Mun/Minmus Gravity Slingshot

[figmentius]

A challenge for the true astrophysicists out there: Use the Mun\'s gravity to slingshot a rocket to Minmus orbital distance. The only requirement is that the rocket\'s initial velocity isn\'t enough to reach Minmus orbital distance without the Mun\'s gravity assist. The absolute most hardcore steely eyed missile-man would be able to not only slingshot it to Minmus orbital distance but actually have it lined up to reach Minmus.

[PakledHostage]

This idea has been my puzzle for the last few days. Having flown two previous missions to Minmus, I was looking for a new challenge. I spent some time on the weekend planning and I flew the mission this evening. Here are the results:

I flew a 'grand tour' of Kerbin\'s system of moons. I didn\'t enter orbit around any of them. I just did fly-bys. I planned my trajectory such that I\'d reach Minmus near the descending node. That way I would minimise the out-of-plane manoeuvres required and use less fuel.

Per the requirements of this challenge, my initial TMI burn resulted in a 200.3 km x 11893 km orbit about Kerbin. This resulted in the boys entering the Mun\'s SOI just shy of 5 hours after the TMI burn. The resulting Munar periapsis was within 200 km of the surface.

From there, they were ejected into an inter-moonar trajectory that intercepted Minmus about 17.5 hours later. An orbital trim manoeuvre was required enroute between the Mun and Minmus to lower the Minmus periapsis, but otherwise the original TMI burn was sufficient to reach Minmus. Another burn was required as I neared Minmus periapsis to prevent the mission from escaping Kerbin\'s SOI, and to return it instead to Kerbin.

Interestingly, reaching Minmus this way actually used more fuel than I used during my previous two missions to Minmus. In those missions, I bypassed the Mun and went there directly from LKO. In this case, the gravitational assist from the Mun was sufficient to put the spacecraft on a Kerbin escape trajectory. That energy had to be dissipated with an ~200 m/s burn at Minmus periapsis if the mission was to return to Kerbin. Had I entered Minmus orbit from my inter-lunar trajectory, I would have required even more Delta-V (on the order of 300 m/s). That\'s substantially more than when I\'d previously entered Minmus\' SOI from a highly elliptical orbit about Kerbin.

Screen shots are attached:

Starting with a fresh Persistence file... We really do need an in-game display of UTC/Julian date (or whatever the Kerbal equivalents are).

I burned a bit of fuel setting myself up for the return to Kerbin. I wanted retrograde hyperbolic trajectory with a fairly low periapsis at Minmus.

I think my next project will be to design a Munar gravity assisted trajectory to Minmus that requires a lower net Delta-V (i.e. sum of accelerations and decelerations) than the elliptical transfer directly from LKO. Having thought about it a bit, I think it should be possible with some minor tweaks to this trajectory.

PH.

Edit: Added paragraph explaining what my next project will be.

[HOC]

Nice log there PakledHostage, I might well try to recreate your mission just simply because it\'d be one of the most advanced mission I\'d have ever done. Thanks!

Also, that\'s a brilliantly minimalistic rocket you have there! How well does it handle without sas or rcs of any kind?

[flaillomanz]

Mind if I refine this a little for those - like PakledHostage - who have achieved this?

Everything is the same up to the point where you slingshot towards Minimus, however, after reaching it you must slingshot around Minimus and return to either Kerbin or the Mun. Or, for those of you looking for more, follow these guidelines:

1. Slingshot around Mun to Minimus (as per original challenge)

2. From there, slingshot around minimus, back towards the Mun

3. On this third slingshot, aim to escape the Mun in the direction of Kerbin... with enough velocity to enter the atmosphere and land.

[Ted]

Mind if I refine this a little for those - like PakledHostage - who have achieved this?

Everything is the same up to the point where you slingshot towards Minimus, however, after reaching it you must slingshot around Minimus and return to either Kerbin or the Mun. Or, for those of you looking for more, follow these guidelines:

1. Slingshot around Mun to Minimus (as per original challenge)

2. From there, slingshot around minimus, back towards the Mun

3. On this third slingshot, aim to escape the Mun in the direction of Kerbin... with enough velocity to enter the atmosphere and land.

I might do this..

Seems like it could be quite a bit of fun.

[PakledHostage]

Nice log there PakledHostage, I might well try to recreate your mission just simply because it\'d be one of the most advanced mission I\'d have ever done. Thanks!

There is one rather critical piece of information missing from my write-up that you\'d need if you wanted to re-create my mission: You need to start with a fresh persistence file then be in position in orbit to make your TMI burn at 1:19:23:00 MET. The tricky part is that the Mun must also be cresting the horizon as viewed from your 200 km high parking orbit at that time. The TMI burn needs to start the instant the centre of the Mun crests the horizon. I estimate that the tolerance is about ±1 minutes but I don\'t know for sure because I\'m limited by the accuracy of my Kerbal astronomy.

This plan probably also won\'t work if you use a spacecraft that has a substantially different thrust-to-mass ratio than my spacecraft.

[Kirkula]

This challenge was the first thing I wanted to do once I knew about Minmus! I have a video of me doing it on youtube. Come check it out!

http://youtu.be/FThBJKbXBik

[Tim_Barrett]

Why is the video sped up?

[Kirkula]

lotsa reasons. one is to make the video file size smaller. another is to keep people from being bored to death (the original unedited recording is an hour, after editing was half hour, after speeding up, was 20 minutes). actually, just 2 reasons I guess! lol

[Kirkula]

The video wasn\'t created for this callenge, but I can upload the original video if requested (unedited, and no commentary, that was recorded outside of FRAPS)

[honolululu]

First. This game is awesome! Got in about a month ago and am loving the update ride as well as the game.

Second. I don\'t even know how slingshots work, but I\'d figured I\'d see if this awesome minimal craft posted by PakledHostage could even take me to the Mun. Boy did it ever, but I didn\'t take any initial shots. To answer HOC towards the end of my journey I found out that enabling the CAPSLOCK control with ASAS enabled seemed to help.

First Mun encounter, accelerate at Munrise.

Had to burn a little here to set up the far flung orbit. I guess that\'s a sling shot? Still don\'t see how it works since how hard you\'re pulled down by the Mun on the way down is how hard you\'re pulled down on the way up. Or no?

Minm

Minmus intersection orbit.

Doesn\'t look too good.

Enough fuel left, unbelievable, to flatten the orbit.

After a few go arounds, finally Minmus contact.

Then the wife calls, who is visiting her Grandfather on another island with the kids and this

We\'ve just lost Minmus

A few boosts later and I\'ve got this

Then after orbiting Minmus, I boosted into a relatively low orbit with Kerbin

Then with atmoshperic drag and saving a bit of fuel for the end, landing!

Everything but the lander blew up.

Again. I\'m only here to sing the praises of PakledHostage\'s awesome creation.

[PakledHostage]

I\'m glad to hear that some others are finding my posts in this thread useful and interesting!

I\'ve learned a lot about the design of gravitationally assisted trajectories in the last week and I\'m working on a more efficient and robust algorithm for getting to Minmus via Munar gravitational assist. First hand experience with this kind of thing really gives me an appreciation for the guys who plan the trajectories of real missions! It\'s what I love about KSP.

Anyway, my new trajectory will involve a burn into a 200.3 x 11180 km orbit about Kerbin which will cause the spacecraft to fly by the Mun and arrive at Minmus about 42:35:00 after TMI. It will require a small burn (~7 m/s) perpendicular to the initial orbital plane along the way, but overall it requires lower Delta-V to get to Minmus than a direct transfer from LKO. My new trajectory is also more tolerant of small timing/burn orientation errors than my first draft.

I\'ll post back once I have a chance to fly the new trajectory over the weekend.

Edit: Corrected a spelling mistake.

[nibula]

i did that on my first minmus go but didnt make eny pictures

[sneakeypete]

People might find it very handy to change the CONIC_PATCH_LIMIT value in the settings to 5 or 6. That way you can see things like this:

click for enlarged image

[PakledHostage]

People might find it very handy to change the CONIC_PATCH_LIMIT value in the settings to 5 or 6.

Kerbal physicists theorise that there are thousands of parallel Kerbal universes, with more being created every day. In my Kerbal’s parallel universe, the map view is displayed on the wall at mission control. The trajectory on it is computed from data acquired from ground radar stations and onboard IMUs. The data isn’t accurate enough to usefully predict an orbital trajectory beyond the next celestial body encounter. The trajectory display has to be updated as the mission progresses and orbital trim burns need to be accomplished to keep the mission on course. (This is the same reason I don’t use MechJeb; it provides more accurate orbital parameters than could be determined from the data that my Luddite Kerbal’s instruments can provide.)

My Kerbal’s civilisation is also rather cash strapped. While they value space exploration, they don’t have the financial resources to fly large numbers of missions. And public support for the space program would dry up if their Kerbal astronaut heros were always being killed in mission failures. I\'m kinda chuffed that, while I have only flown four missions to Minmus (one to Minmus orbit, one to Minmus landing and two fly-by missions), I haven’t aborted or lost a single Minmus mission. Careful planning has made this possible.

The objective for this fourth flight to Minmus was to fly a “grand tour†of Kerbin’s system of moons and return to Kerbin with minimal fuel burn. Starting with a fresh persistence file, the plan was:

- Wait in a 200 km high circular parking orbit until 1:19:39:00 MET.

- While waiting, adjust the orbit so that the centre of the Mun’s disk crests the horizon as close as possible to 1:19:39:00 MET.

- TMI burn at the instant that the Mun’s disk crests the horizon. Burn at 100% throttle to 2871 m/s at 201.5 km. This results in a 200.3 km x 11180 km orbit about Kerbin that intercepts the Mun’s SOI.

- Trim the trajectory for a 295.6 km Munar periapsis.

- Munar periapsis at 2:00:52:00, at 615.5 m/s.

- Munar gravitational assist results in a 7753 km x 54964 km orbit about Kerbin

- Minmus fly-by at 3:14:14:00

This trajectory relies upon the Mun and Minmus being separated by an angle of 77.7 degrees at the time of Munar periapsis. An error of ±7.5 minutes here results in an error of ±1 degree, which would have to be corrected with an orbital trim manoeuvre. Given that my Kerbal astronomy is probably only accurate to this tolerance, I figure this degree of error is inevitable.

Here are the screen shots:

Arriving at the Mun 6 minutes 35 seconds earlier and about 7 m/s faster than planned.

Arriving at Minmus 34 minutes earlier than planned.

The mission expended aprox 70 m/s Delta-V between the end of the TMI burn/trim and re-entry. The TMI burn was sufficient only to yield a 11180 km Kerbin apoapsis. About ½ of the Delta-V used post-TMI was required to lower the Kerbin periapsis to a sub-atmospheric altitude after passing Minmus. Presumably this Delta-V is sufficiently small that it could be achieved with the EVApacks™ that are being proposed for v0.16… Maybe Jeb is has an interesting EVA trip in his future? (What are you doing? Jeb?)

Edit: Fixed broken links to screenshots.

[HOC]

while I have only flown four missions to Minmus (one to Minmus orbit, one to Minmus landing and two fly-by missions), I haven’t aborted or lost a single Minmus mission. Careful planning has made this possible.

I decided a while ago to try and roleplay a little more in the game. It is my personal aim to never again lose a crew, and to put myself more into the shoes of a flight director. Albeit one that doesn\'t have (or need!) a whole mission control to help him make decisions.

Since that decision was made, I\'ve flown about 8 flights and haven\'t lost a single Kerbal. It involved aborting a mission to the Mun due to fuel levels looking too low to complete the mission profile, a mission that took 20 minutes of my time to set up.

It\'s great, I\'m enjoying the game more than ever.