TWR

[James D Kerman]

Hey. I have a question. I am building a Duna mothership and Kerbal Engineer says the NERV stage TWR is .36 with tanks full. This stage is for space only. What will happen? Help!

[Snark]

1 hour ago, James D Kerman said:

Hey. I have a question. I am building a Duna mothership and Kerbal Engineer says the NERV stage TWR is .36 with tanks full. This stage is for space only. What will happen? Help!

Hi, and welcome to the forums!

What will happen is that fire comes out of the loud end and then the rocket goes the other way.  ...By which I mean, you're fine (if I'm correctly understanding your situation). 

This is just an orbital ship that will never be landing, correct?  In that case... the NERV is an excellent choice for interplanetary mothership types of craft, because of its extremely high fuel efficiency (Isp 800).

Yes, it has quite a low TWR.  But TWR doesn't matter (much) for orbital-only craft.  A TWR of 0.36 is plenty.

Where TWR becomes really important is for landers, because the engine has to fight against gravity.  Low TWR = lots of fuel wasted due to gravity losses.  Too low, and you can't even lift off.

But if your ship won't be landing, you're doing just fine.

Just a side note (unrelated to your TWR question, but you need to know this if you're using the NERV):  One thing to make sure of (this is an easy mistake to make, if you're new to KSP).  The NERV engine, unlike all the other rocket engines, runs on liquid fuel only-- it doesn't use oxidizer at all.  So if you're putting "normal" fuel tanks on there, you're wasting huge amounts of useless dead weight and you'll get abysmal performance.  It's also bad if you use regular fuel tanks and just set the oxidizer to empty, because then you're still lugging too much dead weight in the form of half-empty fuel tanks that are twice as big as they need to be.  So be sure that you're using liquid-fuel-only tanks (like the ones that airplanes use) to supply your NERV.

[Palaceviking]

Don't panic!,  in space no one can hear you scream.

Also once in space and orbit a low twr is no problem, however you do realise you can't land on that stage, right?

Only landing and liftoff stages need a twr over 1*

*you can start a landing with twr<1 as long as it climbs fast enough as you burn fuel.

[wumpus]

I'm also assuming that you already slid your slider over to vacuum (not sure how much effect Duna's atmosphere has on the NERV).  TWR of .36 on the surface of Kerbin would be *way* too much TWR in space.

One old trick to use with low TWR (typically much lower than you are using) is to split the burn to Duna into multiple sections.  You could split the Kerbin->Duna burn into two bits of delta-v ~2000m/s apiece.  Just set up an encounter with Duna (so the manuever node tells you when to begin) and fire for a burn of 2000 m/s (the easiest and most effective way would be to have a 2000m/s fuel stage).  Then set up another maneuver node all over again to tell you how long to burn (the node should be at your new Pe).  I wouldn't do this with your rocket (interplanetary maneuver nodes are tricky, and you actually have quite a bit of TWR for interplanetary travel), but keep it in mind if you want to build a bigger rocket and need the efficiency.

This trick is often called the Mangalyaan maneuver after the Indian spacecraft that first did it.  For Duna, gains are limited after 8 burns (I think Mangalyaan did more) since that last burn is going to be at least 130m/s or so out of a total ~1080m/s.  But until you get out to the Mun (which is pretty close to escape velocity anyway) your orbits will be pretty fast (similar to LKO) and shouldn't change your Duna trajectory all that much.  Sometime on the forums it is called "periapsis kicking".

[Kryxal]

With TWR of .36, you're looking at maybe a 5 minute burn to head to Duna ... I wouldn't bother to do more than split the burn in half, and for planning purposes, aim your first burn as if you were making the whole burn about 2 hours later, then reduce the delta-v by 50%.