Thrust and TWR

[Mmmmyum]

So I'm building an experimental Duna craft , and I'm aiming for every stage to have a TWR of 2.2 (Kerbin gravity), including the lander. Now to achieve that I'm going to need 345.312 kN of thrust for the lander (added a few parts after the picture, excluding the engine). I'm trying to stay away from radial engines, but I understand that throttle will allow me to get that amount of trust. What would the most fuel efficient way of getting that thrust be?

First stage will be launch and interplanetary adjustments, second stage will have the 3.75m inflatable heat shield from Deadly Reentry to aerobrake, which will then drop off so the engine can ignite after the craft is in some sort of orbit, I will lower my orbit into a 60km circular one, leaving the second stage in orbit under command of a probe pod (which will also be the docking craft, RCS weighs too much for the lander). The lander will then undock, land and return to rendezvous with the second stage which will then return to Kerbin once the lander has docked with it.

Mods used are (ones that add parts anyway): KW Rocketry, Deadly Reentry, FAR, B9 Aerospace, with a few more that won't affect this

Edited November 9, 2013 by Mmmmyum

[Kasuha]

TWR has only sense during landing and liftoff where you need to fight gravity while transferring between surface and orbit frame of reference. Once in orbit, the only thing that counts for efficiency is delta V.

[Mmmmyum]

I know that... That's why I wrote "experimental" and TWR does play a role. You can have a stupidly efficient ion engine run for 2 hours straight, but for large craft that would probably do only around 1km/s of delta-v as the acceleration from that 0.5 kN of thrust is almost non-existant

[kahlzun]

Keep in mind that Duna's gravity is about 1/3 Kerbins.

The most mass-efficient way to get the thrust you want will always be the engine with the highest TWR, which is the 48-75, followed by the Mainsail.

To get 360 kN thrust, you will need 12 of the engines (surprisingly achievable, put a cube strut on the bottom of the tank and attach the engine to that) which is an outer ring of 8 with an inner ring of 4.

Unsure why you need such a high TWR, but sure.

If you are looking for 'fuel efficiency' you will need to get creative. Abandoning the 2.2 requirement for the moment, the most efficient engines are:

Nuclear engine:

In vacuum gets 800 Isp, and will exceed 390 Isp in any atmo less than 0.7 atmospheres (duna has 0.17, nukes will get ~700 isp on Dunas surface)

LV-909, "Poodle" & aerospike all get 390 Isp in vacuum.

Edited November 9, 2013 by kahlzun

[Mmmmyum]

I made a whole chart on engine's TWR xD It's mostly because I remember reading somewhere that it's good to have a TWR of 2.2 with the rocket equation. And because I'm bored... (Had a ion probe with 4000m/s Delta-v (launch vehicle having 11k delta-v) and used it to get to Ascension with the ion engine braking the probe into orbit, so I wanna try the other side of things)

Also it would be nice if my lander was capable of achieving Kerbin orbit (normally one of the things I do when designing Duna landers)

Edited November 9, 2013 by Mmmmyum

[capi3101]

That 2.2 TWR you read about was referring to Kerbin launch only - it corresponds to an ascent along the terminal velocity curve, which is important because it optimizes fuel efficiency during the launch. Once you're up in space, TWR takes a back seat to delta-V in its level of importance; up there you need to be efficient, not (necessarily) fast.

[arq]

Also, 2.2 is too high in atmo. It's true that you need roughly 2.2 to maintain terminal velocity all the way up, but remember that TWR increases as you burn fuel. 1.7-1.8 is plenty high for the beginning of stages, typically when I build for that I get around 2.3 by the end of each stage.

[Mmmmyum]

Well this is my second stage that's been designed so far (just the two upper stages can get into orbit), and I know efficiency is important once in orbit, I'm just messing around because I'm bored:

[Dispatcher]

A helpful mod I didn't see used (but you might already have it) is Kerbal Engineer. It lets you view stats, including delta V and TWR of your craft for any object (planet) in the KSP system. Press the Duna button, for example, and you will see the TWR per stage and a running total of other stats.

Edited November 9, 2013 by Dispatcher
Clarification.

[JiWint]

you can always hyperedit to a planet/moon to see if a lander works I wish there was some kind of "testing chamber" where you can select gravity and or atmosphere