Orbital rendevous above Minmus

[Plusck]

22 minutes ago, Loren Pechtel said:

Don't worry about orbits?  The first time I tried for the Kerbel in a low orbit about Minmus I just barely escaped with my life--one tiny burn trying to match velocities and my periapsis went well below zero.  (My target was below 5km.)

You were lucky to survive at all, considering there are mountains on Minmus higher than 5000m.

Still, how much you can rely on the map and how much on the navball depends very much on the size and eccentricity of your orbit, how steep the gravity gradient is and how massive the orbited body is.

On a highly elliptical orbit around a smallish body, very slight differences in velocity between you and the target will cause you to take a very different orbital path. Heading straight for your target from 3km out can therefore set you on a course which narrows the gap for the first kilometer then widens it massively afterwards, requiring constant adjustment. Making a similar initial burn using a manouvre node and the navball could easily get you to within 0.1km in half the time and with only a final dead-stop maneuvre at the end, using far less fuel.

Similarly, if you are deep in a gravity well of a massive body, and if those 3km are vertical kilometres, then burning towards your target will take significantly more fuel and cause a significant prograde/retrograde sideslip to occur, and require a large number of big corrections as you get closer, again using far more effort and fuel than if you'd simply plotted a somewhat exaggerated orbital transfer using the map. 

Edited February 25, 2016 by Plusck

[Snark]

25 minutes ago, Loren Pechtel said:

Don't worry about orbits?  The first time I tried for the Kerbel in a low orbit about Minmus I just barely escaped with my life--one tiny burn trying to match velocities and my periapsis went well below zero.  (My target was below 5km.)

I would say that in general, yes, don't worry about orbits, as long as you're close by (e.g. within physics range, under 2.3 km).

Rationale:  You're going to be matching position and velocity with the target.  The target is in a stable orbit.  So you're going to end up in that same orbit.  In other words:  Your orbit will sort itself out.  You just focus on the target.

When you're far away from the target, that's when you need to care about orbits.  That's where you need to deal with orbital mechanics, and counterintuitive principles such as "I have to slow down to go faster."  That's where you'll trip yourself up if you just point your nose straight at the target and accelerate.

But by the time you get really close to the target, you don't have to worry about that, much.  Effectively, both you and the target are just floating in space, and all you care about is your velocity & position relative to the target, and it doesn't really matter whether you're in low or high orbit, or whether you're going around Minmus or Kerbin or Jool or the Sun.  Just pretend that the rest of the universe doesn't exist, and the only things that exist are you and the target.

So, yes, really, ignore the orbit at that point.  Don't even look at the map screen, you'll just give yourself needless worries, anything you see there is irrelevant.  Just look at the target position & relative velocity markers on the navball, and steer by those, and you'll be fine.

You don't have to take my word for it.  If you don't believe me, just hit F5 and try it.  Seeing is believing. 

[Gojira1000]

Oh, and if the toggles are giving you headaches, use Precise Node. That and KER are probably the two mods I'd make stock, immediately.

[Loren Pechtel]

26 minutes ago, Plusck said:

You were lucky to survive at all, considering there are mountains on Minmus higher than 5000m.

The Kerbel I was after survived, the orbit was safe.  At least I assume it enforces terrain on Kerbels to rescue.

[RocketPropelledGiraffe]

2 hours ago, Snark said:

2 hours ago, Snark said:

Yes.

It's a super cool feature, and one of KSP's best-kept secrets.

It's a super cool feature, and one of KSP's best-kept secrets.

I did not know that, it's always awesome with you guys. Thanks!

[radonek]

14 hours ago, Loren Pechtel said:

The targeting jiggles all over the place (precision errors, perhaps??)

1. Go into slight timewarp when finetuning maneuver nodes - that will put you "on rails" (game stops recalculating your orbit all the time)
2. Do not go for perfect "touching orbits"  when planning rendezvous, make it overlap a bit so there are crossing points.
3. For finishing touches forget about nodes, just align ship about cardinal axes, open a map view and work it there. You don't even need to know your translation axes - just push some RCS and you will see if it makes your rendezvous closer or not. 
4. Like colleagues above said, once you are in physics range, ignore map and fly by navball.

Yes, Minmus is kinda poodle when it comes to AP/PE wobbling, but it is also easy on delta-v which makes it very forgiving and good place to practice rendezvous and docking.

57 minutes ago, Loren Pechtel said:

Don't worry about orbits?  The first time I tried for the Kerbel in a low orbit about Minmus I just barely escaped with my life--one tiny burn trying to match velocities and my periapsis went well below zero.  (My target was below 5km.)

That's not "low" orbit, that's a lithobreaking waiting to happen. Some areas of Minus are close to or over 5km high. Safe orbits begins at about 6km, but that still give you very little headroom. IMO closest practical parking orbit is at 13-15km - you have lots of clearance, but still can drop to surface in a minute and you can go to 100x warp.

[Loren Pechtel]

29 minutes ago, radonek said:

1.  

That's not "low" orbit, that's a lithobreaking waiting to happen. Some areas of Minus are close to or over 5km high. Safe orbits begins at about 6km, but that still give you very little headroom. IMO closest practical parking orbit is at 13-15km - you have lots of clearance, but still can drop to surface in a minute and you can go to 100x warp.

The Kerbel I was after was at 4,700 meters.  She's the one who decided to play games with lithobraking.  I figured it was safe to match orbits with her so long as I didn't go lower.

[Malich]

6 minutes ago, Loren Pechtel said:

The Kerbel I was after was at 4,700 meters.  She's the one who decided to play games with lithobraking.  I figured it was safe to match orbits with her so long as I didn't go lower.

Does she have fuel for her EVA pack? If so then ok Minmus you should be able to bring her up to 15k ap / pe easily.might make things a bit easier.

[ElWanderer]

9 minutes ago, Loren Pechtel said:

The Kerbel I was after was at 4,700 meters.  She's the one who decided to play games with lithobraking.  I figured it was safe to match orbits with her so long as I didn't go lower.

Was this a rescue contract? I don't think they should generate those so low, though given how they will sometimes spawn with a Kerbin orbit that'll splat into the Mun sooner or later, perhaps there are no guarantees.

[Loren Pechtel]

12 minutes ago, ElWanderer said:

Was this a rescue contract? I don't think they should generate those so low, though given how they will sometimes spawn with a Kerbin orbit that'll splat into the Mun sooner or later, perhaps there are no guarantees.

Yup, a rescue contract.

17 minutes ago, Malich said:

Does she have fuel for her EVA pack? If so then ok Minmus you should be able to bring her up to 15k ap / pe easily.might make things a bit easier.

You have to get within physics range to get control in the first place.

[KerikBalm]

Here a procedure for quick and easy and fast (within 1orbit) rendevous (not neccessarily the most efficient though):

Set the target object as your target

Matchplanes with the target to within a degree or two.

Have your Pe within theorbit of the target object.

At Pe, set a maneuver node, drag prograde to put your Ap outside of the target's orbit.... keep dragging untilit shows anintercept within about 5km (well, it sort of depends upon the body's size).

Burn prograde as per the maneuver nodes's dV... this increases the time of your orbit, and you just burn the right amount so that as you are coming back to perapsis, you meet up with your target.

As you approach the intercept (its sort of like a suicide burn... typically, wait as long as you can), burn to kill velocity relative to the target, while pushing the velocity vector toward the target indicator.

Within a short distance, you just point at the target and approach.

It doesn't matter if its gilly, or Jool, or wherever.

If you are just entering the SOI of a body, and want to rendevous with something in orbit, then instead of doing a prograde burn, you do a retrograde capture burn.

When I go to the Mun, or minmus, I rendevous with my orbiting station (if that was the plan) after the capture burn and 1 orbit.

[mikegarrison]

1 hour ago, Loren Pechtel said:

Don't worry about orbits?  The first time I tried for the Kerbel in a low orbit about Minmus I just barely escaped with my life--one tiny burn trying to match velocities and my periapsis went well below zero.  (My target was below 5km.)

Wait a minute, are you trying to do the final maneuvering with the rescue ship? Just get the rescue ship pretty close and match velocities within about 1 m/s. Your orbit will be basically circular if you do that (because the orbit you are matching speeds with is basically circular). You can do this from the navball or by using Mechjeb. You should not be trying to do this final speed matching in the orbital map view.

Once you get yourself close and stable (like I said, within about 1 m/s relative velocity to the target), then kill your rotation and switch to the rescue victim.

As long as you are close enough to switch control, you are within a couple km. Just point and shoot. Find the rescue ship. Double-click on it to set it as the target. Then burn straight toward the rescue ship until your relative target speed is at least 1% of the distance. (Ie. if you are 2km away, burn your relative speed up to at least 20 m/s. If you are 200, get it at least above 2 m/s.) If you do that, it will only be about 100 seconds until you are at the rescue ship. That's not enough time to crater, no matter what your orbit looks like.

You can go even faster, but things happen fast when your relative velocity is high, and you don't want to screw up. So if you keep it close to that 100 second rule until you get more practice, it's probably smart.

And don't get right up to the rescue ship before you start your breaking burn. You should start slowing down a little when you get close. If you are going 20-30 m/s relative, close is probably 500 meters. If you are going 2-3 m/s relative, close is probably about 50 meters. Again, think in terms of time.

All this is actually a lot easier in v. 1.05 than it used to be. Earlier versions didn't give you a navball when you were EVA. Use the navball!

33 minutes ago, Loren Pechtel said:

The Kerbel I was after was at 4,700 meters.  She's the one who decided to play games with lithobraking.  I figured it was safe to match orbits with her so long as I didn't go lower.

I've never seen a rescue contract spawn that low. When I get them at Minmus, they are at about 7000 meters. Something sounds odd.

[KerikBalm]

21 minutes ago, mikegarrison said:

All this is actually a lot easier in v. 1.05 than it used to be. Earlier versions didn't give you a navball when you were EVA. Use the navball!

Ah yes, I recall doing orbital rendevous without the navball... it was a pain. EVAing down to the surface of minmus, grabbing an EVA report and surface sample, and EVAing back up to the orbiting ship is much easier now.

Put a pod in a polar orbit of minmus, and with 1 kerbal, you can get EVA and surface samples from every minmus biome, without ever putting a craft down... sort of exploity way to farm science in the early career.

[mikegarrison]

41 minutes ago, KerikBalm said:

Ah yes, I recall doing orbital rendevous without the navball... it was a pain. EVAing down to the surface of minmus, grabbing an EVA report and surface sample, and EVAing back up to the orbiting ship is much easier now.

Put a pod in a polar orbit of minmus, and with 1 kerbal, you can get EVA and surface samples from every minmus biome, without ever putting a craft down... sort of exploity way to farm science in the early career.

I guess we make different choices in careers, because I usually have landed ships on Minmus in about half the biomes before I upgrade the buildings enough to even be able to get surface samples.