Nuclear engines fuel efficiency in multiples

[Claytsuk]

On my way to duna with 6 kerbals of which 3 will attempt a landing. The rest will stay in orbit for science stuff and then I plan to get them all back home. I have 4 nuc engines on my mother ship and am currently doing a 20 min burn real time to get into duna orbit. My question is .... Is it better to burn 2 nuc engines for 20 mins or all 4 engines for 10 mins to get the best fuel efficiency or does it make no difference what so ever...

13 mins left....

[Randox]

If you have the engines there with you, then for sure using all 4 is going to be more efficient. The further you are from the optimal point for a burn, the more fuel is wasted. I'm unsure at what point it becomes really noticeable, I just know the math works out that way.

However, if you really want to get into the math, you would want to look at the wasted fuel from a slower rate of acceleration using two engines versus the delta v lost to the additional weight of using 4. Without looking at the numbers, the heavier the rest of the ship, the less difference an extra engine or two makes. A very small ship with a bank of engines resembling the back of Tantive IV (ship Darth Vader is chasing at the start of Star Wars), the weight of the engines is wasteful. If you have a veritable star destroyer and a single engine on the back, that's a problem too.

Edited May 22, 2013 by Randox

[W3asel]

Since you have them all, use them all. It would be more efficient not to be moving around the extra engine mass by only having one or two engines, but unless you're inclined to try some controlled colisions to remove them that's probably not an option at this point.

[RawChicken]

use all of them, if not they are just dead mass to pull.

On the 20min burn, try burning while you are in kerbin escape velocity, it is much faster and much efficient. Make a maneuvering node and see when and how you have to burn. If you cant burn while in kerbin escape velocity, burning at apoapsis or periapsis is the best o lower or increase your orbit.

[Claytsuk]

Thanks for the replies guys. This story does not end too well.

I managed to burn my engines and enter Duna orbit but used a lot of fuel... I managed to land on Duna and lift back off again using my lander craft but didn't have enough fuel to dock in ordbit with my mother ship, which also now does not have enough fuel to get back to Kerbin.... Sign ) was a fun 4 hours trying though...

Think i need to figure out a way to get more fuel more efficiently to Duna so i can land and get back safely... Also need a rescue mission now for my 6 guys in Duna orbit should be easier since i dont need to land.

will start out by dumping some weight before lift off will dump 2 of the nuc engines i think which wil save fuel, also will add a few more tanks...

Can you lift off from duna with nuc engines? I don't think they would have slowed me down fast enough to land though, i used a lot more fuel than i had planned slowing down for landing so i could deploy my parachutes...

[Uberick]

You can land/lift off of Duna with nukes, depends on how much your lander weighs though. . . My biggest Duna landers usually consist of one nuke engine, one 360l fuel tank, a command module and some RCS, and that lands/lifts off just fine.

To find out, there are several mods that will give you your thrust to weight ratio on various planets, or you can compute your thrust to weight ratio yourself.

The tricky part with using nukes for landers is that nukes are HUGE, so you have to mount the landing gear on struts or structural wings to get them to reach to the ground.