understanding the dV map

[MPDerksen]

Minmus example:
3,400 to 80k KO
930 transfer to Minmus
160 to circularize ~10k MO
180 to land (seems a bit optimistic, IMHO)

All very well.  Until I plan the math to head home.  The return may be the same as 180+160+930, and landing is simply aero-braking with the parachute.  But I'm NOT that good of a pilot, and was able to orbit and return (no landing on the surface) on far less that that total.  

Michael

[OhioBob]

The return is just 180+160, provided you are planning a direct atmospheric reentry and landing.  You don't need the 930 unless you plan to enter into a low orbit around Kerbin after returning.

And, yes, I think 180 m/s to land/takeoff is very optimistic.  I'd budget more like 500 m/s for landing and re-orbit.

[MPDerksen]

14 minutes ago, OhioBob said:

The return is just 180+160, provided you are planning a direct atmospheric reentry and landing.  You don't need the 930 unless you plan to enter into a low orbit around Kerbin after returning.

And, yes, I think 180 m/s to land/takeoff is very optimistic.  I'd budget more like 500 m/s for landing and re-orbit.

That makes far more sense.  Thanks.

again, using Minmus as our example, I assume you mean 500m/s for landing, and another 500m/s for re-orbit?  

how about a ball park for a Minmus Biome hop?  (each)

[Zhetaan]

6 minutes ago, MPDerksen said:

again, using Minmus as our example, I assume you mean 500m/s for landing, and another 500m/s for re-orbit?

You shouldn't need 1,000 m/s to land and re-orbit at Minmus.  500 for both (or 250 each way) is quite a lot of margin, especially when you consider:

6 minutes ago, MPDerksen said:

how about a ball park for a Minmus Biome hop?  (each)

Hopping, at its worst, will be slightly less than double what you need to land or re-orbit.  That's because you need to take off and land in a single manoeuvre, rather than launch to orbit and stay there.

However, the trick to hopping is to hop to the next biome, not the one on the other side of the planet.  You can cut your needed delta-V by quite a lot by choosing somewhere that allows you to visit several biomes with a few short hops, rather than somewhere that is the only biome in sight until the horizon.

Edited July 3, 2019 by Zhetaan

[MPDerksen]

7 minutes ago, Zhetaan said:

You shouldn't need 1,000 m/s to land and re-orbit at Minmus.  500 for both (or 250 each way) is quite a lot of margin, especially when you consider:

Hopping, at its worst, will be slightly less than double what you need to land or re-orbit.  That's because you need to take off and land in a single manoeuvre, rather than launch to orbit and stay there.

However, the trick to hopping is to hop to the next biome, not the one on the other side of the planet.  You can cut your needed delta-V by quite a lot by choosing somewhere that allows you to visit several biomes with a few short hops, rather than somewhere that is the only biome in sight until the horizon.

Right, that all makes sense.  So maybe 5,500 for a round trip with a single landing?  This assumes little/no inclination correction (see other thread from this morning).

Switching to Mun instead, and the map indicates 580 for a landing, perhaps 1,500 for a single landing/re-orbit is a fair calculation?

[Zhetaan]

7 minutes ago, MPDerksen said:

Right, that all makes sense.  So maybe 5,500 for a round trip with a single landing?  This assumes little/no inclination correction (see other thread from this morning).

Switching to Mun instead, and the map indicates 580 for a landing, perhaps 1,500 for a single landing/re-orbit is a fair calculation?

5,500 will get you there and back with plenty of manoeuvring reserve.  I'd suggest a bit more for the Mun until you have more time behind the stick; a good thumb rule is to take half again what you expect to need until you get experience.  In this case, since the Mun requires 580 for a landing and the same for a return to orbit, you should figure on 1,740 for landing, return, and salve for screw-ups.

Granted, taking fifty percent extra fuel gets prohibitively heavy later on, which is why you should focus your efforts on perfecting your piloting close to home before trying manned interplanetary landers.  As you gain experience, you'll find yourself consistently returning with extra fuel.  When that happens, you no longer need to take that fuel.

Edited July 3, 2019 by Zhetaan

[MPDerksen]

All this aligned my personal reality.  My current, very basic Mun Orbital Science Collection rocket (no lander) has 5,400 and I had a ton to spare.  I've gotten pretty good at most stuff, including docking.  I still really struggle with landing.  Specifically near a target (i.e. a surface rescue mission)

[OhioBob]

4 hours ago, MPDerksen said:

again, using Minmus as our example, I assume you mean 500m/s for landing, and another 500m/s for re-orbit?

Zeetan already answered, but I just wanted to confirm that the 500 m/s is the total for both landing and re-orbit.  It's roughly split between the two, though landing typically requires more.  Maybe around 275 landing + 225 re-orbit.  That's because when taking off you just punch the throttle and go.  When performing a soft landing you spend some time hovering (or near hovering), so there's more gravity loss.

[ajburges]

On 7/3/2019 at 1:12 PM, Zhetaan said:

.As you gain experience, you'll find yourself consistently returning with extra fuel.  When that happens, you no longer need to take that fuel.

50% is a bit much, but for landers you should always have extra budget. Sometimes you need to hover to change landing sites or touch down carefully on a slope. That is really expensive in dV. Sometimes you just goof in design or decent. An extra margin can allow recovery instead if rescue/reversion.

The trick is to have a plan for your margin if you are efficient. Mother ship missions can use the margin from successive landings to pay for another or you can bank surplus fuel at depots for future use.

Also: RCS. On a light lander the cockpit can provide over 100 m/s. This could be the extra oomph you need to orbit a lander.

[Snark]

On 7/3/2019 at 11:18 AM, MPDerksen said:

Minmus example:
3,400 to 80k KO
930 transfer to Minmus
160 to circularize ~10k MO
180 to land (seems a bit optimistic, IMHO)

All very well.  Until I plan the math to head home.  The return may be the same as 180+160+930, and landing is simply aero-braking with the parachute.  But I'm NOT that good of a pilot, and was able to orbit and return (no landing on the surface) on far less that that total. 

Just to go back to the original question of how to read / understand DV maps, here's a rephrasing of the numbers you give above, to make the semantics a bit clearer:

1. 3400 m/s from Kerbin surface to low Kerbin orbit
2. 930 m/s to change between Kerbin orbit and transfer trajectory
3. 160 m/s to change between transfer trajectory and Minmus orbit
4. 180 m/s to change between Minmus orbit and landed on Minmus

So to go home, you just play that in reverse.  180 m/s to go from landed-on-Minmus to Minmus orbit, and then 160 m/s to initiate the transfer home.  Then you can skip #1 and #2 on the way home, due to aerobraking.

The key takeaway is that when shifting from "orbit around a body" to "transfer to some other body" (or vice versa), that's a dV requirement that will be very different at the two ends of the transfer-- in the above example, it's 930 m/s on the Kerbin end of the transfer, but only 160 m/s on the Minmus end.  So that was the bit you were missing. 

I'd add that 180 to land from low Minmus orbit seems a bit optimistic to me, too-- I'd allow 200 at least, perhaps a bit more if you're not an expert at efficient vacuum landings.

On 7/3/2019 at 12:05 PM, MPDerksen said:

So maybe 5,500 for a round trip with a single landing?

That should be ample, yes.  I'd also add that biome hopping is far cheaper on Minmus than on the Mun, so if you've gone all that way, it's generally worth doing at least two or three different biome landings to greatly ampify the science obtained. 

On 7/3/2019 at 12:32 PM, MPDerksen said:

I've gotten pretty good at most stuff, including docking.  I still really struggle with landing.  Specifically near a target (i.e. a surface rescue mission)

The two main skills to develop for landing are efficiency and accuracy.  I would say that for most game circumstances I've run across, efficiency is the more important of the two, and good to master first.

The relevant term to search for on the forums is "suicide burn".  Basically, it goes like this:  the most efficient way to land on a vacuum world is to do an initial de-orbit burn to put you on a suborbital trajectory (preferably a fairly small burn, so that you're going mostly horizontal at your projected impact site), and then point and wait until the last possible second to turn on your engines.  Then hit the engines at 100% such that you brake to a halt right at the surface.  The tricky bit, of course, is gauging exactly when that "last possible" second is, thus the name of the maneuver.    There are various techniques for that-- some which work in stock, some which are assisted by mods-- but getting your suicide burns working is the main thing.

As for accuracy... yep, that's another skill.  But how much work is needed is highly subjective-- there are various strategies and techniques, and how much of a refinement you need to your current technique depends on how well that's working for you.  How would you evaluate your current Mun-landing skills for accuracy?  i.e. if you land on the Mun, in a way that's as efficient as you can make it (i.e. without doing a lot of wasteful hovering), how close to a desired target can you land?  i.e. how far off target is your typical landing?  10 meters?  100 meters?  1 km?  10 km?  Something bigger than 10 km?

[MPDerksen]

1 hour ago, Snark said:

racy... yep, that's another skill.  But how much work is needed is highly subjective-- there are various strategies and techniques, and how much of a refinement you need to your current technique depends on how well that's working for you.  How would you evaluate your current Mun-landing skills for accuracy?  i.e. if you land on the Mun, in a way that's as efficient as you can make it (i.e. without doing a lot of wasteful hovering), how close to a desired target can you land?  i.e. how far off target is your typical landing?  10 meters?  100 meters?  1 km?  10 km?  Something bigger than 10 km?

Helpful, as usual.  Thanks.

Not quite there in the new career yet, but when I try to land near a stranded Kerbal for rescue, my goal is to get within 2km, so that I can switch to that Kerbal to start the run over.  I'm in that window about half the time.  Minmus is actually a little trickier, since it rotates faster.  So setting my first retro burn to contact the surface keeps moving by the time I get there, and I end up further away.  This is even more difficult of I'm not on an equatorial orbit.

Since I understand the mechanics, now it comes down to practice.  
To the original question, I figure I'll design DOWN, and then build the final stage to get me out of the gravity well, with the mass I have built to do the rest of the mission.

Michael

[Zhetaan]

5 hours ago, ajburges said:

50% is a bit much

It absolutely is, but provided that the lander can still take off, it eliminates bad fuel budgeting as a cause for a failed mission.  Even better, having fuel still in the tank tells you exactly how much extra you over-budgeted.  Contrast that against empty tanks and the slow fall to crash into the Mun; that only tells you that you didn't have enough fuel, not how much less than what was needed.  I find that it's a lot easier to improve when I have accomplished the task and now need to refine it to do better, as opposed to refining it to do it at all.

3 hours ago, Snark said:

The two main skills to develop for landing are efficiency and accuracy.  I would say that for most game circumstances I've run across, efficiency is the more important of the two, and good to master first.

Agreed.  However,

3 hours ago, Snark said:

The relevant term to search for on the forums is "suicide burn".

Please allow me to suggest an alternative:

Please note that this video is positively ancient by KSP standards.  The engines are different--even the Mun is from a time before procedural craters--but gravity still points downwards and this technique is just as good today as when the video was posted.

2 hours ago, MPDerksen said:

Minmus is actually a little trickier, since it rotates faster.  So setting my first retro burn to contact the surface keeps moving by the time I get there, and I end up further away.  This is even more difficult of I'm not on an equatorial orbit.

In this case, it's important to know the equatorial surface velocity.  Corrections for inclined orbits require either mathematical skill or a good eye, but they can be made.

I should note that there's a sort of uncertainty principle at work here:  increased efficiency tends to come at the cost of accuracy.  Unlike quantum mechanics, though, it is possible to reduce the errors with both provided that you practise your piloting ...

2 hours ago, MPDerksen said:

Since I understand the mechanics, now it comes down to practice.

Which you've already realised for yourself.  Good luck!