destination ship is above you, the only way to get there is to use the Shift key to thrust up. Similarly with down and the Ctrl key.

I really only fly EVA in space by eye -- almost never with the navball.

You zoom the camera out until you see your destination. You swing the camera around until the camera is directly behind the kerbal, facing the destination. Hit Space to force your kerbal to face in that direction. Now you know that W is prograde, and S is retrograde. Now judge whether your destination ship is above your kerbal's head, or below his feet -- you need to fix that with the Space or Ctrl keys. So you hit Shift/Ctrl and W to head toward the destination. Keep your speed down -- there's no reason to go faster than maybe 7 m/s until you are an expert.

If the destination ship is sliding sideways (vs your prograde symbol, or just visually) then use A/D to cancel that movement.

You want to fix the vertical dimension (Shift/Ctrl) before you get to your destination, because it's hard to see in that direction.

Usually, your helmet light is L, and that can really help for the last few meters.

It can also help to turn on the lights on your destination ship.

[Espatie]

I think helmet lights are U by default. IDK I have mine on Numpad *

 

Unlike @bewing I fly long distances almost entirely by Navball in EVA mode. I find flying by eye impossible unless at really close range.

 

Quicksave, try it, revert if failed. You've got about 200 m/s Delta-V in the pack. It's perfectly possible to cross kilometres between ships once you have enough practice with the technique.

[GoatRider]

Helmet light (and ship lighting) is U on my keyboard, and I haven't changed it from default.

[MPDerksen]

You guys are really patient and helpful.  
I’m not really able to get identical orbits, which I’m not sure how critical that is.  Current state is now.

Rescue Ship:
Pe - 77014
Ap - 85328

Target:
Pe - 77065
Ap - 85346

The next Interscet is in 29m, 26s.  Separation of 3.0Km Relative Speed of 13.6m/s

I backed up a few quick saves to adjust my orbits closer (I was within 0.6Km, but only had a few min to EVA before the ships separated).  Right now, with my nav ball on “Target” mode, I see the pink markers mentioned above.  However, I’m not really sure if the 13.6m/s referrs to getting closer or further apart.  I think the first step is a little advice how to get my relative speed down to 0.0m/s.  How close do the orbits need to match?  Is the above close enough just for a simple rescue or docking?

 

Edit Note:  I got to the node.  I pointed at the target, and burned prograde.  Never got close.  Then it got ahead of me and when I went back to the map, my orbit was not significantly different and the next encounter was going to be 110K separation.  So yeah, I’m missing something or I’m not smart enough for this simulation.

 

Michael

Edited February 11, 2019 by MPDerksen

[Starman4308]

The orbital elements don't need to be terribly similar for the intercept. All that matters is that:

You have a close approach

You have enough delta-V to kill your relative velocity at the intercept

The intercept is not so incredibly fast that it's hard to get an accurate velocity matching maneuver. A 40 m/sec relative velocity is easier to eliminate than a 4000 m/sec relative velocity.

The orbital elements will match up quite well once velocities are matched. Before that point, a near match helps, as it'll reduce the magnitude of the velocity matching maneuver, but it's not strictly necessary. 

Edited February 11, 2019 by Starman4308

[bewing]

41 minutes ago, MPDerksen said:

I got to the node.  I pointed at the target, and burned prograde.

Nah nah nah. Gotta go back and reread the initial comments in this thread. You set target mode, and point retrograde in target mode, and burn in that direction.

In target mode, the displayed speed is your relative speed. When you burn retrograde, you are burning exactly against your relative speed -- which makes it go to zero.

43 minutes ago, MPDerksen said:

 I think the first step is a little advice how to get my relative speed down to 0.0m/s.

^That's how you do it. Burn retrograde in target mode.

44 minutes ago, MPDerksen said:

However, I’m not really sure if the 13.6m/s refers to getting closer or further apart.

Both. If you have a close approach, then you are getting closer until you get to the node, and then you start getting further apart. The 13.6 is the vector speed difference when you pass each other.

 

[James Kerman]

48 minutes ago, MPDerksen said:

The next Interscet is in 29m, 26s.  Separation of 3.0Km Relative Speed of 13.6m/s

Ninja'd by @Starman4308

Set a maneuver node a few minutes ahead of your vessels current position and play around with it until you get it to 2.2km or less.  Your relative speed is good but you only really need to get within switching distance of your target (A kerbal on EVA has about 500m/s Dv - enough to go more than half way to the mun and in this case you only need to kill 13.6m/s) so that you have control of the kerbal.  Don't worry about how much time you have to perform the transfer, this changes as you make adjustments.

You are moving towards your target when the prograde indicator (actual direction of your kerbal/vessel) is close to your target marker  .  If you are moving away from your target you will see the retrograde indicator  (opposite direction).

Your relative speed should only be zero when you actually get to the rescue vehicle so you don't need to perfectly match orbits - the fine tuning of the orbits is what happens as a natural consequence of keeping your markers lined up with RCS.  For docking you want to minimize the amount of monoprop your vessel carries so in that case an orbit that varies by 1-2km is desirable for an easy, slow speed dock.

I have rescued kerbals with a vessel that was in an opposite orbit to the stranded wreck around minmus and often perform high speed intercepts to save on fuel up to 200m/s speed difference (but its best not to try this until you have a better understanding of the process).

Edited February 11, 2019 by James Kerman
grammar

[Capt. Hunt]

ok, part of the problem is your rescue ship started from an orbit that is almost identical to that of the target.  That's why it took you so many orbits to reduce the distance at intercept to a reasonable level.  The larger the difference in orbit, the fewer orbits you need to catch up with the target.  I usually go for an orbit with 2:1 or 3:1 resonance, that means an orbit where the lower of the objects orbits 2-3 times in the time it takes the higher to go around once.  The tricky part here is that instead of just precessing into a rendezvous with your target, once you line up you need to spend some delta-v to change orbits to a rendezvous.  It's a trade off of time over delta-v, but it allows you to have more control over the rendezvous itself.

It's the same principal as transferring to a moon.  Wait until the ship in the higher orbit is about a quarter further around the planet then the one in the lower orbit.  Plot a maneuver node to intercept the target as close as possible by burning either retrograde or prograde (depending on whether you're in the high ship or the low ship), when you get close enough, either plot a maneuver node to match orbits, or burn the opposite direction of your previous burn until the target velocity is as close to 0 m/s as possible.

Edited February 11, 2019 by Capt. Hunt

[MPDerksen]

45 minutes ago, bewing said:

Nah nah nah. Gotta go back and reread the initial comments in this thread. You set target mode, and point retrograde in target mode, and burn in that direction.

In target mode, the displayed speed is your relative speed. When you burn retrograde, you are burning exactly against your relative speed -- which makes it go to zero.

^That's how you do it. Burn retrograde in target mode.

Both. If you have a close approach, then you are getting closer until you get to the node, and then you start getting further apart. The 13.6 is the vector speed difference when you pass each other.

 

So here is the clarification I may need.  I was starting with an orbit that was higher (slower) than my target, and letting it catch me.  But if I was in a lower orbit and catching it, then prograde and retrograde get flipped?  Geez, I’m probably over-thinking this.  But I don’t want to get lucky, I want to LEARN the process.  I’ll reread the entire thread before i just stab at it again.  (Plus it looks like there are a few more replies below to consider...)

[Kryxal]

Just remember that prograde and retrograde are relative ... in the case of rendezvous, target-relative.  Thrusting target-retrograde will always bring your velocity closer to the target, which is probably what you need if you're already close to it.  If you're not close, it could be a good way to burn up.  If you're adjusting your orbit to set up a later encounter, you'll want to be working relative to the orbit instead.

[bewing]

Once you have a close approach (less than maybe 15km at Kerbin -- more on a CB with lower gravity), it's best not to think of it in terms of orbits anymore. Just think of it in terms of 3-D space.

You and the other ship are passing each other. It doesn't matter which direction. All you are concerned about when doing a rendezvous is relative speeds.

Target mode on the navball shows you the relative speeds and directions. You don't need to worry about which compass points they are.

You want prograde (the direction you are moving) to be right on top of the target marker on the navball (or equivalently the retrograde marker on top of anti-target). Because that means you are moving straight toward the target.

But you may be going too fast, or too slow, and the prograde marker will always drift over time.

As you begin your pass, you may have time to "push" the retrograde marker on top of the anti-target marker (or pull the prograde marker on top of the target marker).

But at some point, you always want to come to a stop, relative to the other ship -- and you always do that by burning retrograde.

 

[Dan Kerman]

For rendezvous and docking, I found this guide helped me immensely...  explains it very well...  my approaches are much more controlled and I barely use any mono now to line up. Makes rescues simpler and docking easy peasy.

Tutorial

[MPDerksen]

right, so that Tutorial helped, but I'm not getting closer.  In fact, my intersection points get further away with each effort.  I see how I'm supposed to move the pink target to move on top of the retrograde marker.  But the target is passing below me at a lower orbit.  Once that happens, then I end up flipping around and chasing it.  My m/s goes from it getting closer to me, to it pulling away.  In the first picture below, you can see that we don't REALLY "intersect", but I suspect this is our closest point of passing?

Now here is my nav ball.  According to the tutorial, I would guide the pink marker onto the retrograde one by going to the other side, thrusting, and it would move the yellow one on top of the pink one...

Here is a picture from that tutorial.  As you can see, the relative speed between them is much higher, and the markers are much closer together than mine above.

What's happening, is that I get my speed down to about 3m/s, then Vergas flies below me, and starts pulling ahead.  Then, the next encounter is even further away.  I'm generally not very good at this, I guess, so what else can I try?

[bewing]

Ah, you have to make your orbits cross to make it easy to actually get an intercept by only pushing the yellow/green retrograde marker. And you push the retrograde marker around, not the pink antitarget marker. Basically, you waited too long to push your retrograde marker. If you push it around sooner, that will make your orbit cross.

Yes, that orange intercept marker mostly shows the closest approach, but it is not absolutely precise. As you make adjustments to get your closest approach even closer, yes the intercept marker will move farther away from you in time -- but the "separation" number (which is the important one) will decrease to zero.

You can also turn around and point at the target, and pull your prograde marker around in the same way as you push your retrograde marker -- and that will also fix the fact that your orbits don't cross. This is also necessary if your "target" speed gets too low (or goes to zero). Having slightly faster intercepts actually works better. The markers don't have time to drift very far if you are going faster.

 

Edited February 13, 2019 by bewing