Fuel-efficient Mun Encounters

[Sovnheim]

Hi all,

I'm trying to have more fuel efficient Mun encounter trajectories.

I can get there and back. But is there a better way to do it ?

I tend to have my orbits in the same direction as the orbit of the Mun, but then I realized that maybe doing the opposite might be more fuel efficient (despite having to fight Kerbin's rotation) What do you guys think ?

Thanks in advance !

[Starhawk]

I tend to have my orbits in the same direction as the orbit of the Mun, but then I realized that maybe doing the opposite might be more fuel efficient (despite having to fight Kerbin's rotation) What do you guys think ?

It's not. A prograde equatorial orbit is most efficient to get to because Kerbin's rotational velocity is all added to your eventual orbital velocity. If you try to get to orbit turning West rather than East you need about 350 extra m/s of orbital velocity relative to Kerbin's surface.

It would take the same delta-v to reach the Mun from orbit in any case. There is no advantage to being in a retrograde orbit in this case, or indeed in any case other than the need to match a specific retrograde orbit. (i.e. satellite contracts).

Happy landings!

Edited June 9, 2015 by Starhawk

[Sovnheim]

That would make sense. But what about approach ?

Is there a way to use the Mun's gravitational force to pull the craft retrograde relatively from Kerbin's orbit ?

[cybersol]

You definitely want to be on a prograde lower Kerbin orbit (eastward) for efficiency. However, there is also the decision of whether to enter the Mun SOI either in front of it for a retrograde Munar orbit, behind it for a Munar prograde orbit, or over the poles for a Munar polar orbit. Which is best mostly depends on what you are trying to do there and how many times you will be doing it.

EDIT: Enter munar orbit retrograde will give you a little gravity braking action, but a polar orientation will give you more landing site choices. If its a multi-use orbital station you might want to be in a prograde orbit to better service landers taking off from the Mun in the more efficient eastward direction.

Edited June 9, 2015 by cybersol
OP Ninja

[Snark]

If your goal is simply "land on the Mun and return to Kerbin with the least dV possible", then you should do this:

1. Launch to circular LKO as low as possible (i.e. just above the atmosphere)

2. Do a single prograde burn in the right spot, so that you intersect the Mun with a periapsis as low as possible without impact

3. Coast to the Mun

4. When you're at munar periapsis (really low altitude, as low as you can without scraping your toes), burn retrograde to circularize.

[Khazar]

Do you use precise maneuver nodes mod ? It really helps to set good and efficient node. It allows you to move back and forth already placed node. When you place maneuver node, get the encounter you can then change its position.

If you consider only dvs cost, then enter mun SOI in front of it and get into retrograde orbit. Doing opposite is almost twice as expensive ( I belive its around 250 vs 450), and have your mun's periapsis lowest possible (around 6k to be sure you would not hit any mountain), then burn at periapsis to circulize.

After lnding, take off eastward to benefit from muns spin, (you will have prograde orbit) and when you get to point where you are between mun and kerbin burn prograde.

[Ferdoni]

The best way to do a Trans Munar Injection (from LKO to a Mun encounter) when, from the vessels point of view, the Mun is rising just above the horizon of Kerbin.

For returning from a Mun orbit, simply burn retrograde when your vessel is exactly in between Kerbin and the Mun. In this way, you 'plummit' into Kerbin's atmosphere.

(I managed to go to the Mun and back in this way without a manouvre node)

Hope this helps you a bit

cheers!

[wumpus]

First: Kerbin->LKO takes roughly four times the delta-V of LKO->LMO. Make sure your ascent rocket and gravity turn are as efficient as possible.

Second: I'm pretty sure Khazar has most of it, although I suspect I've easily spent more than 250-450 m/s each time I've tried a retrograde munar orbit.

A few more pointers: To fine tune your munar encounter select "focus view" on the Mun. This will let you see where periapsis is. You want to get it as low as possible as soon as possible.

Depending on the adjustments, they should be either right after your initial burn (prograde/retrograde) or halfway to the mun (inclination changes and lowering periapsis via those blue circles).

If your periapsis isn't quite where you want it (i.e. you can go lower and expect to live), just burn enough at periapsis to get an orbit at all.

[edit: If your docking skills are up to it and you have unlocked the command chair, switch to your lander (I'm pretty sure no other module is worth it, but if you can live without F5 I *think* you can stay on just a ladder: there may be other penaties as well (I gave up on it).].

Then adjust your periapsis at apoapsis, and back to periapsis to circularize (according to Obereth, circularizing at periapsis is optimal, but once you get apoapsis below 100k I doubt it makes much difference. At this point you are mostly concerned about finding a landing space).

To optimize descent, see http://forum.kerbalspaceprogram.com/threads/124760-How-to-time-suicide-burns (especially redironcrown's nifty use of manuever nodes. Or just look at kerbal engineer).

To optimize acsent, I'm pretty sure an amazingly fast gravity turn is in order. I tend to turn 45 degrees as fast as my craft can turn, and look to be 90 degrees within a few thousand meters. Not sure what is optimal.

Once you've circularized around Mun, set a manuever node that intersects with the trailing side of Mun's orbit (you will have to zoom out to see it, but it takes a few seconds to fade once you zoom back in). Optimal burn is very close to just barely escape velocity (I give it a touch more while watching periapsis get closer to Kerbin, but I'm not sure that is best. Also actual burn time might be slightly different than on the orbit, but it will be very close). Wait nearly a full orbit to apoapsis and bring periapsis to just under 30,000m (depending on how much you trust your heat shield) [if you have any spare fuel at all, going the extra orbit is likely a silly habit of mine].

Beyond that, I don't think there is anything you can to optimize a trans munar injection (ignoring capture). For a "flyby the mun" mission, any burn that leaves apoapsis just inside the mun's SOI is going to be optimal. The only tricks come in reducing capture/landing costs. You could wait to intersect the Mun at [its own] periapsis, but I think it's pretty circular. The trick with Minmus is to wait till it is above (or opposite) KSC and then launch in its inclination.

Edited June 9, 2015 by wumpus

[Eric S]

That would make sense. But what about approach ?

I'm not sure if people are answering the question you're trying to ask here.

If you're thinking that a retrograde orbit of Kerbin would be a more efficient orbit to reach the Mun because of the direction you'd be approaching the Mun, this is not the case. With a retrograde orbit, your velocity would be added to the Mun's velocity to determine the relative velocity, instead of being subtracted from it, meaning that your insertion burn in Munar orbit would have to expend more delta-v to circularize.

[Lukaszenko]

I'm not sure this makes sense, but can you use a Munar gravity assist to help with getting captured by the Mun? I'm pretty sure that it won't help you on the first pass, but can you use the Mun to get your orbit around Kerbin to more closely match the Mun's, with the result that on the next pass you will need less Dv to get captured by the Mun?

I'm at work, so I can't really check if this even makes sense right now.

[Mic_n]

technically, if you launch at the right time, you don't actually need to establish that LKO, you can just blast straight out from launch straight into the transfer and save the circularization burn.

I'd guess that time is pretty much around 'moonrise', maybe a little before. Haven't really bothered with it that much.. my spaceplane launcher is already efficient enough for me.. to the Mun and back for ~18,000 bucks suits me fine.

[Sharpy]

If your goal is simply "land on the Mun and return to Kerbin with the least dV possible", then you should do this:

1. Launch to circular LKO as low as possible (i.e. just above the atmosphere)

A direct launch at the precise launch window will be more efficient. You spend a whole lot of dV circularizing the orbit, which would be much better spent raising the apoaxis towards the encounter point (and leaving your periaxis near the planet core). Of course it's much more difficult to time it right.

But the real savings are in reaching the LKO altitude. Look into more efficient first stages, e.g. jet engine based.

- - - Updated - - -

I'm not sure this makes sense, but can you use a Munar gravity assist to help with getting captured by the Mun? I'm pretty sure that it won't help you on the first pass, but can you use the Mun to get your orbit around Kerbin to more closely match the Mun's, with the result that on the next pass you will need less Dv to get captured by the Mun?

Yes, you can use Mun gravity assist to better match speed and as result reduce the amount of delta-V needed to land. Of course you vastly increase the time of travel, because your orbit *just* went out of phase with the Mun, and it will take months to catch up with it with the new, better matching speed. (and if it doesn't, that means the speed isn't well-matched, you can instead do another lunar fly-by, gravity assist or Oberth Maneuver, to match it better.

Look up the route of the Rosetta probe; it used three gravity assists against Earth to get the final speed; gravity assists can be just as well used for turning or slowing down. But all that saved Delta-V comes at cost of a lot of time.

[DarkGravity]

How does one do this speed matching via gravity assist?

[wumpus]

The Rosetta probe was going to a comet. Not remotely helpful for using the moon's (or mun's) gravity to get to the moon.

The "obvious" ways are to go retrograde and capture/circularize at a low periapsis.

It may be possible to find a means that involves a mun flyby that kicks your orbit into something that more closely matches the mun's orbit. This may take a bunch of encounters and careful planning of burns during Mun encounters (to set up the next encounter). Ideally you will end up with a process that looks like a [ship to ship] rendezvous encounter (only at munar orbits and seriously slow). Get close enough and you should be almost to a gravity capture (no idea if a true gravity capture is possible with KSP's two-body gravity hack, but it shouldn't take much of a burn at all). Also don't be surprised if this requires going to Minmus and using it to slow down (maybe a few times). Note that this probably doesn't come close to real interplanetary transfer network methods (supposedly you can get to the moon in a couple of years on the cheap) due to KSP's two-body hack.

I really can't recommend this method. If you have KSP set to extremely hard, try it in sandbox to find if it is possible. Otherwise just add moar boosters and get the delta-v needed for a conventional capture (and just tighten up all your maneuvers).

[tutike2000]

Correct me if I'm wrong, but I think that because of the way the sprehers of influence work, the most efficient way to do things may be very different in Kerbal than in real life.

To land on the Mun:

- Start from lowest circular LKO

- Burn prograde at the right time so that your Kerbin Ap barely touches the Mun's SOI.

- Once inside the Mun's SOI, reduce your horizontal speed to zero (your vertical speed should be close to 0 m/s once inside Mun's SOI)

- Suicide burn to the surface

[Nich]

For me the most efficient way to get to the moon has been cutting out the weight and only bringing the fuel you need or at least not returning to Kirbin with fuel. I have an orbital station over Kirbin requires a poodle, 2 rockmax 32s and 5 kickbacks to get into orbit. this has enough fuel for about 15-20 trips to the moon. Then another station at the moon with solar panels and batteries and a lander. The lander has no batteries or monoproplent, 2 mat bays 2 goo containers, 1 temp sensor and 1 pressure sensor 1 Fly by wire (a luxury) and 4 landing gear and 2 90 kg LF tanks. I use 1 kick back and 1 360 kg LF tank with a terrier and a command module. Fill up at LKO taking on 70-80 kgs of fuel. Meet up with the munar orbital station transfer 180 kg of fuel to the lander and 30ish kgs to the station (building up a supply as I only need 21 kg to return to Kirbin. I have gotten so good at docking/intercepting it only adds 30-40 dv but the cost saved on lower stages has been enormous as I used to use a poodle with the rockmax 32 costing 4500 for that stage alone. Times 15 that is 67500 vs 7500 to lift the fuel into orbit once.

ALSO blasting straight out of the launch pad straight to the moon means your fight gravity the entire burn and requires a less efficient TWR engine to take advantage of the oberthe effect. Not to mention you would still need 500ish m/s delta v to capture since you have no horizontal velocity and the moon is moving at what 550 m/s?

[itsmagic2009]

Correct me if I'm wrong, but I think that because of the way the sprehers of influence work, the most efficient way to do things may be very different in Kerbal than in real life.

To land on the Mun:

- Start from lowest circular LKO

- Burn prograde at the right time so that your Kerbin Ap barely touches the Mun's SOI.

- Once inside the Mun's SOI, reduce your horizontal speed to zero (your vertical speed should be close to 0 m/s once inside Mun's SOI)

- Suicide burn to the surface

Suicide burn dv from the edge of the Muns SOI is about 2700m/s with an LV909.

[Acet]

How does one do this speed matching via gravity assist?

The basic principle in KSP (which is much simpler than real life) is that when you enter an SOI, the vector of the planet's speed is SUBTRACTED from the vector of your craft's speed to give the speed of the ship inside the SOI. Once you exit the SOI, the vector of the planet's speed is ADDED back to make the craft's speed outside the SOI.

For example, if you enter the Mun's SOI from the head-on (centre front) your speed inside the Mun's SOI becomes your original speed plus the Mun's speed since the Mun is coming towards you (remember that a vector going opposite yours is negative, so when you subtract it you get a positive value). Similarly, when you enter the Mun from behind, the Mun's speed is subtracted from your speed (a subtraction of a parallel vector takes speed away). When you exit in those positions the reverse happens.

Now, this doesn't sound very useful since if a craft enters from one side and come out the opposite side, the planet's speed gets added and then taken away on exit in an equal amount, which is not at all useful.

However, whilst inside the Mun's SOI your trajectory will curve due to gravity (the lower you go the more it curves) and you can help that even further with some well placed manoeuvring. This means your craft will not actually exit in the exact opposite point as it entered, with the end result being that the speed which is added/removed when entering the SOI is not the same as the one removed/added when exiting. This can be used so that the end result is getting or loosing speed between entering and exiting the SOI purely from the change in your craft's direction under the effect of that planet's gravity.

For example, in the extreme (not really possible in reality, just an idealised "what if"), if you managed to enter the Mun's SOI from the front, turn 180 degrees and exit it exactly were you entered, the following would happen:

- On entering, the Mun is going opposite from you (hence negative to your craft's speed), so when that negative vector is subtracted away from your craft's speed it actually makes you craft go 542.5 m/s faster

- On exiting, the Mun is going the same way as your craft. Because on exit from an SOI, that planet's vector is ADDED, and your craft's speed vector is exactly parallel, you just got another extra 542.5 m/s of speed.

the result of this would be an extra 1085 m/s speed, more than enough to get from Mun orbit to, for example, Duna or Eve.

Whilst in reality you can't do an 180 degree turn around a planet purely from gravity (though two well placed small burns would get you close to it), you can get more than 90 degrees when low & slow enough near heavy planets. Those 90 degrees would get you from no speed change on entering SOI (if coming from the 90 degrees or 270 degrees in the plane of that planet) to getting the entire orbital speed of the planet added (if exiting via the front) or taken away (if exiting from behind).

You can play with this purely in the Kerbin system by getting just enough dV for a ship to exit Kerbin's SOI and then coming back in again (just reverse course) and trying to use the Mun to lower your orbit for a Kerbin aerocapture.

You can also try to return from Minmus to Kerbin via the Mun's SOI, using the latter to lower your orbit for a Kerbin aerocapture or in reverse, go up from Low Kerbin Orbit and use the Mun to push your orbit up to Minmus, although these two are harder to get right since Minmus' orbit is inclined (not to mention not usually worth the time).

In the big bad world of Kerbol, the dV savings which can be had are much more substantial, though as somebody pointed out above, you will be trading time for dV.

Edited June 10, 2015 by Acet
Edited for grammar correction (duh!) and Kerbol system gravity assist gains comment.

[Nich]

Gravity capture is very easy actually, but not with the KSP tools available to you

Enter retrograde and turn counter clock wise so that your exit elipse nearly the same period as the moon.

Then you both do a lap and when you come back the velocity that was added is now subtracted.

Next enter Prograde and turn clock wise and exit in the same manor. The elipse will become shorter and fatter until you are captured at at time infinity. In reality 1 pass reduces a ton of delta v and a second pass doesn't save that much unless the orbit is highly eccentric like a straight Minimus approach. However sling shot off the moon straight to minimus is much faster and possiably just as efficient as a 2 pass gravity capture. In addition the method requires Extreme ACCURACY as you can easily burn off all saved delta v making course corrections.

[ajburges]

Never saw OP elaborate on intent. There are 4 reasons to encounter the mun (or other body):

> Fly by

You just want the SOI change (for contract I assume), so you want as efficient a return as possible. Enter free return trajectory. Plan your maneuver so you enter ahead of the targets orbit (in a retrograde orbit (counter clockwise) for Kerbin -> Mun) with a low PE. This costs 20 m/s more for the encounter, but after that burn you should need no other adjustments for a Munar encounter and return to sub-orbital Kerbin trajectory.

> Landing

One option is a free return trajectory like Apollo used. This gives you a reliable abort mode at the cost of lander dV needed.

The more efficient option as mentioned is to encounter in prograde orbit (clockwise) with a low Pe, circularize, and land. Takeoff in prograde to a low orbit (8-12 km) before the burn to leave Munar orbit.

> Gravity assist (accelerate)

Insert behind target orbit (prograde for Kerbin -> Mun). Lower Pe give more acceleration. Optionally burn at Pe for oberth effect usage.

> Gravity assist (decelerate)

Insert ahead of target orbit (retrograde for Kerbin -> Mun). Lower Pe give more deceleration. Optionally burn at Pe for oberth effect usage.

Efficiency for munar and other burns are best pursued with the following guidelines.

> Never burn radial.

> Modify orbit path while eccentricity is high at Pe and Ap. Sometimes making the orbit more eccentric for future burns is itself efficient.

> Burn prograde (and retrograde) at Ap and Pe.

> There is frequently a trade-off between trip time and efficiency.

[Feradose]

The most efficient way is using gravity assists. Here is how you do it:

1- Get a Jool encounter.

2- Get retro gravity assist from Jool that sends you to a Duna/Ike encounter. You might want to use RCS for fine tuning.

2- After getting your Duna/Ike gravity assist that leads to a Kerbin/Minmus encounter, use Minmus to encounter Mun.

3-Land on mun without orbiting for best fuel efficiency.

4-???

5-Profit!!!

Tip: Use ION engines for most efficientest landing.

[Mic_n]

For reference: Step 4 is "Slingshot around the Sun to break warp 9.0 and travel in time back to 1986 when fuel was cheaper"

[PLAD]

I'm not sure this makes sense, but can you use a Munar gravity assist to help with getting captured by the Mun? I'm pretty sure that it won't help you on the first pass, but can you use the Mun to get your orbit around Kerbin to more closely match the Mun's, with the result that on the next pass you will need less Dv to get captured by the Mun?

I'm at work, so I can't really check if this even makes sense right now.

You can indeed use a gravity assist from Mun to reduce the total dV required to get into a low orbit around Mun from LKO. Since version 0.23.5 or so I have used the stock Kerbal-X to go from Kerbin surface to Mun's surface and back, trying to do it for as little total dV as possible. My best try is here, and another try with a slightly different approach is here. To sum it up, to get from a 72x72km Kerbin orbit to the first Mun flyby is about 846 m/s, tricky manuevers add 15m/s, then decelerating into a very low 6x9km(for efficiency) Munar orbit is 208m/s. LKO to LMO for 1069m/s. A direct flight would normally be about 860m/s (with course corrections) +271m/s or 1131m/s. But it the trip takes 13 days to save those 62m/s! Note that you can use similar methods to get from LMO (8x8km) back to Kerbin for about 230m/s.

This sort of stuff has actually been done in the real world! My favorite was a Japanese moon probe that malfunctioned before completing its trans-Lunar burn and was stranded in a too-low ellipse to get to the Moon directly. They thought it was doomed but an astrodynamcist at JPL figured out how to adjust the orbit's period so it would get little tugs from the Moon that eventually pulled it all the way up and into a low-energy capture.

[Snark]

Correct me if I'm wrong, but I think that because of the way the sprehers of influence work, the most efficient way to do things may be very different in Kerbal than in real life.

To land on the Mun:

- Start from lowest circular LKO

- Burn prograde at the right time so that your Kerbin Ap barely touches the Mun's SOI.

- Once inside the Mun's SOI, reduce your horizontal speed to zero (your vertical speed should be close to 0 m/s once inside Mun's SOI)

- Suicide burn to the surface

You really don't want to do that-- it pessimizes Oberth effect and will cost you dV.

You want to do as much of your burns at as low an altitude as possible. Therefore:

1. Start from lowest circular LKO

2. Burn prograde at the right time so that your trajectory passes the Mun at the lowest possible altitude, grazing the surface

3. At Mun periapsis, burn retrograde to circularize into lowest possible LMO

4. When you're ready to land, lower your periapsis to the surface, then suicide-burn to land.

Actually, in principle, it would be slightly more efficient if, instead of steps 3&4 above, you made it so that your munar periapsis in step 2 is right at ground level, and you just directly suicide-burn on a near-horizontal trajectory directly to the surface. However, the dV savings over the above scheme would be minimal; it would be trickier to set up; and you'd lose a lot of convenience in terms of being able to choose where your landing site is. Therefore, in my experience it's not worth the hassle for the minimal extra dV savings.

[tutike2000]

Isn't the whole point of the suicide burn that it uses the Oberth efect most efficiently? Ie: burn as low in the grav well as possible.

I agree that skipping points 3 and 4 is the best way to go, however I don't understand why you can't just use the method I described.

If you have the smallest possible horizontal and vertical velocity (relative to the Mun) when you enter the Mun's SOI, isn't that the most efficient way to land?

If you're on the edge of the SOI, the best speed that you'd want to have is zero, so that you'll only have to cancel out the kinetic energy you get from falling to h = 0m