The past week in Kerbal Space Program has seen several years go by for Jebediah and the other Kerbalnauts. Through these years, they have been completely in training mode, for the most part. There was a Mün mission as well as a couple of Minmus missions completed by Jebediah, Bill and Bob. But for the most part, the Kerbal's efforts have been focused on building a rocket that could get an unmanned probe to the surface of Duna and return it with a complete science payload.
It took many, many test flights to figure out a rocket system that could not only get the transfer and return stages to a 650 kilometer orbit above Kerbin, but do so with enough fuel in those stages to actually accomplish the mission. The final design incorporated three six-meter rockets with "Skipper" engines and eight two-meter rockets attached directly to them. The entire assembly had to be strut-taped together. The vibrations from so many engines was tremendous, and had the bad habit of shaking the entire first stage apart. The fuel system was set up to maintain an even distribution of fuel between all tanks. This behemoth weighed in at just under 400 tons and had a surface Thrust to Weight Ratio (TWR) of 1.05. It needed all the thrust it could generate!
[caption id="attachment_1554" align="alignright" width="300"] My first Duna transfer, soft lander and return rocket in Kerbal Space Program.[/caption]
And it did what it was supposed to do. It got the transfer and return stages to a 650 kilometer orbit with less than 50 fuel remaining. I even got a picture of the transfer and return stages in orbit.
Because the first stage needed so much fuel to get the upper stages into orbit, I had a real problem with the rocket getting top heavy as the fuel drained out of the first stage. The upper stages needed to be as short as possible to stop the rocket from tumbling at high altitude. This is why there are "lobes" on the upper stages. It made the entire rocket short enough the Advanced S.A.S. could control the entire thing during ascent.
Once all the system checks were complete, commands were sent to the lander to begin it's long journey to Duna. But just because the journey is long doesn't mean it is slow. To get away form Kerbin's gravity well requires a vessel to reach at least 3,431.02 m/s. And the vessel is still pretty much traveling that fast when it reaches Duna's gravity well. It would take a tremendous amount of fuel to attain a circular orbit around Duna at transfer velocity.
Fortunately Duna has an atmosphere. This allows for an aerocapture maneuver. This is done by allowing your fast moving craft to dip into the planet's atmosphere where friction and the resulting drag will slow your craft to a speed that places it into an elliptical orbit. Once in an elliptical orbit, performing an aerobreaking maneuver (it's the same thing as before but you're already in orbit) will lower the craft's apoapsis and allow you to start planning a landing.
[caption id="attachment_1555" align="alignleft" width="300"] My first Duna lander failed it's aerocapture maneuver spectacularly.[/caption]
This aerocapture maneuver is not as easy as it sounds. In fact, it's down right unforgiving. If you make the slightest mistake in trajectory as you pass through the atmosphere, you will either not achieve a capture, or worse, you could end up like the first lander we sent to Duna.
No amount of fuel could have slowed this probe enough for a soft landing. However, we had it expend all it had in the hope the chutes would hold. They did, but the sudden negative g-forces were so high when they deployed the engine and fuel tanks ripped right off the lander body. The amazing thing was, the science section was relatively undamaged. We were still able to operate most of the science package (one instrument was actually under the probe body so we couldn't activate it) and transmit the data back to Kerbin. Thank goodness the Kerbals insisted we put a 1000 volt battery on that sucker because the Blutonium generator was the one piece of the lander that didn't survive impact. There was just enough juice to transmit it all.
Our second lander fared much better with it's aerocapture maneuver. In fact, because of the magic of LouKermanArts™, Mohawk Kerman was able to film the entire landing! Here's that 4 minute landing sequence in glorious Duna red.
But landing is only half the mission. There is the getting back that must also be done. And that's just as hard, if not harder, than getting there. The lander had more than enough Δv to get into an 825 kilometer high orbit of Duna. The return transfer was picture perfect. It went so well no one thought to make any records of it - oops. The lander came into Kerbin's gravity well with a periapsis of just over 50 kilometers. The aero-capture went perfectly. Unfortunately it was done at the wrong time of the month. As the craft came around Kerbin we projected its apoapsis based on the new velocity. We realized it got far too close to Mün. The Kerbals ordered the lander to burn all the fuel it had left, all six seconds of it, to try and stop the inevitable. But sure enough, Mün tugged at our little Duna lander just enough to raise its periapsis above aerobrake altitude. The lander was stuck in a highly elliptical but stable orbit. Damnit! We were so close. It looked like this mission was going to be another data transmit semi-success. But the lander had a docking port. We began contemplating the feasibility of a rescue mission. Jebediah was up for it!
And as we pondered our options, we discovered what Mün taketh away, Mün can also giveth back. On the lander's sixth highly elliptical and useless orbit, it again got very close to Mün after rounding apoapsis heading for periapsis. And instead of flinging the lander out of the Kerbin/Mün system, Mün slowed our lander; reducing it's periapsis to a Kerbin intercept. The lander came home, and it brought over 500 science points home with it. Success! (Yeah, I air punched when it touched down. :-) )