Welcome to Jester's Trek.
I'm your host, Jester. I've been an EVE Online player for about six years. One of my four mains is Ripard Teg, pictured at left. Sadly, I've succumbed to "bittervet" disease, but I'm wandering the New Eden landscape (and from time to time, the MMO landscape) in search of a cure.
You can follow along, if you want...

Sunday, March 2, 2014

Duna mission wrap-up

As promised, I want to cover my recent KSP activities with a Duna mission wrap-up. As always, people not interested in KSP can skip to the next post. Again, the idea here is to share my experiences, maybe give other players some ideas, and almost certainly expose some of my own weaknesses so as to get tips from all of you out there that know way more about this game than I do. ;-)

Enterprise ended up going to Duna with 18 fuel tanks the equivalent of the large diameter "-16" size for the main body of the spacecraft, plus one "-16" and four mid-size "-400" tanks for the Orion lander. And while this was mostly guess-work and instinct on my part, it was also based on my experiences with my Pathfinder series probes to Eve and Duna, plus Enterprise's mission to the Mun. Attached to the ship were a total of three Science Junior mods with all four standard science instruments attached plus two Mystery Goo containers per Science Jr. All three Science Juniors had docking ports mounted at both ends. Also attached were two "fueling shuttles." My intent carrying these was that if the lander was able to get into Duna orbit but with insufficient fuel to negotiate a rendezvous, I could send a fully fueled shuttle to make up the difference.

The overall mission profile was for Enterprise to push the lander into Duna orbit. The lander would then negotiate the final descent and landing with one of the three science packages docked to the top. The second science package would be used in Duna orbit, the third either in Ike orbit or -- if I had a large fuel surplus -- in an Ike landing. When I was ready to head back to Kerbin, I'd stack all three Science Juniors to the docking port at the top of the lander, cut the two outboard fuel tanks loose, and come home. And as covered in my little pictorial, that's exactly how the mission went, with a few quirks here and there.

To date, I haven't used anything except Hohmann transfer orbits in KSP for this kind of thing. I have not played around with gravity assists nor anything more complicated than that. I've been using this orbit calculator to give me an estimated time of burn and ejection burn delta-v. I find it to be reasonably accurate to get a rough estimate. But it can't account for differing orbital inclinations, and on a long burn, even a small error in the maneuver tool's burn marker can result in missing a target by a wide margin. So it's good for an estimate but there's still a lot of work to be done to negotiate an encounter. In particular, I tend to try for one long burn out of Kerbin orbit, followed by a second burn to adjust inclination at the node where my transfer orbit crosses the target's. I usually pre-plan both burns in advance from Kerbin orbit, then tweak and adjust as needed while en route. To date, this has served me reasonably well. Still, I feel like I'm missing a trick or two to make transfer orbits easier. I need to find some time to watch some video tutorials on them.


I got very lucky on my third burn to Duna orbit. My injection burn into orbit swung close enough to Ike that I was going to have an Ike encounter thanks to the burn! This was pure luck, not skill. But unfortunately, I judged that I would not have the fuel to do an Ike landing with the lander. And unfortunately, I hadn't brought any unmanned probes with me. So that was the first mistake of the trip: I should have carried only one refueling shuttle and used the opposite docking port for a light unmanned probe of some kind. That would have gotten me an Ike landing and perhaps even something I could have brought back from the surface. So I now know that trick for next time.

But the flyby was good enough to get some "high above Ike" data from several of my science instruments. Since I'd brought a science lab with Enterprise, I could even cycle the Science Junior and the Mystery Goo container attached a couple of times during the encounter. So the science lab paid for the fuel to haul itself to Duna right there. I also got some good readings from most of the science instruments both while in transit ("high above the Sun") and just before the injection burn ("high above Duna"). I also did kerbal EVAs at all three of these points and transmitted this data back since there was no science loss by doing so.


For the Mun landings, the four "-400" series fuel tanks had been enough. But I was unsure about what orbit Enterprise would end up in, plus Duna has both an atmosphere and higher surface gravity. That motivated me to go with the heavier lander with the additional "-16" series tank. That turned out to be the right decision. The amount of fuel could not have been more perfect and I returned the lander to Enterprise with nearly dry tanks. The lander performed beautifully at every step. In particular, just as with the Pathfinder mission to Duna, I slowed the lander to about 3000m and 200m/s on engines, then popped the four drogue chutes. Then once those opened I popped four radial chutes. After that, only minor burns were needed to keep the lander's falling velocity within the safe landing range. I landed hard enough to break the two downhill struts but was able to repair them.

Duna orbit rendezvous and docking with Enterprise went smoothly. I then detached one of the refueling shuttles and moved all three science modules to the top of the lander, then cut loose the two outboard fuel tanks. That left the center fuel tank nearly full and I needed nearly every drop of that fuel for the Kerbin transfer and injection burns. I find injecting to a planet closer to the Sun to be much more difficult than injecting to one farther away for some reason. So it took me almost an hour to find the right transfer orbit I wanted that didn't burn too much fuel. This was after I accelerated the game to max time warp to let several months pass for Kerbin to get into the proper position for the transfer, again using the transfer tool above for the rough first pass.

Amusingly enough, since I had attached probe cores to both refueling shuttles, both of Enterprise's outboard fuel tanks remain controllable, though empty. The only thing I left behind was 100 monoprop in the tanks of both shuttles, mostly to save mass. Both fuel tanks retain Science Juniors and Mystery Goo containers at the their tops.

Speaking of monoprop fuel, I find it more and more valuable as a way to do fine tweaks on orbits. Usually during a transfer or injection burn, I'll use the main engines to get within 20 or 30m/s of the correct trajectory, then use the monoprop thrusters to do the rest, depending on the mass of the vehicle. You can adjust the orbit of even a pretty heavy vehicle by a surprising amount using RCS thrusters alone.

As noted in my pictorial, once I was in Kerbin orbit I used aerobraking to pull my parking orbit close to Kerbin. My initial burn put my periapsis at about 400km. But my apoapsis was something ridiculous like 50 million kilometers. By that time, I was down to fumes in the tank, less than 400 liquid fuel and about 500 oxidizer. This left me with no other option than to try out aerobraking, the first time I had done so. But it worked just fine: I used about 20% of my remaining fuel to reduce my periapsis to 45km. Amusingly, it took about a dozen orbits or so to bring the apoapsis down to something sane. Then another short burn at the new apoapsis brought peri back up to about 125km.


I had already tested soft-landing the Orion lander from Kerbin orbit and it worked fine. The only quirk was that I needed to repack the eight parachutes on the lander before it left Duna. I made this easy by mounting ladders to the sides of the fuel tanks. Once your kerbal gets within a certain range of a used parachute, you just right click to repack it. This worked like a charm. Then it was just a matter of rearranging the parachute stages once the lander was in Kerbin's atmosphere. I had no problems with the three Science Junior modules breaking off the top of the lander.

And that's pretty much it! Lots and lots of notes here! I hope they're useful to someone. Since this mission, I've done a few other things but I'll save them for another update. I also have a crazy idea to do a post: "Can EVE players learn anything from KSP?" Turns out, I think the answer is "yes." ;-) I'll get that published sometime soon as well.

5 comments:

  1. I know there have not been a lot of comments on your KSP posts, but I, for one, love them. Keep it up!

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  2. KSP posts and CSM updates are just boring...

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  3. I'm so bad at this game that I wouldn't mind some more post explaining some things ;)

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  4. MechJeb is an excellent mod, but a lot of people knock it for making the game too easy. The key thing if you're wanting to learn about orbital mechanics and such is actually paying attention to what MJ's doing and figuring out why.

    Eventually you'll get to the point that you can do a better job piloting, and that's when I'd recommend Kerbal Engineer for the readouts.

    Cheers!

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