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Thread: Economical descent profile for Mun landing?

  1. #1

    Economical descent profile for Mun landing?

    So, I have no problems getting to the Mun from a Kerbin parking orbit. I can land pretty easily. The problem is, I use a LOT of fuel on the descent path because I end up freaking out and trying to cancel out some of the orbital velocity way sooner than I probably should, which basically leads to me fighting physics when I could be using the high velocity to my advantage to speed the landing process up. Theoretically, I know this. When I see the Mun below me and my distance toward it decreasing, I start trying to fight it anyway. This leads to me having very little fuel to establish a Munar orbit that's efficient, and leads to me having to find creative ways to make it back to Kerbin with little fuel after I reach a stable low Munar orbit.

    Basically, at what point do you start bleeding off some of the orbital velocity during your descents?

  2. #2
    Junior Rocket Scientist
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    The optimum descent profile is the time-reversed optimum ascent profile with a negative propellant flow rate. It'll look mostly like a good takeoff in reverse.

    However that's hard to do without teh maths so I found a method that's nearly as good ad a lot easier. The idea is to place your periapsis about 3-5km above your intended landing site. Upon approaching your landing site (how far determined by TWR) you should be near your 3-5 km close scrape height. Burn retrograde with enough radial positive component to cancel any falling but not enough to gain height. As you decelerate pitch up more and more to direct more of your thrust to maintaining altitude. When you're over the landing site and your horizontal speed is manageable then let down at 10-30 m/s vertically until close and land per normal. I budget about 60 seconds hover time (Mun's gravity times 60 = hover dV) for the landing.

  3. #3
    Rocketeer Wesreidau's Avatar
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    I land by burning nearly all of my orbital V off after a circular 10km orbit.

    Edit: Derp. I should probably have mentioned I get to that orbit in a service module from which my lander decouples after the pilot EVA's over to it. I should probably also have mentioned I come in at about 45 degrees but level off at about 1km, from which I descend vertically. 3 AM posting fail.

    Come down nearly vertical and use RCS to keep the prograde pipper centered on the ball. Don't tilt the rocket to correct horizontal drift if you can help it at all, and avoid the mistake of false halts as you descend. You may also want to drop your landing gear and any empty tanks upon liftoff from the Mun (not gonna need legs in space) and rendezvous with a craft orbiting the Mun that performs your orbital maneuvers and transfer burns so the lander can be reserved for landing and takeoff.
    Last edited by Wesreidau; 29th July 2013 at 10:55. Reason: 3AM Derp Correction


    After placing Neifel Kerman into orbit at Apo 129,319m @ 2200.3m/s and Peri 129,005 @ 2201.2 equatorial,
    flying IVA/Orbit Map with stock parts, someone went and told me MechJeb and Engineer existed. Spoilsports.

  4. #4
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    Quote Originally Posted by Wesreidau View Post
    I land by burning nearly all of my orbital V off after a circular 10km orbit. Come down nearly vertical and use RCS to keep the prograde pipper centered on the ball. Don't tilt the rocket to correct horizontal drift if you can help it at all, and avoid the mistake of false halts as you descend. You may also want to drop your landing gear and any empty tanks upon liftoff from the Mun (not gonna need legs in space) and rendezvous with a craft orbiting the Mun that performs your orbital maneuvers and transfer burns so the lander can be reserved for landing and takeoff.
    This is nearly the least efficient descent profile that you can manage. About the only way to make it less efficient is to decide to hover at 1m altitude above the Mun rather than actually landing. It is also the hard way to land, because your velocity is always vertical, so if you wait too late for your burn you're out of luck.

    The reverse gravity turn method is easier, more efficient, and much more forgiving to mistakes (if you wait too late, you just wind up burning a bit more fuel rather than testing the viscosity of your spacecraft).

  5. #5
    This is a good example of what Frederf described. I have tried to re-create this, but can only watch in awe and hope to be this good one day.

  6. #6
    The (theoretical) optimum would be similar to a Hohman transfer from your parking orbit to an orbit just above the surface:
    At the apoapsis of your parking orbit you burn retrograde till your periapsis almost touches the ground. At periapsis you then burn retrograde again (parallel to the ground) until l you lost your orbital speed. Since you're burning at the lowest possible altitude you get the best use of the Oberth effect.
    How low you can put your periapse (i.e. at what height you start the break) will mainly depend on your TWR. Just autosave and try some values. For your average mun lander 500m should be a good guess.
    This also much easier than the "vertical drop" method, since errors tend to make you over- or undershoot your target position. Using a vertical drop you're more likely to be hovering in the air wasting fuel or vertically overshoot the lithosphere.

  7. #7
    Quote Originally Posted by Frederf View Post
    The idea is to place your periapsis about 3-5km above your intended landing site.
    Beware that there are features on the Mun's surface above 3km, and the 0.21 Munar surface may have features above 4km [citation needed]. Frederf's plan is what you want to aim for, but be prepared to add some altitude during your final burns if you see you're flying too fast straight into a mountain.

  8. #8
    I design my ship with a supplmentary stage for retro burning on Mun landing. I eject the stage between 5000m and 2000m before lading so that I burn a very little with the ship itself and i get enough for take off, docking on an orbital station for refuel or even to eject from mun to kerbin and land to Kerbin.
    The sup stage is too big for Mun and Minmus and I usually burnt half tank before jettisson, I could use a smaller one, but my design is supposed to be used on farther planets and this stage could be use also for establish orbit and then retro burn before landing
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  9. #9
    Okay so I'll admit I personally use the inefficient kill orbital velocity all at once and plummet straight down, then cancel vertical velocity when I think I have little time left.

    Now the approach in the video a few posts up is extremely impressive, but let's say you want to land somewhere specific. Like if perhaps you're using Kethane and need to come down over a deposit. How would some of you more skilled / crafty people land somewhere very specific?

    I've watched MJ do it, but I prefer manually doing things. Like the OP though, I tend to see the ground coming up mighty fast and jam on the throttle...

    I shall resolve to practice the elegant landing in that video at least today sometime practice practice practice is how I learned to dock... That and watching ORDA do it. Eventually stopped using it and even MJ I mostly only use for info screens now, but landing is something I'm still relatively bad at. Half the time I end up rolled the wrong way. And during the final few meters I pitch the wrong way and smear over the landscape.

  10. #10
    Quote Originally Posted by Immashift View Post
    ... let's say you want to land somewhere specific.
    The shallow descent method works pretty well for hitting a target in no atmosphere. Adjust your orbit until it intersects the surface at your target. A shallower descent is more efficient, but runs the risk of hitting higher elevation areas. You burn retrograde as you get close, pitching up and down slightly to keep your intersect at your target location. As you near the target, you can switch to vertical descent, which is practically necessary in rough terrain. How close you are when you switch to vertical is up to you and the surface.

    The only concern is when you have long burn times to reduce velocity. The same shallow descent method works, but needs more care. Scott Manley showed a cool way to do it. Set periapsis [edit: see note below] directly above the target by only a few km. Set a maneuver node that kills all horizontal velocity so you drop down directly on the target. Take the reported burn time, and when you're that far away (in time) from the maneuver node, that's roughly the closest you can get before you need to start burning at full throttle. That keeps you from overshooting when you follow the shallow descent method.

    [Note: doesn't have to be periapsis, but the less vertical speed you have in the orbit above your target, the more accurate the estimate of your burn time will be. At periapsis, the vertical speed is zero.]
    Last edited by Somerled; 29th July 2013 at 09:08.

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