More Delta V than expected

[Pupu]

Hello,

i am trying to get back from Duna to Kerbin.

This Delta- V map suggests it would only take 110 Delta V. http://i.stack.imgur.com/bNAIZ.png

This wiki says i need 600 delta-v to rach kerbin orbit (from duna orbit): http://wiki.kerbalspaceprogram.com/wiki/Duna

When i set my intercept waypoint i get this:

Thats more than half of my delta v only for intercept. Not even get into orbit at kerbin. Am i doing something wrong?

[ddenis]

Have you considered the variant of crashing straight into Kerbin atmosphere? To make aerobraking or land without going into orbit around Kerbin?

For this you will have to adjust your direction somewhere in the middle of trajectory, but it will require less dV than burning retrograde.

Edited November 20, 2013 by ddenis

[Pupu]

Yeah that was the first plan. The problem is, that the lander cant do that:

I forgot to rearrange the landerlegs after several redesigns. So it would hit the ground with the engines first. And it only has 2 of even landing on the Engines wouldnt work. I want it to get into kerbin orbit and then dock some kind of landerleg-module. I dont wanna give up that first Dunalander ^^

[ddenis]

In this case you still can use aerobraking, but you will have to find such Altitude in atmosphere, which will slow you down enough to circularize orbit, and then you can use rest dV to rise your Pe over atmosphere.

[sgt_flyer]

You can aerobrake both at duna and kerbin (just using the slowdown by drag by dipping low enough in atmosphere to lower your apoapsis to 120 / 150km), then, after your orbit got you back out of atmosphere to your apoapsis, you will need only a handful of delta-v to raise your periapsis out of atmosphere to establish a stable low orbit. (Most given delta-v to reach out atmospheric planets assume you will at least aerobrake into orbit)

[voneiden]

So where is your ship now? Landed or Duna orbit? Ah erm, nevermind. Good altitude periapsis altitude for Kerbin aerobraking is 25-30 km

Edited November 20, 2013 by voneiden

[Superfluous J]

Yeah that chart tells you it takes 110 delta V to get from the very edge of Duna's SOI to the very edge of Kerbin's SOI. If you're in orbit, you need to add that 370. And even that seems low.

Also, that's for a PERFECT transfer at the exact window. From your image it looks like you missed the window and so are using extra dV to catch up to Kerbin after the fact (by dipping your orbit under Kerbin's). Also, Moho's orbit is a little eccentric so sometimes it costs more, sometimes less.

All of these factors affect dV needed, and 800 really isn't that bad considering what it cost just to get into space in the first place

[Pupu]

yeah iam not the best at interplanetary travel yet. I only sent some probes and this lander farther than minmus. So i try to get some fuel to duna and refuel my lander to reach kerbin orbit. i dont think iam ready for aerobreaking into orbit yet. But i`ll try that. Thx for the tipps.

[Bishop149]

Areobraking into orbit is somewhat variable in its difficulty.

For most returns to Kerbin you won't be far wrong with the 25-35km Pe height, if in doubt over do it a bit and then you'll end up on descent anyway which may be what you want anyway. I usually try to areobrake into orbit before descent so I can pick my landing site. . . I prefer landing on land, as close to KSP as possible.

A direct intercept with Laythe however is more tricky. . . . coming in at 6-7000m/s a difference in Pe height of 500m can be the difference between hitting the ground hard at fire and death speed, get into a nice orbit or flying off into space again.

Aerobraking at Duna I wouldn't recommend because the atmo is two damn thin. . . the one time I tried it I set Pe height to 3km. . . . . still needed to thrust to slow down enough.

[numerobis]

I have no faith in that chart; many of the number clash violently with what I calculate, and you almost never want to hop around like the solar system is a subway system -- it's much cheaper to go direct to your destination.

Say you're in a circular orbit about the sun at 20.7 million km (Duna's semi-major axis; not exactly where you are, but close enough for illustrative purposes) and you want to go to 12.6 million km (Kerbin). That'll cost you just over 800 m/s. You also need 900 m/s at Kerbin, but the atmosphere can do most of it for you, no need to burn rocket fuel (maybe ~30 m/s to lift periapsis back out of the atmosphere). This is what you're seeing right now; so I'm going to say the chart is wrong and you're in trouble.

I should note though that you're wasting nearly 600 m/s by being where you are.

To get from LDO to where you are currently (just barely escaped from Duna) should have cost you about 375 m/s. So if you start from LDO, and you do two burns to get to Kerbin, it'll cost you about 1175 m/s.

Say instead you're in low Duna orbit, and you want to get to Kerbin in one burn. Then you only need to burn a smidge over 600 m/s -- as the wiki claims.

[Kulebron]

There's a better Delta V map http://forum.kerbalspaceprogram.com/threads/41652-A-more-accurate-delta-v-map

a launch window calculator http://alexmoon.github.io/ksp/

and aerobraking calculator http://alterbaron.github.io/ksp_aerocalc/

Hope with these three tools you can make the cheapest approach and get into orbit.

[Kulebron]

Aerobraking at Duna I wouldn't recommend because the atmo is two damn thin. . . the one time I tried it I set Pe height to 3km. . . . . still needed to thrust to slow down enough.

It's usually 14 km height. If you went so low and still needed thrust... I guess you were flying past Duna like a bullet.

[Soda Popinski]

There's a better Delta V map http://forum.kerbalspaceprogram.com/threads/41652-A-more-accurate-delta-v-map

a launch window calculator http://alexmoon.github.io/ksp/

and aerobraking calculator http://alterbaron.github.io/ksp_aerocalc/

Hope with these three tools you can make the cheapest approach and get into orbit.

Any significant changes made to aerobraking when using FAR and/or Deadly Re-entry (besides needing heat shields)?

[numerobis]

With FAR, you'll need to go deeper in the atmosphere, since generally drag is lower; with deadly re-entry, you'll die. Apart from that, it's all the same.

That definitely argues for reducing the amount you do with aerobraking, and instead just using rockets.

[Soda Popinski]

With FAR, you'll need to go deeper in the atmosphere, since generally drag is lower; with deadly re-entry, you'll die. Apart from that, it's all the same.

So is it possible / practical to aerobrake with Deadly Reentry? So far I've managed to aerobrake into Duna orbit in vanilla KSP, but do want to try these two mods. Does anybody ever try expending 1/3 or 1/2 their delta-V before hitting the atmosphere first? I've only flown 2 missions to Duna, and on the second one, the Aerobraking wasn't quite enough, so I had to expend a little Delta-V to stay in orbit. I don't see why I couldn't do it in reverse.

Edited November 20, 2013 by Soda Popinski

[Kulebron]

I don't know about FAR. Calculations get more complex with this mod.

With deadly reentry you can try aerobraking a little once, and get from escape trajectory to orbit. Then aerobrake a couple of times more to adjust trajectory. Which is kind of a chore.

[Kulebron]

With Deadly reentry all you need is a piece in front of all the ship (maybe 2x2 plate) to shield everything else.

[numerobis]

The faster you hit the atmosphere, the more energy you deposit in the atmosphere through drag (linearly so: drag grows quadratically with speed, but the time through the atmosphere falls linearly with speed). So, energy-wise, you shouldn't use rockets until at least periapsis.

The ability to survive re-entry is going to add constraints to this analysis.

[Pupu]

ok, i just have to practice this interplanetary travels. my burns rides are very unefficient. i quickloaded several times. i never use the same amount of delta-v. not even close.

but the solution with the refuel ship was succesful. maybe not the smartest and cheapest way, but it worked