A Question about Deorbit Burns / Parking your Lander?

[Turk_WLF]

I’m having problems with knowing when to start a deorbit so my Lander will land near a set landing site like at a MunBase, I’m using Mechjeb to help me land. I have 2 Crewmembers stuck on the Mun I need to rescue them.

Thanks,

[bewing]

Do you have fuel limitations, and is your engine overpowered or underpowered?

[Spricigo]

First of all: Define "near". For my typical needs anything bellow 5km can be considered near, but I doubt that will be enough for you.

More info in general certainly help us to understand your situation. You said you are using \MJ to help you, any problem using the features it have for it (landing site prediction, autolanding, ....) or figuring out how those works? Any reason to not quicksave and try until you are satisfied? Or landing not so close and hoping close? (i think @bewing was about to suqgest it)

 

 

[Kerbal101]

@Turk_WLF Well, if your Lander is capable of having at least 1.5 TWR on Mun surface (which can be calculated from TWR on sea-level Kerbin x 0.16), then you are looking at approximately 1000 dV for descend from and 900 dV for ascend back to 15km/15km orbit. You can easily measure this by sending a very lightweight lander (unmanned probe) to specific routes and then writing down the m/s values. You'll have reliable data, using which you would be able to design stages accurately.

To descend, start killing horizontal speed by burning retrograde direction - but at horizon line, at about 15km away from your goal (triangle to orbit height), then adjust it looking at map view.

However, be careful that your vertical speed does not slip behind -70m/s until you have finished dropping horizontal speed - and shift direction radial-out to hold it at that value.
After you finished with horizontal, switch speed indicator from orbit to surface, aim sas-hold at radial-out and let vertical loose so long its within the 1/100 of your altitude. So, for 10,000m - it should be max 100m/s, for 5,000m - max 50m/s.
Of course, it all becomes easier if you have separate digit values of horizontal and vertical speeds, altitude from surface (not only sea level) and even suicide burn distance with KER or DMagic mods, allowing to reduce deltaV requirements by reducing burn duration.

Happy landings.

[Spricigo]

2 hours ago, Kerbal101 said:

 900 dV for ascend back to 15km/15km orbit.

And why are you telling him to land in the most fuel inefficient way? 

He is using MechJeb and consequently can use its features to make a eficient and precise landing near the target. Its not only the info necessary (e.g. time to suicie burn) but the actual autopiloting thing the mod is about.

 

[Pand5461]

I tend to start deorbit burn 90 to 60 degrees from the target. While descending, I'm trying to keep ground intersect point a bit in front of target and keep reasonable vertical velocity. BetterBurnTime mod shows estimated time to impact and time to kill the velocity, keeping the latter not greater than the former ensures you're on the safe side.

[The Aziz]

You use MJ, so you might as well take a look at another mod called Trajectories. It pretty much predicts your trajectories (duh), even on atmospheric bodies, so you know exactly where you hit the ground. If you set your target and deorbit burn maneuver node, the mod window will show you how far away from target your landing will be. It helped me much, I could land within 100m from my target.

[NewtSoup]

This is not the most efficient way of landing at a target.  But it will certainly help you land accurately.  I'm pretty sure had Scott kept burning on the retrograde marker it would have been fairly close to a suicide burn.  The way he does it uses more fuel but affords you more control and more room for error.  And does it with no mods at all.

I don't know anything about MechJeb  but I gather it can do the flying for you on a suicide burn. Either way I'm sure this short video will be instructive all the same.
 

 

[psychogre]

I tend to do things more by the seat of my spacesuit.  The method I use could be called a 'reverse gravity turn'.  I first tried this out a few times on Mun, but then had a lot more times to practice and perfect it when I set up a mining operation on Minmus (ferrying the fuel up to ships in orbit).  I've since moved the whole operation to Ike.

I made an imgur album showing my procedure - hope this helps

If there's an atmosphere, you would have to make a larger overshoot to take account of the drag.

[DerekL1963]

Use F5 to perform a quicksave before doing anything....

You can use MechJeb's Surface Info window to find the location of the crew you're trying to rescue.  Write that info down, and then swap to the lander.   Open the Landing Autopilot and enter the coordinates you wrote down, and then click on the arrows by the Lat and Long to offset your landing by thirty to fifty meters - then click on "land"  (or whatever the button is, I don't have MJ open at the moment).

Once you've landed and recovered your crew, you can use MJ's Ascent Autopilot to return to orbit.

If anything goes pear-shaped, use F9 to reload your quicksave, then come back here and let us know what went wrong and we'll help troubleshoot.