Least deltaV to the Mun

[numerobis]

The challenge is simple: take a spacecraft from a 71-km Kerbin orbit to an 11-km Mun orbit for as little delta-V as possible.

Why? Because I'm curious how much better can be done than the usual straight shot. The bi-elliptic should save a bit, perhaps with gravity assists. Perhaps there's tricks I don't know about.

How to play: The easiest way to get a spacecraft into the requisite starting orbit is to go into your persistence.sfs and replace the ORBIT section of a vessel with the following:

Spoiler

                       ORBIT
                        {
                                SMA = 671000
                                ECC = 0.0001
                                INC = 0
                                LPE = 0
                                LAN = 0
                                MNA = 0
                                EPH = 0
                                REF = 1
                        }

 

When you get to Mun, your target orbit must have Ap less than 11km and Pe above 10km.

Score: how much delta-V did you use up? Demonstrate that by showing the spacecraft resources panel at the start and end, as well as mentioning the Isp of your engine.

Requirement: tell us what burns you did.

If you want to use autopilot mods, go for it. Tell us what settings you used. Any mod that properly models physics is fine in fact.

Preferably don't use RCS or a mix of engines, because it's going to be hard to account deltaV accurately if you do. You really don't need much: a probe, oscar, and ant are more than enough (oh, don't forget solar panels).

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Here's my initial submission: 1181.31 m/s by doing the typical two burns:

1. When Mun rises, burn prograde until we get an intercept. Keep burning until the intercept falls to under 11km.
2. When I reach Mun Pe, burn retrograde until the orbit circularizes.

It should be straightforward to beat that -- that was entirely by hand!

 

 

Edited July 24, 2016 by numerobis

[dire]

Delta-V is a little tricky because if you make a smaller, more efficient craft, you just wind up with more Delta-V. I seem to pretty consistently use 850-870 delta-V for munar capture, another 200 or so to slow down to high munar orbit, and then probably another 200-300 for the orbit you're describing (which is close to the orbit I use to de-orbit and land). Then another 600 (but leave yourself 700) to actually land.

One trick I hardly ever use, is that if you only burn out to about 9-10m AP in just the right spot, the mun will grab you and reverse your orbit; I've had one case where I did that and it captured me into high Munar orbit, but I don't know that I could duplicate that. That trick would save you a couple hundred delta-V though.

Edited July 24, 2016 by dire

[numerobis]

How efficient your spacecraft is has little bearing on this challenge -- that's why I made it be about deltaV to the Mun, not fuel usage or cost. Whether you fly a whackjobian monstrosity or a tiny ion-powered probe doesn't affect much.

The cost to transfer to Mun is normally 868 m/s, and capture directly into low lunar is 278 m/s, 1145 m/s all told. By flying it manually I wasted about 36 m/s -- not bad actually, I impress myself!

If you go to high lunar orbit you can capture for 200 m/s or so -- saving 70-80 m/s. But then for this challenge (or, not coincidentally, for a landing) you need to transfer to low lunar, which loses you all the savings and then some. That's just the Oberth effect speaking up.

[PLAD]

Oh my, I did a mission very close to this some time ago when I was trying to get a Kerbal-X to the Mun's surface and back without refueling (which I demonstrated was impossible in 0.23.5). The thread is here, or you can shortcut to the Imgur album here. I tried to show every manuever with Mechjeb's dv counter so that others could copy the path. This mission had a parking orbit in LKO of 71.0x71.1km, I figure that gives about a 0.2m/s advantage over starting in an exact 71x71. Unfortunately I braked into a 6.3x8.8km Munar orbit, I went as low as safely practical to get that maximum Oberth effect for the landing. The key parameter is to note that the V-infinity when entering Mun's SOI was 129m/s, this cost 207.8m/s into my 6.3x8.8, I figure it would cost 206.3m/s to get from a Vsoi of 129m/s into a circular 11km orbit, so this mission cost a bit more than the one specified in your challenge would cost when using the same path.
Summary of this method:
LKO start boost: 845 m/s.
Total of course corrections during first three Munar flybys: 3m/s
Deep Space Manuever: 13m/s
Braking into 6.3x8.8 Munar orbit: 208m/s.
Total: 1069m/s.  However it took 326 hours!  I guess this cannot be a formal entry because of the low Munar orbit, but perhaps it can give people ideas. The only way I can think of to reduce it is to add another Munar flyby or two before braking into Munar orbit, but that would not save much and the course correction costs would make it iffy. I kind of don't want to fly this again because it took a lot of time to execute. Setting up that DSM is tricky as I note in the album.
That mission continued to a Munar landing for 575m/s, 584 to get into a 9.3x9.3 orbit, (refuel), 205m/s Munar departure (heading out from Kerbin), 4m/s course corrections, to another DSM of 20m/s and on to Kerbin landing. 2457m/s for the whole LKO-Mun surface-Kerbin surface.

[numerobis]

Sweet! I should try replicating your success.

But it shows how little there is to save: I eyeballed a flight at 1181, you did multiple assists to achieve 1069.

I bet with a higher landing site and very slightly more optimal landing and liftoff, you'd have made it. You were so close!

[PLAD]

It's fun to run the numbers to find a theoretical minimum. If you start in a 71x71km orbit around Kerbin, I believe an 842.2m/s boost is the absolute minimum to get you to an orbit that just touches the inner edge of Mun's SOI (= Kerbin apoapsis of 9571km). Let's then assume that some combination of flybys allows you to finally enter Mun's SOI with a Vsoi of 0m/s. (One might point out that you can't enter the SOI unless you are moving relative to it, so let's assume it's a number arbitrarily close to zero but not zero). Using the vis-a-vis equation and knowing Mun's SOI is 2429.56km in radius tells us that the resultant speed over Vcircular at 11km altitude would be 195.3m/s. So the absolute theoretical minimum for getting from a 71km Kerbin orbit to an 11km Mun orbit would be 842.2+195.3= 1037.5m/s. In practice course corrections, DSMs and the fact that you need some sideways motion at Mun's SOI to 'miss' it by 11km will always add to that but I cannot think of any way to beat that number.

And if instead of entering an 11km orbit around Mun, one was to arrange to pass over the highest point on Mun (7.061 km) and do an explosive suicide burn at that periapsis it would take a 758.7m/s burn, so the theoretical minimum for going from a 71km Kerbin orbit to the Mun's surface is 1600.9 m/s. (Since the high point is near the pole its surface velocity is roughly zero.) Edit-actually it is cheaper to go for the 6000m mountain near the equator, the lower surface requires about 0.8m/s more for braking to zero, but the Mun's surface motion of 9.0 m/s give an overall benefit of 8.2m/s, so KLO to Mun surface for 1592.7m/s.

And you are right, it is a lot of effort for relatively little gain to squeeze these last few m/s out of the mission.

Edited July 27, 2016 by PLAD

[herbal space program]

13 hours ago, numerobis said:

But it shows how little there is to save: I eyeballed a flight at 1181, you did multiple assists to achieve 1069.I bet with a higher landing site and very slightly more optimal landing and liftoff, you'd have made it. You were so close!

I think there's not much to be saved because the only parameter that can really be optimized here is the dV required to get captured into a Munar orbit with the Target Pe and its Ap near the edge of the SOI. Going from LKO to the edge of the Munar SOI is basically a fixed quantity, as is what it takes to drop your Ap from the edge of the SOI to target height (edit: as I see now @PLAD has just thoroughly explained). If however you were to extend the part of the mission covered by the challenge forwards or backwards, I think there would be a lot more room to get creative. Measuring total dV required from the ground instead of LKO would greatly expand the possibilities for economizing, as would extending the mission forwards to an actual landing. I understand of course if these are not the questions you're interested in, but for my part I think a surface-to-surface minimum dV challenge could be quite entertaining.

Edited July 27, 2016 by herbal space program