Duna and Back

[Poky22]

I need help at building a ship. Ive gone to Duna 3 times but couldnt get back. What should I do my building skills are very poor. TY

[Plusck]

You need about 600 m/s dV to circularise at Duna.

Since getting back from any other planet or moon orbit always needs exactly the same dV as it took you to get there to start with (assuming you're inserting efficiently, then burning back home efficiently), that means you need about 600 m/s to get home.

Assuming you have a single pod, parachute and heatshield (which you can easily reduce to about 20% ablator for a return from Duna), plus the usual antenna, science, battery and solar panels, that means you need (1) a Terrier or Spark engine, and (2) either exactly an FL-T100 or only slightly more fuel, to get home. With an FL-T200, you have about 1500 m/s: enough to circularise and return with a very decent safety margin.

That means that whatever else you do while at Duna, on leaving Kerbin you want to have a full "return stage" consisting of the above Mk1 pod, FL-T200 fuel tank and Terrier engine, ready to take you back.
Or if you are using the 3-man pod, you want the smallest 2.5m tank plus a Poodle.

You say that you're getting there easily enough. Therefore the obvious answer is droptanks. A Mk1 pod (plus parachute, batteries, solar panels, battery and 20% heatshield), with an FL-T200 and Terrier underneath, flanked by a pair of jettisonable T200 tanks (with nosecones front and back), will give you a massive 3100 m/s dV. If you can get that to orbit, you can orbit Duna, land on Ike and return to Kerbin, easily. I'm sure you've sent heavier ships to the Mun.

As always, the thing is to shed as much mass as possible. If you can throw empty fuel tanks away, do it. If you use boosters, place fuel tanks on them and make sure they drain fully through your main engines at as close to exactly the same time as you have to drop the boosters.

 

I'm sure you've already been to Minmus and back with a lot of fuel to spare. Going to Duna shouldn't be a problem, as long as you keep the mass down. One easily understandable tendancy is to try to do a lot more than what you've done for Minmus, just because it's so much further away. Don"t. It's not a huge leap from Minmus to Duna, it just looks like it

[Aegolius13]

It might help to have a screenshot of your current craft, and/or more detailed description of what you're trying to do.  For example, by going to Duna, do you just mean going to Duna orbit, or actually landing on Duna?  The latter is going to impose a lot of more requirements than the former.  As @Pluscksays, a Duna mission is not that different in terms of total requirements than a trip to the Mun, but due to the extra steps and time requirements, it['s a lot easier for something to go awry.

But in a general sense, if you need your rockets to go farther, I think the best framework to work around is the Tsoilkovsy Rocket Equation.  (Not sure if you're familiar with this, or with the concept of delta-v).  There's quite a bit of math involved, but in general, the Rocket Equation says a rocket's total delta-v (i.e., effective range) is a function of only three things: the dry mass, the wet mass, and the engine's specific impulse.  So, to get more delta-v, you can do a few things (these are very general tips, so they might or might not be applicable to your current issue):

• Reduce dry mass.  This means getting rid of any parts you don't absolutely need, including excess engines. 
• Increase wet mass. This simply means adding more fuel.  But of course, adding fuel hurts your TWR and adds dry mass in the form of empty tanks, so it can hit diminishing returns pretty quickly.
• Increase specific impulse.  Assuming you're able to get you ship to orbit, this means picking engines with good vacuum specific impulse. For a Duna mission, NERVs, Poodles, Terriers and Sparks are all decent choices, depending on your craft size.
• Make your staging more efficient.  (Technically outside of the Rocket Equation) This is a bit more of a complicated topic, but generally you probably want each stage to have about the same total delta-v.  As a general rule of thumb, if a new stage has 1/4 to 1/3 the thrust of the previous stage, you're probably in the right ballpark.  For example, it;'s common to have a Poodle powered stage, followed by a Terrier, followed by a Spark.  
• Or, if all else fails, add MOAR stages, starting at the top/end of the rocket and moving downward. 

One other thing that will help extend range on a Duna mission - on the way in, use Duna's atmosphere to aerobrake (i.e., slow your ship enough to get into Duna orbit without needing an engine burn).  You'll probably have to use a little trial and error to figure out the right periapsis to aim for.  

 

 

[Pecan]

Apart from the ship design remember there is a specific transfer window for the return, just as there is for the outward trip.  There's generally quite a while before it's efficient/easier to come back, rather than trying to do it immediately.

[Snark]

21 hours ago, Poky22 said:

Ive gone to Duna 3 times but couldnt get back.

There are a lot of ways to "not be able to get back".    For example, it could be a problem with ship design, or with piloting technique, or with interplanetary navigation, to name a few-- or it could be some combination of all of the above.  It's hard for us to offer specific advice if we don't know which of the many possible problems you might be experiencing.

So let's start by narrowing this down.  The first and most important question:  Does your ship even have enough dV to get home from the surface of Duna?  If it doesn't have that, then you won't be able to get home no matter how you fly and navigate the ship.

So... could you please post a screenshot of your ship sitting on the surface of Duna?  With that, we can look at what sort of dV it has available.  If it turns out that "yeah, that's way short of what you need", then that's your problem right there and we can move on to discussion of how to tweak your ship design to give it more dV to work with.    On the other hand, if your ship looks like it ought to have plenty of dV, then we can ask some questions about how you're flying and/or navigating your way back.

[GRS]

Get like over 10000 m/s Delta v and your mission should be secured, also prepare parachutes, Ike assists may help you out.