Moho slow-down burn requiring over 5000 m/s of Delta-V

[techgamer17]

Okay so ive sent two missions to Moho, both probes with ion engines in an attempt to gather orbital data from the planet as ive had several contracts to go there and do orbital surveys. 

The most recent rocket had around 14500 m/s of delta-v in total, which is WAY more than sufficient to get to any planet and just orbit. The payload consisted of a survey scanner satellite, with the resource scanner and all of the SCANsat things to do an altimetry survey. 

I got to Moho with around 4000 m/s of delta v left, hoping that not much would be required to do a full-on retrograde burn to slow down into a polar orbit (apoapsis around 250km). At this point I was on to the ion engines. I plotted a manoever to get into a decent polar orbit, and the total cost would be like 5000 m/s of delta v, which was too much given the amount I had in the tank. 

Both missions have had this problem and ive literally just ended up doing two flybys of Moho and then just sending them back to kerbin with the huge amount of fuel they had (just not enough to orbit Moho).

How has anyone else managed to intercept Moho and orbit with sufficient delta-v, let alone land? 

Any suggestions would be appreciated as this is bugging me. Thanks.

[1101]

Not been to Moho recently so not the most knowledgeable on the subject.

However - you say the rocket had 14500m/s in total, and then had 4000m/s left at Moho - with around 3500 to get into LKO, I wonder how your ejection burn used 6000m/s of dV.  That may be the issue - the initial ejection gives you an excessively fast encounter, and then it is difficult to capture afterward because of the excess velocity.

Maybe try the transfer planner:

http://alexmoon.github.io

 

[Scotius]

The main problem of Moho missions is in the huge difference in relative velocity between rocket and planet. That deep in the Kerbol's gravity well maneuvers require a lot of dV on the principle. If you aren't careful enough Moho will zip past your ship like a bullet, and fly-by will be too short to brake into orbit if your TWR is low. I had problems with it even when my ships carried multiple nuclear engines. So, unless you are sending lightweight probe forget about ion engines - their thrust is too low for this task. Plenty of dV paired with relatively high TWR should get you to Moho - it is totally doable.

[something]

Oberth. The faster you are going the more efficient your burn is. That means, the closer you are to a gravity well (Moho), the less fuel you need to get into a specific orbit. Just place a maneuver node at Periapsis and circularize the orbit, then move it close to the SOI change and see what happens - the orbit will become elliptic and your Apoapsis increases. So try to do a small correction burn to get your Pe down to a reasonable altitude (depends on the planet) while you are still hohmann transferring. That usually costs around 3-30m/s depending on how accurate your initial burn was. But 4.5km/s seems about what the dV maps predict for Kerbin SOI to Moho landing... When doing your approach correctly, you could as well use the gravity assist to slow you down. Or get yourself in a resonant orbit and do a series of gravity assists - never did that personally but read about someone who did here on the board.

Also, see this Oberth Effect, Wikipedia

Edited January 1, 2017 by something

[Foxster]

Confirming other suggestions...

1. Get a better transfer. Don't go with whatever is available at the current time. Use a transfer planning mods to get this right. Otherwise you will have a high velocity when encountering Moho and will have wasted dV getting there too. 

2. Ions are not the best for capture and orbit at Moho. The TWR is low and you end up burning a long time before and after Pe. Have a look at nukes instead.

 

[Waxing_Kibbous]

Here is some more food for thought

 

[obi_juan]

I send a huge ship to Moho.

60tn with fuel and 5000m/s delta-v alone.

The transfer ship had around 7000m/s delta-v (2000 in the transfer ship and 5000 in the drone ship)

Thanks to AluxMun's Launch Window Planner I found a position that allow to use only 1300m/s to send the ship. Thanks to the fuel left on the final phase to put the ship in orbit around Kerbin I managed to reach Moho with 6400m/s delta-v

I land with the fuel that you can see.

These are my tips, don`t follow as perfect... 

0º) Choose the lower delta-v window.

1º) Don't use a high burn time, around 30 min is the maximum. (for this reason I choose nukes)

2º) If you go with Ions is better if you start the burn before the capture with Moho, down the apoapsis down to the Eve orbit.

[Carl]

If your set on using Ions a practical method would be to setup so your going into a Kerbol orbit just inside Moho's then match plane, then rendezvous from thee, d/v to kill once you hit moho SOI should be low if you did it right, (and my memory of orbital mechanics is good, usually it is, i'm just awful at applying that knowledge).

[Camelotking524]

Do an Eve flyby, that lowers your solar orbit and then do a burn to Moho.  Takes longer but it will lower the cost of the burn to get into orbit around the planet.

[Vanamonde]

How-to question moved to the how-to subsection. 

[DD_bwest]

if i use ions for moho,  i often just over budget on delta v, and plan slowdown/inclination burns outside moho SOI, so that the eventual encounter will have a much lower capture burn. 

I find that having enough  delta v isnt the problem, its having the time to complete the capture burn.  

also remember that as close as you are to the sun, solar panels will be pumping our the juice, so you may be able to get away with adding extra ion  drives to increase twr

oh and make sure when you get there your Pe is on the"inside" of mohos orbit with the sun.  it can be frustrating when you get to moho but then pass behind it and have no power for the most efficient time to burn lol

[Aegolius13]

4 hours ago, Waxing_Kibbous said:

Here is some more food for thought

 

I usually do it basically the same way, but with one change  Once I hit the periapsis of my Hohmann transfer from Kerbin (which is hopefully tangent with Moho's orbit), I do a retrograde burn so that the next time I get back to the same spot, Moho is there also.  The amount of this burn depends on where Moho is relative to my ship at its periapsis, of course.  

This has the advantage of shortening the duration substantially, since you only have to make one more trip around the sun and you apoapsis will be closer than Kerbin (meaning that trip will go faster).  It also reduces the amount of burn you have to do when you actually capture around Moho, which is particularly nice if you have a low TWR ion or nuclear rig.  The disadvantage is that, by doing this retrograde burn in deep space, you lose the Oberth advantage of doing the whole injection burn around Moho.  But I think this is a sacrifice worth making (Moho does not generate the biggest Oberth at any rate).  

 

There's yet another incremental step you can do: split your ejection burn and do your plane change at the slow end of your orbit in between the two burn phases.  This add complexity but saves a little delta-v, and also breaks the ejection process into more manageable chunks if you have lower TWR.  See additional explanation here:

 

[techgamer17]

On 1/1/2017 at 3:56 PM, 1101 said:

Not been to Moho recently so not the most knowledgeable on the subject.

However - you say the rocket had 14500m/s in total, and then had 4000m/s left at Moho - with around 3500 to get into LKO, I wonder how your ejection burn used 6000m/s of dV.  That may be the issue - the initial ejection gives you an excessively fast encounter, and then it is difficult to capture afterward because of the excess velocity.

Maybe try the transfer planner:

http://alexmoon.github.io

 

Okay thanks I was using Kerbal Alarm Clocks transfer window planner to get the encounter. It seems I was still going to fast though so Ill check out the Transfer Window Planner mod....

[Birdco_Space]

Since I regularly visit Moho, here are my suggestions:

1)  Use a transfer window planner, and only use the lowest delta V transfers.  Some optimal transfer windows in Moho's orbit are much higher Delta V than a later transfer, and that difference can add up to as much as 1000 m/s of delta V on different transfer windows.  Eve assists are useful, but optimal assists don't occur all that often.

2) Ions are perfectly acceptable for capture burns, but don't use them for ejection from kerbin, accuracy on ejection burn is essential when trying to capture at Moho, and trying to eject with a  .1TWR can result in hundreds of m/s of mid course correction and added capture burn time.  Note, on ejection from kerbin, put a 500ms burn 2 orbits before your ideal transfer window to increase ejection accuracy by reducing the magnitude of the final ejection burn.

3) when capturing with ions, remember you will need about a quarter of the solar cell power generation at moho then you need in kerbin orbit, so spare on the solar cells for more TWR and delta v for a given Xenon mass

4) With ions, or very low TWR with nukes, Start your Capture burn a couple of minutes early from the half burn time, and adjust burn vector to keep periapsis where you want it.  it ends up being a more efficient and controlled insertion because there is a lot less burning after perapsis.

5) with low TWR captures, budget an extra 10-20% delta v on the capture burn over what the Transfer window planner says.  Low TWR is inherently less efficient

 

I routinely put probes in Moho orbit with only 6000m/s of delta-V when in LKO, launched with 1.25 meter parts and using Ions.  Landing a probe takes about 800 m/s more (though the ion deceleration stage is quite a bit beefier to slow down the LF/ox for the probe lander).  Manned landing and return missions...  Eve gravity assists and asparagus staged nukes with a .1TWR (or less), and a docked lander are your friend 

 

Edited January 11, 2017 by Birdco_Space
correct spelling error

[Tyko]

13 hours ago, Birdco_Space said:

Since I regularly visit Moho, here are my suggestions:

2) Ions are perfectly acceptable for capture burns, but don't use them for ejection from kerbin, accuracy on ejection burn is essential when trying to capture at Moho, and trying to eject with a  .1TWR can result in hundreds of m/s of mid course correction and added capture burn time.  Not, on ejection from kerbin, put a 500ms burn 2 orbits before your ideal transfer window to increase ejection accuracy by reducing the magnitude of the final ejection burn.

Building on @Birdco_Space advice on ejection burns - if your burn exceeds 4-6 minutes you're getting a lot of inefficiencies on the outer portions of your burn. One method I've used is to do "periapsis kick" burns on several orbits in a row. Each one raises my apo and gives me more velocity on my subsequent orbit. Let's say you have a 12 minute burn. Do two 4 minute burns on your first two orbits, on the third orbit complete the ejection.

It takes a bit more work to do this, because after each burn you have to create another maneuver node. Also, you have to plan a bit so that your final burn occurs at the right time, but I've found I save a lot of fuel doing it this way - as much as 25%.

Edited January 6, 2017 by Tyko

[Kryxal]

My "planning" for a periapsis kick consists of setting up the full burn for the transfer, then reducing it to maybe 420-430 m/s prograde and moving it 3 orbits back in time.  You end up coming out of that burn with an orbital period of about 90 minutes, very close to 3 times the previous period.  Note that you'll want some sort of node editor.

[Tyko]

2 hours ago, Kryxal said:

My "planning" for a periapsis kick consists of setting up the full burn for the transfer, then reducing it to maybe 420-430 m/s prograde and moving it 3 orbits back in time.  You end up coming out of that burn with an orbital period of about 90 minutes, very close to 3 times the previous period.  Note that you'll want some sort of node editor.

that's a great technique. I'll have to try it