Jump to content
  • 0

Required Delta-v to Land table?


Zosma Procyon
 Share

Question

Does anyone have a table or list of calculator with the upper limits of the Delta-V require to go from say a 50-100km orbit to a safe landing. I get it wrong too often. I just landed a base segment on Minmus with literally 10 times the Delta-V it needed. I ended up just throwing away all that fuel because it was late at night and I had to get to bed. It would also help if the resource you supply me with also listed optimal acceleration figures for landings. That's been on of my problems in these 155 years too. Help me improve my game, please.

Link to comment
Share on other sites

4 answers to this question

Recommended Posts

  • 1

@Zosma Procyon, there are a lot of variables that go into it.  I don't like using delta-v maps for it.  Delta-v maps are probably OK for ascent, but I think they underestimate descent.  On ascent you just hit the throttle and go, but on descent you have to approach the surface more slowly and with caution.  This means fuel is burned effectively hovering.  It can be minimized with a suicide burn, but some people prefer a stop-and-drop method.  I generally do what is sort of a hybrid.  I'll do a burn to lower my periapsis to maybe 5-10 km over the landing site (low as I can while assuring I don't impact any mountains).  At periapsis I do a big burn to kill most of my velocity.  Then when I have descended to about 1 km altitude, I kill most of my remaining velocity and slowly drop to the surface.  This isn't the most efficient, but it's not the worst either.

Several years ago I plugged a bunch of numbers into a spreadsheet to compute how much delta-v this method used for each of KSP's airless bodies.  I was somewhat conservative in my numbers, and included an extra 30 seconds of hover time to play it safe.  Even still, there are many things that can change the results, like height of initial orbit and TWR.  Nonetheless, the numbers form a basis for budgeting delta-v.

Moho - 1300 m/s
Gilly - 45 m/s
Mun - 885 m/s
Minmus - 275 m/s
Ike - 595 m/s
Dres - 615 m/s
Vall - 1260 m/s
Tylo - 3360 m/s
Bop - 320 m/s
Pol - 210 m/s
Eeloo - 915 m/s

Landing should definitely be doable using these numbers.  But even this might not be enough if you are really inefficient.
 

Link to comment
Share on other sites

  • 0
2 hours ago, Zosma Procyon said:

Help me improve my game, please.

Learn to suicide burn. : D

I'm sure everybody has their preferred method. The latest versions of KER even give you a big red target and the exact countdown and dV requirements to do it. Still, I find it much more satisfactory to do it all manually. So here goes:

First, get into a low-ish orbit. Look at your orbital speed. That speed will rise the lower your orbit is, so just add a bit and you have essentially the dV requirement to come to a stop down on the surface.
From that, your requirement to come to a dead stop on the surface of Minmus should be obvious: somewhere in the 170 to 190m/s range.

Next, start a quarter orbit ahead of your landing zone. Plot a node to hit the surface. Plot another node where you hit the surface and drag the retrograde vector until the resulting Ap starts to move back up your trajectory.
Then burn that first node at full power. Don't try to get it exact, just watch the map and cut off the engines when it's more or less right. Delete the node. Look at the node on the surface and make sure it's just above the surface.
You should now have a time to that surface node, plus a number which should be in the ballpark figure of 170-190 m/s, plus a burn time. If you did the last burn at full power, that burn time will be exact (if you have the latest version of KSP, that burn time should be exact even if you didn't burn at full power, but I prefer to be safe than sorry).
When you are about 2/3 of that time away from the node, point retrograde (with retrograde hold) and burn at full power. When under 10m/s, stop and let the craft swing towards the vertical - the ground should be just a few dozens of metres below. Ramp up power slowly until you fall at 3-5 m/s constant.

Why 2/3? Well, that comes from the basic relationship between distance and acceleration. Under constant acceleration from 0 to x m/s, you will travel exactly half the distance that you would travel if you were going at x m/s to start with. S = ut + 1/2 at2 and all that. On deceleration it's exactly the same thing, you'll travel half the distance that you would have travelled if you stayed at the same speed.
So basically you could wait to exactly half the time to your surface-striking node and burn, but the problem is that as you slow down, you start heading towards the ground as gravity kicks in. Therefore you need to add a bit of time to avoid hitting the ground. But any additional time will be time that you need to hover and waste fuel. Therefore 2/3 is roughly the right amount: if gravity is strong, you survive with minimal gravity losses; if gravity is weak, you hover for longer but you don't need to care so much about it since gravity losses will be small.

With that technique, your total dV requirements to land from low orbit will be just a few percent more than the classic "dV map" figures. My hardest landing was on Moho when I had no other refuelling options and about 950 m/s left on my mining ship. It took a few tries but I made it...

Link to comment
Share on other sites

This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...