In Kerbal Space Program, Duna and Ike, counterparts of Mars and Phobos. Dreams of Mars missions come true here. In this chapter, I build the lander and the main ship. Interplanetary flight is described in Chapter 6. Interplanetary Travel on Budget, part 1 and Chapter 8. Interplanetary Travel on Budget, part 2.

Chapter 5. Planning the Duna Mission

As I was writing this tutorial, I thought that a simple one-planet mission was too easy, so I decided to see a new planet and try it out: Duna. When I was making these missions, 0.22 came out, and suddenly I had a well-planned ship, but no plugins, and had to try if what I do were possible with “vanilla” KSP. Turned out it is.

The easiest way is to visit Duna only, of course. The plan is simple, see the picture.

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Sun is in the bottom, and we approach from the left, because we are on the tip of a very eccentric orbit, and orbital speed is lower than on a circular orbit at the same height.

Since a trip to Ike is cheap, I thought to take the bigger challenge, visiting both Duna and Ike.

Initially thought there are two choices: ether go to Ike first, or to Duna. In both cases one can aerobrake in Duna, both to Duna itself or to Ike. I thought I’d visit Duna first, and Ike on the way back. From my experience, launching from a low orbit around a satellite saves a lot of fuel. I started studying the map and launching probes, the optimal mission is different and more intriguing.

The simple mission configurations are in the chart. Let’s call them A (Duna->Ike) and B (Ike->Duna) for a later analysis. Guess which one is more fuel efficient?

_images/mission-a-b.jpg

I built a lander for Duna, which can be used twice, if staged properly. The command-service module is scalable: you can add more tanks if needed without changing the configuration.

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Duna lander

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Duna lander, top view

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Duna lander in Duna orbit

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Tiny landing module (can be adjusted into a refuelling tank)

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Tiny landing module in assembly. Attach 180 kg (weight of 2 Kerbals) of pods or struts to get a correct estimation of 𝛥v.

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Command module with other modules

The answer is: A (Duna -> Ike) mission requires 4.68t of fuel, B (Ike -> Duna) mission takes 5.46t of fuel. Looking at the chart we think that Duna->Ike transfer costs a lot (250 m/s). But even though it does cost a lot, we save one circularization or escape burn (130 m/s) we land a lighter lander on Ike, and we conserve the 𝛥v in Ike orbit, to reuse it later when departing to Kerbin (saves 200 m/s).

Optimizing Further

But look at the charts again: Lander has to land at both planets, but Command-Service Module and fuel for different parts do not necessarily need to go together with the lander. We can park it wherever needed. I thought of a different profile:

  1. Fuel for trans-Kerbin injection and for Duna landing weigh 2.7t, so I can circularize this fuel directly in Duna orbit, and CSM with lander would go to Ike, then to Duna. A simple calculation shows that I could save 2700 \times (e^{\frac {400} {390 \times 9.81}} - 1) = 297 kg of fuel.

  2. C. But then, why send CSM for this? If departing from Ike is cheaper, park CSM there right away, and let lander go to Ike from Duna by itself (it will require more fuel tanks, but still is doable).

  3. D. The next plan was to not extend lander, but park CSM at Duna and let lander do the trip to Ike, which is much cheaper, and refuel it at Duna before landing:

    _images/mission-c-d.jpg
  4. Another option is to use a tank with a remote-controlled pod. CSM goes to Ike, lander lands on Duna, then is refueled by a tank in orbit, then lander flies to Ike and lands there, E.

  5. A reverse of this, with CSM at Duna and refueling in Ike orbit, F, although, Ike trip takes little fuel, and this refuelling saves very little.

    _images/mission-e-f.jpg
  6. Lastly, the lander for Duna itself is quite heavy: 2 pods, metal plates, parachutes, struts and decouplers. A lighter lander could use much less fuel, so why not, instead of a fuel tank make it a tiny lander? Remember the space bicycle idea? I made it. To let Flight Engineer plugin calculate 𝛥v correctly, I had to attach pods for the weight of 2 kerbals (180 kg). Its full mass is 793 kg which is less than 1500 kg that the big lander would use for Ike trip. CSM and the big lander park at Duna, the tiny lander goes to Ike, then down to Duna. This is mission profile G.

  7. A reverse profile, H, where CSM stays at Ike, is more complex: both landers go to Duna. When the big lander returns from landing and docks with the smaller one, kerbals transfer to the small lander and just go to Ike and land there.

    _images/mission-g-h.jpg

Summing Up

I wanted to be sure I had enough fuel for maneuvers in the main ship, because it would need to correct orbit with both landers and then circularize with the big one. I had to make a spreadsheet in which there were all the mission profiles with all thinkable configurations and maneuvers were calculated. This is a bit too much for playing for fun, but I have to confirm my assertions, so here it is: the spreadsheet, and the results:

id Profile Fuel used Total mass Mass reduction
A Duna, Ike, together 4,678 14,055 5.46%
B Ike, Duna, together 5,458 14,867  
C CSM @ Ike, lander goes to Duna first 4,316 13,742 7.57%
D CSM @ Duna, lander goes to Ike first 4,684 14,180 4.62%
E CSM @ Ike, lander & a tank go to Duna, refuel after landing, lander goes to Ike 4,108 14,129 4.96%
F CSM @ Duna, lander & a tank go to Ike, refuel after landing, lander goes to Duna 4,548 14,447 2.82%
G CSM and lander go to Duna, tiny lander goes to Ike 3,718 13,590 8.59%
H CSM parked at Ike, lander & tiny lander go to Duna, then kerbals go to Ike in tiny lander 3,442 13,340 10,27%

Surprisingly, when the CSM stays at Ike, and the lander – with bigger tanks – goes to Duna first (C), less fuel is used than if you park the CSM at Duna and go to Ike first (D). Comparing the maneuvers, I saw that not only we save 400 kg on Kerbin return, but we save on Ike landing. Landing and return take 800 m/s and are a significant maneuver, so sending a lighter lander (without parachutes and without fuel for the trip down to Duna), we save 400 kg more. We lose about the same on carrying extra fuel down to Duna and up.

As you may understand, the mass is reduced through increased complexity. The last mission profile, where kerbals have to get into the tiny lander, requires complex redocking in orbit and rendezvous of ships that have no RCS fuel. Then the crew would need to EVA to the ship that’s freely floating around.

As I write this, I flew the mission in D configuration (without these calculations), and then in G, which is still quite simple compared to H. The second mission was calculated with a spreadsheet, so I was sure I had enough fuel at any point.

Duna take-off takes 1380 m/s, but I added 600 m/s for the round trip for soft touchdown (takes 200 m/s!), deorbiting (100 m/s), rendezvous (100 m/s) and some more unforseen circumstances. They did happen.

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Lander went down too fast, and the parachute tore off its decoupler. You need to slow down to 150 m/s or less with parachutes. The lander survived landing and could return into orbit.

Fortunately, the lander has its engine inside, shielded with pods and tanks, so I lost only the struts.

In the next chapter, I’ll show how the rocket with this ship flied. See you soon, fly on budget!

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See also

One Giant Leap
a photo gallery of Apollo program (with comments in Russian)