Tips on interplanetary travel?

[Duck McFuddle]

So I have sent some probes to Jool and Duna, but I was wondering if anybody has some tips on getting Kerbals back from Duna, which I am going to try first. Whenever I get there, I never seem to have enough fuel to get back to Kerbin. I sent a probe Rover to Duna that I successfully landed, but there was no hope of getting back with the fuel and dV I had left. Any tips on making ships more efficient? Thanks! 

 

[Geonovast]

The key is to build backwards.  Start with the lander.  Make sure it has enough dV to get down to Duna, then back into orbit, with some juice for a rendevous.  Then build the orbiter, make sure it has the dV to return from Duna with the lander attached, if you're taking it with.  Then build the transfer stage to get from Kerbin to Duna, etc.

A good thing about Duna is its moon Ike, which is a really good place to mine fuel.  So if you build your ship with a mining drone, you can go up to Ike and refuel while your lander crew is doing their thing.

[The Aziz]

If you have any sort of D-v display, you may find this useful: 

So basically around 1500 for the lander plus roughly the same for Duna transfer and orbit insertion, and another, I think, a thousand for return (when you don't aerobrake at Kerbin). However, for efficiency, I'd just leave the lander in Duna orbit after you transfer whatever you need because that's just unneeded dead weight on the way back home. Next time you'd only have to refuel and use it again.

[Gargamel]

The simple answer is to just add more fuel.  Leave most of it in orbit around Duna, take what you need and land, return, then use that fuel to come home. 

[Snark]

On 6/9/2018 at 1:24 PM, Duck McFuddle said:

So I have sent some probes to Jool and Duna, but I was wondering if anybody has some tips on getting Kerbals back from Duna, which I am going to try first. Whenever I get there, I never seem to have enough fuel to get back to Kerbin. I sent a probe Rover to Duna that I successfully landed, but there was no hope of getting back with the fuel and dV I had left. Any tips on making ships more efficient? Thanks! 

Going to Duna is easy:  assuming you aerobrake and use a parachute, it actually takes considerably less dV to land there than it does to land on the Mun.

From LKO:

• Landing on the Mun:  860 m/s transfer burn, plus 310 m/s braking to orbit, plus 580 m/s to land = 1750 m/s from LKO
• Landing on Duna:  1080 m/s transfer burn.  Aerobrake (free), land with parachute (free).  Ta dah! 

 

Coming back from Duna, though, is more expensive than coming back from the Mun:

• From the Mun:  580 m/s to orbit, plus 310 m/s to escape and head homeward:  890 m/s
• From Duna:  about 1200 m/s to orbit, then 610 m/s to head home, total 1810 m/s.

So, from LKO, you can do a round-trip Mun landing for around 2640 m/s, and a round-trip Duna landing for around 2890 m/s.

So, the Duna mission only needs about 250 m/s more than a Mun mission! 

 

Thing is, though... that it's different.  And so you need to adapt your strategy accordingly.  (The following advice assumes you're simply doing a Duna-and-return mission without involving Ike, just to keep things simple.  If you mine fuel from Ike, then of course it's a whole different ball game.)

First, make sure you don't waste fuel landing on Duna.  It's easy to faceplant there, since the atmosphere is so thin that if you Do It Wrong™, you can hit dirt at speeds much higher than parachutes can cope with.  There are a variety of landing strategies, but my personal favorite is this:

• Put a pair of tiny drogue chutes on the lander, and one regular parachute.
• Drogues are set to open at the highest possible height, 5000 m.  Regular parachute set to open at 1500 m or so.
• Drogues kick in at around 600 m/s when low enough, which then slows the ship to the point that the regular chute opens.
• The regular chute won't slow it anywhere near enough to land-- it'll be like 30 m/s as it descends to the surface.  But then give it just a very brief blip of rocket power right at the moment of touchdown to cushion the landing.

It's actually possible to spam enough parachutes to land on Duna on chutes alone... but I like doing the above, instead.  Reason:  you need so many parachutes that they're heavy.  The fuel you burn in that little-blip-right-at-landing is a lot lighter than all the extra parachutes, so it saves mass.

Second, take off efficiently from Duna.  Its atmosphere is a lot thinner than Kerbin's, and your local TWR is likely to be higher, so you can do a significantly more aggressive gravity turn.  Saves dV.  Do it right, and you can get to low orbit for around 1200 m/s.

One decision point is whether you want to do a direct land-and-return mission, or do an Apollo-style orbital rendezvous.  Both of these can be made to work, and it's not that big of a deal either way.  It's a matter of your personal preference and play style:

A direct land-and-return means that your lander has to have at least 1810 m/s of dV remaining, after landing.  So that's a respectable chunk, and it means your lander's going to have to be fairly good-sized, which in turn means it's going to be a fairly big (and therefore more expensive) craft upon takeoff from Kerbin.

In the Apollo-style mission, you leave an orbiting vehicle around Duna, and the lander only carries enough fuel to descend to the surface and then return to orbit.  There are a few ways to slice this.  You could make the orbiter be just a fuel tank, i.e. the lander is also the Kerbin return vehicle, and it simply refuels from the tank upon returning to orbit.   Or you could make the orbiter be the return vehicle, with its own crew pod and engine, so that you leave the lander parked in Duna orbit when you go home.  Either way works.

The Apollo-style mission allows for a smaller craft, because that's a big chunk of fuel that you're not schlepping down to the surface of Duna and back again.  This means that the overall mission can be smaller and therefore cheaper.  However, it does present more of a navigational challenge.  First, it means that on initial approach to Duna, you need to aerobrake to low orbit rather than descending directly to the surface, which is certainly doable, but it's challenging-- Duna's atmosphere is so thin and so vertically shallow that the difference between "aerobrake too much and go down to the surface" and "aerobrake not enough and go flying off into space" can literally be only a few hundred meters of altitude.  So it's finicky and will probably take a fair amount of trial-and-error to get right.  Also, it means that you need to be able to do orbital rendezvous and/or docking.

So, what it boils down to is:  Apollo-style is smaller and cheaper but needs more piloting and navigational skill; direct land-and-return is bigger and more expensive, but simpler to do.  Pick what works best for you. 

Also, make sure you have a good transfer window, both going to and returning from Duna.  The numbers quoted above assume an ideal transfer window.  If you've got a bad window, you would need a lot more dV.  There are a variety of tools out there to help with this-- my personal favorite is http://ksp.olex.biz because I like its simple interface and graphical display.  The TL;DR is that you should launch to Duna when Kerbin is 44 degrees behind it, and return from Duna when Kerbin is 75 degrees behind it.

 

And finally, the emergency-when-all-else-fails thing to bear in mind:  Kerbal EVA packs have a whopping 600 m/s of dV in them.  So, if you find that you've shaved it a bit too close and you don't quite have enough fuel to make the return-to-Kerbin burn from Duna... you could always go EVA, grab the science out of the ship, and then just thrust your way home that way.    If you can get to low Duna orbit, a kerbal has nearly enough EVA juice to fly all the way home to Kerbin.  Of course, there's no way the kerbal would survive Kerbin reentry... but you could always put them on a near encounter with Pe just above atmosphere, and then send up a rescue ship to snatch them as they go whizzing past.

[CrashyMcCrashFace]

Are you leaving at a planned time or just whenever? It will make a big difference. A way to leave without planning is to leave Kerbin with just enough to escape SOI and go into a solar orbit. Then plan out your maneuver way ahead of time and >>> to it. You can come back this way too.

I'm going to venture a guess that going direct is still more efficient.

[Duck McFuddle]

I succeeded in landing on the red planet, and have a drilling rig for mining and maybe a refuel at Ike. Thanks for the help!