Difference between revisions of "Talk:Tutorial:How to get to Duna"

Latest revision as of 17:26, 27 March 2019

Launch to Kerbin Orbit

I'm a little confused by the following in step 3, launching into Kerbin orbit:

…heading east instead of the usual west.

The normal ascent to orbit from KSC is going east, taking advantage of the small amount of rotational velocity you start out with from Kerbin's own rotation, into a counter-clockwise orbit (this is also mentioned just after the above quote). So, either step 3 is incorrect in some way, or it needs more explanation to clarify the intention.

I'm not claiming expertise here, but I'm struggling to see why you'd do anything other than a standard east / 090° ascent from KSC, at least as your initial launch to orbit when keeping it simple. An advanced departure might try to do something like matching the solar orbital plane, I guess, but that's easy enough to do either as part of the later Hohmann transfer, or fine tuning the intercept once in solar orbit. Matching Duna's plane wouldn't account for an opposite launch.

So, should this just be a standard launch to orbit heading 090°, or what am I failing to see here?

--Murph (talk) 02:30, 23 November 2013 (CST)

I had the exact same question and came to discussion to see if there was a discussion/explanation. I'm going to edit it; if we're missing something, please do add a clarification while reverting.

Kenberto (talk) 02:05, 14 January 2014 (CST)

Edit: Very helpful instructions. Just wanted to say that the numbers described are accurate only if your orbits and alignments match up, which isn't exactly possible without a way to measure the angles and accurately drag the maneuver nodes into intended positions. I think the angles/clock references mean the sun as the center and the planet of origin as the 3 o'clock position for alignment of planets, and the planet of origin as the center and the sun as the 9 o'clock position for when to make the burn. When using maneuver nodes, I had to drag them back and forth in the general area to find a good balance of delta v and time to intercept. I've also found that making course adjustments as soon as possible to get the desired periapsis saves the most fuel. The maneuver node seems buggy and wont appear for me until the ship has escaped the planet of origin. I adjust all 3 axes to try to get the best periapsis. Sometimes I have to make a 2nd adjustment because the first one's periapsis changed by itself midway into the transfer. The RCS was helpful with making tiny adjustments, especially at the end of the burn. I also found that if my velocity into Duna was higher than ideal, I had to get my periapsis on Duna aerobaking to as low as 11km in order to produce an apoapsis. As the instructions stated, 10k and below seems to brake into a landing. Username is Hodor, October 15, 2014

Hello Hodor, first of all you can sign your posts by adding --~~~~ (or the pencil like icon in the bar above the edit box). Now yes the planetary alignment is seen with the Sun in the center. Also your observations are correct: If you do orbital maneuvers lower you save fuel due to the Oberth effect. And KSP only shows orbital predictions after a certain amount of sphere of influence changes. Usually this is about 3, so after three changes it doesn't predict the orbit anymore, which would be after leaving Kerbin's SOI with crossing Mun's or Minmus's SOI (enter moon + leave moon + leave Kerbin). There is a setting in a configuration file which you can edit manually to change that.

Also yes burning on the far side of your current body to reach a target farther out is the way to go (more or less). This applies to all transfers: So you'd burn on the night side to get to Duna, Dres, Jool and Eeloo and on the day side to get to Eve and Moho. But the same happens when you want to transfer from Mun to Minmus: You need to be on the far side (which isn't necessarily the night side for moons). This is so, because then you can use the velocity of the moon/planet and “just” need a bit more to leave it. But generally you need to burn before reaching the closest/furthest point.

Now obviously the numbers are usually the best case (plus a bit tolerance) because there isn't really a worst case (or at least not realistically) which would be to fly directly at your target. — xZise [talk] 04:10, 16 October 2014 (CDT)

Delta-v budget

The delta-v budget seems inaccurate and even internally inconsistent. The inaccuracy might possibly reflect just being out of date; the internal inconsistency is a bit harder to figure. First, it cites 4700 m/s for ascent to Kerbin orbit. That's pretty pessimistic. I normally budget about 4000 m/s, which actually leaves me quite a bit to spare; I see that the wiki page on Kerbin claims that it can be done with 3400, though I'm not sure I could do it with that little. This number might be just out of date.

For return from Duna orbit to Kerbin the budget shows 500 m/s, which won't even get you to Duna escape. Duna escape needs somewhere around 800 m/s. Again, that might just be out of date.

But the most confusing part is the description of building the upper stage. It talks about needing 2000 m/s in that stage, that's at least consistent with the numbers for ascent plus transfer (1500+500), though too low in current reality. But then it describes this as being "almost capable of reaching Kerbin orbit". I don't know how to interpret the "almost", certainly 2000 isn't "almost" 4700, to use the figures from the prior section. And I wouldn't call 2500 almost 3400 either, if I use a more realistic upper stage requirement and a very optimistic number for Kerbin orbit. Heck, my current Duna rocket upper stage uses a Terrier engine for its light weight and high ISP. The Duna gravity is low enough and atmosphere thin enough that the Terrier can do the trick. It wouldn't even be able to budge from the launch pad on Kerbin; that's a pretty far cry from almost reaching orbit. --Rmaine (talk) 17:26, 27 March 2019 (UTC)rmaine