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[Tutorial] Duna For Dummies: How to get there without math


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DUNA FOR DUMMIES: A guide for the rest of us

By now you've gotten the update, built some big rockets, watched them fly into space and/or explode into a firey mess. You've also probably seen all the photos of people visiting the cool new planets.

If the posts I've seen are any indication, however, a lot of you haven't managed to visit them yet, or if you have, lucked into a chance encounter.

So you went looking for help, and found some great guides by Kosmo-Not and Olex. But they were heavy on math, and math's not your thing. Or you're young and just haven't gotten that far yet in school. You just want something simple to understand, as easy as 'point orbiting rocket at rising Mun and fire'.

Well, I can't make it quite that easy. I can, however, make it much easier and eliminate the math.

This guide is a how-to on reaching Duna - and returning, with a 3-man crew. We are going to cover what you need to know and do to reach another planet, step by step, covering everything from designing your rocket to splashdown back at Kerbin.

Edited by sal_vager
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ROCKETSHIP TO ANOTHER WORLD

People built some amazingly tiny rockets to get to the Mun and Minmus. Most of us constructed larger but easier to fly spacecraft, with more margin for error built in. Others built enormous computer-taxing monsters that probably would burn KSC to the ground taking off, if it were only modeled.

We don't really need a giant monster rocket to reach Duna, but it will need to be decent sized. This is intended as a primer for smart rocket design. If you already have something reasonably sized and Duna-capable, but just have issues with actually intercepting it, skip ahead. I also have a rocket pre-designed for Duna missions here which will help you understand what I'm saying here (or you can just grab it and go flying, if you like)

There are a few basic rules of thumb we will want to keep in mind here:

- THRUST is more important than FUEL CAPACITY for taking off from Kerbin but the opposite once in orbit.

- Never use small tanks where a big one can go

- Never use nuclear engines where there is thick air or lots of gravity.

- Discard the biggest stages first

- Use as few nuclear engines as possible. They are heavy.

Let's go over those in order and see how they apply to rocket design.

(1) Thrust is more important than fuel capacity on Kerbin takeoff

This is probably a rehash for just about everyone, but...

Gravity stinks. It constantly pulls you down, robbing you of your upwards speed. If we were doing math, I would bust out some vectors here, but we're avoiding that, so we'll just say that it would be nicer to fly mostly sideways instead of straight up, so we don't lose so much of our forward speed to gravity.

Kerbin has an atmosphere. The Kerbals are happy for that because it lets them breathe, but it also means you can't just fly off sideways, because the air robs you of speed too.

So, missions take off vertically, clear the dense air, and THEN turn sideways. "But Treb, I already knew that. I know all about gravity turns." Yes, but the implication for rocket designs must be kept firmly in mind. You want to get out of the lower atmosphere as soon as possible. That means accelerating fairly hard, which means more engines pushing less weight.

(2) Never use small tanks where a big one will do

This is for two reasons. One is strictly performance-related: keep the parts count low and make your computer happy. The other is for stability reasons: tall spindly rockets with lots of parts tend to be more wobbly and unstable in the upper atmosphere. One short fat tank is worth four tall skinny tanks. The tall fat one is worth eight. Wherever possible, make the switch.

(3) Never use nuclear engines where there is a lot of air or gravity.

The entire point to the nuclear engines is their incredible fuel efficiency. However, they're only efficient in very thin air or vaccuum. In atmosphere, they're horrible and burn fuel faster for their thrust level than most engines, making them pointless.

In high gravity, the lack of thrust makes them waste a lot of power fighting gravity. You're usually better off with a high thrust rocket to get you up high and moving fast. On an airless world with lots of gravity (ie, when you reach Tylo) the nuke engines are more useful lower down (because you can make your gravity turn immediately) but you still want something to provide an initial burst of acceleration.

(4) Discard the biggest stages first.

This comes with an asterix, because there is an exception for solid boosters, which will burn out before the liquid stages. Apart from this, though, it certainly applies. This is my most violated rule. Even people who know better, like most of the people who have been to Duna, violate this one. Even professionals do (the Falcon Heavy violates this rule)!

Around here, this is mostly because of improper asparagus staging. Now, if you know what asparagus staging is, skip this paragraph. If you don't, this involves making multiple radially-mounted stages linked by fuel lines. The simplest asparagus staging goes like this: you have a central stack of tanks and an engine underneath. Mounted radially are two more stacks of tanks and engines. Fuel lines go from the two stacks on the side to the center engine. The fuel tanks for the side stacks feed all three engines. When they are empty, the side stacks are separated and the central stack of tanks, which are still full, feed the center engine. A more complicated asparagus booster would add another pair of tank stacks which would feed all five engines before being discarded, after which the 'simple' asparagus stack would take over.

Where people violate this rule is by making all those stacks of tanks the same size. Remember, we want thrust, and thrust means engines. Those tanks of fuel will get lighter and lighter as the fuel is burned off, which means the thrust-to-weight ratio of the rocket gets better and better as it goes. So, we want the stack of tanks to be discarded first to be the largest and the last, central stack of tanks to be the shortest. That way, we have the most engines firing for the longest period of time during the launch. In my Duna stock rocket, the first pair of stacks are each three large tanks tall, the second pair of stacks is two tall large tanks and one short large tank tall, and the central stack is two tall large tanks tall.

As a side benefit, the taller side tanks make great places to attach struts to your upper stages/lander to reduce swaying when launching.

(5) Use as few nuclear engines as possible, because they are heavy.

As mentioned, you shouldn't be using them in heavy air or in heavy gravity. This means that, on Kerbin, you light them up only after you've reached orbit. This means you have to haul them up out of the planet's gravity well. And they aren't made of feathers and styrofoam; the nuke engines are heavy.

With their low thrust, the temptation is to stick a bunch of them on the vehicle in order to quickly accelerate. Fight this temptation. Hauling a bunch of extra NERVA stages into orbit adds a lot of weight that the stage beneath has to lift, making it heavier, making the stage before that heavier... don't start down that path! You are in orbit. Thrust to weight no longer matters much. Be patient, and let less engines fire for longer. ONE nuclear engine suffices for flying to Duna or wherever - just be patient. If you don't already know, physical warp is toggled by RightAlt + > whenever you want it, which will speed up the amount of time you will take.

DESIGNING THE ROCKET

As KSP makes you build rockets from the top down, let's build our rocket lander-first!

Pick the 3-man capsule. Put a big parachute on top and a big decoupler on the bottom. Obviously, this is for the big finale, when your brave Kerbals detach and re-enter Kerbin's atmosphere. Would that all our stages could be this simple.

Next, the landing and ascent stage. Duna does not have thick air or high gravity. This means nuclear engines are a viable means of propulsion (they get about 750 ISP or so on Duna's surface!) so we'll use them and save a lot of fuel. Put a small fat tank right under the decoupler there. This will provide the bulk of our fuel. Now, we can't put the nuclear engine on the bottom, unfortunately, because nuclear tanks are very tall and our lander legs won't reach. That means we have to put them to the side. We also need to be balanced, so the minimum number we can use is two. Put two of the short, small tanks high up on the sides of the middle tank and put the nuclear engines under them. Duna might not have much air but it does have some, so put a pair of big parachutes on top of those side tanks. Strut them securely to the big tank so they don't get ripped off. Add struts from the parachutes themselves to the little tanks if you are cautious (might come in handy at Duna). Now you've got a lander than can take off from Duna and make it all the way back to Kerbin, thanks to the nuclear engines. Put another decoupler under the center tank. Finally, add the finishing touches - lander legs and ladders. Make sure the legs are placed low enough to keep the two engines clear of the ground!

The next stage will carry us from an orbit around Kerbin to Duna. You really only need one of the taller fat tanks to do this, given nuclear engines. Now, in the spacecraft I linked up above, this has its own nuclear engine, but the truly weight-frugal can eliminate it with a bit of cleverness. Instead of providing it with its own engine, just use the ones in the stage above! Run a pair of fuel lines from this tank to the two nuclear engines on the lander above. Remember to move the engine icons from the lander stage to this stage in the VAB, otherwise they won't light! Put another decoupler under this stage.

The bottom part is all booster, and needs to be able to get everything above into orbit. I used asparagus staging with two pairs of radial stacks, with 12 of the tall fat tanks and 2 of the short ones, with a total of 5 'Mainsail' engines firing at takeoff. Feel free to experiment here, keeping in mind the lessons above. As long as you can get the lander and that one big tank of fuel in orbit, you're good!

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Edited by Trebuchet-Launch
removed random spaces
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FLYING TO DUNA

Now, before we go any further, I must drive home an important point.

DO NOT SEND ONE SHIP AT A TIME.

Just don't, especially if you don't know the math. Instead, launch multiple spacecraft. There is a very good, practical reason for this. We are going to be eyeballing it. Duna has a tiny SoI and we can't tell, at a glance, if we've missed it because we're going to pass slightly in front of it or slightly behind it.

If we send, say, three spacecraft at a time, they won't all be on exactly the same trajectory, either. MechJeb we are not. Odds are, however, fairly likely that one of a group of spacecraft leaving during the same departure window WILL intercept Duna's SoI. And - just like that - now we do know where it will be when our spacecraft arrive! This means that we can adjust our course on the other ships which missed, because now we do know how they're missing Duna.

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As an additional benefit, of course, you get to explore multiple locations on Duna. Or, for that matter, you can divert one of the fleet to Ike instead!

There are three parts to our trip to Duna. One is initial departure, the second is making course corrections to off-target rockets, and the third is capture into Duna orbit.

Leaving is reasonably easy. Let's do it in an organized fashion.

First, put up all of your spacecraft save one into orbit already. Let them hang out in orbit for a while. Now, with the last one on the pad, go into map view. Zoom all the way out so you can see Duna and Kerbin slowly circling their sun. Now, imagine the orbit of Duna as a pizza with eight slices. You can probably picture the width of a slice well enough, right? We want to leave Kerbin when it is one slice behind Duna in its orbit. This will put us on an intercept path.

So, fast forward until you are a slice behind. Now, when to launch exactly depends on how much thrust you have in orbit, once you've discarded the chemical stages. Launch in the late afternoon to give yourself some time to burn the engines in orbit. Once in orbit, pick up speed and accelerate prograde until you escape, passing behind Kerbin's night side into Kerbol orbit. Zoom out in map view. Keep burning prograde, watching your apoapsis slide outwards towards Duna. Once it touches Duna's orbit, if you don't have an intercept then, burn a little more, pushing the apoapsis beyond Duna a bit, to see if you can get an intercept.

If you don't, switch over to one of your orbiting craft. Warp to the right spot in orbit (late afternoonish to the dusk line) and escape Kerbin orbit. Push that orbit out to Duna's and a bit beyond, hoping for an intercept. If not, try the third, etc - however many you have in orbit. It shouldn't take too many, unless you used a very uneven pizza slice.

So, now one of your brave Kerbals has managed to get an intercept. What about his buddies? Well, we're going to adjust their orbit towards his! We can see the trajectories of other spacecraft as gray orbits in the map view. We know which one crosses Duna's SoI when it crosses Duna's orbit. Now we need to bump everyone else's orbit so it goes there, too. You are probably thinking, Treb, I don't know how to do that. No problem, that's the purpose of this section on midcourse corrections.

2FdwRh.png

First, we should look at inclination. Perhaps you left in a slightly inclined orbit and are missing Duna above or below. Zoom out and rotate the view. Look at where the successful intercept is. Is the yellow line (you're still in Kerbin's SoI, right?) heading above or below Duna's orbit at that point? Determine which it is.

If it is ABOVE Duna's orbit at that point, twist your rocket around to heading 180, elevation 0. If it is BELOW Duna's orbit, turn your rocket to heading 0, elevation 0. Light the engine up gently. This should slowly change your orbit's inclination. When it reaches Duna's orbit, hopefully you get an interception. If not, we'll go to the next section.

OK, you have the correct inclination. Are you passing in front of or behind Duna? Let's take a look at that orbit again. Look at the successful intercept again, and then look at your ship's trajectory. Is it in FRONT of the successful intercept, further along Duna's orbit, or BEHIND the successful one, closer to Duna?

If it is IN FRONT of that intercept, turn you rocket to elevation 90 (straight up, directly away from the surface of Kerbin). If it is BEHIND the successful intercept, turn to elevation -90 (straight down). Turn up the thrust gently. This will move where you intercept Duna's orbit in the correct direction slowly. You should get an interception at some point.

(In the picture above, the off-course Kerbals were behind the successful intercept)

If, after your orbit crosses the same spot as the successful intercept and you still don't have a Duna encounter, switch to the next non-intercepting rocket and repeat.

Each additional successful interception pins down Duna's location a bit more, and lets you tweak the courses of the remaining ones more as well. Eventually, everyone should have a Duna encounter. If for some reason you still don't, get started on the way to Duna anyhow. Warp at high speed until you're halfway there. Now, you should see where the non-intercepting rockets have gone astray. Either they are going too fast and ahead of their buddies, or too slow and behind. Burn retrograde or prograde to fix their speed problem and tweak their orbit as indicated above. NOW you should have intercepts for everyone! Break out the champagne in Mission Control and get ready to explore.

Capturing into Duna orbit is easy. It's the same as any other orbital capture, although the relative speed is higher. Switch to the craft closest to Duna. When it enters Duna's SoI, burn retrograde until it captures into an orbit. Do this for all the rockets you sent out, then lower their orbits in the normal fashion until everyone is in Duna (or Ike, if you want to send someone there) orbit.

P6Yleh.png

You'll probably end up in assorted crazy orbits. As I said, we're not MechJeb here.

DIpuoh.png
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SEVEN MINUTES OF TERROR

If you're landing on Ike this is a straight-up Mun mission, pretty much. Nothing you haven't done before, so decelerate and land. Use the transfer stage until its fuel runs out or you are fairly close to the surface - never waste lander stage fuel - and then land as normal. Enjoy the view.

Duna is different. Very different. You will need a different way of doing things than on the Mun or Minmus, because there is a lot more gravity. On the other hand, you don't have Kerbin's conveniently thick atmosphere. Parachutes alone are not going to cut it here.

First, don't plunge vertically towards Duna like a lawn dart. Lower your orbit in such a way that you do kind of a grazing landing, so as to maximize the ability of its puny atmosphere to help you. Also, make sure you do this on the daylight side, to preserve your sanity. Once you are in readiness, quicksave (because you will likely goof this up on the first try) and get ready for the plunge.

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Keep your TDI stage for a while and use its fuel in the beginning of retrofiring. Once you are at 30000 m or so, begin firing. DON'T fire directly retrograde. While this seems reasonable, based on Mun and Minmus experience, it doesn't leverage the atmosphere to full extent. Instead, keep your rocket thrusting something like halfway between retrograde and straight away from Duna into the sky. This will make you slide sideways through the air more and bleed off a bunch more velocity.

tUrjXh.jpg

Even if you have more fuel, discard the TDI stage and burn hard once you pass 10000 m. You need to get your speed low enough that your parachutes won't be ripped off when they fully inflate. You'll see them release as drogues soon. Burn like crazy. Hit G to lower your lander legs - you might forget later. You want that speed to get down to less than 300 m/s if possible. When they fully open, it will violently and your lander will bleed speed at an incredible rate. Soon after you will be headed generally straight down but swaying all over the place violently. Get that under control quickly and get your engines pointed straight down. Once you've gotten the lander stabilized the hard part is over. Land as if landing on the Mun.

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Relax. Enjoy. You've made it to Duna. EVA for a bit if you want. The Kerbals can run around fairly quickly on Duna, yet the gravity is light enough that the jetpacks can let them fly.

After taking a little break, go switch to one of your orbiting craft and land it somewhere else.

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RETURNING HOME

Assuming your vehicles are in one piece, you should be happy to know that going home is pretty much the same as going to Duna, with only a few exceptions:

(1) Duna has very thin air, so you can make your gravity turn very low, almost as if taking off from the Mun.

(2) You want Kerbin to be behind Duna, not ahead of it, in its orbit.

(3) The spacing for a return trip isn't 'one pizza slice'. It's somewhat more than that (maybe a piece of pepperoni wider)

(4) You don't need to deal with the hassles of a Duna landing - your capsule's parachute will do all the work. All you need to do is hit the air.

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Let me give a suggestion. MECHJEB BABY.

Why do you keep bragging about mechjeb. The only thing it is good for during interplanetary travel is keeping you aligned. I also find it rude to have someone write up an amazing guide and then have someone wallow in and shout that this amazing tutorial is obselete because of a mod. Some people play stock you know.

Edited by KasperVld
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That's a really good guide! One thing though, do not retroburn when in Duna's SOI. Use it's atmosphere to do it for you, for free! Adjust your Pe to about 10 km and watch your craft get into an orbit. From there wait until Ap and do a circularizing burn. A lot of fuel saved:)

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That's a really good guide! One thing though, do not retroburn when in Duna's SOI. Use it's atmosphere to do it for you, for free! Adjust your Pe to about 10 km and watch your craft get into an orbit. From there wait until Ap and do a circularizing burn. A lot of fuel saved:)

Yes, you save a ton of fuel this way. Here are some pictures of my MapSat probe's aerocapture into Dunar orbit.

Also, to keep your craft from swinging around all over the place when you deploy your parachutes, open them in stages... have a single parachute all the way up at the highest, center-most point of the lander. Open it as a drogue. It will stabilize your craft nose-up. Once any swinging has stopped (turning ASAS on while nose-up will speed this process), then open the rest of your 'chutes to slow your descent.

Edited by RoboRay
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Let me give a suggestion. MECHJEB BABY.

There are those of us who take pride in accomplishing things the hard way.

If I do something using MechJeb, I do not post that I accomplished it- because I didn't actually do anything.

The computer did all the work and I just sat around and watched it happen.

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Wow really helped me much, i actually made to duna but well i used even less math than you *ahem* yes thats possibel but it takes me 14223 days and it was without returning

and also i used one of these gaint monster rockets who burn down ksc AND my poor little computer so thank you very much ;)

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I do thank everyone for their kind comments and appreciation for this how-to guide. I do plan on making guides for Jool and so forth in the future as well, with stock parts and 3-man crews. This is actually slightly different than how I do it in my 'campaign' save gave, as I use a variety of mods. Also, my mission profiles are a bit different, as well - I send my Kerbals back in a different spacecraft than the one I sent them in, using the orbital rendezvous technique. (Here is a picture of such a missions safe at home on Kerbin)

I did not cover aerocapture because, frankly, using this method of getting Duna encounters tends to get you fairly distant ones. If you do get the chance to aerobrake, go ahead, for practice - although since we're planning on disposing the TDI stage anyways, it's not that much of a benefit.

Don't expect a Jool guide right away. At the moment, I want to get back to building my Duna base in my main save file.

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an easy way to eyeball the 45 deg needed for the planetary transfer is to use the 6am terminator (sunrise). If Ike/Duna line up with that in the map view, you are at about a 45 deg angle.

I'm not certain I understand that terminator thing. You just mean aligning with sunrise on the map view or should I be looking for some marker?

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I did not cover aerocapture because, frankly, using this method of getting Duna encounters tends to get you fairly distant ones. If you do get the chance to aerobrake, go ahead, for practice - although since we're planning on disposing the TDI stage anyways, it's not that much of a benefit.

If you do it far enough out, you can adjust your trajectory into an aerobraking one with just RCS, or at most just a few puffs of the main motor. But even if you do have to make a huge adjustment with the main motor, you'll still come out way ahead on fuel savings.

You may not need to save that fuel, if you're about to discard that stage anyway, but that just means you don't need to bring so much along next time.

Edited by RoboRay
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