Oops....today I learnt what "LAP" is.....and how do I fix my mistake?

[paul_c]

Hello all, new user on these forums but I've admired KSP for ages and finally got a decent PC and installed it a few weeks ago. Its additcive! I love the career mode, I am "enjoying the journey" and resisting the temptation to just build massive rockets, instead thinking strategically etc. So far I've been to Mun and Minmus several times, sometimes successfully.... Anyway...

I had a contract to build a space station and put it in orbit around Minmus. I also had a contract to launch a probe into Polar Orbit there. So I built the space station (about 5-6 tons of payload), a big rocket underneath it (about 150 tons all in to lift itself and the payload) and then I realised, for a few kg more, I could make a minimal little automated probe to do another assignment - to put one into Polar Orbit with specific details. Its not much more than a tank of monopropellant, an automated command module and the science goo thing.  So its happily piggybacking for the ride and I'm "nearly there"....

Or so I thought.....look carefully at the screenshot. Look at my LAP (228 deg) vs the contract requirement of 47.9 deg.....I'm going the wrong way round the orbit I think??? I'm 95% sure that's what's going on.....and all after a "funky" angular entry into the orbit around Minmus and about an hour of tweaking the orbit to match!!

 

Fortunately, Minmus is tiny, and the orbit is "far out" so I am only going 63m/s, albeit the wrong way. I can just "do a U turn" can't I? I have 171m/s of fuel; and a few tanks of monopropellant and RCS thrusters too I could detach the little probe and use its onboard mono; or use up the liquid fuel in the "mothership", since I have enough of it?...

Or is there a better/more elegant way of getting out of the hole I've found myself in?

[theAstrogoth]

I think you've correctly identified where you went wrong; you are orbiting in the opposite direction to what the contract is asking for. 

I don't think there is a more efficient way of turning around than the one you have already thought of. At your apoapsis (or anywhere because the orbit isn't too eccentric), burn retrograde until you've reversed your velocity. It should only take 63*2 = 126 m/s, so you should be able to pull it off.

[paul_c]

Phew! I thought so. Fortunately, I'd built the thing with a bit of reserve fuel (not much though), and I did a beautiful gravity turn on the way up so probably saved about 50-100m/s on my usual calamitous flying attempts. I even got the initial trans-Minmus injection spot on, I normally do a correction about 5-10 mins afterwards; and a mid-course check and tweak but no further burns needed, just got into the encounter nicely and a nice de-orbit burn too. I shall try a retro burn, and in the middle (since I am still in the sphere of influence of Minmus) I guess I will fall (dis)gracefully towards the moon, so I'll spend another half hour tweaking the orbit until I'm happy. Thanks!

[radonek]

Usual way to handle this is to raise AP to something eccentric, change inclination at AP and then recilcularize down at PE. Idea behind this is that your orbital energy is very low at top of eccentric orbit, so it does not take much to alter it. Google bi-elliptic transfer. But at 63m/s it's probably not worth the fuss. Just burn retrograde until it turns prograde and you are good.

Edited November 13, 2020 by radonek

[Superfluous J]

You're moving 63m/s and need to move -63m/s, so the whole burn should cost you 126m/s. No problem.

Also note, I never, ever, EVER look at the numbers. I look at the orbit in map mode as I'm approaching the world, and just make sure I come in so my ship's trajectory is touching the target orbit in (as close to) the same direction as the orbit is going (as possible).

You can tell "which way should I go?" by the swirling going on on the orbit itself, and "which way am I going" in your target line because there is a little circle denoting when you'll leave the SOI.

 

[bewing]

If you want to be really super clever about it, there is one special point in your orbit (one of the two crossing points between the orbits), where you can complete your contract instantly with just one minimum cost burn.

 

[paul_c]

25 minutes ago, bewing said:

If you want to be really super clever about it, there is one special point in your orbit (one of the two crossing points between the orbits), where you can complete your contract instantly with just one minimum cost burn.

 

You've got me scratching my head, I'll have to have a think about it!

Its not the same as the conundrum when I landed at the North Pole of Minmus, then my little Kerbin pilot spent ages scratching their head wondering how to fly East?

[Spricigo]

5 hours ago, paul_c said:

I'm going the wrong way round the orbit I think???

...

I can just "do a U turn" can't I?

As pointed out by @theAstrogoth you correctly identified the problem and a viable solution. You also have enough deltaV left to put it in practice. In other words: Yes, you are going the wrong way and can indeed just "do a U turn".

5 hours ago, paul_c said:

Or is there a better/more elegant way of getting out of the hole I've found myself in?

The method mentioned by @radonek saves fuel but takes more time and effort.  However, you don't seems to have a use for the fuel savings.

About @bewing comment: You may reverse the initial orbit anywhere and later adjust it to match the target orbit. But if instead you let to do the reversal when the initial orbit crosses the target orbit you can combine the reversal and the matching in a single maneuver.

 

[paul_c]

8 hours ago, radonek said:

Usual way to handle this is to raise AP to something eccentric, change inclination at AP and then recilcularize down at PE. Idea behind this is that your orbital energy is very low at top of eccentric orbit, so it does not take much to alter it. Google bi-elliptic transfer. But at 63m/s it's probably not worth the fuss. Just burn retrograde until it turns prograde and you are good.

I just Googled bi-elliptic transfer. It looks interesting stuff, and I'll definitely try it at some point. However, it appears in real-life, this hasn't actually happened (not that we know of...)  I imagine if it did, it would be something of a double-facepalm moment at the space control center and the retrograde rockets wouldn't be the only thing fired...

[AHHans]

In my experience the best method to determine if your craft is orbiting in the correct plane (and orientation) is to look at the ascending (AN) and descending (DN) nodes displayed on the target orbit in the map view. These actually show the AN and DN of your craft's current orbit to the target orbit, so when hovering over the AN / DN markers it shows you the difference in the inclination. And if it says 180 deg then you are orbiting the wrong way around! 

P.S. These AN / DN markers show the difference in the inclination for your current orbit, not any orbit after a maneuver node - in contrast to the AN / DN markers for a "regular" target. So planning is a bit more complicated.

Edited November 13, 2020 by AHHans
added P.S.

[paul_c]

Yay success! Well....partial success. Top tip: if this ever happens to you, if you use the button next to the nav ball to do it THEN TURN IT BACK TO "STABILITY ASSIST"!!!

I didn't, I left it on the retrograde setting and the nav ball toppled and went crazy as I was giving the T45 rocket motor the beans, sending it into a random crazy orbit, including at one point having an escape velocity and another point, not having a periapsis (ie...the line going into the surface of Minmus....which is never good for longevity). Fortunately I was able to recover from this, but with little fuel left I fired the probe off and let it do its own subsequent corrections to the orbital path.

I got it nicely matching the orbit, the right way round this time, with 4x well controlled burns. 1x to close up the inclination difference from about 70deg to 20deg; then 2x at apoapsis to match the inclination and retrograde to match the periapsis; then a final burn at periapsis to match the apoapsis. 

Next brain teaser is....what's the best altitude for a space station? It also has a pair of RA15 relay antennas, so its nice to have it high up? But not too high so that its reachable for craft possibly landed at Minmus on low fuel?

[king of nowhere]

2 hours ago, paul_c said:

Next brain teaser is....what's the best altitude for a space station? It also has a pair of RA15 relay antennas, so its nice to have it high up? But not too high so that its reachable for craft possibly landed at Minmus on low fuel?

it depends on what you want to achieve, really.

for fuel considerations, it's better to put them low. it costs less to reach them from the surface, but also - for oberth effect, look it up if you're not familiar - it's going to be cheaper to reach them when coming from outside the planet, and it's also going to be cheaper to leave the planet from the space station, unless you are going very close.

still, they should be up high enough that a ship trying to make a rendez-vous can have a smaller orbit, to catch up to them (it can always go into a higher orbit and have the station catch up, but it's going to take very long if the station was ahead and have to go all the way around the planet). so, for minmus that good height should be around 20-30 km.

on the other hand, as you correctly pointed out (btw, nice to see a new player who already has a good understanding of stuff like gravity turn and antenna placement), on a low orbit minmus is going to block a lot of your antennae's path. so, to have the station act as relay, you would want to put it up as high as possible, so it won't be obstructed.

it really depends on what you want to prioritize.

[paul_c]

I must have read your mind; I chose 25km, although I've left it at an elliptical of 25-28km for now. I haven't done space station docking yet but will do it once my new joystick arrives (tried and failed with a keyboard many times). I already have another comms satellite at Minmus so the issue is not so bad; I will stick a relay antenna onto any satellite for Minmus which comes up in the contracts, until I have a nice collection of about 6-8; and also think about the best plan for orbital placement to ensure 100% coverage. 

[radonek]

1 hour ago, paul_c said:

… I haven't done space station docking yet but will do it once my new joystick arrives (tried and failed with a keyboard many times)…

Joystick is strictly optional. Did you switch camera to "Locked"? Using freecam is a common mistake.

[king of nowhere]

5 hours ago, paul_c said:

 I haven't done space station docking yet but will do it once my new joystick arrives (tried and failed with a keyboard many times).

 

2 hours ago, radonek said:

Joystick is strictly optional. Did you switch camera to "Locked"? Using freecam is a common mistake.

Nah, using the camera in the first place is the mistake - unless you have some complex design or are doing something else unusual.

you only need the navball. unless... how big are your stations? they can't be that big, if you haven't done docking you must have launched them all in one piece. then you can still rotate them freely, and the best way is to treat them like a normal ship docking. for which i refer to the tutorial on docking. Basically, once you are close you have to select for each ship the docking port of the other ship as target, and point your navball to the target to align. do it for both ships, and the docking ports are aligned. I have some vague recollection of finding it difficult at first, but with some practice it becomes trivial. unless you have vehicle that are very hard to turn around.

docking becomes much easier once you unlock the high level probe cores that can point the taerget authomatically; with them, you don't even need a kayboard except to throttle the engine.

Aside from that, docking is much easier if the ship can be turned around easily. reaction wheels are recommended. A low TWR also helps with fine manuevers, and you can get it by limiting the engine's power. using rcs is... complicated. it's easier to dock without rcs, but once you learn rcs it can improve docking with complicated shapes. especially when you have your docking ports on the sides and can't just accelerate towards the target with your main engine

[Mikenike]

20 hours ago, paul_c said:

Fortunately, Minmus is tiny, and the orbit is "far out" so I am only going 63m/s, albeit the wrong way. I can just "do a U turn" can't I? I have 171m/s of fuel; and a few tanks of monopropellant and RCS thrusters too I could detach the little probe and use its onboard mono; or use up the liquid fuel in the "mothership", since I have enough of it?...

Yeah, you have 171m/s in your probe, thats plenty to make it reverse orbits.

9 hours ago, paul_c said:

Yay success! Well....partial success. Top tip: if this ever happens to you, if you use the button next to the nav ball to do it THEN TURN IT BACK TO "STABILITY ASSIST"!!!

I didn't, I left it on the retrograde setting and the nav ball toppled and went crazy as I was giving the T45 rocket motor the beans, sending it into a random crazy orbit, including at one point having an escape velocity and another point, not having a periapsis (ie...the line going into the surface of Minmus....which is never good for longevity). Fortunately I was able to recover from this, but with little fuel left I fired the probe off and let it do its own subsequent corrections to the orbital path.

I got it nicely matching the orbit, the right way round this time, with 4x well controlled burns. 1x to close up the inclination difference from about 70deg to 20deg; then 2x at apoapsis to match the inclination and retrograde to match the periapsis; then a final burn at periapsis to match the apoapsis. 

Next brain teaser is....what's the best altitude for a space station? It also has a pair of RA15 relay antennas, so its nice to have it high up? But not too high so that its reachable for craft possibly landed at Minmus on low fuel?

Park it in a stable 100k by 105k orbit equatorally

[Spricigo]

8 hours ago, paul_c said:

and another point, not having a periapsis (ie...the line going into the surface of Minmus....which is never good for longevity)

This is expected (when you burn retrograde your periapsis drops, them it start to raise again after the velocity drops to zero and start to increase in the opposite direction ). The escape trajectory is a bit odd but the fact you were holding retrograde kind of explain it.

8 hours ago, paul_c said:

....what's the best altitude for a space station? It also has a pair of RA15 relay antennas, so its nice to have it high up?

For Minmus RA2 are enough. And since you are planning to deploy a few extra satellites, I wouldn't bother with the ones in the station.

 

1 hour ago, king of nowhere said:

using rcs is... complicated. it's easier to dock without rcs,

It may be conter-intuitive and a bit overwhelming at first glance but if one keep practicing it will "click in" at some point and become second nature. 

Granted that a well setup RCS goes a long way to facilitate the process but that is also something expected to be figured out at some point 

[paul_c]

You'll never believe what happened again......

Fortunately, it was once again Minmus so the speeds were low. I tried the "go a thousand kilometres away" then turn then come back down, because it was a contract for a satellite at 300-400k up. I hadn't precisely matched (well....I was 179.9deg out) on the LAP, so during the turn around the periapsis dropped into the centre of the moon then curved round with no drama.

I will scratch my head and read up the wiki a bit more, in the mid term. I am keen to learn how to do a "lean" mission with only just enough dV for the task, an economy rocket and equipment. So, I need to get a good grip on the delta V. LKO, TMI, orbital insertion etc are all fine but there's savings to be made in what style of insertion you choose; and if carrying 2 (or more) satellites, which is first to deploy etc.

[king of nowhere]

1 hour ago, paul_c said:

I will scratch my head and read up the wiki a bit more, in the mid term. I am keen to learn how to do a "lean" mission with only just enough dV for the task, an economy rocket and equipment. So, I need to get a good grip on the delta V. LKO, TMI, orbital insertion etc are all fine but there's savings to be made in what style of insertion you choose; and if carrying 2 (or more) satellites, which is first to deploy etc.

for the aforementioned oberth effect, the best insertion is close to the planet. it saves fuel. so much so, in fact, that even when you target a high orbit, it's often cheaper to make an insertion close to the planet and then raise the periapsis. for an equatorial orbit you should enter equatorial, if you need an inclined orbit you enter with the orbit's inclination.

changing inclination is easier when your ship is slower, so away from planets. so much so, that sometimes it's cheaper to heighten your apoapsis, make the inclination change at apoapsis, and lower it again, rather than making the change immediately