2 questions re ongoing Duna operation

[Progressm]

I recently set up a Duna base at the rim of the large crater just E of Duna's highest peak (Dunampus Mons? Olympus Duns?)

The plan is to rotate a 4-kerbal crew in and out every few launch windows. I thought I would send a dedicated orbit-surface-orbit shuttle to land at the base and fly back to a station in Duna orbit (parachutes, enough delta-v for one return trip, little else).

The Kerbin-Duna-Kerbin trip could be done with a separate craft, mainly consisting of a nuke engine, enough delta-v for a one-way transfer and a few 1-man lander pods for the crew. No reentry apart from aerobraking, just dock to orbital station in each end for refuel & crew transfer.

Now. Two questions, if I may.

1. Equatorial or polar orbit for the Duna 'transport hub' station? Polar would mean easier trips to the surface base, as it is on a northerly latitude... but I don't know what the best way to return to Kerbin from a polar orbit station launch. Equatorial orbit would mean easier Kerbin launch solutions but harder trips to surface base.

2. How does one effectively use the maneuver node to choose equatorial / polar orbits for distant encounters? I can "aim" well enough to Mun / Minmus but can't quite get the technique to do the same with interplanetary transfers. I just get an encounter, capture, go for a high orbit and match/change inclinations from there to save delta-v... but getting aligned right in the first place would be better.

Many thanks and kind regards

Edited September 24, 2014 by Progressm

[KerikBalm]

#1), Normally, you want to eject retrograde to the motion of the planet... this only occurs twice per year for a polar orbit... I don't know how well that would align with the transfer windows.

When your polar orbit is perpendicular to your ejection angle, this will cost you about 930 m/s (if i remember the orbital velocity for low duna orbit correctly).

How far north is your base? If I read your description right, its the crater with a "channel" running towards the main lowland depression?

According to http://www.kerbalmaps.com/ The crater seems to span 6-26 degrees lattitude?

I don't think its worth doing a polar orbit for that. In fact, you really don't need your orbit to go to any higher lattitude than a point oyu want to "service" - you also save some dV from the planets rotation that way, but for Duna, its quite slow. The slow rotation also means the launch windows from the base to the transport hub will be less frequent, but still more frequent than kerbin launch windows.

I think an orbit with a ~10 degree inclination should serve you just fine, or an equatorial orbit since a day on duna is quite long.. 18 hours ie 3 "kerbal days". (day length is related to transfer window frequency from the surface to an inclinded orbit)

If you have NEAR/FAR, which makes aero much better, gliders work better in duna's atmosphere, and you could easily reach that lattitude when dropping from an equatorial orbit. I'm not sure how well it could save you dV on ascent though (if your craft had negligible parasitic drag, and a high L/D, you could use the atmopshere to make a plane change at the equator for "free", assuming your apoapsis is still within the atmosphere, or you are still in enough atmosphere when you cross the equator)

#2) You should do these manuvers at the ascending/descending nodes.

Burn normal to the ecleptic to come in at a higher lattitude, antinormal to come in at a lower lattitude, you should be able to come in right over a pole if you wish (I don't bother with that)

I also use modded "electric ducted fans" for moving about duna (since a rover would take far far too long), and they should work just fine at .1 atm. I only "space" and "de-space" (space=orbit ) from the equator, and then travel by solar powered means from there

[Yasmy]

2) A maneuver node editor such as Precise Node makes fine-tuning maneuver nodes very easy.

A) Drop a node near your current location.

Hit Tab repeatedly until your intended target is your current focus. (Backspace to focus on your ship again.)

C) Zoom in until just your target and your encounter trajectory are visible, ie, well within the target's SOI.

D) Adjust your node until it is where you want it. Zoomed in like this on your target, it is very easy to see the effects of adjustments in the three velocity components (prograde, radial, normal), and thereby easy to judge how to set up the correct burn.

E) If the adjustments are too small to perform accurately with your engines or RCS ports, move the node forward in time and try again.

[Superfluous J]

I doubt it's wroth the effort, but you could orbit polar (or at least put the northern maximum of you orbit over the base) and then when it's time to go home, burn out to about Ike's orbit. Using Ike, you can cheaply align your orbit to equatorial and then you can, on the next orbit, do your ejection burn.

To get back to your base on the way in from Kerbin to Duna, essentially do the same thing backwards. Takes some timing and trickery but you can easily modify your entry time +/- one ike orbit (or any fraction thereof) from Kerbin when you're setting up your burn to Duna.

Like I said, it's a ton of extra work but it'd be a cool technique to get down, IMO.

[Sirine]

Below link should fix your problem, if you play vanilla stock game. Otherwise, many mods can do that.

http://forum.kerbalspaceprogram.com/threads/48342-How-to-warp-into-Eve-Gravity-Influence-in-the-correct-angle

[Progressm]

Thanks guys, input much appreciated!

KerikBalm, that's the crater in question - the first expedition has just set up base at the SE rim above the crater, with nice views of the peaks NE and NW on the other side and the darker area (crater lakebed I guess) below, about 10km North.

The distant mountains in the picture are the SW rim of the crater. The primary habitat module, 2 recon rovers and one mobile emergency shelter are visible - the second emergency shelter is still making its way towards the base, 7 kms away at the crater basin. That's all I could fit in one launch - my two brave explorers seen inspecting the rover are yet to have any way back home until I have designed the Kerbin-Duna-Kerbin shuttle...lucky they brought snacks.

So far the only mod I have is Kerbal Engineer...perhaps I should get Precise node. Also quite like the idea of those electric ducted fans...! I am yet to properly explore with the Rovers but already it seems rather tedious!

[Landge]

Awesome! I've been in the process of setting up a base in the northern latitude of Duna myself. It's situated about 25 km east of a "peculiar" rock formation.

I sent the habitat module out a week ahead of my guys, Bill, Jeb, and Bob. They finally made it to the surface yesterday.

I managed to land the ascent/descent vehicle less that a kilometer away. It landed next to this boulder seen in the distance in both top and bottom pics.

Here is their first sunrise seen from the surface of Duna.

In my case I decided on an equatorial orbit for my orbital station/transfer stage. My lander's 4 LV-Ns has enough fuel to land and return to orbit so it can refuel at either it's transfer stage or the habitat's transfer stage that are still in orbit.

Edited September 24, 2014 by Landge

[Progressm]

Nice! Your mission looks good - a lot heavier than mine, haha!

My rovers, emergency shelters and habitat module total less than 20t payload on surface...I need to gear up my operation and bring some heavy metal to the red planet like you have. :-)

[cantab]

1) Put the station in the lowest inclination that allows the base to pass underneath it. Depending on exactly where you are something like 30 degrees should be fine. That will allow your launches and deorbits to be straightforward and efficient, just be sure to make them at the right time - you'll have one window per Duna day, when the base passes under the station's orbit. It will also mean your interplanetary departures and captures stay straightforward and efficient too - a steeply inclined orbit can be difficult to reach when it's plane is at the wrong angle to the planet's direction of motion.

2) Make a midcourse correction when you're in solar orbit. I think the ideal is when you're at the Duna-relative AN/DN, but anywhere about halfway should be fine. The earlier the less delta-V you need in general, but needing too little makes things imprecise. For fine-tuning try focussing the map view on Duna and using RCS, watching how the trajectory changes with different directions to refine things.

[KerikBalm]

Well... hmmm, I haven't taken any screenshots of my setup on Duna... although I only have one "simulation" - ie a duplicated game that I edited the craft into duna's orbit.

Mods:

NEAR

Custom biomes (helps motivate me to go all over Duna)

SpacePlane+ (which will soon be stock)

Self-made mods:

Nosecones and part adaptors are liquid fuel/Liquid fuel+oxidizer tanks with a 9:1 full:empty mass ratio

Xenon tanks have a 9:1 full:empty mass ratio

Added larger xenon tanks (just the small can, scaled up)

Added a "IntakeAtmosphere" resource + intake module to circular intakes

Added a "ducted electric fan" part, massing 0.4 tons, producing 30 kN of thrust, consuming electric charge in a 45:1 ratio with intake air, with a thrust curve: 0 m/s = 100% thrust; 175 m/s= 70%, 250 = 30%, 275 = 0% thrust.

- Uses the version .18 basic jet graphics (so its visually distinguishable from the current basic jet engine)

ISP: 1800s at 0.3 atm, dropping to 1000 in a vacuum

Science lab boosts transmission science by 2x instead of 1.5x

1st part of the mission, hefting up the tug/crew vehicle, and the massive fuel depot. I didn't take any pictures, but it's quite similar to this one (which is intended for laythe, the tug lacks the hitchhiker container, and carries a large supply of xenon that is not present on the Duna fuel depot, it also carries a little more fuel)

The 2nd part of the mission, was lifting up the all in 1 Sciencelab-habitat-rover, the "ike experiment package" and the lander.

Yes, the Lab-Rover has wings, to allow it to steer to a more appropriate landing site when de-orbiting, also to give it more time to do atmospheric analysis (although this was later rendered redundant by the 3rd launch). It also has a small monoprop supply to deorbit, and to complement the parachutes for a soft landing.

The 2 person lander also has wings to allow it to steer closer to the rover (I don't use mechjeb, and don't want to waste time save scumming, also, NEAR/FAR help those wings actually work). Its also got a small supply of xenon for some extra dV if needed, its designed to be able to go from an equatorial orbit, to the poles, and then back to the depot in equatorial orbit

The 2 person lander was also somewhat redundant, as the 3rd launch was a SSTO space plane, with 2 rapier pods on detachable pylons (with probe cores and parachutes, to allow recovery), it has a very high aspect ratio wing, 2 lift fans, 2 propulsion fans, and a nuke with enough fuel to get it to duna. In duna's low gravity, the lift fans actually allow it to act as a VTOL (yet another reason I'd like ISP curves to scale thrust, not propellant consumption), and with a refuel at the depot before de-orbiting, it can function as a duna SSTO to ferry crew to and from orbit.

It masses more than the 2 person lander, and requires 2 trips, but it can get to roughly 250 m/s and relatively high altitudes in Duna's atmosphere on its electric fans alone.

Also, as it can just fly to the poles, and back to the equator, it can save a lot of fuel for polar expeditions.

For the moment, I plan to use the lander only fuelled enough to make trips to and from the equator, and the electric fan craft for expeditions elsewhere.

I'm also planning another heavy lift spaceplane to drop one (or more) larger, stationary, surface habitat + surface fuel depot (I love the new Claw part, it makes linking things on the ground so much easier) + better rovers (that science lab rover is rather meh... I gave it a pretty low CG and wide wheelbase... but I could do much better with a smaller rover)

Duna wil be Kerbalized!

If you think about it.... much of the surface is over the armstrong limit... it gets to nearly .2 atm, which is easily enough for liquid water to exist, temperature readings suggest it should get warm enough to melt ice, and its got plenty of ice.... its a much better terraforming candidate than mars.

[Landge]

My equatorial orbit is 150 km. I eyeball my orbiting target to be 20~30 degs ahead of passing my lander's longitude for my time to launch. Bearing 90 degs east, I aim for an alt of 100 km. There I circularize orbit, with maneuver node, to intersect, within 5 km, on the opposite side of the planet. My orbit is still out of plane with my target at this time. Once my navball changes to target mode I make my retro burn to zero out my velocity to target, aligning the orbits at the same time. Then it's just a matter of closing the distance and dock.

Edited September 24, 2014 by Landge

[Skorpychan]

#1: Neither. Prograde orbit over the base site, so you use less tug fuel.

Incidentally, nuclear engines are inefficient for landers. Yes, they have a high ISP, but they're HEAVY, and have piddly thrust. An LVT-454 will work fine.

[tsotha]

A polar orbit would probably be cheaper than a gravity launch and a inclination correction. And, as 5thHorseman pointed out, you can use Ike to fix your Kerbin inclination on the way back.

[Landge]

#1: Neither. Prograde orbit over the base site, so you use less tug fuel.

Incidentally, nuclear engines are inefficient for landers. Yes, they have a high ISP, but they're HEAVY, and have piddly thrust. An LVT-454 will work fine.

Do you mean the LV-909? I tried the 909 first because of the low profile. But ultimately went with LV-Ns for better fuel economy. Even in Duna's thin atmosphere I think I was able to retain more fuel getting back into orbit.

[-Velocity-]

#1: Neither. Prograde orbit over the base site, so you use less tug fuel.

Incidentally, nuclear engines are inefficient for landers. Yes, they have a high ISP, but they're HEAVY, and have piddly thrust. An LVT-454 will work fine.

Depends on where you're trying to land. Nuclear engines are the best lander engines there are for low gravity worlds with no atmospheres. Duna is somewhere in-between, but even on Duna I think nuclear engines are likely to be very, very good. Even if Duna has enough of an atmosphere and enough gravity to make nuclear engines not the best lander engine, the fuel savings you get during orbital maneuvers may make up for it.

[-Velocity-]

What about putting your orbital base on something like a 6 Duna-sidereal day (or whatever is appropriate) polar, highly elliptical, synchronous orbit with the apoapsis being in-plane with the plane of the solar system. The orbital base will pass almost right over the surface base once every orbital period, as the orbit is some integer multiple of Duna's sidereal rotation rate. Even though the orbit's apoapsis is outside Ike's orbit, because the orbit is polar, the spacecraft will never cross into Ike's SOI as Ike's orbit will reside entirely within the orbit.

As the orbital base's periapsis only just above the atmosphere (say, maybe 60 km), then you only need to provide a small amount of delta-V to land on the planet from the orbital base- all you need to do is to dip the periapsis of a lander from like 60km to like 15 km and you should aerobrake away your orbital velocity and land.

To leave Duna's SOI from the orbital base, you might want to switch from a polar orbit to an equatorial orbit first. This won't cost very much delta-V, as you will be moving very slowly at apoapsis and additionally, as already mentioned, you'll be in the equatorial plane of the solar system. Once you are in an equatorial orbit, you can then do any number of things to return to Kerbin- Ike slingshot/orbital modifcation, or, easiest option- simply providing a little extra prograde delta-V at periapsis.

So the advantages of this are several:

1) It takes almost no delta-V to land;

2) You've got launch and landing windows to/from your orbital base every X number of Duna days- every time the orbital base passes overhead of the surface base;

3) It takes very little delta-V to leave Duna's SOI from the orbital base.

Disadvantages:

1) Rendezvous with the orbital base can be somewhat time consuming as the orbital period is rather long (but isn't that what time compression is for anyway?)

2) As already mentioned, when departing Duna, you might find it advantageous to do a plane-changing maneuver at apoapsis to take you from a highly elliptical polar orbit into a highly elliptical equatorial orbit. This will cost a small amount of delta-V (100 m/s? You won't be moving very fast at apoapsis, as you are so far away from the planet).

Edited September 24, 2014 by |Velocity|

[Progressm]

Hmm for single purpose crew transfer orbit-surface-orbit vessel I expect 48-7s to do well...not much orbital maneuvering needed so to minimize mass is my gut feel.

Edit: as to the above post, holy sidereal orbits, batman! I need to read that through at home tonight to internalize the idea, I think.

One option not discussed yet would be to have the base on equatorial Ike orbit.. unsure if that would make any sense, can't see why it would but who knows.

Edited September 24, 2014 by Progressm

[KerikBalm]

Regarding the LV-N vs other rockets - some might bring up Tavert's excellend optimal engine charts... but they have one flaw: they are mass optimal, not fuel optimal.

If one is going to re-use a lander over and over again, you want a fuel optimal engine.

Considering the best "chemical" ISP is 390, and the LV-N is 800, then if the crafts dry mass with the LV-N is less than 800/390 times the dry mass with the "chemical" engine, then for *any* delta-V, the LV-N is fuel optimal.

If its not, then it becomes more complicated, as you need to use the rocket equation to determine the mass ratio you need to acheive a target delta-V.

To get to orbit from Duna, you need a delta-V over 1300 m/s. In all my lander designs, this strongly favors the LV-N.

Even more so if I make a 2+ kerbal lander (even using 2x 1 seat lander cans, which are mugh liger than 1x 2 seat lander can).

Even on the Mun, I used a LV-N, but that was because I was also carrying up goo and mat bays to the orbiting fuel depot+ science lab - when farming biomes, I very much consider fuel optimal solutions.

Yes, the LV-N is ungainly... its too long... but I get around this with some structural elements (two side mounted T-200 tanks with structural fusalages beneath them, and then the tiny girders at the base, before adding struts), which add to the dry weight even more, but for the dV needed to get to orbit from duna + my requirement for a capacity of 2 kerbals (or rather, considering fuel use per trip * trips needed to get all the kerbals back), the LV-N comes out way ahead for me for use on Duna.

[Progressm]

That makes sense. Fuel optimal is key, at least until my kerbals discover ISRU. (I have been eyeing up kethane mod, I have just been introducing mods very slowly, as I wanted to get good at stock first - so far only using kerbal flight engineer for info)

Oh, on a semi-related side note, your thoughts on terraforming (kerbaforming?) Duna reminded me of the Mars Trilogy of books by K.S. Robinson. I'm up to the second part now and liking it - check it out of you haven't.

[-Velocity-]

Hmm for single purpose crew transfer orbit-surface-orbit vessel I expect 48-7s to do well...not much orbital maneuvering needed so to minimize mass is my gut feel.

Edit: as to the above post, holy sidereal orbits, batman! I need to read that through at home tonight to internalize the idea, I think.

One option not discussed yet would be to have the base on equatorial Ike orbit.. unsure if that would make any sense, can't see why it would but who knows.

It's incredibly simple. It's just an orbit that takes some integer multiple of the planet's rotation period to complete. The object in such an orbit orbit will always pass over the same point during its close approach. The catch is you need to make the time to complete the orbit an integer multiple of the sidereal day, NOT the solar day. The Earth spins once every 23 hours and 56 minutes; however because the Sun is slowly moving from west to east against the background stars (due to Earth's orbital motion around it), it takes 4 extra minutes each day for Earth's rotation to "catch up" with the new position of the Sun in the sky. So a SOLAR day lasts 24 hours.

[Laie]

Incidentally, nuclear engines are inefficient for landers. Yes, they have a high ISP, but they're HEAVY, and have piddly thrust.

The piddly thrust is enough to lift itself plus 16 tons at Duna -- make that 10-12 tons if you want a reasonable TWR. Duna is perhaps the heaviest place where LV-Ns can still be useful.

Do you mean the LV-909? I tried the 909 first because of the low profile. But ultimately went with LV-Ns for better fuel economy. Even in Duna's thin atmosphere I think I was able to retain more fuel getting back into orbit.

As always, it depends a) on the weight of your vessel on how much delta-V you get out of the engines. If the engine would double the weight of your lander, it's kinda pointless. If the engine only ever gets used for landing and takeoff, it's usually better to use something more lightweight.

However, if your lander carries a lot of science gear (or maybe even a whole lab); and/or if the lander can go idependent for a while, visiting both Ike and Duna before returning to the mothership; and/or if the vessel is built in such a fashion that the lander engines can also power the interplanetary part of the voyage; then, by all means, the LV-N shouldn't be discarded outright.

[Skorpychan]

My landers are built around either four LV-909s or a single LV-454. Works great, right up to fairly hefty landers with two-kerbal cans and science gear and so forth. The absolute limit is a decent-size kethane tank on a Mun lander; it just about makes it to the low-orbiting station on full tanks.

I prefer a high TWR for my landers over a high ISP; Efficiency won't help when you're still going 100+m/s at a few thousand meters. Plus, as interplanetary engines, they're long and unwieldy, and more than a little fragile.

Compare being able to stick a lander down on Kerbin at ~5m/s onto the -454 engine bell without issue, and my 'landing' at Gilly with a cruise stage that simply snapped the LV-N off the bottom of the fuel tank.

Quote marks on 'landing' because it was more of a docking maneuver than anything else.

[cantab]

There's no engine called the LV-454. Do you mean the LV-T45?

[Progressm]

In the Duna surface/orbit shuttle's case, you don't need high TWR, just enough to operate reasonably; the parachutes take care of most of the deceleration. I would be surprised if someone proved a different engine to LV-N / 48-7s to be the best for the job.

Thanks for the sidereal explanation, that mostly makes sense to me now.

One thing, though; does a rendezvous with a station on highly elliptical orbit cost more delta-v? I'm showing my hazy understanding of orbital mechanics here, but oh well; in order to dock, I need to match speeds and stationkeep before docking, right. Do I not end up accelerating to a high elliptical apoapsis in the process if that is the station's orbit?

Edited September 25, 2014 by Progressm

[-Velocity-]

One thing, though; does a rendezvous with a station on highly elliptical orbit cost more delta-v? I'm showing my hazy understanding of orbital mechanics here, but oh well; in order to dock, I need to match speeds and stationkeep before docking, right. Do I not end up accelerating to a high elliptical apoapsis in the process if that is the station's orbit?

Yea, of course you'll have to match its orbit, and it will take more delta-V to do so since the orbit is elliptical. However, that is not necessarily detrimental because like 90% of that delta-V is part of the delta-V you have to expend ANYWAY to leave Duna's SOI. It does matter if you're doing a bunch of ferrying between the orbital base and the surface- but the benefit of synchronous orbit, and Duna's relatively weak gravity (which means that the extra delta-V you need might only be like 300 or 400 m/s) should outweigh that downside even in this case.

The synchronous orbital period of the orbital base doesn't necessarily make launch windows so much as landing windows. To launch and rendezvous with the base, you would probably launch your lander from the surface a day before the base reaches periapsis, and go into a circular orbit that is co-planar with the orbital base's orbit. Next, you would modify your lander's orbit so that its periapsis (or apoapsis) is the same as the orbital base's periapsis. To complete the rendezvous (minus the final velocity matching maneuver), your lander would make a prograde burn at the orbital base's periapsis, making your orbit increasingly elliptical until it reaches the periapsis point at the same time as the orbital base. You'll end up rendezvousing with the orbital base as it passes over the equator, just before it passes over your northern hemisphere surface base.

Also, for aerobraking and landing, for a northern hemisphere base, you'll want the orbit of the orbital base to come in over the south pole so that it is travelling from south-to-north as it makes its closest approach. If not, you'll be coming in rather steep when you land.

Edited September 25, 2014 by |Velocity|