Eeloo and Moho

[miklkit]

I'm trying to find out how much DV it takes to get from solar orbit near Kerbin to landing on Eeloo and Moho.  Experience so far says it takes me every bit of 8200dv to land on Moho which makes it really dicey at my skill level.  I'm hoping that Eeloo will be a bit less demanding.

[king of nowhere]

eloo is a lot less demanding. the closer you move to the sun, the more expensive maneuvers are, you are fighting against higher gravity.

you can optimize trajectory to moho, anyway. with gravity assists, I got from landed on ike to moho orbit and back with 5 km/s.

for eeloo, starting from jool in the transfer window is really cheap, less than 1 km/s.

[miklkit]

Gravity assists?  Nope.  Not at my skill level. 

I have thought about stopping at Ike ot Dres, but the Jool gravity well is huge.

[Vanamonde]

Moho is the hardest. It's small, fast, and has an inclined orbit. Eeloo is small-ish and far away, and so is more forgiving even if the burns are large in absolute terms. For guidance you can use the classic dV map, 

https://www.reddit.com/media?url=https%3A%2F%2Fi.redd.it%2F89f2wjwndxz91.png

but keep in mind that it assumes ideal conditions and does not include inclination changes. 

[Hotel26]

5 hours ago, miklkit said:

8200dv to land on Moho

I think you will find this post by Laie to be very helpful.  It links also to a tutorial Laie wrote.   The claim is 4,600 m/s dV (low orbit to low orbit^[1]), if you leave at the right time.

The gist is that Moho is so inclined (so deeply in the gravity well), that a different approach is required.

(Whereas the standard method is to leave when the target will arrive at your transfer destination (so-called transfer window), this approach targets arrival at Moho's DN, which is its node closest to its PE.  You can also make (most of) the inclination alignment on Kerbin ejection, which should(?) be cheaper than doing it at Moho capture.)

The tute is The annual and reliable Moho Transfer Opportunity.  And will definitely "expand horizons".

If you read these a couple of times carefully and still have questions (still likely), I recommend you return to this post of yours and ask those questions here.

[1] and refueling in LKO is highly recommended

Edited September 25, 2023 by Hotel26

[king of nowhere]

15 hours ago, miklkit said:

Gravity assists?  Nope.  Not at my skill level.

don't sell yourself short. and don't conflate the difficulty of the task.

 

there's gravity assist and there's gravity assist. some are easier than others. getting a chain of multiple gravity assists is relatively difficult. using a gravity assist to change inclination and intercept a comet is very difficult. but that's not what you are doing here. a basic gravity assist is simple. in fact, you may have done them without even realizing.

from low kerbin orbit, make a normal transfer to mun. to go to mun, you will have to make a capture burn at periapsis.

stop there. do not make the capture burn. you will pass close to mun, and then you will be shot out of the kerbin sphere of influence. it normally takes 930 m/s to escape kerbin's gravity, but you spent the 850 m/s of a mun transfer, and you still got out. you have taken a gravity assist. basic gravity assists are easy. similarly, when you arrive at jool, aim at tylo or laythe - they are so big, you can find them easily - and you will notice your trajectory change after passing close to one of the big moons. there, that's a gravity assist. with a bit of trial and error you can use that to get captured for free.

sure, there's a lot more about gravity assists, and to perform the complex ones you need practice and you need to know what you can and cannot do with them. but don't let yourself be intimidated, it's something you absolutely cannot do.

 

in this specific case of reaching moho, try instead going for eve, and pass close to eve. depending on which side of eve you pass, your solar orbit will get higher or lower. choose the side that will lower your orbit. and there you go, you have lowered your solar orbit without using fuel, with a gravity assist.

a single passage like that will not get you to moho. for that you would need to know about intercept speed, and you would have to chain multiple assists. however, your single passage already brought you closer to moho than you were before. it will save maybe 1 km/s, and it's not overly difficult; all you need to do is intercept eve, and you already know how to intercept planets. so it's worth doing, isn't it?

 

P.S. 8.2 km/s is very high, even for moho. did you by chance eject from kerbin into solar orbit, and then lower solar periapsis from solar orbit? that would explain why it's so expensive for you.

[miklkit]

@Vanamonde I have that chart and I need 3-4 times that much fuel. 

@Hotel26 I have the tute saved but do not have that other link.  I am trying to time it so I use less fuel.  So far fuel use is very random.

@king of nowhere Yes I first go to Minmus and refuel, then go to solar orbit and turn it over to MechJeb.  I am a poor pilot and rarely try to do it meself as MJ does it better.  I just monitor it and then do corrective burns after it is done.  Gravity assists are for the experts.

[Hotel26]

25 minutes ago, miklkit said:

but do not have that other link

https://forum.kerbalspaceprogram.com/topic/196048-when-is-the-annual-moho-transfer-again/?do=findComment&comment=3830984

This link is directly to the comment by Laie within the topic.

Good luck.

[king of nowhere]

2 hours ago, miklkit said:

@Vanamonde I have that chart and I need 3-4 times that much fuel. 

@Hotel26 I have the tute saved but do not have that other link.  I am trying to time it so I use less fuel.  So far fuel use is very random.

@king of nowhere Yes I first go to Minmus and refuel, then go to solar orbit and turn it over to MechJeb.  I am a poor pilot and rarely try to do it meself as MJ does it better.  I just monitor it and then do corrective burns after it is done.  Gravity assists are for the experts.

I see. it's really a terribly inefficient way to make interplanetary transfers, only partially mitigated by the refueling on minmus. if you do things like that, going to moho does indeed cost 8 km/s, and going to eeloo will cost only slightly less.

with proper navigation, going to moho will cost less than 5 km/s. with a single simple gravity assist you can push this down to 4 km/s. with more difficult chained gravity assist you can do it all with 2.5 km/s, or even less.

the difference is even more dramatic going out. arriving to eeloo your way is going to cost maybe 7 km/s. going to jool first, with just a simple assist to get captured there, and it goes down to 3 km/s. with chained gravity assists, you can make the whole trip to eeloo with 1.5 km/s, even less if you can manage the inclination.

are you sure you don't want to try and improve your navigation? perhaps first thing try a launch for jool from low kerbin orbit, as jool is a bigger target to hit and far enough to get good savings this way?

with those premises, the answer to your original question can only be

On 9/25/2023 at 8:19 PM, miklkit said:

I'm trying to find out how much DV it takes to get from solar orbit near Kerbin to landing on Eeloo and Moho.   I'm hoping that Eeloo will be a bit less demanding.

it's not a matter of the target being demanding. it's entirely a matter of how you get there.

[Grogs]

I normally just brute force it to Moho. It sounds like a lot, and it would be for a single stage but it's no problem doable if you break it up and use multiple stages, drop tanks, etc. For my Mohollo mission, I scoped out the trip to Moho and back like this. Each letter (except E) is a stage or stages. I designed the ship starting at the end then added enough tanks/engines to provide the necessary fuel for each step.

Quote

 

Here's the mission profile:

A. Lift ship into low-Kerbin orbit [3400 m/s]

B. Transfer the ship to Moho and put it in a low orbit [5000 m/s]

C. Land on Moho and return to orbit [1800 m/s]

D. Return the ship to Kerbin [3800 m/s]

E. Capture around Kerbin and enter orbit before landing [2000 m/s]

That totals about 15,000 m/s dV. Part E is optional. If I have the dV left I'll do it, but if not I'll do a direct re-entry. Also the dV for part E may be reduced by aerobraking in Kerbin's atmosphere.

 

You can see the full report for that mission here if you want to see details. You can also find the Eeloo mission a couple of posts above this one with a similar mission analysis.

 

Edited September 26, 2023 by Grogs

[miklkit]

As I said before gravity assists are for the experts.  I am done with Moho now and am working on getting to Eeloo.  If I ever go back to Moho it will be to pick up the crew there and bring them home.  They are busy doing science now.  Hopefully Eeloo will be a little less stressful.

[Superfluous J]

You should visit the planets/moons in this order, or at least one similar to it:

1. Mun
2. Minmus
3. Duna and Ike, either one mission or 2 separate ones.
4. Eve SOI. Don't even think about trying to land with Kerbals.
5. Dres
6. Eeloo
7. Jool and all its moons. Do them in 1 mission. Do them in 10 missions. Doesn't matter. Explore and enjoy.
8. Gilly (Eve's Moon)
9. Moho
10. Land and return from Eve. If you dare.
11. Go on the forums looking for challenges.

Edited September 27, 2023 by Superfluous J

[Hotel26]

On 9/27/2023 at 12:18 PM, miklkit said:

gravity assists are for the experts

About this, you are perfectly correct.

Spoiler

[Obscure reference] similarly to when someone suggests the solution to your problem is 'regex', the correct response is 'then I would have two problems'.

We all know Moho is difficult.  The key question is why is it difficult?

                                                                

In the standard interplanetary transfer, you leave your orbit when the transfer time matches when the target will arrive at the same place you do, when you arrive in the target's orbit.  (This is the "transfer window".)

A consequence of this is that any inclination change must (very likely) be performed as a mid-course maneuver (without the aid of an Oberth effect).

In the case of Moho's 7-degree inclination from the ecliptic, this inclination change is a major expense.

                                                                

Therefore, the strategy Laie suggests is to make the transfer, targeting not Moho, but one of its nodes, AN or DN.  The one at the lower altitude (nearer its PE) turns out to be more efficient in terms of matching orbital speeds of Moho and vessel, requiring a lesser burn for capture.

Upon arrival at that node, a retrograde burn is performed to adjust the orbital period (for one more short orbit, something >102d), to synchronize an (inclined) encounter with Moho upon the next loop.

This leaves one with the question of where to perform any required inclination change.

The obvious, or default, choice is to perform the capture at Moho and then adjust inclination within Moho's SOI, at a much lower orbital speed.  If you intend anyway to land out of low orbit, you may not even have to bother with the inclination.

The other choice is to include an inclination change in the transfer injection burn at Kerbin.  This can be done to quite easily lower the inclination differential to very nearly 0.  This might make ultimate rendez-vous with Moho easier, but would be a personal choice whether worthwhile or not.

                                                                

I think Laie's strategy reliably makes Moho a quite accessible target.  The chief (and only) disadvantage I see is that your arrival will include at least an extra 102 days for the rendez-vous sync orbit -- plus the Kerbin annual wait for departure (average 213d; whereas the usual Moho window would be every 102 days)  I feel this is offset simply by the good frequency of Moho opportunities.

                                                                

I have no doubt repeated a lot of the content in Laie's tutorial, but my slant is to compare his strategy with the conventional transfer.

 

Edited September 29, 2023 by Hotel26

[miklkit]

The order that I am doing my exploration is:

1. Mun

2. Minmus

3. Eve SOI: 1 failure, 1 lucky success, another failure, then 2 successes.  That thing is never where it is supposed to be and it sucks to arrive and find only empty space.

4.  Ike, then Duna

5. Dres.  The Kraken lives there. 

6. Jool SOI, Bop and Laythe

7. Moho  5 successes, 2 failures, 1 success.

8. Eeloo  So far 3 successes with another en route.  Gotta find time for the 2 hour burn time. This is my largest rocket at 4,485 tons at liftoff.

I am not sticking to my guns but am recognizing my limitations.  I am a lousy pilot who has never learned how to do rendezvous, so am limited to space planes. 

My last success was using that AN, DN method which MJ did automatically.  Still used a lot of fuel.

Taking off from Minmus and heading for solar orbit.

coming down

refueling for the return trip