Did first interplanetary trip, have some questions in order to improve

[phemark]

Understood now, thanks:)

In one of my tests yesterday, I exited Kerbin's SOI and orbited the Sun. Then I was looking for a point where I would need 130 deltaV to reach Duna - never happened Now I understand why:)

[KerikBalm]

Those staged burns would be called "perapsis kicks", and each one would have to be done exactly at perapsis to avoid changing the perapsis (of course, they don't need to be exact to get it to work, close enough is good enough)

Regarding #4, one person replied:

"4) The most efficient way is a suicide burn. Just drop down and burn at the very last moment, so that you lost all your velocity when you reach the ground."

What you want to avoid here, is gravity drag. Imagine hovering using your rocket - zero change in velocity, all your fuel used. Every second you spend fighting gravity, is lost dV equal to the gravity's strength. So you want to burn pointing at the planet as little as possible - ie at apoapsis to lower your perapsis to just above the surface, at perapsis to kill your horizontal velocity, and then right before touch down to kill your vertical velocity (or do this in one continuous retrograde burn).

You don't even need mechjeb.

What I do, is set up a perapsis just below the surface. Then I eyeball a manuever node somewhere low but before the trajectory intersects the surface, and drag the retrograde pointer until the post maneuver elispe looks more like a straight line (ie, a very very very high eccentricity, so it is like two mostly straight, parallel lines segments, joined at the end).

If the estimated burn time is 1 minute, I start burning at 30 seconds from the maneuver node - (note, I make sure the estmate dburn time is more or less accurate with a brief full throttle pulse prior to the main burn).

Since the maneuver is meant to kill my horizontal velocity, and it assumes a constant acceleration, a 1 minute burn, 30 seconds before the node, will stop you before the node, because by the time your burn finishes, your average velocity has been half of what it was at the start of the burn (ie, due to your deceleration it took a minute to get there, not the 30 seconds it would have taken before the burn) - but since you use up fuel, your acceleration actually increases throughout the burn, for an additional safety margin. Lastly, its the velocity relative to the surface that you want to reduce to zero, but I set up the manuever node to get the "orbit" velocity to zero, and of course, I picked a node above the surface.

Combined, these mechjeb-less suicide burns give me plenty of margin for error

[Starstrider42]

Understood now, thanks:)

In one of my tests yesterday, I exited Kerbin's SOI and orbited the Sun. Then I was looking for a point where I would need 130 deltaV to reach Duna - never happened Now I understand why:)

There's actually a disclaimer about burning at periapsis in the lower right corner of the map. But yes, the key point to remember for any interplanetary transfer that if you add delta-V to your (low orbit) escape burn, it will disproportionately increase your speed in interplanetary space, far more than if you did a burn *in* interplanetary space. More mathemagical details at http://en.wikipedia.org/wiki/Hyperbolic_trajectory#Velocity, if you want them.

As other people have said, for actual maneuvers it helps to use an interactive calculator (be it Alexmoon's, MechJeb's, or another). That has the benefit of helping you nail both the timing and the burn direction. The delta-V maps are more useful for deciding how much fuel include in your spacecraft design.

Edited April 9, 2014 by Starstrider42

[Sirrobert]

I also want to give a final advise, asuming you have no further questions:

All the confusion you had, and may still have (such as how to exactly use the deltaV map. You probably won't really understand until you see it in practice), will eventually fade as you gain experience.

Eventually, it'll start becoming easy.

Because of this, Mechjeb can actually be a great teacher. If you are confused on how something is done, just let Mechjeb do it, and watch HOW he does it. After a couple of times you'll start to understand why, and how. And eventually you'll just take over and do it just as easy as the computer does.

That's how I learned landing and docking

[phemark]

Thanks guys, really appreciate your time in helping me.

Will now (try to) return from Duna and then will start learning docking.

Changing to "Answered".

[KerikBalm]

I'd say learn docking first.... orbital rendevous interplanetary missions are just so much cooler and more fun (at least for me) - although it can become much more difficult if your interplanetary transfer was done poorly, and you arrive in something more like a polar orbit instead of equatorial... that makes the orbital rendevous for the return mission so much harder (the timing is harder, and the launch windows for the rendevous are much less frequent)