Help with low Kerbin orbit rescue mission

[Snotshot]

I need some help rescuing stranded kerbal please. I have only succeeded rescuing kerbal once which was in high circular orbit but really struggling this time.

This one is in very low oval orbit AP 88km and PE 83km and I am really struggling with target navigation due to fast orbit.

Do I need to match target AP and PE orbit first before attempting rendezvous? I got 0.6 km intercept marker however, my orbit does not match target orbit and when close to it I do not have enough time to maneuver before I loose sight of my target. 

All of the orbital rendezvous tutorials I have seen show matched orbits, is this a requirement for successful docking/rescue?

 

Many thanks 

[DD_bwest]

you  dont need to, but there are thing that can make it easier.  when you get alot better it can be done without much problems, but while learning the ropes it can help alot

match inclinations before you try to intercept with a rendezvous. this will make your burn to match velocities shorter.  also try to get a circular orbit around 100 or 150km.  the closer you are to its orbit. the smaller your burn gets again.

if you can get within a km, match velocities and then nudge yourself cliser with a series of burns towards it then cancelling out your speed and doing it again.

[Foxster]

If you want a good tailored demo of how to do it then install Mechjeb and use the Rendezvous Autopilot. Once you have seen how it achieved it then F9 and try to repeat it yourself. 

[Aegolius13]

You don't want to match orbit before your rendezvous point, because then you can't get closer to your target.  For example, if you're ahead of the target, you want to be in a slower orbit until you get close.

If you got within .6km, you've done the hard part.  All that's left is to select target/ retrograde and kill your relative velocity.   By definition, this will put you in the same orbit as the target

[GoSlash27]

Snotshot,

 You don't have to precisely match the target orbit, but it's easier if you do.
 Match the inclination, then...

if you're ahead of the target, raise your apoapsis until you have an intercept on the next orbit
If you're behind, lower your periapsis until you have an intercept on the next orbit (be careful not to hit the atmosphere)

About 1/8 orbit away from the intercept, you will begin to match your closure rate (m/sec) to 10x the range to target (km), while "pushing the marble" to keep your retrograde marker (green) on top of the target's retrograde marker (purple).

This will park you right on top of the target. I usually park within 20m for the EVA.

 You need to practice this intercept/ rendezvous maneuver until it becomes second nature. It's a critical skill in this game.

Best,
-Slashy

[DD_bwest]

Slashy's works very well.  

if im looking for a close intercept the first try without having to nudge myself closer,  ill plop a node down around that 1/8th mark, and use it to both move a AN or DN as close as i can to the given intercept point, then tinker with the prograde/retro ever so slightly as that almost always will let you get your intercept on the map to show 0.0 km.  if im using something with data read outs i like to aim for 20-30m from target.

 

[Waxing_Kibbous]

Here's a good video- one important concept is to keep the navball in target mode and to use only the retrograde (yellow) and prograde to target (pink) marker. I like to use a one star pilot to keep my vessel on retrograde so I can focus on the velocity. When you get more comfortable you can start using a method where you push or pull the yellow and pink markers towards each other. Go to around 4:00 to get to the launch

 

Edited November 2, 2016 by Waxing_Kibbous

[Plusck]

For low orbit missions, it's easier just to meet as soon as you reach orbit. Start with a basic slightly overpowered orbit-capable craft (parachute, probe core, command pod, 1x100 battery, decoupler, FL-T400 fuel tank, Terrier, decoupler, FL-T800+FL-T400 tanks, swivel, 2x radial decouplers, cross-feed enabled, with Hammers and an FL-T200 on top of each one).

Lift off when the target is 320km away. Follow standard gravity turn, heading a touch higher than perfect if anything. Drop the Hammers when they run out (you'll probably drop a bit of fuel in the side tanks too, but it's no big deal and you have more important things to worry about).

Once you get to about 25 km, switch to map view,  and watch your orbit lines. When you see that they intersect with the target vessel, set NavBall to target mode, and burn retrograde (which should be slightly under the horizon) until you need to stage. Stage and come back to map view. Now, retrograde and anti-target (pink cross) markers should be relatively close to one another. Burn on the other side of retrograde (compared to anti-target) until it slides over the anti-target marker. You should now have a very close encounter coming up in a couple of minutes. Divide your relative velocity to target by 15 (with this configuration, the Terrier's acceleration will be about 15 m/s²), that's the time you need to nullify relative velocity.

Now wait until you're about 2/3 of that time away from target, and burn hard retrograde. Again, pushing it over antitarget if it's a bit off. Keep watching the map view and time to target. It should count down very slowly. Ideally, you want it to say you have 10 seconds to target when your relative velocity is very low - 10 m/s or so.

If you do all that.. congratulations! You should be a hundred metres or so from target, without ever having had to plot a maneuvre node. From the instant you switched to target mode, you were also following a near-optimal trajectory to get into orbit alongside your target.

[Plusck]

4 hours ago, Snotshot said:

I got 0.6 km intercept marker however, my orbit does not match target orbit and when close to it I do not have enough time to maneuver before I loose sight of my target.

If you had a 0.6km intercept, you should have no difficulty matching orbits as long as you understand what "target" mode on the NavBall does.

On target mode, you have your relative velocity marked. You need to know how long it takes to accelerate by that amount, so drop a node anywhere and drag it to that value to see the burn time. Call this Time T

If you already have that close an intercept, the NavBall retrograde marker (in target mode) is really all you need. When you're about 1/2 to 2/3 of that time T away from the target, burn hard retrograde until velocity is very low (a few m/s).
That's it! You shouldn't need to do anything more to be hovering at about a kilometer or less from your target.

 

If you want to do it better, learn to "push the marble": if you point to one side of the retrograde marker (in target mode) it will push it away from the middle of the NavBall. If you point to one side of the prograde marker, you will pull it towards the middle of the NavBall (but the effect is necessarily less unless you increase velocity to target by a huge amount).
Use this to put yourself on a perfect collision course with your target. While you're still a couple of minutes away, thrust to one side to push retrograde over the anti-target marker. Your intercept point should have reduced even more. Now wait until you're less than time T away and reduce relative velocity by burning retrograde.
If you so this in stages as you get closer to target, your 0.6km intercept will rapidly turn into a 0.1km intercept and, with good timing, you will finally kill velocity with your target about 100m away from you.

Edited November 2, 2016 by Plusck

[bewing]

Beyond the good advice above -- if you can get within 2.2km of the victim (even just for an instant!) then you can switch control to them, and EVA them, and then have them fly themselves to a more convenient orbit or place for you to pick them up. You don't have to do it all in one step (with just one ship) if you don't want to.

[Snotshot]

Thank you very much for such speedy replies guys. 

My bad Aegolius13, I meant to ask if I needed to match target orbit after getting intercept markers close enough. I achieved that from higher orbit.

That tutorial video is pretty much a copy of my current rescue mission but with oval orbit. Quite informative, thank you for that.

I will give it another go when I get back home. I have a quick save like -T 6 mins on that 0.6 km intercept marker. 

Bewig, I didnt think of that! I assume it is safe for kerbal to orbit in EVA if you miss your first try?

Thank you for your advise everyone. I will update on my attempt later today :]

[ForScience6686]

8 hours ago, bewing said:

Beyond the good advice above -- if you can get within 2.2km of the victim (even just for an instant!) then you can switch control to them, and EVA them, and then have them fly themselves to a more convenient orbit or place for you to pick them up. You don't have to do it all in one step (with just one ship) if you don't want to.

This Is helpful in doing multiple rescues.   this way you can keep the rescue ship in an elliptical orbit and just eva  over.   that way your  rescue ship is in a slower orbit  for the next rescue to catch up,  and you're saving fuel.   but keep an eye on intercept points,  it should give you relative velocity to target,  which you want to keep somewhat reasonable or you'll just go flying by.

[Plusck]

39 minutes ago, Snotshot said:

Bewig, I didnt think of that! I assume it is safe for kerbal to orbit in EVA if you miss your first try?

The EVA pack has about 600m/s dv. Obviously a bit less if your Kerbal makes a lot of corrective movements (so beware of the transition between suborbital and orbital trajectories, if you try to go too fast retrograde to catch a ship behind you).

However, once you've taken control of your new Kerbal you can leave him / fly him around in orbit as long as you want until you get an encounter.

[Sharpy]

Note that 600m/s is enough to go 2/3 the way to Mun from LKO. That's really a lot.

Your relative velocity will be usually below 200m/s. So just make your encounter as close as possible (don't overdo it though - you CAN crash into the stranded pod!) and when within range, switch vessels, put the kerbal on EVA, go to map and set the rescue craft as target. Make sure your speed indicator is in Target mode, then orient your kerbal to the retrograde marker, and accelerate till your speed drops to within some 20m/s. Use shift/ctrl to reduce vertical component (if the retrograde drifts too far up/down the navball).

If your relative speed is in its 'teens' and you have more than 1/4 of EVA propellant left, you're as good as home on Kerbin. Don't worry the rescue pod is 10 kilometers away You're in a good orbit and that's all that matters.

Rotate the kerbal to the Target market, and using forward, left, right, shift, control - bring the prograde marker squat into the target marker, bringing your speed to some 20-50m/s. Then just watch as the target approaches, rarely correcting as it drifts a little.

Reduce speed to 4-5m/s when you're 300m from the pod, less than 1m/s when 50m away, then gently maneuver to the hatch. Switch vessels once more and rotate the pod for the hatch to face sideways, as with it facing down kerbals sometimes get launched away and that may cost more propellant than you have left. But with gentle maneuvers you should easily board the pod with a good 100m/s to spare.

[Snotshot]

Success guys! Thank you very much to all of you, Debemma has been rescued and joined my team :]

Oh boy I had frustrating time catching her!  Not sure if it is by default or I accidentally pressed F4 but I was not seeing my target even 500 meters away. Dont laugh but I even fully upgraded my tracking station thinking that it might be the problem lol :]

I had 2 batteries on board but no solar panels thinking I will not need them being in such a low orbit, had 2.3 electric charge left when Debemma boarded my interceptor! :]

I do feel quite confident now and will happily accept new rescue contracts.