The Big Red Target. (A Duna Mission.)

[Alias72]

Hello everybody! I have a problem. before I begin I should probably list my mods.

DRE

FAR

KIDS (set to FAR adjusted)

Bunch of parts packs (most not unlocked)

Ultimate Storage

KAS

TAC

Engineer

KAX

Ok back to the matter at hand. I am planning a manned mission to Duna. The mission will take three astronauts on a multi-billion dollar journey through the solar system to the big red planet and land two of them on it's surface. Note TAC means I must also bring food water and oxygen for my intrepid victims. My current issue is weight.

The First part of my craft is the Duna Lander. It has a mass of 14.2 Metric Tons (hereafter Tons). It carries 2 Kerbals to the surface and back. It has 3 experiment bays, and enough endurance for a few weeks on the surface.

The second Part is my Command Module. The CM comes in two parts, a capsule (with spaces for experiments) and a reentry module (the segment on the back with the engines) It has a mass of 12.4 Tons.

The Third Part is the Service Module. This SM is for the interplanetary craft and has a mass of 30 Tons. it can sustain the crew for 640 Days.

I need a Propulsion module capable of providing 5000 m/s Delta V for the combined craft. This may require cutting down my parts, which is unfortunate. Also any tips you can provide me would be welcome.

Please help me get to DUNA!

[Pecan]

Get rid of those huge RCS tanks. They're enough for an entire space programme, let alone a single mission. The monopropellant in the lander and command pod alone should be sufficient.

[Jovus]

Get rid of those huge RCS tanks. They're enough for an entire space programme, let alone a single mission. The monopropellant in the lander and command pod alone should be sufficient.

"Should be" and "is" are different things, though ultimately I agree. With those huge RCS tanks, you stick one on a station module if you're going to use it as a refueling bus, or an orbital tug you never ever want to bother refueling. The tiny 50 unit RCS tank is more than sufficient for your one-time docking needs, and if it isn't you're doing something gravely wrong.

In addition, you can drop the third strut and move that blue-and-white whatsit up. Further, you can drop all those batteries on the service module - a crewed module can do just fine without electricity during Duna's night period.

Beyond that, stick a Mainsail on it to get to LKO, and then an LV-N with appropriate fuel to get to Duna.

[r_rolo1]

5000 dV? I assume you will not aerobrake in Duna or Kerbin then ...

I agree with Pecan: you don't need that much RCS. I've used far less than that in a full biome munar science grab. Cutting that would decrease a lot the payload mass you have ... and, without wanting to mess with your planning, if your lander has RCS manouver ability, maybe you don't need a proper CM detached from from the Service Module.

[Alias72]

Thank you for the suggestions. I don't have access to LV-N's at the moment, which is unfortunate because I really would love the ISP. I currently have 166.5.

[Jovus]

You can still get there by using a couple Rockomax 48-7S engines; you just have to plan your fuel budget accordingly. Oh, and it really doesn't hurt to aerobrake at Duna and Kerbin when you get there; it cuts down your dv requirements considerably.

[Red Iron Crown]

Assuming you have access to an engine with 390s of Isp and want your propulsion module to be a single stage, you will need approximately 300t of tanks to get 5000m/s of delta-V. I would consider reducing your payload as much as possible and possibly staging your propulsion module.

Edit:

Oh, and it really doesn't hurt to aerobrake at Duna and Kerbin when you get there; it cuts down your dv requirements considerably.

He's using Deadly Reentry, it could possibly hurt greatly. Though a suitable heatshield is probably worth it to allow safe aerobraking at both destinations.

[Endersmens]

You could make a series of drop tanks. This would give you maximum efficiency in that area. It would be a bit tricky to get the fuel to drain from the back, but it's possible. With fuel lines especially. use a 2.5M quarter tank, and put 8 48-7S engines on it. Then radially attach a series of tanks and decouplers onto the side. When the bottom tanks runs dry, drop it. lather. rinse. repeat. This will leave nice little loops of debris in LKO, but who cares. delete it if you want. Just an idea i had in my head. Only one of many possible ways of using drop tanks. Might be ridiculously over complicated, but oh well.

[Hodo]

The only suggestion I have is make the lander a two stage design. You really don't need the dead weight of the extra engine on the return to Kerbin trip and you could reduce as much mass as possible.

[Alias72]

I am using KIDS. I have an ISP of 166.5. The ships is supposed to be reusable. drop tanks are out. The lander has no heat shield. this may be a bad idea as I am using parachutes to land the lander.

still all good ideas. realism mods make everything harder!

BTW if you have any ideas on reducing payload I'lld love to hear them.

[Zylark]

Many ways to Duna one can go! - Koda

Though if it were me, I'd just as well wait until I'd gotten the DockingPort Sr or through Near Future Construction have the Octo-Girder Docking Connector. Thing is, ordinary docking ports are wobbly. Stick three vessels plus an interplanetary stage together as you intend to in order to make a little train heading for Duna, your combined craft will behave very much like a slinky.

Now, using KAS you can apply liberal amounts of Space-Tape (aka struts) after assembling the thing in orbit. But I've had only varied success with that approach. For one thing it is a very tedious job and for a second, any significant twisting stress on those struts and they will snap.

Other notes to heed - as others have said, no need for all that RCS. One such tank is plenty - a bit overkill actually, but if not too used to docking better bring a bit more than needed Speaking of RCS - I see your lander only got one set of RCS thrusters. Each module you intend to manuever using RCS need two sets of RCS thrusters. One above Center of Mass, and one below CoM. Preferably equally spaced so that CoM also becomes Center of Rotation.

Your lander should be much lighter. And be powered by more than one little LV909. Two will do, three is better, four is overkill. You should have a Duna TWR of 1.5 to 2.5 ideally. Below that and you'll burn a lot of fuel going not very far, above that and you got dead weight in engines. With a light enough design, some 400 units of LFO (equal to the smallest Rockomax tank + a little bit more) should give you between 1700 and 1800 dV. You'll need roughly 150dV to deorbit and 1500dV to get back to orbit.

The interplanetary stage will need roughly 3500dV for the round-trip when all hooked up to your other modules. Doing it with chemical rockets (like say the Poodle), you'll need to use drop-tanks most likely. To loose some mass during the trip so as to get the most out of the fuel. With nuclear rockets, it is easy enough as you get over twice the bang for the buck, uhm, fuel. With nuclear propulsion two Jumbos should be plenty. With chemical rockets you might need as many as four - certainly three.

And last - try for a puller interplanetary stage rather than a pusher. With a puller (locomotive) you put stretching tension on the rest of the train, any sideways stress will equalize itself under the tension when you are burning. the construction still need be stiff though, as turning will become nigh-on impossible if not.

If you put contracting tension on the other modules by going for a pusher interplanetary stage, your design need to be *VERY* stiff. If not, it'll just start wavering to one side or another, and as you do your burns that sideways motion will get reinforced, eventually breaking up the entire thing.

[Alias72]

Good news everybody! I have managed to reduce the craft mass to 36.5 T through tweakables. now I only need 790 T of fuel to get the 5000 m/s delta v I need... well any more suggestions?

Interesting things to note: there's a reason US hasn't released a TAC pack yet. all TAC units are in days requirement, and all US units are in liters. If I don't fix this then the craft is going to be wonky indeed! specifically the mass is presenting issues. I need thousands of litres of oxygen for my fuel cell, yet only 640 for my crew. I'll try fixing the configs and see how that goes. Another problem, also related to fuel cells, Is that I do not know there input rate vs my requirements. I'll need to do a lot more math there.

also:

1. I shouldn't need 8 quad RCS for full orientation. 2 coupled forces provide a coupled moment and I can produce all three moment as well as translations with just 4 quads. technically I could do it with 3 but it would be wasteful as I would be using fuel canceling forces in directions I don't need.

2. The poodle is fine. I have a Duna TWR of 1.78. I also have 1442 m/s delta V. which brings me to...

3. Re-entry Delta V's. how much delta V is required to reenter:

-Kerbin from 80km

-Duna from 60km * answered above but DV to reorbit seems low compared to my numbers.

this is important because I don't want to thrust all the way down. I just need to lower my periapse so that the atmosphere grabs me and brings me down.

4. Does my Duna lander need a heat shield? It will only be going 1.2 km/s in a sparse atmosphere.

BTW I removed the second RCS tank and temporarily added the second set of thrusters *grumble grumble grumble*

Edited August 25, 2014 by Alias72

[Zylark]

Uhm, ooops. Made a little error in my previous answer. you'll actually need about 800 units of LFO/2100dV to get down to and up to orbit again from Duna. Been a while since last I visited that place *cough*. And whilst you won't burn through more than 100-150dV to deorbit, you will burn through 3-400dV more to get the speed down to a reasonable 300m/s before you fully deploy parachutes. The (stock) atmosphere of Duna do not have the required density to slow down your lander enough before popping the chutes. Meaning if they fully deploy whilst you are still screaming down at 1km/s, the braking force of the chutes will rip your lander to pieces. Once speed is down a bit, the atmosphere is just dense enough at lower altitudes that once the chutes are fully deployed, provided you got enough of them you won't go too fast when getting close to the ground. At which point you just need a little bit of thrust to get speed down from say 8-10m/s to 4-5m/s as you touch down.

As long as you got around 1500dV left when landed on Duna, you are good for getting back to orbit again.

Regarding the RCS thrusters, one group is as you say enough to make you turn and stuff in all directions - but try docking where you need to move laterally or vertically - with high precision. RCS thrusters don't weigh much, and having them well balanced in two groups on your vessels will make life much easier.

If using Deadly Re-Entry, then I'd hazard to guess you'll need a heatshield. I've not used that mod however so I can't say for sure. But I do know that your craft will heat up significantly when entering the Duna atmosphere. With nice flame effects and all.