Moho...A step too far

[Smallish]

Well, 4 brave souls were lost today.

I have no idea what mistakes were made, but it seems that Moho is not to be taken lightly...

I wonder what I could do better, perhaps someone can tell me where I failed?

I waited in orbit over Kerbin for Protractor to get to around 1 hour to zero hour for Moho with around 13kDv in the tank and hooked up to a lander with all intentions to grab some science.

I left heading in Kerbin retrograde direction to try and decrease my Kerbol orbit speed as much as possible.

I waited until I left Kerbin's gravity well and then fired up retrograde until my Kerbol orbit was brushing with Mohos.

After this I waited until I was at the Moho descending node and made my anti normal burn to 0.0 out the planes. This was the start of my woes, it took something like 3kDv to level off with Moho, maybe more...

I then did another correction to intercept with Moho, but it was really odd, Moho was way off, I had to make another 4kDv correction or so to get an intercept..

After this, as I was approaching the planet, I could see that my velocity was going to be VERY fast, as I hit the gravity well I did a quick node and saw I needed 6K to make orbit!

Obviously it wasn't going to happen, I wasn't going to make orbit, land the lander and make it back to Kerbin, so I decided to head straight back to Kerbin - which I screwed up badly also, it ended with me sending a rescue ship, and with that and the original ship being humanley terminated.

So I would really like to know...why didn't my intercepts match up correctly after the plane change, did I do something wrong (as written above) on leaving Kerbin do you think, and how much Dv do you need for a return trip to Moho?!

[regex]

I've never used a mid-course plane change on the way to Moho, I always use a direct burn plus correction after leaving Kerbin SOI. That method takes roughly 2.5km/s all told, plus braking delta-V (at least 2.5km/s). Even without mods this method would have worked out better for your trip (6~7 km/s sloppy best, I bet).

If you still want to do a plane-change, a better way to do your initial burn is to burn prograde (your ship's prograde) on the inside of Kerbin's orbit (sun side, Kerbin retrograde), which will put you inside Kerbin's orbit initially. You can even keep dropping your periapsis down to Moho orbit before stopping the burn. Then plane-change as normal once out of Kerbin SOI.

[capi3101]

Glad I gave maccollo reputation or I wouldn't have found his post on the topic of getting to Moho again...

http://forum.kerbalspaceprogram.com/threads/61478-Oh-bugger-Injection-burn-at-Moho?p=835667&viewfull=1#post835667

Kashua's post two down from maccollo's is worth considering. The sat in my signature took ~8000 m/s for a one-way trip.

I've had some real issues with Protractor over the last few versions of KSP. It's still useful for determining ejection angles for the most part - phase angles not so much.

Edited January 13, 2014 by capi3101

[Smallish]

Thank you very much for the comments, Maccollo's writings are very helpful too!

I think protractor is indeed a bit iffy at times, I have had a couple of odd readings I think.

Sorry that I didn't take more time to search for the previous thread also!

[Tank Buddy]

I waited until I left Kerbin's gravity well and then fired up retrograde until my Kerbol orbit was brushing with Mohos.

From what this sound like, you waited until you left Kerbin's SOI to finish your transfer orbit burn. To make use of the Oberth Effect, you should do your burn at once in Kerbin's SOI. If you were using a mod to help you with launch windows, you shouldn't have had to do any retoburns out of Kerbin's SOI until the final adjustments.

All I could recommend are the following: http://alexmoon.github.io/ksp/ and http://ksp.olex.biz/

P.S. Using the phase angle, I used a real life protractor to make the appropriate angle, with Kerbol at the center. I then could set Kerbol as the focus in map view, and put the paper on my monitor to find the exact launch window. Also, you will not have to preform the inclination change if you launch from Moho's ascending or descending node.

[Smallish]

From what this sound like, you waited until you left Kerbin's SOI to finish your transfer orbit burn. To make use of the Oberth Effect, you should do your burn at once in Kerbin's SOI. If you were using a mod to help you with launch windows, you shouldn't have had to do any retoburns out of Kerbin's SOI until the final adjustments.

All I could recommend are the following: http://alexmoon.github.io/ksp/ and http://ksp.olex.biz/

P.S. Using the phase angle, I used a real life protractor to make the appropriate angle, with Kerbol at the center. I then could set Kerbol as the focus in map view, and put the paper on my monitor to find the exact launch window. Also, you will not have to preform the inclination change if you launch from Moho's ascending or descending node.

Thank you, I have used ksp.olex before, but "upgraded" to Protractor although I feel like I've lost a bit of connection to what's happening a bit now. I will still use Protractor for now but double check the data before leaving..

I didn't realise that you could hit your main burn straight from orbit. The way the vectors with the encounter vectors look, it seems like you should wait...I have always cleared the SOI and then made my main orbit burn. Clearly I've been making some mistakes there! I'm not sure how I completely managed to ignore all the talk about using the Oberth effect, live and learn though : )

[Kasuha]

I have always cleared the SOI and then made my main orbit burn.

I was using this approach earlier and I don't find anything really wrong on it. I even visited Moho this way successfully. If nothing else, it allows you to place the maneuver on the trajectory and fiddle with controls and its position until you get a decent transfer without using any other tools or window planners. The only downside is that it costs about 30% more dV when transferring from Kerbin. Which does not necessarily have to be an issue.

What often helps a lot is to avoid separate plane change and merge it with either ejection or injection burn. Or both - that's what Alexmoon's "ballistic" trajectory gives you anyway. Good thing about using this maneuver is, you get your intercept right after the first burn so you know you've done it right. And it is the optimal transfer 90% of the time.

I definitely recommend Alexmoon's calculator over the other one, too.

[Bengsch85]

Both Protractor and launch window calculators figure their numbers out based on a single burn. If you wait till after SOI change to make the main burn you will miss the window.

[Kasuha]

Both Protractor and launch window calculators figure their numbers out based on a single burn. If you wait till after SOI change to make the main burn you will miss the window.

Um... no, you will not. Not even for Moho. It's not that narrow.

[Bengsch85]

Um... no, you will not. Not even for Moho. It's not that narrow.

I'll reiterate, it can still be done but not for the dV price you were expecting. I've tested it both ways and for outer planets its manageable but for Moho it can be the difference between a 5000 m/s and a 20000 m/s trip.

[EtherDragon]

Step 1 - The transfer:

To land on Moho you really have to be at the top of your optimal Kerbin-Moho transfer game. Suboptimal transfer courses literally cost thousands more Dv to pull off.

Try using a tool like the Interactive illustrated interplanetary guide and calculator for KSP

Step 2 - Mid course correction and encounter:

To land on Moho you have to make a really optimal mid-course correction to get your periapsis around Moho as low as you can during your encounter. Try waiting until you are about 10x the distance away from Moho's SoI to make this correction. Burn Radial-In (the circle with four pips pointing in) to bring your Periapsis down low. Moho has no atmosphere, so no aerobraking is available.

Step 3 - Capture:

Get ready for a long burn if you are using nuclear engines - although the blue maneuver pointer will say "go that way" burn directly retrograde instead at the time indicated; this is the most efficient way to apply thrust (and you need all the efficiency you can get!) since a fair bit of your burn will be outside of the optimal Oberth effect range. Remember, you are at the fastest point in your Sun-centered orbit and will be whizzing by Moho at nearly ludicrous speed.

Step 4 - Return:

Get ready for a trick...

You don't need enough Dv to get all the way back to Kerbin, you only really need enough to get as far as Eve, from Moho. It may take some waiting, but look for a time that allows you to use Eve as a gravity assist to get to Kerbin; this will save a ton of Dv.

Step 5 - Capture / reentry:

In Kerbin's SoI, burn Radial-In to get your Periapsis to about 25km, which should be plenty deep enough for full reentry.

Good luck! (Hey! You should capture it on Video and post it!)

[ArmchairGravy]

I just failed at a Moho mission myself. I thought I had enough for return, but didn't even have enough to make orbit. Ran out of gas with a velocity of 750 m/s, so I was close! That 7 degree plane difference...

[nimbim]

Inclination changes at the furthest point of your orbit. Making an inclination change close to the sun will waste your fuel just like you saw.

[Smallish]

Ok, I managed to pull off a visit yesterday. I got my lander down, took the science back to the inter planetary ship to transfer the data and got back to Kirbin with Dv to spare. I took a lot of fuel though...Before Leaving I was in Kirbin orbit with around 13kDv with the Lander attached (that would be about 16k-17Dv without the lander).

I used Macollo's technique posted earlier, although I missed the ideal time and couldn't get the full inclination change, I had to make a 0.3 correction on the next node in mid sun orbit, but this was fairly small in Dv. Getting the main inclination burn done with the Kirbin escape burn helped a lot.

I have taken to looking at my encounters way out and seeing what time my entering and leaving time is, for Moho it was really short, something like an hour or less, I can't exactly remember, but I tried fiddling with my mid orbit burn to get it as long as possible, either way, it was going to be a long burn - again, I can't remember but I'm pretty sure it was something around the 3.5kDv (with a heavy lander).

The orbit/landing/take off/rendezvous and science data exchange all went well, I dumped the lander and then headed home. I made the burn for Kirbin from a similar position to the leaving Kirbin setup, with the Moho Apoapsis lining up with Kirbin on the far side and got an encounter straight off. Again I managed to take off most of the inclination on the initial burn and had only to make a 300 Dv or so burn mid way. Of course the final kirbin burn was long, but I had plenty of fuel left.

Thank you for your advice.

I'm learning slowly...very slowly : |

(just over 900 science)

You can right click and copy the links from the broken images below if you wish to see my ship setup

Hmm, maybe you can't copy and past from the broken images.. so here:

http://www.flickr.com/photos/101998334@N02/11961768213/

http://www.flickr.com/photos/101998334@N02/11962309846/

http://www.flickr.com/photos/101998334@N02/11962310606/

http://www.flickr.com/photos/101998334@N02/11961768433/

http://www.flickr.com/photos/101998334@N02/11961914464/

Edited January 15, 2014 by Smallish

[maltesh]

I was using this approach earlier and I don't find anything really wrong on it. I even visited Moho this way successfully. If nothing else, it allows you to place the maneuver on the trajectory and fiddle with controls and its position until you get a decent transfer without using any other tools or window planners. The only downside is that it costs about 30% more dV when transferring from Kerbin. Which does not necessarily have to be an issue.

If you leave aside the mid-course plane change, the costs of coasting to the edge of the SOI, then doing the transfer burn in interplanetary space are about 75% higher for nearby destinations like Eve and Duna, and about 100% higher for distant destinations like Moho or Eeloo over a direct burn from LKO.

http://forum.kerbalspaceprogram.com/threads/61387-Kerbin-SOI-versus-Sun-SOI?p=834183&viewfull=1#post834183

[The_Rocketeer]

A neat trick u might try is using the gravity of Eve and Gilly to slow down and drag your trajectory down to Moho's orbital altitude. This'll save you a lot of fuel if you set your maneuvers up right. Matching planes with Moho isn't necessary as long as you can make an intercept at a sensible velocity - remember Moho's SOI is tiny, so u don't have a lot of time to drop the anchors once you're inside. Try to slow down plenty before you get there.

[Smallish]

A neat trick u might try is using the gravity of Eve and Gilly to slow down and drag your trajectory down to Moho's orbital altitude. This'll save you a lot of fuel if you set your maneuvers up right. Matching planes with Moho isn't necessary as long as you can make an intercept at a sensible velocity - remember Moho's SOI is tiny, so u don't have a lot of time to drop the anchors once you're inside. Try to slow down plenty before you get there.

I have wondered about all this using other bodies to slow you down, does anyone have a link to anything that might explain how this is done? I think it might just be witch craft.... {:-/

[The_Rocketeer]

I have wondered about all this using other bodies to slow you down, does anyone have a link to anything that might explain how this is done? I think it might just be witch craft.... {:-/

Haha, I actually once made an figure S-trajectory between Eve and Gilly to angle for a very low altitude pass on Kerbol!

I'm afraid I don't have a link to give you, but in practice passing close to a planet will have a drastic effect on your velocity, seriously altering the shape of your orbit. Whenever you enter a gravity field, the gravity effectively encourages your ship to travel with it - accelerating you towards it's centre (the planet). If you get your approach angle just right, you can use this gravitational acceleration to 'rob' you of orbital momentum, effectively slowing you down. The maths is a little over my head (I only have a mediocre GCSE!), but I strongly advise you to experiment as the results can be very rewarding (and fun!)!

[Kasuha]

I have wondered about all this using other bodies to slow you down, does anyone have a link to anything that might explain how this is done? I think it might just be witch craft.... {:-/

It is very simple. Get an intercept with Eve, lower your periapsis as near to Eve as possible and then watch what your trajectory after exiting the Eve system is doing. By minute changes to your intercept you can achieve large differences after leaving. Inclination changes, accelerating, decelerating - they all come almost for free.

You may need to increase your conic patch limit in your settings.cfg to see it, though.

In maneuver phase it's the easiest if you set a maneuver for intercept first with as close intercept as possible, then another maneuvers right before the SOI to perform those minute changes (the original maneuver might be very hard to manipulate).

With actual implementation of the maneuver, use RCS to perform your minute corrections. And don't forget to correct again after each SOI change.

Edited January 15, 2014 by Kasuha

[Smallish]

Ahh ok, I see, but surely the times when an "in-between" body is in the right position to help must be a rarity? But I'll have a play and see how it looks from now on, thank you.

[EtherDragon]

Ahh ok, I see, but surely the times when an "in-between" body is in the right position to help must be a rarity? But I'll have a play and see how it looks from now on, thank you.

You are correct, these alignments are somewhat rare - but not impossibly rare.

[Kasuha]

You have two options if the body is not in the right position.

1/ wait till it comes there

2/ perform a "blind date" - set up your trajectory to just touch the body's orbit. Then, when you get to that point, use a maneuver to change your orbital period slightly to get the intercept the next orbit. If it's too far, wait one more orbit and try again.

You can get a lookup what happens in further orbits by placing maneuver nodes with 0 dv on your trajectory (you need to place them so there is almost whole orbit between them, not a tiny fraction of orbit). That allows you to see that e.g. if you do a correction for 33 m/s now, you'll get an intercept in 3 orbits - instead of passing two orbits with no maneuver, and then doing a 100 m/s correction.

[Smallish]

You have two options if the body is not in the right position.

1/ wait till it comes there

2/ perform a "blind date" - set up your trajectory to just touch the body's orbit. Then, when you get to that point, use a maneuver to change your orbital period slightly to get the intercept the next orbit. If it's too far, wait one more orbit and try again.

You can get a lookup what happens in further orbits by placing maneuver nodes with 0 dv on your trajectory (you need to place them so there is almost whole orbit between them, not a tiny fraction of orbit). That allows you to see that e.g. if you do a correction for 33 m/s now, you'll get an intercept in 3 orbits - instead of passing two orbits with no maneuver, and then doing a 100 m/s correction.

This is really enlightening stuff, thank you!

I had a few hours messing about around Jool today, and with the quantity of bodies around in close proximity it was helpful to see the effect of using near misses..However, when trying to use planets en-route to another planet, you would only normally have the target planet showing when you setup your nodes, so how would you even know if you have a chance of an encounter with a helpful planet. The only way I could imagine is if you see your orbit crossing another non targeted orbit, then you would have to target that particular body and try and tweak your node to see if you get any interaction. Would that be about right? Because obviously, you couldn't just rely on waiting till you got chance encounters that clearly showed up on your predicted path. - that would mean missing many chance encounters that are helpful but are literally just outside of your "A" path.

I hope that makes sense, and again, thank you for all the helpful information...This game has been extremely educational!

Oh, and a tip for anyone of similar rocketing skills as myself; make sure you enter Jool's influence going in the right direction! (unless you like having moons coming at you like trucks on the wrong side of a motorway that is)

Edited January 15, 2014 by Smallish

[Kasuha]

Yes, if you want to check if there aren't any possible near misses along your trajectory, you need to select all planets whose orbits you cross and check if there aren't any.

Except chance to get random encounter along your trajectory in interplanetary space (unlike around Jool) is extremely small. Most often your trajectory misses the body's orbit far enough that the game won't even show you nearest approach indicators. But truth to be told, some time ago somebody posted here on forums that he accidentally crashed to Dres on his way to Jool. So it may happen, eventually.

Also, Jool, Eve, and Kerbin are about the only bodies worth seeking a gravity assist. Not that other planets can't be used but their effect is very small at interplanetary speeds.

Oh, and a tip for anyone of similar rocketing skills as myself; make sure you enter Jool's influence going in the right direction! (unless you like having moons coming at you like trucks on the wrong side of a motorway that is)

Hehe yeah, very true. It's always good idea to fine tune your intercept from the very beginning - if you use ballistic trajectory, then from the initial burn at Kerbin. And if you use mid-course plane change then from this plane change.

The exception is if you're going to land on Laythe. Because it does not really matter from which direction you aerobrake, atmosphere can eat a few more km/s with no consequences in current version of KSP.

Edited January 15, 2014 by Kasuha

[Vicomt]

Also, Jool, Eve, and Kerbin are about the only bodies worth seeking a gravity assist. Not that other planets can't be used but their effect is very small at interplanetary speeds.

I wouldn't say they're not worth it, even Ike can help

I can't remember exactly how much DV this save me, but it was significant enough.