Most Efficient Duna Ascent

[davidpsummers]

I have Jeb on the surface of Duna. (Yeah!) The possible fly in the ointment is that fuel is tight. He has 1788 m/s and the listed requirement is 1300 m/s. So one is keen to be as efficient as possible doing this. Any orbit is fine (the oribiter can come get him) so I just need to get above 50km. So what is the most efficient acsent?

I'm landed at 3,500 km altitude. That is about the same as ~9,000 km on Kerbin? So I should pitch over something just under 45 degrees on launch? I'm also thinking that since I don't have any velocity at that point, compared to the same pressure on Kerbin, I should have a higher thrust? Maybe a TWR off 2?

Then as usual, I just pitch over a bit more as I get higher, get my apoapsis high enough, and circularize?

Edited October 10, 2015 by davidpsummers

[Randazzo]

The most efficient profile is always the flattest possible trajectory. On airless worlds, you only need to go high enough to clear ground obstacles (such as, you know, mountains.). In the atmosphere you have drag to contend with as well, but the atmosphere on Duna is extremely thin. The last time I lifted off from the surface of Duna, I was pointed at the horizon by 20k and it took a lot less than I expected (definitely less than 1800m/s).

[Ripper2900]

The analogy with 45 degrees sounds fine to me, but note also that Duna's atmosphere runs out quicker than Kerbin's. So an even lower angle is better, provided you have the twr, which in your case, you do.

[A_name]

So it's no secret that the most efficient ascent for an atmospheric body is to perform a gravity turn. I have a pretty well established ideal ascent profile for Kerbin, so what I do on other atmospheric bodies (because I'm too lazy to look up the ideal launch profile) is to use the atmospheric density gauge at the top of the screen (right below the altimeter) to approximate my profile as much as possible to what I would do on Kerbin. So for example, the first blue bar on the atmosphere gauge ends at around 8km on Kerbin. If I was launching on Kerbin, I would be pointing around 60° at that time. So to lift off from Duna, I'll try to be pointing at 60° when I reach the altitude at which the first blue bar ends. I don't need to know how many km up that is, I just need to keep an eye on the blue bars. Similarly, on Kerbin I'll be going pretty much horizontal by the time the second bar ends, so I'll try to do the same on Duna.

[Laie]

the atmosphere on Duna is extremely thin.

On my last visit (1.04) I found the atmosphere to be surprisingly thick, at least in it's upper reaches. There was considerable drag when moving at near-orbital speed in the upper atmosphere, a lot more than in a similar situation on Kerbin. So my recommendation is to make the ascent steep enough and/or aim for an apoapsis well above the atmosphere.

[Randazzo]

On my last visit (1.04) I found the atmosphere to be surprisingly thick, at least in it's upper reaches. There was considerable drag when moving at near-orbital speed in the upper atmosphere, a lot more than in a similar situation on Kerbin. So my recommendation is to make the ascent steep enough and/or aim for an apoapsis well above the atmosphere.

Odd, I was there not long ago with a Lithobrake Exploration Systems lander pod, and I imagine that's quite draggy. Didn't experience (or notice) much at all once I got out of the 15-20 range.

[davidpsummers]

Thanks. So I used ~50 degrees on take off. TWR about 2-2.4. I was maybe 30 degrees at 11 km and raised my TWR to 3. I think I came up a bit too shallow. I was at 20 degrees at 25 km when I reached the desired apoapsis and cut the engiens. From the losses as I coasted up to 50km I think I lost ~50 m/s to drag from there. Maybe a bit more. OTOH, I made it up in less than the dV chart said. 1200 m/s vs. 1300 m/s.

I was worried that my actually performance would relate to the theoretical value like it does on Kerbin, where I always need 30% extra, which would have made it tight and given little room for error. But clearly Duna isn't Kerbin. It's like all red and stuff.

[Tatonf]

Duna's acceleration is 2.94 m/s^2 so as long as you don't lose that much due to air resistance it's better to go horizontaly than verticaly.

Since Duna's atmosphere is quite thin and the orbital speed quite low it's maybe better to accelerate the gravity turn compared to Kerbin.

But on the other hand, Kerbin's acceleration is much higher so the need of doing it is more important...

I don't really know what the perfect ascending profile looks like actually. It requires some testing I think.

[magnemoe]

Duna's acceleration is 2.94 m/s^2 so as long as you don't lose that much due to air resistance it's better to go horizontaly than verticaly.

Since Duna's atmosphere is quite thin and the orbital speed quite low it's maybe better to accelerate the gravity turn compared to Kerbin.

But on the other hand, Kerbin's acceleration is much higher so the need of doing it is more important...

I don't really know what the perfect ascending profile looks like actually. It requires some testing I think.

Depend a bit on your lander, is it aerodynamic or an pancake. do you use pure vacuum engines as Poodle, 909 and LV-N works well on Duna.

if so treat it as an Kerbin launch but with an faster gravity turn.

Something aerodynamic with all round engines I would treat it as vacuum launch.