Orbits to Intersect Minmus

[Bosun]

Hi,

Slightly New here, running a science campaign (so funding no issue), I'm trying to get to Minmus, and have done a fly by at high speed with a suicide into Kerbin, but I've had a hard time finding a good vector to approach minmus without being at ludicrous speed and having to burn a lot to slow down.

Is there a time of day to launch, or a position of the mun and Kerbin relative Minmus, that makes an intercept more efficient? Has anyone used Mun gravity to assist in Minmus orbit? Also, how can you calculate trajectories to other planets without trial and error of moving a node around until you get lucky? I'm familiar with watching the orbits and placing the node in front of the orbiting target, but it's crude and imprecise.

If anyone has shots of a good Mun assist or trajectory too, screens are fun.

Cheers for any advice,

Bo'sun

[legoclone09]

Well get into LKO, do an inclination change(burn normal/antinormal at AN/DN), and then set a manuver node to intersect the orbit, and then move it around until you get an encounter. You should only be moving a few hundred m/s.

[heng]

Well, according to the charts, the d/v difference between going to Mun and going to Minmus from LKO is 10%... 860 to 930 m/s according to the one i use. while doing a Mun gravity assist to Minmus would be a cool maneuver in itself, the potential savings are minimal.

What legoclone says.

a- have a circular orbit

b- set a node anywhere you like

c- increase AP to target orbit (for Minmus that is roughly less than 47million km)

d- slide node around until you get an encounter

e- fine tune by lowering AP until you have no encounter

f- return to d)

Sorry. I forgot.

PS:

welcome to the forums!

Edited June 11, 2015 by heng

[ElWanderer]

It's more efficient to burn at the ascending or descending node, but that requires waiting until Minmus is in the right place such that you'll get an intercept. It's more efficient because you don't need to make a plane change. There must be a phase angle between the nodes and Minmus that is the best time to burn, but I don't know it. Making a plane change in low Kerbin orbit is more expensive, but makes setting an intercept easier. In the middle is plotting two burns - an intercept then a mid-course plane change at the AN/DN. Trickier as you may have to fiddle both manoeuvre nodes to get an intercept in the first place, and.the first burn never quite goes to plan so you may need to re-plot the plane change.

Your burn from Kerbin orbit should put your Kerbin apoapsis close to the distance that Minmus orbits (about 46-47Mm) - that way you'll be going very slowly when you reach Minmus. If your burn puts you in an orbit that takes you out to 80Mm such that you meet Minmus going out or coming back, you'll be going much faster at intercept and it'll cost more to get into orbit.

[rofltehcat]

Because minmus has no atmosphere you'll have to use fuel to slow down enough. So the best ways to save fuel are:

1. Hit minmus at the ascending/descending node to save 200 or so dv in plane change. But hard to do because it reuires good timing.

2. Fiddle with your transfer maneuvre so you accelerate as little as possible to do it. You'll probably have to pay back every dv you overshoot twice.

3. Once on the way to a contact you can achieve massive changes in the position of your minmus orbit with very little dv. Accelerating or slowing down 2 dv can make the difference between near-equatorial and near-polar orbit or between orbiting minmus clockwise/counterclockwise. Changing This one is probably the best thing to do and mods like mechjeb's maneuvre node editor or precise maneuvres (i think it is called but i use mechjeb for that) will help with that because you can change the nodes by 0.1 and 0.01 increments easily. Set a maneuvre node 10 or 15 minutes on your path. Focus view on minmus and look where you'll pass. Adjust all 3 axis of the maneuvre until your pass happens in a inclination/altitude/direction that can easily be changes to your target orbit.

Edited June 11, 2015 by rofltehcat

[OddFunction]

Everyone else here has a lot more intelligence than I have to offer. But my Minmus strategy is the same as any other planet. I just adjust the screen so that the target planet (in this case, Minmus) is rising on the horizon while you are in orbit. Then you just make a maneuver node exactly where Minmus rises over the horizon, adjust the node prograde, and you should be able to get an intercept trajectory for about a minute of burn.

Works everytime sometimes

[Prepper-Jack]

Well, I think it's all been answered - though I might add that it's more efficient to try and match the angle of Minmus while you ascend from the planet into LKO. Takes a bit more doing, and frankly I'm too lazy/forgetful to do it myself, but it is what it is.

[ElWanderer]

Well, I think it's all been answered - though I might add that it's more efficient to try and match the angle of Minmus while you ascend from the planet into LKO. Takes a bit more doing, and frankly I'm too lazy/forgetful to do it myself, but it is what it is.

That would definitely work, though to match inclination fully you'd want the launch site to line up with one of the ascending/descending nodes. Hard to judge when on the pad, but if you have any other craft in an equatorial orbit you could target Minmus with that and so find out where the nodes are.

[cephalo]

I don't see how burning at the ascending/descending nodes saves dv. You still have to burn normal/antinormal in order to not miss correct? Otherwise your ap will be very far from Minmus orbit path right?

NM I get it now. You meet Minmus at the other node.

[Prepper-Jack]

I don't see how burning at the ascending/descending nodes saves dv. You still have to burn normal/antinormal in order to not miss correct? Otherwise your ap will be very far from Minmus orbit path right?

The ascending and descending nodes are where Minmus orbit intersects with your current (presumably equatorial if you follow a typical Kerbin launch) orbit. Burning at those nodes is pretty much the only place you'll be able to intercept Minmus, unless you either align your Kerbin orbit to Minmus or adjust your maneuver to take such inclinations into account. Both of these cost in terms of dV. So yes, unless you match the angle on launch, it's most efficient to burn towards Minmus at the ascending and descending nodes.

[ElWanderer]

I don't see how burning at the ascending/descending nodes saves dv. You still have to burn normal/antinormal in order to not miss correct? Otherwise your ap will be very far from Minmus orbit path right?

NM I get it now. You meet Minmus at the other node.

Yes (to your edit). To be more precise about "burning at the AN/DN", you burn near (not necessarily precisely at) one node such that your apoapsis is at the right altitude at the opposite node, at the right time to intercept the target. It's where your apoapsis ends up that is important, though you'll only need to burn far from the node if your starting orbit is very eccentric.

[legoclone09]

Getting to Minmus is easy if you mess around with manuver nodes enough. Try it in sandbox a few times and if you can do it multiple times, do it in career with the same strategy.

[moogoob]

I do a flat intercept burn from an equatorial orbit (IE only a prograde burn, no angle to it), then do a plane change en route - I find that saves me hundreds of meters per second of delta v versus doing a plane change first then burning to intercept. An alternative it launching into Minmus' plane to begin with, but that requires planning and patience beyond what I've got.

I've done many, many Minmus missions, manned and otherwise.

[mrklaw]

Get into LKO then target minmus and adjust inclination at the AN/DN points.

then I zoom the map out, turning the camera so minmus is at around 4 o'clock (with kerbin at the centre) then I put a manouevre node at the bottom of the kerbin orbit and drag prograde to overlap with minmus orbit. I overlap it so the AP is a short distance outside of minmus orbit. If at doesn't give me a minmus encounter and periapsis, I'll drag the node circle a little in each direction to see if that helps (adjusting the amount of prograde so that the orbit always crosses minmus orbit)

[Brunzio]

If anyone has shots of a good Mun assist or trajectory too, screens are fun.

Cheers for any advice,

Bo'sun

Hi all

first post here

after many tries I managed to do what you say

Well, according to the charts, the d/v difference between going to Mun and going to Minmus from LKO is 10%... 860 to 930 m/s according to the one i use. while doing a Mun gravity assist to Minmus would be a cool maneuver in itself, the potential savings are minimal.

yup, total maneuver dV was 862.4 so saving a little

burning 100 more m/s retrograde at minmus PA got an orbit

The game offers little help to find the exact WHEN for these maneuvers, I just tried many times, at firs eyeballing and then using time warp, to know the mun/minmus phase angle.

Now if I may ask, I know there are spreadsheets over the internet with all the phase angles to transfer to and from planets, but there is such a thing for slingshot? or maybe a mod?

Would be nice to be able to see the planets positions at a certain time without actually timewarping, a kind of preview of future time to try maneuvers.

[cybersol]

In the middle is plotting two burns - an intercept then a mid-course plane change at the AN/DN. Trickier as you may have to fiddle both manoeuvre nodes to get an intercept in the first place, and.the first burn never quite goes to plan so you may need to re-plot the plane change.

I highly recommend learning this method of getting to Minmus once you have nodes and conics unlocked. Efficient interplanetary transfers to anywhere essentially come down to this same technique plus timing the right transfer window.

- Get in LKO

- Set target to Minmus

- Add node with enough prograde dv to send your Apo just past Minmus about 45 degrees in front of it (~900-940 m/s)

- Add a second node mid-way with enough normal/anti-normal dv (~10-50 dv) so that your Apoapsis "height" occurs right on the plane of Minmus's orbit

- Spin first prograde node around until you get a closest encounter marker to Minmus

-- Tune node location so that the closest enounter marker gets even closer

-- Tune prograde amount so your apoapsis happens right on Minmus's orbit

-- Tune second node normal/anti-normal amount to get a closer encounter

- Do the three above until you get an encounter, then focus view on Minmus to see your zoomed in path and periapsis

- Keep tuning the above in smaller amounts to get a retrograde, prograde, or polar orbit with your desired periapsis

- Execute the first burn and re-adjust the second

Now learn about launch windows and ejection angles and you are ready to set up a couple of nodes to go to any planet in the game.

[OddFunction]

Now if I may ask, I know there are spreadsheets over the internet with all the phase angles to transfer to and from planets, but there is such a thing for slingshot? or maybe a mod?

Would be nice to be able to see the planets positions at a certain time without actually timewarping, a kind of preview of future time to try maneuvers.

Is this what you are looking for?

Ignore the number for the angles, just center on the sun in mapview, then timewarp while keeping Kerbin at 3 o'clock. When the destination planet is at roughly the correct position according to the chart you should have a reasonably good transfer window.

^Read that and click that pic below

http://i.imgur.com/Uo3N6Tu.png

Edited June 11, 2015 by OddFunction

[ajburges]

Just do the plane change in route to for 40 m/s. Set Ap to match target altitude and change plane (and fine tune Pe) when it will have maximum plane change per dV (somewhere near half way)

Seriously. While not the most* efficient, this is pretty low cost for dV and requires no extra orbits of waiting before departure burn.

Once you become practiced at it you can insert into any minimus orbit efficiently in 2 or less orbits. (Great for sat and rescue contacts)

-Use your two burns to get an escape trajectory with a low Pe on an ascending/descending node.

-Burn retrograde at Pe for a highly elliptical orbit.

-Burn at Ap to correct plane alignment and set Pe to orbit Pe.

-Circularize (if desired orbit has different phase)

-Burn to set Ap correctly (incomplete burns can be used to force a quicker target intercept)

*Most efficient requires planetary alignment, some real gravity assist shenanigans, and crazy precision. That .... is not worth 6% dV savings outside of challenges. Even intercept at node only 4% savings over my approach.

[Brunzio]

*Most efficient requires planetary alignment, some real gravity assist shenanigans, and crazy precision. That .... is not worth 6% dV savings outside of challenges. Even intercept at node only 4% savings over my approach.

sure, but you miss the fun!

Is this what you are looking for?

not quite, but thank you

interesting method, but limited to a single tranfer at a time

I have some of that numbers, found them empirically or by fiddling with nodes

what I would like to do is to have a sort of "map timewarp" without actually going forward in time, just to have planets in position and then try with nodes to discover the best routes

that would be useful to plan slingshots and tours

[match]

If delta v is of no concern, you can set your Apoapsis a bit further out and catch Minmus before your orbital speed drops at Apoapsis. It can save a few days if you are trying to meet a deadline or are simply impatient. You will of course, have to spend more to make orbit.

I have found the cheapest plane change to be around 2/3 to 3/4 of the way there. Close enough to Apoapsis so that you are slowing, but far enough away so that the correction isn't too abrupt.

Edited June 11, 2015 by match
prepost jitters

[mrklaw]

Is this what you are looking for?

^Read that and click that pic below

http://i.imgur.com/Uo3N6Tu.png

what point in your kerbin orbit do you burn prograde though?

Some one of you are talking about matching inclination mid-way to minmus to save dv. But how do you know you're on an intercept with minmus in the first place? The map won't show an intercept if your inclination isn't already correct. Do you just know based on experience?

[ElWanderer]

what point in your kerbin orbit do you burn prograde though?

Some one of you are talking about matching inclination mid-way to minmus to save dv. But how do you know you're on an intercept with minmus in the first place? The map won't show an intercept if your inclination isn't already correct. Do you just know based on experience?

I'll admit I usually guess where abouts to place a burn, then slide the manouevre node around. On your second question - Set up two manoeuvre nodes before you do any burns. First an ejection that takes you to the right Ap (46.4Mm) then a mid-course plane correction at the AN/DN. The game will then show the resultant orbit as if you had done both burns perfectly. You will probably need to slide these nodes around before you get this orbit to intercept Minmus. Also, its likely your first burn won't be perfect - you may need to recalculate the second one afterwards. As said above, this is a fairly standard way of getting to other planets, so worth learning.

[Sharpy]

You use less fuel by braking at Minmus periapsis to enter its orbit after an encounter at the apoapsis of Kerbin orbit meeting Minmus, than you would burn to circularize an orbit at Minmus level, which would allow you to "enter" at a speed that needs nearly no dV to enter the orbit.

The only optional burn is equalizing the orbits, which is a huge time-saver and doesn't cost all that much. Anything else is pretty much "all things equal". Increasing Kerbin periapsis reduces your "braking" speed. "Braking" replaces circularizing. If you really don't care about time, you can try several Oberth maneuvers/gravity assists from Mun, to bring your Kerbin periaxis higher (Kerbin orbit more circular = less braking), but then getting Minmus encounter would be tricky to say the least.

[Bosun]

thanks all for the tips!

Just returned from first minmus mission! I finished off my Kerbin intercept with less than 5 units monopropellent (because I ran out of fuel to burn retrograde.) When I saw the camera angle change as my orbit officially deteriorated, I thanked the Lords of Kerbol for watching out for my almost stranded crew.

They made it back for over a 1000 points of science.

Score.