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As a part of a contract I'm bringing a surface outpost of ~10t to the surface of Minmus. I want to land that thing intact and for that I need to calculate how much engines/thrust/fuel I need on the landing device, so that I can bring impact velocity down to minimal numbers.

I would really like to bring some kerbalnauts on board, so trial & error does not seem a viable option.

How can I calculate how much delta-v/thrust/etc do I need to allow my craft to slow down fast enough?

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  • Note that the hardest part can often be needed thrust. I remember fiddling around for landing on the mun and needing basically a suicide burn from a 10K orbit with a thrust of 4 times mun's gravity. So that is where I would start, assume you need at bare minimum 4 times the gravity to manage any type of landing (and far more is way better, ideally I would like to see >8).
    – Jonathon
    Commented Sep 2, 2015 at 18:45

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tgharold answered pretty well how much delta-v you need in the ideal case. To get from orbit to ground you need at least as much delta-v as the orbital speed at ground level. But he didn't say anything about how much thrust you need.

You can look up the surface gravity acceleration of a body in the tracking station or on the wiki. For Minmus, it is 0.05g or 0.491 m/s². When you have a craft which can accelerate at least that fast, it is capable to hold its speed constant while descending. But to land, you need to reduce the speed to zero before surface contact, so you need more acceleration. How much more acceleration? That depends on how efficient you want to land. The later you start your deceleration burn, the less delta-v you need. That means the more thrust you have, the closer to the surface you can dare to start your break maneuver, which means you will be closer to the ideal delta-v budget.

So how do you calculate the acceleration of a vessel? Simply divide the thrust by the mass. A 10 ton vehicle with a LV-909 "Terrier" (60kN) engine has an acceleration of 60,000 / 10,000 = 6 m/s². Subtract the gravity of Minmus and you have a remaining deceleration of about 5.5 m/s². That means the vessel can stop a 200 m/s descent with a 36 second burn.

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  • Thrust part was exactly what I was looking for. Thank you!
    – Sergii Zaskaleta
    Commented Sep 2, 2015 at 20:56
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This becomes trivial to calculate if you have Kerbal Engineer Redux installed. It will tell you how much dV you have in each stage. Then you can refer to a delta-V map to figure out the minimum dV needed to get from here to there.

For Kerbal -> Minmus, you need:

  • 4550 to get into low orbit (80km)
  • 930 to get to Minmus intercept
  • 160 to get into orbit (10km)
  • 180 to land

It doesn't matter whether you are moving 100t or 10t, you need to calculate the dV for each stage.

Delta-V map links:

http://forum.kerbalspaceprogram.com/threads/120030-1-0-Delta-V-Maps-and-Atmospheric-Charts

http://forum.kerbalspaceprogram.com/threads/96985-1-0-4-WAC-s-Delta-V-Map-continued-1-2-2-(August-7th)-Vacuum-Space-Good?p=1481940&viewfull=1

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    But if I have a very slow engine with a lot delta-v but not enough force, it won't be able to slow me sufficiently fast enough, would it? I suppose its very unlikely for Minmus, but might get an issue for other celestial bodies.
    – Sergii Zaskaleta
    Commented Sep 2, 2015 at 6:16
  • The 4550m/s to get to orbit is an estimate based on expected losses due to atmospheric drag. I find I use much less than that, and tend to use closer to the 3200-3400m/s end of the scale when launching.
    – MBraedley
    Commented Sep 4, 2015 at 0:36
  • @SergiiZaskaleta The only time you need to worry about how powerful your engine is is when you're landing. If your thrust to weight ration is above 1, you can land (although not necessarily easily). If you're just concerned with the time it takes to perform a burn, don't worry. Long burns will cost you some dV but that cost is easily made up by using higher Isp engines.
    – MBraedley
    Commented Sep 4, 2015 at 0:40
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    Since the atmosphere mechanics overhaul it takes less dv to Kerbin orbit; it's closer to 3500m/s now.
    – SevenSidedDie
    Commented Oct 15, 2015 at 5:38

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