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To the Mun and back, for 5060 m/s


camacju
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I'm probably being too petty here but I had someone argue that a rocket with 5200 delta-v cannot go to the Mun and back. It was your standard fare of "It takes 3400 m/s to get to orbit" and "you need 800 m/s to land on Mun" etc, etc. But this is what made me want to actually do this mission:

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So, tac, here you go!

QzJzthf.png

Craft in VAB. 5114 vacuum delta-v and 4510 surface delta-v. There's some part clipping and aero trickery going on here, but no sources of free energy.

Journey to Mun:

Spoiler

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On runway. Low drag means I can fly a very shallow ascent and lose very little delta-v to either gravity or drag.

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"Ascent" is really more just accelerating horizontally.

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I throttle down because of heat and also because apoapsis increases very rapidly here

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Coasting to space

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Circularization. Note that Mechjeb says 2372 m/s of delta-v expended. I'm not quite sure how it calculates this.

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In orbit with 2546 m/s of delta-v. I know that 2500 m/s should be enough to complete a Mun mission from LKO, but the margins might be tight.

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Using a test craft, I note down certain orbital elements. This will be useful later.

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Mun transfer. This series of Mun assists puts me into an orbit similar to the one from the test craft.

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Fine tuning final periapsis

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Deep space maneuver at apoapsis to encounter Mun with lower relative velocity

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And here we see the purpose of that initial test craft. I can capture around Mun for 203 m/s capture burn plus 17 m/s deep space maneuver, for 220 m/s total, as opposed to 280 from a direct transfer.

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Descent over Mun. High TWR means a very effective suicide burn.

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Burning

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At this point, KSP ate a bunch of memory on my laptop so I used Mechjeb for terminal descent. Normally I can hand-fly a descent more efficiently, but lag made it impossible here.

0stekVP.png

Landed on Mun with almost 900 m/s of delta-v remaining. More than enough to get home

Return trip:

Spoiler

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Ascent

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Coast to Ap

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In orbit

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KSP crashed a couple times and some funny stuff happened corrupting my save. I took this opportunity to fly a slightly more efficient ascent.

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Since I have ample margin at this point, I plot a direct return instead of using more gravity assists. However, had I really wanted to, I could have saved around 50 m/s over this.

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Aerobraking

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Chute reefed

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Chute deployed

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Landed back on Kerbin! Margin looks tight, but as explained above, it's probably more permissive than it seems.

Total vacuum delta-v spent is 5114 - 54 = 5060 m/s. (Thanks to @Poppa Wheelie for the correction - 46 m/s as shown in the last screenshot is from sea level).

My best estimate for delta-v actually spent (based on Mechjeb's readings) is 2372 + 70 (ascent and circularization) + 2546 - 54 (vacuum dv spent) = 4934 m/s.

 

 

 

Edited by camacju
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Your aerobraking screen shot shows 54ms of dV at 38km ASL.  You had at least this much (probably a little bit more) before reentry began.  The 46ms you had upon landing is not vacuum dV.  I think your "total vacuum delta-v spent" should be no more than 5114 - 54 = 5060.

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8 hours ago, Poppa Wheelie said:

I think your "total vacuum delta-v spent" should be no more than 5114 - 54 = 5060.

Good catch, I forgot that the Spark took that much of an Isp hit at sea level

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