Moho transfers: me vs. delta-v map not even close

[EdFred]

I'm obviously doing something wrong.

Kerbin 80km orbit ~4500dv: Check

Kerbin escape+Moho transfer intercept:~1700dV: Check

Moho burn to stay in low orbit: ~2200dv: Hahahaha, not even close.

It doesn't seem to matter whether Moho is near its apoapis, periapsis, whether I'm coplanar, intercept at my periapsis, or have an "abrupt" encounter, my burns always seem to be in the 4000dV range. I've tried having a periapsis at Moho of around 10km, up to over 1000km. Doesn't seem to matter. I'm no where close to that 2200dv that all the maps have.

So what's the trick to using only ~2200dv for insertion/injection?

Answering my own question: Make the transfer at the correct window.

Edited June 5, 2014 by EdFred

[Dkmdlb]

Do some braking at Eve and catch Moho at Ap.

[Taki117]

There was a thread around here somewhere (I have since lost it ) that had a very good explanation on how to get to Moho, Basically you ignore Phase angles and what not, Perform your Inclination change during your ejection burn and perform the burn when Kerbin is at Moho Apoapsis.

[EdFred]

There was a thread around here somewhere (I have since lost it ) that had a very good explanation on how to get to Moho, Basically you ignore Phase angles and what not, Perform your Inclination change during your ejection burn and perform the burn when Kerbin is at Moho Apoapsis.

That's not the issue. Getting to Moho I'm dead on with the dV (even better that the dV maps sometimes) The issue is after I enter Moho's SOI my orbital velocity shows as being well over 4,000m/s, last time it was over 5000m/s Orbit around Moho at 10k, is around 800m/s. Not even taking into account Moho's gravity accelerating you as you get closer to LMO, that's 4200dV needed to slow down. SO my question is, how does 5000-800=2200?

[XiXLLAMAXiX]

I'm not sure where you're getting your 2200m/s value from, in my experience a direct one way trip from kerbin to moho requires at the absolute minimum 4400m/s (not counting dv to get into kerbin orbit), and normally that value is closer to or above 5000m/s.

I would doubt that anyone could beat 4.4km/s without some fancy maneuvering such as gravity assists or aerobraking around eve

source: http://alexmoon.github.io/ksp/

[Geschosskopf]

So what's the trick to using only ~2200dv?

According to the cognoscenti (I've never tried this myself), the delta-V maps assume you're doing a bi-elliptic transfer with your initial Ap somewhere out near Jool's orbit. Personally, I'd think this would make the situation worse but I'm no expert.

In any event, the amount of delta-V needed to capture into a circular orbit at any planet is the difference between how fast you're going at Pe and the planet's orbital velocity at that Pe altitude. To minimize capture delta-V, therefore, you need to minimize how fast you're going when you arrive at the target planet. You do this by making your approach orbit as shaped as much as possible like the target's orbit before you get there, so you're going not much faster than the target. IOW, instead of going direct from Kerbin to Moho, you 1st put your Pe down near Moho, then pull your Ap down after you to near or inside of Eve. Problem is, this is just moving the delta-V expenditure from the capture burn to the transfer phase so unless you can brake at Eve as suggested above, it probably won't make the total delta-V much less, it just makes less of it happen at capture which is often useful.

There are several ways to lower your Ap. The cheapest is to brake at Eve, which means you're leaving on an Eve window, not a Moho window. The simplest is to just burn at your Pe down near Moho. And the 3rd is to brake at Moho by NOT capturing the 1st time you go by, but escaping and intercepting again about 1 Moho year later.

Note that all of these methods (especially going out to Jool) take a lot of travel time, which can be an issue if you're using a life support mod.

[EdFred]

I get it from the Kerbal Wiki page.

One of the maps:

http://wiki.kerbalspaceprogram.com/w/images/7/73/KerbinDeltaVMap.png

Shows a 2200m/s injection dV

[EdFred]

According to the cognoscenti (I've never tried this myself), the delta-V maps assume you're doing a bi-elliptic transfer with your initial Ap somewhere out near Jool's orbit.

If that's the case, it makes perfect sense. Moho orbits Kerbol in the neighborhood of 12km/s. I arrive with a Kerbol velocity higher than that using a standard Hohmannn transfer. If the dV maps are not for Hohmann transfers that answers my question.

[godefroi]

According to the cognoscenti (I've never tried this myself), the delta-V maps assume you're doing a bi-elliptic transfer with your initial Ap somewhere out near Jool's orbit. Personally, I'd think this would make the situation worse but I'm no expert.

I'm not part of the cognoscenti, but I do watch Scott Manley videos. Also, I read the Wikipedia article on bi-elliptic transfers. According to it, the savings are minimal (as in, 2% savings in a transfer scenario where you extend your Ap out to 30x the distance to the moon, which it labels as "impractical" and "extreme". This example transfer would take 4.5 years, where the equivalent Hohmann transfer would take 15.5 hours. All to save 2% of your dV.

I doubt doing the bi-elliptic transfer out near Jool would cut the dV requirement nearly in half. It's my experience that the dV maps grossly underestimate the Moho insertion requirements. That's why I ended up burning all my return fuel, my ascent fuel, and my descent fuel just to get into orbit when I went there the first time. I ended up sending not one but THREE ships full of fuel to get the landing done and get back to Kerbin.

[Mr Shifty]

If you could do a Hohmann transfer to Moho from Kerbin, the insertion burn would be about 2400 m/s and the total delta-V from LKO would be about 4100 m/s. (I can show calcs for this if you want.) Also, when Moho was introduced in 0.17, it had a higher mass, which might have made it easier to get captured. The delta-V map you're using might be from then.

You can't do a Hohmann transfer because of Moho's eccentricity and inclination relative to Kerbin. You can use Alex Moon's launch planner to see possible launches, and it shows Moho takes at least 4900 m/s to reach, which matches my experience. Note that it is possible to have a longer ejection burn from Kerbin and get a smaller insertion burn. See, for instance, the minimal transfer from LKO to Moho on Year 2, day 87, which has an insertion burn of only 1900 m/s, but a total delta-V of 4740 m/s. Regardless, the delta-V map you're using is wrong regarding the necessary energy to reach Moho.

Edited June 5, 2014 by Mr Shifty

[EdFred]

OK, after some more experimenting and getting my transfer window more precise, I'm within 10% of the dV required according to the dV maps. I was off on when it looked right, but wasn't and I wasn't intercepting smoothly. I was over burning on ejection, and coming in waaaaaay too fast. In another attempt my insertion burn came in at ~2500dV, which is acceptablish.

[cantab]

Bear in mind the capture burn only considers this part: with a low Moho periapsis already established, burn retrograde until you have an apoapsis *just* within Moho's SOI, so you're on a very elliptical orbit. To then get down to a low circular orbit requires another big slice of dV. If you don't have a low enough initial periapsis just capturing will need more than the maps say.

And I think it also matters whether you hit Moho at its periapsis or apoapsis (or in between), but I can't remember which is better.

[Tank Buddy]

If you don't have your Moho intercept at Kerbol periapsis, you can have braking DV values upwards of 3000 m/s. I had one at 5000 m/s.

[metaphor]

The problem with Moho is that it's going around the Sun really fast. So if you come in with even a small radial component, it's going to take a lot to match Moho's orbit and get captured. Make sure you're crossing Moho's orbit as close to tangent to it as you can, and your capture delta-v will go down by a lot.

This transfer:

will result in a lot lower capture delta-v than this transfer:

It's also more efficient to encounter Moho when it's at its periapsis, then your capture burn can be as low as 1500 m/s (your Kerbin departure burn will take a bit more delta-v though).

Edited June 5, 2014 by metaphor

[EdFred]

And I think it also matters whether you hit Moho at its periapsis or apoapsis (or in between), but I can't remember which is better.

An intercept at Moho's Pe will take a bigger burn at Kerbin, but with its velocity relative to Kerbol higher, your insertion burn would be less.

The problem with Moho is that it's going around the Sun really fast. So if you come in with even a small radial component, it's going to take a lot to match Moho's orbit and get captured. Make sure you're crossing Moho's orbit as close to tangent to it as you can, and your capture delta-v will go down by a lot.

And that was the problem. The burn from Kerbin was in line with the dV maps, but I was closer to the second one than the first one when using one mod's transfer window vs KAC's. Using KAC's, I'm much closer to optimal.

[Jouni]

I launched a quick probe to Moho, using the same kind of transfer as I would use for manned missions.

The probe started from a 90 km equatorial orbit around Kerbin. I waited until Kerbin was at the ascending node relative to Moho, and then planned the escape burn (2050 m/s prograde, -1120 m/s normal, 2336 m/s total). This burn was supposed to place my periapsis to touch Moho's orbit, while matching the inclination. Because I started from a low orbit with initial TWR 0.6, the burn didn't go perfectly. I ended up using 2434 m/s for it.

After leaving Kerbin's SoI, I noticed that the periapsis was too low. I burned prograde to raise it, spending 87 m/s.

The interception burn was done at the periapsis. A 480.5 m/s retrograde burn would give me an encounter with Moho after one orbit. This went more or less perfectly.

My final course correction was done at the ascending/descending node relative to Moho, which was conveniently near my apoapsis. By using 2 m/s, I got the closest approach to 33 km.

I entered Moho's sphere of influence from retrograde at a relatively slow speed. The insertion burn was supposed to be 1520.9 m/s, but the burn time was long enough that I actually used 1529 m/s. I ended up at a 39x26 km more-or-less equatorial orbit around Moho, after spending 4532 m/s.

[mhoram]

I did some tests with hohman-transfers from a Kerbin-like Orbit around the Sun to the other planets.

Since Moho is on an eccentric orbit, the ÃŽâ€V for circularization depends on where Moho is on it's orbit when you reach it.

If Moho is near its Periapsis around the Sun, I needed ~2000m/s for the circularization burn.

If Moho is near its Apoapsis around the Sun, I needed ~3100m/s for the circularization burn.