Duna³ - A direct descent mission to Duna and back 3 times

[Stratzenblitz75]

Greetings all!

Two weeks ago, I decided it would be a fun challenge to build a Duna² mission. The goal, of course, was to go to Duna, come back to Kerbin, then go off to Duna again, then come back home again. All with one rocket on a direct descent mission (No docking or refueling). However, after building this rocket, I realized there was a crippling flaw: 

Do you see it?

Its only 1672 tons and only 168 parts! And thats with a heavy Mk1-2 capsule on top! That's way too small! Not even worth my time to try!

Suddenly, a crazy thought entered my mind: what if I added another phase to it? What if I transformed this into a Duna³ mission? After running some calculations, I realized a Duna^3 rocket could be created with a mass of less than 30,000 tons. Surely, a 30,000 ton rocket was possible, right? With 1.1 and proper application of more boosters and more struts, anything is possible. Well, turns out, it wasn't that simple.

 

--- Building Big ---

For designing the mission, I broke it down into three phases. Each phase represents a full mission from Kerbin to Duna and back. Breaking the mission into pieces like this helped me focus and prevent the mission from growing into a convoluted, unmanageable mess.

Building the last phase was one of the most important steps of designing the mission. The mass of it single-handily determines how massive the final rocket is. If I built it too large, then the rocket carrying that phase would be larger, and the rocket that would carry that rocket would be even larger. It exponentially grows out of control. Keeping this in mind, I arrived at this design:

It uses an aerospike coupled with 3 boosters, each running a 5x cluster of "spark" engines. While an aerospike with just 2 boosters would be more efficient, the three booster setup allows me to land safely and (somewhat) reliably. A service bay is used as a heat shield for the final reentry (it has the same mass as a heat shield with no ablator, plus, you can put batteries and other draggy parts inside of it.)

Notice the lack of landing gear on the Duna lander as well. Turns out, its more mass-efficient to simply let the lander fall over and use a separation to push the vessel upright for launch. It also produces far less drag, which is very important for a rocket this small.

The second phase was more straight forward. It didn't have to be as efficient, so I didn't focus much on optimal engine configurations or anything like that. I only ensured that it stayed within the mass requirements that I determined during my calculations. Instead, I focused my time into making it as tough and reliable as possible. After all, it needed to survive a Kerbin reentry from interplanetary speeds, and land safely afterward. The inflatable heat shields were a god-send here: they produce a ton of drag when deployed, and are much lighter than the other heat shields per square meter. For the landing system, I used a combination of parachutes and engines to perform a powered landing. To take the impact of landing, I used the MK3 to 3.75m adaptor. It holds fuel and has an impact tolerance of 20 m/s, making it perfect for the job.

 

The first phase was by far the most interesting. The Duna lander of the third stage had to safely land over 2000 tons of lander onto the Dunatian surface. Naively, I took the same approach to building the second stage: more boosters, and more struts when it breaks apart. However, KSP had other plans:

I added more struts, same thing happened. Even more struts. Same thing. No matter how many struts I applied, the rocket would not stay together. It always broke apart. Clearly, the ole saying of "more struts" wasn't working. I needed something different.

So, I scrapped the design and thought about the problem. Perhaps the issue was in the way the craft was built. After all, the craft always seemed to fall apart at the same failure points: the boosters furthermost from the core would break first, causing rapid dissassembly of the entire rocket. Perhaps, if I attached all the landing boosters directly to the center stack, then they would be stronger. It would result in a smaller landing base, but the COM was low enough that I didn't think it would be a problem.

It worked.

In fact, it worked so well I almost couldn't believe what I was seeing. In all of my play time of KSP, I've never seen a rocket that massive survive a 5 m/s impact. Yet, here it was, taking the impact like it was nothing. I checked the alt-12 menu. Nope, no cheats, it was actually working under stock physics. 

The mission was almost done. All that was left was to build the first stage. The rocket that would carry the entire mission off the ground. The 30,000 behemoth that I thought I could build so easily with "more boosters, more struts".

I figured that since this stage wasn't supposed to withstand the stress of landing, I went back to this more boosters, more struts philosophy.

It didn't work:

 

After adding more struts again, and watching it fail again, I realized that I was running into the same problem as my Duna lander. The answer does not lie in more struts. Its how you build the craft!

So, scrapped the design and moved from the "booster block" design:

To one that more resembles a sky scraper, with the boosters branching off from the core boosters:  

After some tweaking the fuel lines and struts, I arrived at my final design: the Duna³ mission.

At 1373 parts and over 20,000 tons, it is the largest and most complicated rocket I've ever launched in KSP:

I Hope you all enjoy seeing it in action!

 

Signing off,

-Stratzenblitz75