Eve Gravity Assist vs Direct to Moho

[Mars-Bound Hokie]

I'm trying to plan a mission to Moho. While I have MechJeb's Delta-V gauge to tell me if I can or cannot fly directly to Moho - fleet construction and organization is another problem - it does not tell me if I have enough Delta-V to do an Eve Gravity Assist to Moho.

Here's the story. While planning transfer orbits for my Eve Fleet two months ago, I saw that the planned maneuver would have taken me in an escape trajectory out of Eve and into a somewhat low circular orbit around Kerbol - I forgot the specifics of the orbit, but it would have enabled me to put the crafts in some designated path for a contract. Now it got me thinking "Hey, maybe I can do that for a trip to Moho." 

If that's the case, here's my plan for a Eve gravity assist to Moho:

1. Wait until the next Kerbin->Eve transfer window to escape Kerbin's sphere of influence (I have Kerbal Alarm Clock)
2. As soon as I reach Eve's SOI, set my periapsis to 100-125 km and (possibly, it's been months since I was last at Eve) get in an escape trajectory out of Eve.
   1. Low periapsis requires less delta-V.
   2. Anything lower than 90 km will be dangerous (atmospheric burn-up, aerobraking messing up my escape trajectory, etc.)
3. After leaving Eve's SOI (and in solar orbit), set up a Hohmann Transfer to Moho.

 

On one hand, I get to do a really cool physics-based maneuver and (possibly) save delta-V (I don't say "fuel" since it's not the only factor used in MechJeb's Delta-V gauge) on my way to Moho. On the other hand, here are some numbers that may convince me that flying directly to Moho is just as good as using an Eve gravity assist.

1. DIRECT FLIGHT
   1. LKO to Moho's Surface = 5490 m/s
   2. LKO to Low Moho Orbit = 4530 m/s
2. EVE GRAVITY ASSIST
   1. TO SURFACE
      1. LKO to Eve Intercept = 1140 m/s
      2. Eve Intercept to Moho's Surface = 4350 m/s
         1. 1140+4350 = 5490 m/s
   2. TO LOW MOHO ORBIT
      1. LKO to Eve Intercept = 1140 m/s
      2. Eve Intercept to Low Moho Orbit = 3390 m/s
         1. 1140+3390 = 4530 m/s

NO DIFFERENCE, IS THERE

 

What do you think I should do to save Delta-V? Should I just stick with direct flights to Moho, or do you recommend I use the Eve Gravity Assist?

• If the direct flight is the best bet, fine. 
• If the Eve assist is the best bet, then:
  • How do I time it right to have the most efficient flight? Remember: I have Kerbal Alarm Clock.
  • Any instructions for such a flight, or does my plan sound solid?

Either way, off to the drawing board (I mean V.A.B.) for me.

 

Your input is greatly appreciated. Thank you.

[Foxster]

Do it if you fancy a challenge but otherwise it's a bit of waste of time. The dV saved could likely be made up with optimising your craft a little. 

If you are struggling to find the dV you need then say something about the mission or post piccies. 

[4x4cheesecake]

There are some tools like Flyby Finder or KSPTOT to help you to get the correct timing and angles for a flyby.

Just by running a quick search through Flyby Finder, I found a route from LKO to LMO via an gravity assist of Eve for just 3316m/s dV:

Start Planet: Kerbin
Orbit Departure Time:
   14320800 seconds UT
   664 days UT
   Y2   D238   H0
Start Orbit Inclination: 14,3 degrees
Start Boost from that incl.: 1291 m/s
Start Equatorial Z velocity: 888 m/s
Start Equat. Prograde velocity: 1179 m/s
Start Boost from Equat. Orbit: 1476 m/s
V Infinity Leaving Start Planet: 1557 m/s

1st Encounter Planet: Eve
Time from Start to 1st Encounter: 200 days 0 hours
Vinf in: 3400 m/s
Brake to Orbit?: 2449 m/s
1st Encounter Periapsis:
   864 days UT
   Y3   D12   H0
   907 km altitude

2nd Encounter Planet: Moho
Time from 1st to 2nd Encounter: 135 days 2,1 hours
Vinf in: 2556 m/s
Brake to Orbit?: 2025 m/s
2nd Encounter Periapsis:
   999,36 days UT
   Y3   D147   H2,1

Total delta V expended: 3316 m/s
Total Travel Time: 335,2 days
Total Travel Time: 0,7 years

I'll explain a few values here:
You start from LKO (75km) in year 2, day 238.
You can start from an 14.3° inclined orbit and burn prograde for 1291m/s to get your Eve encounter but since the calculator doesn't tell us the longitude of ascending node, this is pretty useless (in the linked thread, there is a tutorial on how to use it in combination with mechjeb but if you want to do it on your own, don't try this).
So we should take a look at the equatorial values: Burn prograde for 1179m/s + in normal direction for 888m/s (this will sum up to 1476m/s for the whole burn).
You 'll encounter Eve in year 3 day 12 and flyby in 907km distance to the ground. 135 days later, you'll arrive at Moho and you can break into an orbit by burning retrograde for 2025m/s (75km orbit).
Well, these are the calculated values so far.

You can get some serious dV savings there just by hitting the correct transfer window. A flyby in general is still complicated though and it may take some time to set up the maneuver node. I would highly recommend an additional mod to fine tune maneuver nodes like precise maneuver or something similar...there are a few of them around.
Also, is pretty much impossible to reach the calculated values and you will definitly need at least one correction burn but you will still get better results than the one in your example.

[Mars-Bound Hokie]

1 minute ago, Foxster said:

Do it if you fancy a challenge but otherwise it's a bit of waste of time. The dV saved could likely be made up with optimising your craft a little. 

If you are struggling to find the dV you need then say something about the mission or post piccies. 

 

Craft construction is another issue, and I just want a trajectory solution that works. I'm not going to do an Eve assist just for the heck of it (unless it serves a practical purpose). I take it you're saying I should just stick with a direct trajectory to Moho, then.

• Even if an Eve gravity assist is the most fuel-efficient solution, I still plan to have enough Delta-V for a direct flight in case I miss my Eve launch window - or if I mess up the trajectory somewhere in the middle.

 

• I already have a craft in LKO that's capable of getting this single-launch base to Moho's surface

 

• I think this weather satellite has enough Delta-V to make it to Moho (directly) and pick my landing spot for the base. Doesn't it?

 

• A ground relay that I will put on the other side of the planet from my base landing spot

 

• A xenon-powered "Ultimate Relay Antenna" that I will leave in Moho's orbit to maintain long-range communications to KSC.

 

I just need to get a single-launch seven-man space station (with science) to Moho's orbit. It should also carry LOTS of liquid fuel in reserve for the nuclear-powered return pod. It should also have Liquid Fuel and Oxidizer left over for a two-way moon lander that I plan to send the crew in (I'm sending the permanent base empty)

 

• This five-man rover with refueling capabilities (and Bon Voyage) can be of some help.

 

Does this satisfy your need for more information, @Foxster?

 

[OHara]

2 hours ago, Mars-Bound Hokie said:

On the other hand, here are some numbers that may convince me that flying directly to Moho is just as good as using an Eve gravity assist.

The JavaScript you linked to is finding shortest paths through the community delta-V map (forum post) and that map does not show all the shortcuts.  Specifically the locations on the map called 'Intercept' are orbits that touch the associated planet and also touch Kerbin's orbit; the map is Kerbin-centric in this way.

Decelerating from Eve-intercept-Moho-Intercept to Moho orbit can cause even less than the 2410m/s shown on the map between a Kerbin-intercept-Moho-Intercept and Moho orbit. How much less depends on what shape of Eve-Moho leg can be made with a gravity assist of Eve; 4×4cheesecake's example got it down to 2025m/s.

Edited December 29, 2018 by OHara
mistaken identity

[4x4cheesecake]

14 minutes ago, OHara said:

FleshJeb's example got it down to 2025m/s

I guess it's time to change the avatar....

[Kryxal]

If I take a return trip to Moho, I usually go there direct, but slingshot on the return trip ... it just seems less touchy aiming for Kerbin that way than Moho...

[Aegolius13]

I find it's hard to hit the predicted delta-v figures when going to Moho.  Little deviations from the optimal transfer (including that wicked plane change) can make for very large increases in requirements.  I think once I thought I had a pretty good transfer going, then got hit with a 5 or 7km/s burn required to capture.  (That mission turned into a flyby.).  

With that in mind, I think that to maximize prospects of mission success, I'd go direct.  Even if there is some minor difference in optimal delta-v budgets (and your numbers suggest there may not be), gravity assists add a lot more potential for things to go wrong.  

You also may want to look into the alternate method for direct transfers, where you launch at Kerbin's ascending or descending node w/r/t Moho, rather than at a certain phase angle.  I find it cheaper and more accurate.

 

 

 

[Laie]

Most of the trouble when getting to Moho is the inclination change: 7 degrees at a low solar PE.

The easy, reliable way of getting to Moho is to treat it like orbital rendezvous. "Capture burn" is just a fancy word for  "matching velocities", isn't it?

1. leave the Kerbin system just as it crosses it's AN/DN with Moho. Try to fix most of the inclination difference on departure, and make for a PE around the Sun that touches Moho's orbit.
2. at PE around the Sun, do a retrograde burn. Fix your orbital period such that you encounter Moho the next time you come around.
3. profit!

This is *not* the cheapest way of getting to Moho, but you have two launch windows per year, usually arrive within one Kerbin year after departure, and it requires ~4600m/s regardless of when you depart.

Launch opportunities where you can get directly from Kerbin to Moho for less than that crop up only once every four years or so. Otherwise, matching inclinations ahead of time is cheaper than using a launch window calculator and going for a straight encounter.

Why am I telling you all this when you're asking for an Eve assist? Because I want to make one very specific point: in order to save delta-V, the Eve assist you're looking for should fix your inclination first; actually encountering Moho after the assist comes as a distant second. If you want both from the same assist, you're probably looking for a once-in-a-decade event.

[Foxster]

This thread has reminded me of the flexibility in playing KSP...

If it's your thing then you can put effort into finding a clever maneuver to save dV. Or you can engineer your craft to give you enough dV that a relatively brute force transfer will work. Depends where your interest and skill lies. Both will get the job done. 

[Kryxal]

4 hours ago, Laie said:

Most of the trouble when getting to Moho is the inclination change: 7 degrees at a low solar PE.

The easy, reliable way of getting to Moho is to treat it like orbital rendezvous. "Capture burn" is just a fancy word for  "matching velocities", isn't it?

1. leave the Kerbin system just as it crosses it's AN/DN with Moho. Try to fix most of the inclination difference on departure, and make for a PE around the Sun that touches Moho's orbit.
2. at PE around the Sun, do a retrograde burn. Fix your orbital period such that you encounter Moho the next time you come around.
3. profit!

This is *not* the cheapest way of getting to Moho, but you have two launch windows per year, usually arrive within one Kerbin year after departure, and it requires ~4600m/s regardless of when you depart.

Launch opportunities where you can get directly from Kerbin to Moho for less than that crop up only once every four years or so. Otherwise, matching inclinations ahead of time is cheaper than using a launch window calculator and going for a straight encounter.

Why am I telling you all this when you're asking for an Eve assist? Because I want to make one very specific point: in order to save delta-V, the Eve assist you're looking for should fix your inclination first; actually encountering Moho after the assist comes as a distant second. If you want both from the same assist, you're probably looking for a once-in-a-decade event.

Actually, one of the launch windows here is a fair bit better than the other ... it's best if you meet Moho at periapsis, since you'll need a bit of extra delta-v for the transfer but considerably less for the capture.

Also, consider doing as much of your burn as possible while adjusting the timing, a long burn to capture is a real pain.