Fix an Orbit?

[Huojin]

I\'ve got an orbit around the Mun. It\'s close enough to being a circular orbit, but I can\'t be bothered to adjust it further. The issue is that, if you look at the picture below, it\'s not an orbit around the equator.

I really should know this, but my experimentation has yielded no visible success. How do I adjust this orbit?

[Gojira]

Not sure. Try boosting at a 90º angle from the Mün\'s surface when you\'re at the highest point of inclination.

[Kosmo-not]

Burn at 90° to your vector as you\'re crossing the equator.

[Hypocee]

Even more precisely, burn horizontally and at 90 degrees between prograde and retrograde at the 'instant' you cross the equator. Ideally you\'ll be adjusting that burn vector over the course of the burn until cutting off when it points exactly north or south, meaning that your orbit is now purely east-west, i.e. equatorial.

Be advised that plane-change maneuvers are enormously fuel-hungry, especially in low/fast orbits. There are real-world examples where it\'s been less costly overall to burn up to a high orbit, do the plane change there, then burn back down to the low orbit again.

[MaxMurder]

Even more precisely, burn horizontally and at 90 degrees between prograde and retrograde at the 'instant' you cross the equator. Ideally you\'ll be adjusting that burn vector over the course of the burn until cutting off when it points exactly north or south, meaning that your orbit is now purely east-west, i.e. equatorial.

Be advised that plane-change maneuvers are enormously fuel-hungry, especially in low/fast orbits. There are real-world examples where it\'s been less costly overall to burn up to a high orbit, do the plane change there, then burn back down to the low orbit again.

Interesting theory... higher orbit = less speed = easier to change velocity vector = less fuel spent. Also by planning to have your perapsis at the spot you want to preform the burn you could waste less energy fighting gravity. This calls for some !!Science!!

[MaxMurder]

I launched some flights to test if changing orbital inclination at a higher altitude would be more efficient.

There were three tests, each starting at a 100,000m orbit, using an identical spacecraft for every flight.

Weight (fully fueled) : 8.83 (tonnes?)

Fuel : 1000(L?)

Trust: 100

Burn Rate: 3

The first two stages bring the craft to a 100km circular orbit westward around Kerbin\'s equator.

Then the orbital test vehicle then breaks away and is ready for science.

The test is to measure how much fuel is used to change the inclination 90 degrees from an equatorial orbit to a polar one.

Each test was conducted at a different altitude (100km, 200km, and 500km).

Test 1)

Single burn at 100% throttle (Jeb was happy about this one) at 100km altitude.

Burn time: 5min 30sec

Fuel used: 850

Test 2)

Burn at 50% throttle to a 200km transfer orbit.

Burn time: 1min

Fuel used: 50

Inclination burn at 100% throttle and 200km alt.

Burn time: 3min 30sec

Fuel: 700

Burn at 50% throttle to return to 100km circular orbit.

Burn time 1min

Fuel used: 50

Test 3)

Burn at 50% throttle to a 500km transfer orbit.

Burn time: 1min 30sec

Fuel used: 60

Inclination burn at 100% throttle and 500km alt.

Burn time: 2:30

Fuel: 600

Burn at 50% throttle to return to 100km circular orbit.

Burn time 1min 30sec

Fuel used: 60

Results:

Altitude Fuel used

Test 1: 100,000m 850

Test 2: 200,000m 800

Test 3: 500,000m 720

So the higher the orbit, the less energy you need to use to change the inclination. However, this advantage will be canceled out eventually by the cost of the energy required to reach a higher orbit.

[Nibb31]

Inclination changes are the most costly manoeuvres in terms of Delta-V (and therefore fuel), which is why you want to get it right from the launch. In RL, it isn\'t uncommon to raise the orbit, perform the correction burn, and then lower the orbit to save fuel.

The easiest way to achieve a perfectly equatorial Munar orbit is to start with a perfectly equatorial Kerbin orbit. The key is to keep the yellow marker right on the 90° mark. Then you do the same during your trans-munar injection burn and your munar orbit insertion burn. If any corrections are needed to get the yellow marker on the 90° line, do them as early as possible.

Once you get the hang of it, you can practice pinpoint landings. I love landing on the north pole of the Mun.

[Huojin]

Inclination changes are the most costly manoeuvres in terms of Delta-V (and therefore fuel), which is why you want to get it right from the launch. In RL, it isn\'t uncommon to raise the orbit, perform the correction burn, and then lower the orbit to save fuel.

The easiest way to achieve a perfectly equatorial Munar orbit is to start with a perfectly equatorial Kerbin orbit. The key is to keep the yellow marker right on the 90° mark. Then you do the same during your trans-munar injection burn and your munar orbit insertion burn. If any corrections are needed to get the yellow marker on the 90° line, do them as early as possible.

Once you get the hang of it, you can practice pinpoint landings. I love landing on the north pole of the Mun.

I\'m probably screwing something up then, because I always get an orbit around Kerbin that is on the equator when I\'m heading for the Mun, but when I get out there the Mun always catches my in its SOI at a dodgy angle. I\'ve no idea what\'s happening.

I\'m also finding difficult to adjust the inclination of my orbit, still. Maybe a little less jargon? It\'s been a long time since I did any physics... or maths...

[Nibb31]

I\'m probably screwing something up then, because I always get an orbit around Kerbin that is on the equator when I\'m heading for the Mun, but when I get out there the Mun always catches my in its SOI at a dodgy angle. I\'ve no idea what\'s happening.

If your Kerbin orbit is at a nice 90°, then you must be screwing up your injection burn (Apollo called it the TLI burn = Trans Lunar Injection = the burn that shoots you to the Mun/Moon). This burn must be perfectly centered on the 90° line on your nav ball. A slight 1° error at this stage will make you arrive several kilometers above or below the Mun\'s equatorial orbital plane.

So for the this burn, aim your rocket above or below the prograde marker (the yellow marker with 4 branches) to move the marker respectively up or down on the nav ball so that it hits the 90° line. After the burn, switch to the orbit view and you should be able to pan and rotate to make sure that your green trajectory intersects as close as possible with the grey orbit line of the moon. If it doesn\'t, then correct the inclination before you leave Kerbin\'s SOI.

Once you get into the Mun\'s SOI, you should do an insertion burn (what Apollo called a LOI burn = Lunar Orbit Insertion = the burn that slows you down and gets you into a nice stable lunar orbit). This is the occasion to correct the inclination by aiming slightly above or below the yellow marker to bring it back onto the 90 or 270° mark (depending on which direction you are going to orbit the Mun).

For this burn, you want to slow down, so aim your rocket above or below the retrograde marker (the yellow marker with 3 branches) to move it respectively down or up on the nav ball. Once it hits the 90° or 270° degree line, you should be on a pretty decent equatorial orbit.

[Hypocee]

I\'m probably screwing something up then, because I always get an orbit around Kerbin that is on the equator when I\'m heading for the Mun, but when I get out there the Mun always catches my in its SOI at a dodgy angle. I\'ve no idea what\'s happening.

Nothing unexpected. Limitations on instrumentation and human reflexes mean you\'re almost always going to be somewhat off a perfectly in-plane burn. Thus, we introduce the world of...mid-course corrections! Very small burns along the way can tune in your orbit - the earlier they are the more efficient but more demanding on precision. An easy technique with our Magic Videogame Eyes is to fly in map view, position the camera so the mün\'s orbit is edge-on, then adjust your elliptical orbit up or down (probably with RCS) to match it.

I\'m also finding difficult to adjust the inclination of my orbit, still. Maybe a little less jargon? It\'s been a long time since I did any physics... or maths...

burn horizontally and at 90 degrees between prograde and retrograde at the 'instant' you cross the equator

Is the best I can do, sorry. I guess I can add that 'prograde' and 'retrograde are the two green markers on the NavBall.

The more general case is that, as in all astronavigation techniques, you burn at the point where your current orbit intersects your desired orbit.

[Huojin]

The map-view line of my orbit usually overlaps with the path of the Mun around Kerbin. I think I\'m probably just screwing up how I\'m burning to enter a Munar orbit. Nevermind, eh?

[WX_Echo]

I launched some flights to test if changing orbital inclination at a higher altitude would be more efficient.

My calculator will solve all of this in closed form for you! You put a lot of unnecessary effort into this analysis. I highly encourage everyone attempting to perform orbit cranking (orbit inclination changes) to check the 'Plane Changes' section in KSP Orbit Mechanic.

Please check out the optimal plane change reference graphics for a detailed comparison.

Hope this helps!