Lets talk about where I went Right and Where I went Wrong

[thiosk]

After the loss of the MoHo lander and the stranding of Jebediah and Bob (and the subsequent EVA loss of bill, RIP), I've realized that I needed to finally separate the planet return lander from the transfer stage. Design Concept 1: The transfer stage is a ship. It has plenty of fuel. Thrust is unnecessary once in orbit. Design concept 2: Moderately heavy lander. 650 dV.

So I launched the transfer stage, and it was a god awful launch, but I got it up with clustered engines. Jolly good, full tanks.

Its got a measly 1883 dV. When I went to Moho, I needed 3k dV just to slow into orbit. So, my TWR is 0.082, do I simply have too much fuel for two nuclear drives to push, or is everything just too heavy, or what would be a more effective interplanetary launch and return ship? I had thought I had enough fuel in this rig to get out to eeloo, at least based on my previous journey to Dres.

I'm not even sure if its safe for me to do a sample return mission to Duna. I brought side chutes on the lander for home, so they're repackable...

[Markus Reese]

At the end of the day, you have so much mass to get there. If you have two ships, you just split the work in half, but then need the capsules etc to fly them so each rocket is smaller, but it takes more. There is a trick I use however to slow down. It adds a little bit more weight, but is what I like most.

1. Bring along braking solid rocket boosters

2. Affix braking engines to your booster tanks. example, the four outer tanks there to burn off their fuel. Remember, as your fuel is consumed, less dV needed.

The latter works when you plan for obsolescence. Example, you only need the orange engine to escape, so use a different less efficient engine for braking. Ditch some interstellar boosters on way or something possibly as well. The 2m smalls aren't too heavy.

In addition, there is something else to do in conjunction. Don't try to decelerate all your speed. Instead, as you just enter the solar system or get close, adjust your flight path for atmospheric braking. For Moho and the other small bodies, not really effective with their low/no atmo, but it can help. Is how I always capture Duna with it's lower gravity. Just skim down nice and low to save alot of fuel.

Edit: And returning to Kerbin while really low on fuel, saves lots of burn time the further out you make your tweaks. Catch it right and you can capture orbit without needing a burn at all.

Edited November 17, 2013 by Markus Reese

[breakthrough]

The craft you are not controlling during docking can't use its RCS, remove the mono-propellant from the ship that is going to just sit still and receive the docking craft.

Consider mounting engines to the bottom of the Science Jr pods and remove the poodle so the lander can attach the other way and use its engines to help the transfer vessel brake.

Start burning retrograde before entering the target's SOI, LV-N efficiency will probably be good enough to justify not having Oberth effect, and while in solar orbit you'll have plenty of time for 0.08 TWR to get the job done. If you wait until you are 'only' one day out you should be able to do a lot of braking without losing the encounter.

[Dun Kirk]

2x 3 man lander cans is heavy. ( remove 1 lander can and add a probe + ASAS instead)

750 RCS on a lander? You need like 80 units max.

2x 750 RCS on the transfer again why?

you have way too many chutes on the lander,

add a decoupler to the lander, so once you return the the transfer you can then dump the dead mass

if you need more engines for breaking, then fine but design it so that you can then dump them

if you have 4 drop tanks, then do asparagus staging on them. you'll burn 2 sooner and drop them saving on mass.

I suspect this is a two launches joined together? Its too wobbly from experience.

[wasmic]

NEVER use solid rocket boosters in space, they have way too low I_sp. You could, however, add liquid fuel boosters using either skippers or LV-T30 engines. They greatly decrease transfer time, and can add a lot of delta-v if you asparagus them with a nuclear-driven central stage.

[Johnno]

Yeah you simply have too much mass and not enough fuel.

Since you're using engineer you can use it while building to check your dV, then check dV charts or online calculators to figure out how much you need. A lander doesn't need a huge TWR, at TWR 1 you can land but it takes skill/time, but at the same time you don't need bucketloads beyond that either. And lander dV you can check on charts as well, a good rule of thumb is that you need to kill your orbital velocity and then extra for soft landing. If you're in low Mün orbit for example you're going around 550m/s. You can pull off a landing around 600m/s dV but a bit more is healthy, seem to remember dV charts say 640m/s.

Looking at your lander you have way too many engines, the poodle alone is almost overkill for both Moho and Duna. And yeah that huge RCS tank is just dead weight. And you'd be better off using a 3 man pod that stays in orbit and landing with a landercan mk2, which could be unpopulated at liftoff and then discarded before return to Kerbin. You're really just dragging extra mass around. You could potentially design a 3man pod lander than then uses the interplanetary return stage (or just gets refuelled for return) to get back to Kerbin, so you don't need the landercan mk2, but with double up on the pods yeah you're dragging heaps of mass.

Interplanetary stage shouldn't need that much RCS, as previous posters have mentioned already. And yeah, with just 2 nuclear drives you're looking at a 34 minute burn, for 1800 dV. Burn time doesn't really matter if you have patience for it, dV is the really important part. You'll need around 4000 dV for the optimal trip to Moho orbit, 1400 dV to land and 1400 dV to return to orbit. Then nearly 2000 dV to return to Kerbin.

The way I personally design interplanetary trips, and which I can recommend as an easy 'foolproof' starting point, start from the opposite end and work your way back to the start;

1. Figure out what you want to return to Kerbin: reentry pod with chutes, docking port, bit of RCS for corrections, fuel and engine. As lightweight as possible with enough dV to get from your location's orbit back to Kerbin (for aerobraking/direct atmospheric descent entry).

2. Figure out what you need to land: Landercan Mk1, Mk2, multiple Mk1s or even just external seat(s). Docking port, bit of RCS for corrections and docking, fuel, engine(s) with TWR around 1.5-2. Again as lightweight as possible with enough dV to land and return to orbit, potentially in multiple stages if necessary/desirable.

3. Once your return craft and lander are done, start building your interplanetary stage below them. Alternatively if you're doing several launches and docking up in orbit before your trip, just check their masses and simulate above your interplanetary stage with a dummy weight, fuel tanks for example. Build your interplanetary stage with enough dV to take you from Kerbin orbit to stable orbit around your target location, a bit of extra dV is always good for corrections and mishaps. If you don't have the patience for long burns add enough engines so that your total burntime is sensible.

* Of course add power, batteries, probecores and lights etc as needed, a single RTG and battery keeps everything running so you don't have to rely on solar power, if you're not against a bit of radiation.

I hope some of that helps, I'll leave you with an inspirational picture from one of my own interplanetary trips:

Manned Eeloo landing mission, with rover (which had surface experiment packages as well), a polar orbit satellite and a unmanned lander probe as well.

[Dogface]

try one nuclear engine. takes longer to do the burn, but its more efficient.

maybe use a 2 stage lander?

unless you are trying to get six kerbs to moho, you dont need two Mk12 command pods.

less monopropellant, or practice docking more.

that said, I am impressed you got that into orbit with full tanks!

[Markus Reese]

NEVER use solid rocket boosters in space, they have way too low I_sp. You could, however, add liquid fuel boosters using either skippers or LV-T30 engines. They greatly decrease transfer time, and can add a lot of delta-v if you asparagus them with a nuclear-driven central stage.

Shrug, Never bothered to calculate the tweaks. Liquid engines used to be worse for fuel. SRB in space used to be viable due to their low mass to power output. Sure not efficient, but used to be better than hauling high power and heavy engines when you wanted to stop fast. Comparatively, the T800 with a T-30 engine. The weight exceeds offset of its efficiency. Is the price for control. Poodles would work better for that but who knows. In my mind, there is more to it than just dV. I never calculated it ever. I do enough of that at work so I just do staged trials and go by feel/look

[Borogove]

That dV seems way low. Are there fuel lines from the orange tank to the engine outriggers?

[DMagic]

That dV number might be wrong, even with all that excess RCS mass it shouldn't be that low. Your staging is setup a little weird, the four radial tanks seem to be decoupled at the same time as the lander engines are ignited, this might be causing Engineer to give you wrong numbers.

Just a quick calculation, using 149 tons, 8640L of fuel and an ISP of 800 gives around 8000m/s (and a burn time of over 100 minutes!!), dropping empty tanks would bring that number up even higher. So you should be fine to get to Moho.

As for the long burn time and low TWR, just start the burn really early. The dV saving from the Oberth effect around Moho are almost nothing, so don't even worry about where you burn. Just setup a maneuver node for some point before you get too close to Moho and try to burn off most of your excess velocity.

And yes, there is a lot of mass in monopropellant and capsules that you could safely get rid of.

[numerobis]

You don't need any RCS on the lander, nor any orbital maneuvering fuel; leave all that on the orbiter. Decouple lander from the orbiter, then move the orbiter out of the way using the orbiter's RCS and engines. Land the lander, play around down then, and return to an orbit in the same plane as the orbiter's. Then switch to the orbiter, and make *it* go and dock with the lander. That way you don't need to spend all the fuel landing and re-orbiting the stuff you need for docking.

You might also want the orbiter to hold the atmospheric re-entry vehicle (parachute and a proper capsule), and switch the lander to a lander-can, which is much lighter. For an Apollo-style mission you need to leave a kerbal in the orbiter anyway.

I calculate you have about 8km/s of deltaV on that thing from the nukes. Even if you used the poodle you'd get nearly 4km/s. Is Engineer being "smart" and noticing the engines point at each other, so your net thrust is pretty low if you burn from both directions at once?

[Kerolyov]

Poodles are inefficient engines even in vacuum, tavert has shown instead that the small Rockomax 48-7S or LV-N (nuclear) are generally the best engines for fuel efficiency. Exact choice of engine that needs least fuel depends on how much dV you need, the mass you're trying to shift and how much TWR you want.

See http://forum.kerbalspaceprogram.com/threads/45155-Mass-optimal-engine-type-vs-delta-V-payload-and-min-TWR

Edited November 17, 2013 by Kerolyov

[numerobis]

The poodle is dominated by the LV-909 (5 of which have more thrust than a Poodle, and equal or better Isp), and in most applications the aerospike is better yet. This isn't the OP's main problem though.

[thiosk]

Both vehicles have been tossed into atmo. I tried to get my monster fuel tank docked to my new Zanzibar hotel, but that got annoying, so I cranked on the engines and jumped overboard, leaving the crew on the space station. The lander went back to kerbal.

So now let me show the next step of what I've tried to do and see if I can't make it more clear.

This is the assemblotron 1000. He was designed to be VERY balanced, very easy to manuever, and perfectly switchable so I could use RCS and regular drives for convenient docking and tugging.

What I found was that he was even harder to control than the last little tug I made.

I especially like some of the discussions on this page and intend to use them. But I really need to get it so docking is really easy. I look at the plugin for the docking computer, and I see that guy line up his pictures and slam directly into the correct, but his ships are merely thin segments. I always seem to need a gigantic margin of error, especially in fuel, to get lined up and attached.

So what are the principles behind SAS? Do I have too much torque for an item this size, or not enough? whats the optimum postion for reaction wheels-- at the center of mass, or evenly spread between ends of the ship, like with rcs?