Trying to get to Duna, and back

[Wachman]

So, my story. In career mode, with ~2/3 unlocked. Having just unlocked nuclear engines I decided to get ambitious and send Jeb to Duna (and bring him back too.) I went with a two ship design, a Tug ship to go there and back, and a lander to land on both Duna and Kerbin.

The Tug consisted of 4 LN engines and Fuel tanks, an SAS wheel, monopropellent, RCS thusters solar panels and lots of struts. A docking port at the bottom of the centeral fuel tank is where the lander attached. If all went according to plan the Tug would end up in orbit of Kerbin, just needing to be refueled for later missions.

On lander I ran into my first issue, how much power would I need to get from the surface to orbit and docking. I could find the dV, but wasn't sure which engine would be required to lift the 8-10 ton lander. I decided to go big and used the LT-45, just to be sure. The lander was designed so I would only need the engine to deorbit. On Duna I would need a short burst right before touch down to ensure a safe landing, but most of the slowing would be by parachute. On Kerbin, the parachutes could do all the work.

And here it is.

OK, some words about me. First, I play stock, no mods, not sure if I'm just to lazy to install them, or to stubborn. Next, I am not the most efficient flyer. I always seem to burn more fuel than probably most others. This leads me to bring along more fuel, which leads to needing more power, which leads to needing more fuel.....yes it's a viscous circle. Also I think I tend to overbuild things, part of the fuel thing I suppose. But I never seem to trust that something so small, with so tiny engines and so little fuel will actually get there.

And now my talk begins. I had a surprising successful first launch. I launched it all as one unit, and with the Tug so much heavier than the lander I worried about the stress on the docking port. But struts and bracing down to the launch vehicle took care of it first try. I even had enough fuel left in the launch vehicle for a 2 second burn on the escape.

But here our first issue was encountered. Duna was not in the optimal position. I was too late. Instead of ~45° it was closer to 30°. Oh well, go for it, I should be OK, I always bring extra fuel. OK, after initial burn I was close, my mid course correction got me an intersect, at this point I was starting to worry about the fuel, I had burned an awful lot. OK, inside Duna SOI, but way off, didn't realize my inclination was so off, I was way below the planet and not going anywhere near it. Another long burn to get into orbit, and a second to circularize it. At this point, nope not going home, doubt I could get back out of Duna space.

Well lets do the lander and see how that works since were here. Unlocked, deorbit, landed, everything worked right, yeah. Took off, got my orbit close, got the inclination matched, tried to get close to Tug. Oops ran out of fuel still no where close to Tug. Also noticed small design flaw, the lander was suppose to have 2 sets of RCS thrusters, one at each end of the ship so I didn't have to worry about unintended torque if they didn't line up with the CoM. The bottom set is missing.

So After all this fustration I reloaded and earlier save and set the whole thing to a fiery death. I did end up having pity on Jeb and brought the lander down safely just before the water claimed it.

So was it all me? Was the ship never going to fulfill it's mission? Some of both? How do I get to Duna and back?

[tutrakan4e]

http://prikachi.com/images.php?images/409/7291409U.jpg

How I got to Duna+back

(You can't see the giant tug because I crashed it into Duna because the lander was too big and the tug didn't have enough fuel, my overkill lander could make it back easily, though)

I though I was the only one around here who did that. Just never give up. I once ran out of rcs and had to dock with liq. fuel. Or maybe you could go out and push? That's always an option. Just never give up.

[Taki117]

are you trying to get there during the appropriate transfer window? That will save you tons of dV (I calculated that a full round trip could be done with just a hair over 2km/s dV using the proper transfer window both ways)

Also, get out of the habit of building bigger. Build only what you need and get better at flying. I know it's counter intuitive that less is more, but trust me, it's worth it. You could probably shave your lander down a few tons by using a more efficient engine, Duna's Gravity is less than Kerbin's, so you don't have to have all that thrust. THe atmosphere is so thin that that doesn't make much difference either way. (All told you need about 1300m/s dV for ascent) Another thing to note is that while on the edge of the SoI it is much cheaper dV wise to do your plane changes and course corrections than if you wait until Periapsis. Also, duna has an atmosphere, you can Aerobrake and save tons of dV that way.

Here are some useful links

For initial planning and rough estimation: dV map

For More in depth planning: Transfer Window Calculator

Aerobraking Calculator

Edited April 24, 2014 by Taki117

[Dkmdlb]

That tug should have way more than enough fuel to get you there and back, so if you're running out of fuel, you're doing something very wrong. Also, you could probably cut a lot of weight (5 tons in engines alone!) by using 2 LV-Ns instead of 4. Also, instead of using that big engine on the lander, use 5 little Rockomax 24-7S engines, one under each fuel tank. And use 4 legs instead of 8. And use lighter legs if you have them. And not so many lights.

[Wachman]

I went with 4 LV-N instead of 2 for 2 main reasons, 1: If all went well I would be able to use it for later, longer, bigger missions. Just plug some fuel tanks below it and off we go. 2: I really hate waiting through long burn times.

Yes, the transfer window was not optimal. I had missed the optimal time but thought it was close enough anyway. I guess I guessed wrong.

I went with the LT-45 on the lander because I didn't know how much power (TWR?) I'd need to get off Duna, wasn't sure if say a 909 would do it. Still not sure.

The big engine required the heavy landing legs because they were long enough to reach the ground from the fuel tanks. The lighter landing gear would have required a strut. Also I went with a single engine because those sides tanks can be jettisoned when empty.

The eight legs is my bad, a hold over from my early days landing on Mun when I had a bad habit of breaking legs. I got in the habit of bringing extra.

SO ship design probably OK, piloting not so good.

Edited April 25, 2014 by Wachman

[demian_travis]

How fast are you taking off? it does not worth going above 200 m/s on thick atmosphere. What you could also do is send the ship in two parts - orbit the propulsion thing first, with a docking port on top of it, and then send the lander. Set them together on an orbit around kerbin, send another ship to refuel your beast and try again. I really recommend using protractor. It helps quite a lot!

[Zad Fnark]

I was amazed how relative cheaply you can get to Duna and back. I saw on the Wiki page for Eeloo, a pic of a ship. A Mk1-2 capsule, a couple X-200-16 tanks behind it, and a four pack cluster of nukes. I sent that to Duna on a rescue mission and easily made it back. Granted adding a lander and such will strain things a bit.

I tend to favor the brute force approach lately, using the KR-2L semi-asparagused (4 radials feeding the center at once) just to cut down the burn time.

ZF-

[Seret]

I went with the LT-45 on the lander because I didn't know how much power (TWR?) I'd need to get off Duna, wasn't sure if say a 909 would do it. Still not sure.

Easily. If you swap the LT-45 for an LV-909 on a 10 tonne lander it suddenly becomes a 9-tonne lander. It would have a TWR of 1.89 on Duna, so it would go like the clappers. You'd probably have to throttle back a bit to avoid too much atmospheric loss.

Once you start cutting weight like that it becomes a virtuous circle. With a lighter, lower lander you can drop the 8 heavy legs and go for 4 of the medium ones. That's another 0.6t gone right there, and with the lower weight you can probably afford to drop a fuel tank or two, etc, etc.

[KerikBalm]

My 1st 2 person lander used a large size lander can, and carried 9 tons of fuel+tanks, it was designed to be asparagus staged, 3 aerospikes, 2 on the outer FL-T400s, 1 on the inner...

I forgot decouplers, and very soon was running only 1 aerospike, it had plenty of TWR, and fuel. The next iteration was for it to be a reusable lander, left in duna orbit, the central aerospike removed, and a docking port added to allow it to sling rovers underneath and such.

I preer the aerospike to the lv-t45 for landers, you often have enough torque, its the same weight, has plenty of thrust, and higher ISP (its atmospheric ISP is pretty much irrelevant on duna, I basically just use it as a bigger LV-909), and importantly, it is short, a very good thing for landers.

Then I got more efficient, using 2x 1 person lander cans and 2x FL-T400 tanks with a LV-909 underneath (5 tons of fuel+tanks, 1 ton of engine, 1.2 tons of command pod, about 8-9 tons total with chutes and panels, legs and such)

I think I also tried an iteration with a central FL-T100 tank underneath and 2x FL-T200s, carrying 1 LV-909, and two 48-7s. The 48s fired on liftoff, and then I shut them down and used only the higher ISP 909, IIRC, that was also capable of reaching orbit

I think the biggest problem you had, is you didn't set up a close flyby of duna. You save a ***LOT*** of dV if you set your Duna perapsis to 12km just as you leave Kerbin, instead of waiting until you enter Duna's SOI.

And of course, if you don't set up that perapsis upon entering Duna's SOI, you must spend a ***LOT*** more dV to do an orbital insertion.

Lastly, despite the greater efficiency of the LV-Ns, if you find your ship is low on fuel, don't try to bring it back on the nukes - instead transfer all the fuel into the lander, and send the much lighter lander back (or transfer all the fuel into the lander, burn the remaining fuel in the nukes, then undock and burn with the lander.

That lander, if it can get back to orbit, has at least 1,300 dV, which is more than enough to get back to Kerbin when fully fueled.

Sure, your ISP in the '45 is about 46% that of the tug, but with 4 nukes, that tug's dry mass is more than a proper landers empty mass.

I also overbuild my rockets, but I build them so that I can dump a tank as a fuel depot in orbit -> so at the start of the mission, I've got a large margin, I see how much fuel is used in the burn to get to Duna - towards the end of the mission, when its just the one final burn to come back, I take that same amount of fuel in my tug, and leave the rest in orbit as a fuel depot (being much lighter, I still come back with a large margin, if I also leave my lander in orbit around Duna for the next mission, I will guesstimate how much lighter I am and take less fuel, future missions use that same lander I left from the previous mission, and thus its only a consideration once)

[FREEFALL1984]

when Im going on manned interplanetary missions I tend to send a resupply probe, basically a small probe body with a single docking port, an FL-t800 then I attach a bunch of FL-T800s to the sides and a single LV-N on the bottom, then I stick a bunch of RCS tanks and some RCS blocks. depending on the final DV i might run a couple of fuel lines from 2 or 3 of the outside tanks to the middle tank and of course a few static solar panels. Then this gets sent to the planet ahead of my main ship but in the same launch window, it also allows me to practice my entry and aerobraking before the main ship. once my lander leaves the tug, I then send the resupply ship to dock with the tug and fully refuel, before allowing the resupply vessel to burn up or lithobrake onto the planets surface.then once I've redocked my lander, I ditch its surplus fuel tanks and any useless parts such as legs and RCS tanks which I always attach to the radial tanks for this very reason., now I have a fully fueled tug with a command pod and a bunch of science and parachutes ready for the return flight, doing this I usually end up with about 8000m/s Dv for the return, which is more than enough to get back from anywhere with ease,

[radonek]

I dont understand why you aborted. You could send automatic tanker, top up tanks and head home. You dont even need a tug, you can wait where you are and head back with tanker. If you really want it and can't get to it, well, why dont you move tug?

BTW If you want to test your TWR without mods, look up numbers on wiki and use Mun or Minmus as benchmark. Dune has 0.3g, ergo, if you can get off Mun's 0.15g on half throttle, you are set. You could make it dress rehearsal of whole mission.

[Redshift OTF]

But here our first issue was encountered. Duna was not in the optimal position. I was too late. Instead of ~45° it was closer to 30°. Oh well, go for it, I should be OK, I always bring extra fuel. OK, after initial burn I was close, my mid course correction got me an intersect, at this point I was starting to worry about the fuel, I had burned an awful lot. OK, inside Duna SOI, but way off, didn't realize my inclination was so off, I was way below the planet and not going anywhere near it. Another long burn to get into orbit, and a second to circularize it. At this point, nope not going home, doubt I could get back out of Duna space.

This bit worries me. It sounds like you made it to LKO, then escaped Kerbins influence and then did a course for Duna? I never use launch windows myself but I was always under the impression that you used them for burning straight up out of the atmosphere and out of Kerbins influence and straight for the planet you are aiming for. If you make LKO and then escape Kerbins influence you don't need to worry about how many degrees Duna is; you just make a Hohmann transfer at the appropriate point in your solar orbit. If you didn't do this then the course you plotted may have been pretty extreme and there fore you used up way too much fuel.

I attempted to make a replica of your craft to see what the DeltaV values were. I switched the lander to the top just to get the correct values. Apart from that it is fairly accurate I think:

Your lander should be OK I think but, as others have said, the engine is too big. Your TWR starts at about 6 and ends up at 20. You could use 5 x 48-7S's, one on each fuel tank and one on the main lander and save yourself 1 ton of weight. You could then use smaller landing legs and probably save another 0.5 tons. That would give you at least another 750 DeltaV which would easily allow you to dock with your tug.

The tug itself has about 6000 DeltaV and that's while pushing a fully fuelled lander. That is loads of fuel for a mission to Duna. If you didn't make it then, as I mentioned above, you must be performing the initial burn to Duna very wrong. Can you provide more details of how you did that? Hope this helps as it would be a shame if you lost interest in KSP missions due to maybe one mistake.

[Chronin]

Well, I've managed to get 37 ton lander to Duna and back with no docking involved, so I'm sure you can make it. If you want, I can send you my ship link, it has plenty of fuel to get to Duna and back, and if you have good piloting skills (unlike me), you can send it to Ike and Duna in one trip. One tip for making a ship: do what you're good at. I tried thinking small, and I failed miserably. When I built big, I created the largest ship I've ever made and it worked great. If you're good at building big, do it. No one is stopping you. Is it not as efficient as it can be? probably. Yet you had more fun, and that's the important part. If you have fun and it works... then you're set. To all the people who are going to complain and tell me to think simple and smart, here's my lander: 217 parts of pure inefficiency, and a lot of useless junk. Worked great.