Delta-V to reach Moho orbit

[Temstar]

That delta-V map with the lines and circles says from LKO it's about 2500m/s to get an encounter with Moho then 2200m/s to slow down into a Moho orbit. But repeated attempts by yours truly seems to show that while the delta-V to Moho is correct, the delta-V to get into a Moho orbit is more in the area of 4600m/s. I've tried reaching Moho both at its perihelion and aphelion (perikerbolion/apkerbolion???) and it doesn't seem to make much difference in this delta-V requirement, certainly not by 2400m/s.

Has anyone managed a 2200m/s Moho orbit insertion? Where does this number come from?

[Fried]

2200m/s I would assume is the amount excluding human error, and or is just the amount to make orbit not a 100km orbit.

[creator1629]

hey Temstar i have also had the same problem, been trying to get to moho but the delta v requirement to get captured by moho is usually around 4500 m/s and as a result of that massive burn i don't have enough fuel to bring jebediah back home. may i ask what ship design you're using? how many atomic engines and how much fuel you bring?

[Temstar]

hey Temstar i have also had the same problem, been trying to get to moho but the delta v requirement to get captured by moho is usually around 4500 m/s and as a result of that massive burn i don't have enough fuel to bring jebediah back home. may i ask what ship design you're using? how many atomic engines and how much fuel you bring?

I was trying to land a robotic rover on Moho:

Rover + lander at launch on top of a Zenith IV booster. The lander is attached upside down due to form factor, it has 6949m/s of delta-V which I thought was enough.

Transfer to Moho. When I setup that burn to insert into Moho orbit I realized that the delta-V map numbers are either very optimistic or flat out wrong. Fortunately I had just enough delta-V to insert the craft into a 51km x 180km elliptical polar orbit around Moho.

I had the foresight to include a docking port on the lander, to salvage the mission I decided to send a refuelling tanker to Moho

The tanker is similar size as the lander and has just a hair under 10,000m/s of delta-V

After successful insert into Moho orbit, the phase and inclination adjustment was fortunately not that high as I was smart enough to do it before circularising, despite an inclination difference of more than 100 degrees.

Tanker approaching the lander for refuelling. I managed to deliver over 800L of fuel to the lander, giving it slightly over 2000m/s of delta-V. The tank is deorbited to crash into Moho using the remaining RCS fuel.

Lander firing up engines for powered descent to near the north pole.

Managed to land with 766m/s of delta-V left as well as loads of monopropellant.

Rover deployed and on its way to investigate the north pole "Mohole".

Edited April 30, 2013 by Temstar

[saik0]

The large range of delta V requirements for Moho comes from it's orbital eccentricity. If you meet up with Moho at it's periapsis you're going much faster, so you need to slow down more to achieve an orbit. The inverse is true when encountering Moho at it's Ap.

[Conarr]

hey Temstar i have also had the same problem, been trying to get to moho but the delta v requirement to get captured by moho is usually around 4500 m/s and as a result of that massive burn i don't have enough fuel to bring jebediah back home. may i ask what ship design you're using? how many atomic engines and how much fuel you bring?

I had the same issue: Poor Bill. We'll rescue him someday.

[Temstar]

One good thing about rescuing from Moho is how often the launch window comes up and how quick the transfer is. I think you get a Moho transfer window about once every 30 days and it's taken me as short as 25 days to reach Moho from Kerbin. So if you have someone stuck on Moho you could just send tanker after tanker every month until a big enough stockpile of fuel is built up in Moho orbit for a return trip.

[azazel1024]

Dunno. I never did the math or paid super close attention, but, yeah, it seemed to be in the 2km/sec dV range for proper capture once I got there. I did change my orbital inclination in advance though and hit it at its AP. I also had reduced my orbit somewhat, so my AP was not still at Kerbin. My AP was maybe, roughly somewhere in the region of Eve's orbit with my PE at Moho's PE.

If I had to take a WAG I'd say my dV to get to Moho was more in the 3-3.5km/sec range and proper capture and 150km circular orbit was in the 2km/sec dV range. I am pretty sure my overall design did not generate more than about 5km/sec of dV.

[Deep silence]

I am trying to efficiently get on Moho too... From what I can understand, delta-v depends on the eccentricity and the orbit inclinations :

In order to minimise the delta-v, one must reach a planet its apo (less planet's speed, less delta-v required to get into orbit), and/or at its ascent or descent node (to avoid the orbit alignement burn). Luckily, it looks like both apo and ascending node of Moho are at the same place (approximately...).

So the transfert burn must be done when Kerbin is at the opposite of Moho's apo (the orange thing is the 'perfect' transfert orbit) :

The problem is that the combination of the Kerbin position and the Kerbin to Moho phase angle happens only every 5 years (approximately : phase angle increments by 70° every year). I am trying to obtain this combination to check the effective delta-v used.

[Deep silence]

My best score right now : 1500m/s to intercept Moho, and 3300m/s to get into orbit. This is really tricky though... I had to use Mechjeb to trim the maneuver node, sometime by 0.05 m/s steps.

[Oggula]

What the guy above said. Reaching Moho at its periapsis should minimize the fuel needed. Frankly I have no idea how you need 4500m/s though. My best transfers have required on the order of 2500m/s and my worst, which crossed Moho's orbit drastically, still required only 3800m/s.

[maltesh]

What the guy above said. Reaching Moho at its periapsis should minimize the fuel needed. Frankly I have no idea how you need 4500m/s though. My best transfers have required on the order of 2500m/s and my worst, which crossed Moho's orbit drastically, still required only 3800m/s.

4500 is definitely within the realm of possibility.

The following is a graph of velocity at reaching Moho orbit. The horizontal axis is distance from the sun in millions of kilometers, the vertical axis is velocity in kilometers per second. Click the image to go to the Desmos Graph, and its calculations.

The orange curve is the sun-relative velocity of Moho as a function of its distance from the sun.

The blue curve is the periapsis velocity of a transfer orbit from Kerbin's distance as a function of periapsis distance.

The green curve is the difference between Moho's velocity and the Transfer Periapsis velocity.

The red curve is the relative arrival velocity at Moho, taking into account Moho's eccentricity (though not its inclination.)

For a perfect hohmann to Moho periapsis, given the simplifying assumptions, arrival relative velocity is about 2.4 km/s. For a Hohmann to apoapsis, arrival relative velocity is about 3.7 km/s.

The arrival relative velocity peaks at about 4.45 km/s, at a distance from the sun of about 5.3 million kilometers, slightly further from the sun than Moho's semimajor axis (5.26 million kilometers, represented by the black vertical line.)

And of course, Moho's relative inclination would crank that arrival velocity higher, as well as if the player cut inside Moho's orbit to catch up to it.

[cxg2827]

I must be doing something pretty wrong with my launch. I've done the transfer burn, and followed up with a 1300 delta-v burn a few hours outside kerbins SOI. But, when i get into Mohos SIO, its showing that i needed 4800 m/s for the capture. The interception is maybe 20 degrees leading from mohos Periapsis.

does the capture delta-v have any major variation depending on where in the range of the SOI you get the PE?

[tavert]

does the capture delta-v have any major variation depending on where in the range of the SOI you get the PE?

Yes, definitely. If the periapsis of your incoming hyperbolic encounter is low, much of your Moho-relative potential energy will be exchanged for kinetic energy. The orbit speed at low altitudes is higher, but the speed you gain by dropping lower in altitude outweighs the increased target orbital speed. However if you plan on landing, you do want to come in with as low a periapsis as possible (assuming you do the course correction far enough away that its cost is negligible), because the net increase in the capture burn is smaller than the increased cost of landing at a higher speed from a high orbit (or transferring down from a high capture orbit to a low pre-landing orbit).

Maltesh, those plots are great. I might have to learn to use desmos... I think the moral of the story with Moho, Dres, and Eeloo is don't trust the transfer maps, just do the math to be sure you're getting a sane number.

Edited May 18, 2013 by tavert

[TheNorthFace]

Yes, definitely. If the periapsis of your incoming hyperbolic encounter is low, much of your Moho-relative potential energy will be exchanged for kinetic energy. The orbit speed at low altitudes is higher, but the speed you gain by dropping lower in altitude outweighs the increased target orbital speed. However if you plan on landing, you do want to come in with as low a periapsis as possible (assuming you do the course correction far enough away that its cost is negligible), because the net increase in the capture burn is smaller than the increased cost of landing at a higher speed from a high orbit (or transferring down from a high capture orbit to a low pre-landing orbit).

Maltesh, those plots are great. I might have to learn to use desmos... I think the moral of the story with Moho, Dres, and Eeloo is don't trust the transfer maps, just do the math to be sure you're getting a sane number.

Sorry for necroing this thread, but this information is very useful and I just want to make sure I understand this correctly:

- The higher the periapsis of your capture burn, the less deltaV you need to get captured?

If so, is this very significant when performing a non aerobrake capture with Jool? (I use ferram aerospace).

[AmpsterMan]

Sorry for necroing this thread, but this information is very useful and I just want to make sure I understand this correctly:

- The higher the periapsis of your capture burn, the less deltaV you need to get captured?

If so, is this very significant when performing a non aerobrake capture with Jool? (I use ferram aerospace).

I am no rocket scientist, but I believe this to be the case. Unless I can aerocapture at my destination, I usually try to enter as far away as possible.

[maccollo]

I am trying to efficiently get on Moho too... From what I can understand, delta-v depends on the eccentricity and the orbit inclinations :

In order to minimise the delta-v, one must reach a planet its apo (less planet's speed, less delta-v required to get into orbit), and/or at its ascent or descent node (to avoid the orbit alignement burn).

Your approach is almost correct, but it is optimal for a flyby, not capture. To get captured around Moho with the smallest amount of delta V you want to approach it at periapsis.

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

If you're really OCD about doing everything as efficiently as possible, like getting into a Kerbin orbit that will allow you to burn prograde into the correct ejection angle, you can probably save another 100 m/s or so.

Edited September 15, 2014 by maccollo

[Aethon]

Yep the link above is the way to go and I agree this (and the link above) are too important to not necro.

http://steamcommunity.com/sharedfiles/filedetails/?id=313807386

I landed on Moho last night using macollos' method and it's slicker than snot on a broom handle. Basically you leave LKO on Mohos' AN/DN.

The first time this occurs ( IIRC the AN ) is ( Earth ) Year 1 Day 21, on or about Hour 17. Set Moho as your target and drag a dummy Kerbin escape node out to see where the AN/DN is. I combined my Kerbin escape burn with my inclination burn at a total cost of 2300 dv. At my solar periapse ( which was also Mohos' solar periapse ) I burned 1800ms retro ( as Kashua suggests. See link ) to make sure I encountered Moho on my next orbit, which lowered my solar apoapse and slowed the eventual Moho encounter down dramatically. My capture burn at Moho was around 1000ms.

Didn't know this thread was going to pop up or I would have documented better.

Use it! Props to macollo.

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

Edited September 15, 2014 by Aethon

[Hannu]

Moho's orbit is eccentric and inclined. Needed dv from Kerbin's orbit to Moho's orbit depends on geometry and can vary more than 1000 m/s between launch windows. Overall dv-maps are almost useless. It is much better to use a porkchop plot calculator, for example Alexmoon's one.

Burns are very sensitive to errors. You must be very careful, because you can get a Moho encounter quite easily even if your parameters are slightly inaccurate. However, when it is time of the Moho orbit insertion, you notice that required dv is 1500 m/s more than calculated. You should use Mechjeb or some other precision node editor and adjust your burn very carefully. Needed accuracy is 0.1 m/s and one second in time. Calculated values are typically inaccurate by some tens of m/s because the KSP's patched conic approximation is inaccurate, so you have to iterate values by hand. I have found that it is better to leave from a quite high orbit (300-600 km), because calculators assume instantaneous velocity change and practically it takes minutes and causes the more severe inaccuracies the lower the initial orbit is.

You should make trajectory checks and possible corrections immediately after leaving Kerbin's SOI, about halfway, about 10-20 days before encounter and last check couple of days before encounter. I have noticed sometimes strange anomalies in trajectories to and from Moho with versions 0.20-0.23 and do not know are they due to KSP or mods. They are difficult to repeat. Be careful and check the situation often. My best transfers have needed about 100 m/s more than calculated optimal. 200-300 m/s more is still quite a good transfer.

[Red Iron Crown]

Sorry for necroing this thread, but this information is very useful and I just want to make sure I understand this correctly:

- The higher the periapsis of your capture burn, the less deltaV you need to get captured?

If so, is this very significant when performing a non aerobrake capture with Jool? (I use ferram aerospace).

The lower the periapsis, the less delta-V required to capture due to the Oberth effect. Use radial and normal burns as you approach (the sooner the better) to get your periapsis in the plane you want and as close to the atmo as you dare to minimize dV expenditure. Use your capture burn to set the Ap to the desired altitude (likely matching whatever moon you wish to visit).

[SRV Ron]

2200m/s I would assume is the amount excluding human error, and or is just the amount to make orbit not a 100km orbit.

That amount would be based upon a direct intercept from Kerban at the point where Moho crosses the Orbital Plane. Those windows are far more rare then 30 days.

[FacialJack]

Hi all,

At my first attempt, i reach 3400 m/s dV for Moho Orbital transfer,

I easily make my orbit (have enough dV), but as i plan to land a probe on it, and play wiz remotetech which add delay to com, i failed a lots of landing due to delay !! (Really hard to make a correction burn wiz 1.20 min of delay ). I was finally forced to accept Mechjeb as my friend (grrr, but i use it just for landing, no other action, as im all new, i dont want mechjeb steal my pleasure ), when i read that even curiosity have a flight computer for landing as mars atmo cut radio links !!

But, wiz these multiple try, i made several orbital insertion, and my best score was close to 1700, far lower from 2500 !!

I just respect the launch windows give by kerbalarmclock, one burn for exitinig Kerbin SOI, a An manoeuvre for matching plan and manage encounter wiz Moho, and an orbital insertion final burn.

My best technique, is burn backward of Kerbin move (lowering Pe relative to Kerbol), for just exiting Kerbin SOI, not even cross my target orbit, then plan a manoeuvre at the An (or Dn), and then see if i encounter or not my target (i mean, even if u match plan and cross targets orbit, it can be too early or late)

The good way of that technique, is that if i cant reach target, i just have to make a correction burn when still in Kerbin SOI, for going a few quicker or slower, in way to authorize encounters.

Sry, im not eng, and thats hard too explain for me. Try again:

- One burn exiting Kerbin SOI (must be 950m/s i think), no plan match, no orbit cross, just out of Kerbin SOI, backward kerbin move for inner planets, forward for outer

- Plan a manouevre at Dn or An, which permit to cross orbit, and match plan

- If u come too quick/slow, adapt ure speed when ure still in Kerbin SOI (on the still blue part of ure trajectory), and play wiz the An/Dn node for matching target, playing wiz these two spot permit too easily reach anything

Not sure i help, but i try

Edited September 15, 2014 by FacialJack

[TimberWolffe]

I mean, I've gotten to Moho with a spaceplane. But I had 2xSabre-M and 8xSabre-S so I'm not sure just how well I can help here

[magnemoe]

Sorry for necroing this thread, but this information is very useful and I just want to make sure I understand this correctly:

- The higher the periapsis of your capture burn, the less deltaV you need to get captured?

If so, is this very significant when performing a non aerobrake capture with Jool? (I use ferram aerospace).

Not sure how important it is for Jool who has an pretty circular orbit, Gilly is probably the most extreme example, less than half the cost at Pe.

[John FX]

Hi all,

At my first attempt, i reach 3400 m/s dV for Moho Orbital transfer,

http://cloud-4.steampowered.com/ugc/27339016374466087/A43F5C5FEFF1D62C07A09F2ADB8CAFEB4E6130B7/

I easily make my orbit (have enough dV), but as i plan to land a probe on it, and play wiz remotetech which add delay to com, i failed a lots of landing due to delay !! (Really hard to make a correction burn wiz 1.20 min of delay ). I was finally forced to accept Mechjeb as my friend (grrr, but i use it just for landing, no other action, as im all new, i dont want mechjeb steal my pleasure ), when i read that even curiosity have a flight computer for landing as mars atmo cut radio links !!

But, wiz these multiple try, i made several orbital insertion, and my best score was close to 1700, far lower from 2500 !!

I just respect the launch windows give by kerbalarmclock, one burn for exitinig Kerbin SOI, a An manoeuvre for matching plan and manage encounter wiz Moho, and an orbital insertion final burn.

My best technique, is burn backward of Kerbin move (lowering Pe relative to Kerbol), for just exiting Kerbin SOI, not even cross my target orbit, then plan a manoeuvre at the An (or Dn), and then see if i encounter or not my target (i mean, even if u match plan and cross targets orbit, it can be too early or late)

The good way of that technique, is that if i cant reach target, i just have to make a correction burn when still in Kerbin SOI, for going a few quicker or slower, in way to authorize encounters.

Sry, im not eng, and thats hard too explain for me. Try again:

- One burn exiting Kerbin SOI (must be 950m/s i think), no plan match, no orbit cross, just out of Kerbin SOI, backward kerbin move for inner planets, forward for outer

- Plan a manoeuvre at Dn or An, which permit to cross orbit, and match plan

- If u come too quick/slow, adapt ure speed when ure still in Kerbin SOI (on the still blue part of ure trajectory), and play wiz the An/Dn node for matching target, playing wiz these two spot permit too easily reach anything

Not sure i help, but i try

http://cloud-4.steampowered.com/ugc/27339016412179446/5B2215AB9C8FD2FFCA1FF3A3EBD7959FE7368082/

What mod gives those info displays next to your navball?