How do i Make a duna capable ship and how to make it intercept duna

[The disaster]

Im using version 1.8 and im looking to get a rocket to duna in a direct landing approach but I dont know what engine would be good to use in vacuum 

Edited November 22, 2019 by The disaster
Misspell

[Geonovast]

Moved to Gameplay Question from Tutorials since this isn't a guide on how to do something.

 

What you want to look at for vacuum engines is the engine ISP (Vac).  This is essentially a measure of how efficient the engine is in a vacuum. The higher the number, the better, as long as all you care about is efficiency in space.  You'll need to balance that against the engine's weight.

If you're using the engine to leave or enter a SOI, or using it to land, then actual thrust will matter as well.

[Yeet_TheDinosaur]

1 hour ago, The disaster said:

what engine would be good to use in vacuum 

Like @Geonovast said, an engine with high ISP is what you need to look for. If you have the making history expansion, the skiff engine or the wolfhound engine is a good option. If you do not have the DLC,  just use the poodle or terrier engine.

 

[Snark]

1 hour ago, The disaster said:

Im using version 1.8 and im looking to get a rocket to duna in a direct landing approach but I dont know what engine would be good to use in vacuum 

So, the good news is that going to Duna doesn't require much more dV than going to the Mun, so it's not too difficult to build something with enough oomph. 

Short answer:  Build a ship that's like a Mun lander, with just a bit more dV, slightly more powerful engine, and a parachute.  Then launch it at the appropriate launch window with just a bit more dV than you'd need to go to the Mun.

The overall process looks like this:

1. Wait for a good launch window
2. Launch to LKO (just as though you were going to the Mun)
3. Do a single burn of around 1000-1100 m/s to head to Duna
4. Do a small course-correction burn halfway there, to adjust for inclination and such
5. Aerobrake and land.

Details below:

 

Waiting for a launch window

This isn't an absolute requirement-- you can launch even when the window's not great-- but doing your launch fairly close to the ideal window will mean you need less dV, which makes building the rocket easier.

For an optimum transfer from Kerbin to Duna, you'll want to launch from Kerbin when Duna is roughly 45 degrees ahead of Kerbin.

Here's an interplanetary transfer tool that I find really helpful:

https://ksp.olex.biz/

There are other tools like it, but that one's my personal favorite because it's so easy to use, and I like the graphical display.  Just put in where you're going from (e.g. Kerbin) and where you're going to (e.g. Duna), and tell it what's the height of your parking orbit above Kerbin (e.g. 100 km or whatever).  Then you push the button and it tells you how big your burn will be, and also gives you little diagrams showing what the relative positions of the planets should be (i.e. the launch window) and what direction you should make your ejection burn from Kerbin.

 

Launch to LKO

This is the same as if you were preparing to go to the Mun or wherever.  I'll assume you've already got this down pretty solid. 

 

Ejection burn to Duna

Using the info you got from the launch planner (see "waiting for a launch window", above), drop a maneuver node at the appropriate place in your orbit (in the spot the tool indicates), and give it an amount of burn equal to what the tool suggests.

Then zoom out until you can see Kerbin and Duna in the solar-system view.  Hopefully at this point you should be able to see the closest-approach markers for when you'd get near Duna.  Your goal is to tweak the maneuver node until you get a Duna intercept.  Play around a bit with , and also try moving the node veeeerrrry slightly forward and backward in its orbit until you can get an intercept to show up.

Once you've got it showing an intercept, you can go ahead and do the burn.

 

Mid-course correction

Once you've ejected from Kerbin, you'll want to fine-tune your path so that you'll hit Duna's atmosphere so you can aerobrake on arrival.  This step assumes that you already have a Duna intercept from the previous step.

1. Drop a maneuver node roughly halfway along the path to Duna.  Doesn't really matter exactly where it is.  Don't give it any dV yet.
2. Still in map view, click on Duna and choose "Focus Here".
3. You're now looking at Duna, and you should be able to see a trajectory line showing what your path past Duna is going to be.  Probably this will be out somewhere near the edge of the SoI, unless you got lucky in the previous step.
4. While still focused on Duna, rotate the camera around until you can see your maneuver node.
5. Very gently, nudge that maneuver node to adjust the path past Duna.  Give it a little bit of as needed to make the trajectory go past Duna the way you want it to.  You want to get your Pe down to an altitude of 12 km or so.  Note, this shouldn't require very much dV at all, just a little bit.  It'll be a small burn.

Once you've done that, do the burn when it's time.

 

Aerobraking and landing

It's really easy to aerobrake at Duna.  Assuming you launched with a good launch window, you won't be arriving very fast from interplanetary space, and the combination of Duna's gravity and atmosphere makes reentry much gentler than at Kerbin.  So if you launched with a good launch window, you probably don't even need a heat shield for this.  (If your launch window wasn't great, you could be coming in a lot faster, in which case you might need a heat shield.)

There are various ways to design a ship for landing on Duna.  My own preferred approach is to just build a vacuum-lander type of ship (e.g. similar to what you'd use to land on the Mun), though with more dV and somewhat higher TWR to cope with Duna's higher gravity.  Then give it at least one drogue chute, and one regular chute.  Set the drogue chute's opening altitude to be as high as you possibly can, i.e. 5000 m above terrain instead of the default 2500.  Set your regular chute's opening altitude to a bit higher too, say 1500 m or 2000 m instead of the default 1000 m.

Having just a minimal amount of parachute means that the chute will slow you down a lot, but not enough to land safely-- you'll likely be descending at 30-40 m/s even after the regular chute opens.  But that's fine, you can then just use a brief burst of rocket thrust just a second or two before landing, in order to slow to a safe landing speed.

Note that that's not the only way to do it; it's just how I like to.  You could, for example, spam a large number of parachutes, so that you can actually slow to a safe landing speed with parachutes alone even in Duna's thin atmosphere.  It's totally doable.  The reason I prefer not to do that myself is that it needs a lot of parachutes, and that mass adds up.  The little bit of fuel I burn before landing (in the above-described technique) likely weighs a lot less then the mass of all the extra parachutes I'd have to bring if I didn't want to use the engine, so it's a savings. 

Another approach is to build something with wings that does an airplane-style landing, but that's pretty challenging on Duna (because the atmosphere is very thin), so my advice would be to leave that until later, when you're more of a Duna expert.

 

[Fierce Wolf]

I don't know your tech level but try a couple of Cheetas, maybe Terriers. They will suffice both in space and for a lander in Duna to get in orbit again since they have enough thrust in the thin athmosphere. Sparks are useful too, if your lander is super lightweight.

If you are going for something fancier, separate your lander engines from space faring using Atomic engines to get to Duna's orbit and back to Kerbin.