Optimal Altitude For Mun Landing

[DChurchill]

What's the consensus for the most efficient orbit altitude for a Mun landing, generally speaking?

Lower means faster so more dV to come to a stop, plus more dV expended to insert into Munar orbit, right? Or am I going to pick up that velocity on descent anyway, so the only consideration is dV expended on orbital insertion? Or is there a sweet spot?

[Kegereneku]

There's not really an optimal altitude and your logic is wrong. I'll address both point :

First, the closer you do the insertion burn, the more your profited from Oberth Effect .... to save dV (meaning that it's best to burn as close as possible)

Being closer from the Mun don't make you "faster", quite the opposite, the linear speed is lesser than for high orbit.

Second, what truly define the optimal altitude is those three point :

- How high is the relief (you can hardly stay in orbit under 20km )

- How fast you can accelerate. (because the theoretical best descent is one deorbit and one last minute landing burn (nicknamed "suicide burn"))

- How efficient is the thruster.

In short : The sweet spot must be calculated based on your lander design.

One can land from a 20km orbit, other might need a 150km orbit

[tavert]

You want to come in with a periapsis right at the surface, get into a circular orbit at the lowest possible altitude, then burn slightly upwards to keep your vertical speed near 0 during your braking burn. Landing cost is slightly lower for a higher-altitude landing site, I can run more specific numbers if you want examples.

Edited July 28, 2013 by tavert

[Cheaterman]

If your main concern is efficiency, you will want to:

- Circularize as low as possible around Kerbin (75km)

- Do a Hohmann transfer that puts you on low Munar altitude (10km, even 5 if you're badass like Jeb) and circularize there

- Then aim for your landing site - it doesn't matter how far from the landing site you do it, further will give you more time to react though - however if you make it too far your path will come across some craters...

- Suicidal burn is best, leave that to MechJeb though - at this point you'll have saved enough dV to be able to do a manual landing without considerable waste.

- Enjoy!

[karolus10]

I in most cases transfer to 5km orbit before landing, It's the lowest altitude allowing for time warp, so I can pass over target and made minor inclination correction before descent.

I recommend not rushing with getting rid of lateral speed (I prefer shallow angle of descent) and try to keep descent rate at 10-20m/s, so you won't loose too much fuel, killing vertical speed and you had good chances to abort if necessary.

Pre-landing checklist:

• Before 5 kilometers from target, slow down to 60m/s and be bellow 2km altitude, keep 10m/s descent rate if bellow 2km, 20m/s if you are higher.
  
• bellow 1000 meters (radar) You should switch to hover position (~20-0 degrees) slow down to 30m/s
  
• Check Your radar and slow down descent rate to ~5-10 m/s, You should be less than 2000 meters from target
  
• Slowly descent and kill lateral velocity before touchdown, mission accomplished !
  

[Mr Shifty]

Being closer from the Mun don't make you "faster", quite the opposite, the linear speed is lesser than for high orbit.

For a circular orbit, orbital velocity is proportional to the inverse square-root of the radius from center of gravity. Lower orbits are faster.

[Kermunist]

But for an elliptical orbit with periapsis close to the surface of the mun (which is probably what you want for landing) then your speed at periapsis is lower if your apoapsis is lower. I think that is what Kergereneku meant.

[leforian]

is a pretty awesome relevant video.

[Scarecrow88]

is a pretty awesome relevant video.

It is quite an awesome video, and the first time I've seen a landing performed like that, but I'd like to see how that approach would work with the bigger contour changes we have on the 0.21 Mun.

[bobcook]

It is quite an awesome video, and the first time I've seen a landing performed like that, but I'd like to see how that approach would work with the bigger contour changes we have on the 0.21 Mun.

Did anyone else notice that he was skimming the surface pretty closely while decelerating? Watch the "Altitude (true)" value in the left-hand status display. Around 4:23 he is at 180m going 210m/s. At 5:36 he is at 80m going 125m/s. Either he knows the terrain very well, or was exceptionally lucky on this descent. Wow.