Delta V on Multiple Engines

[SteveD80]

I have a long range reusable lander that has 4 terriers and 1 poodle.  Generally I just need either the poodle or 2 terriers, but some maneuvers require all 5.  Do I get extra delta-v shutting off unnecessary engines or should I just keep all the thrust unless it is a hindrance?  

[Snark]

9 minutes ago, SteveD80 said:

I have a long range reusable lander that has 4 terriers and 1 poodle.  Generally I just need either the poodle or 2 terriers, but some maneuvers require all 5.  Do I get extra delta-v shutting off unnecessary engines or should I just keep all the thrust unless it is a hindrance?  

The dV you get per fuel expenditure depends only on your Isp, and not on how many engines.  So you'll get the optimum dV for your fuel if you turn off all the Terriers and run only the Poodle, since it has a higher Isp than the Terriers do.

That said... it has only a tiny bit higher Isp, less than 1.5% better (350, as opposed to 345), so it doesn't really make much difference in practical terms.  A Poodle-only burn will be less than 1% extra efficiency than burning a Poodle with four Terriers.

So unless you care about that tiny smidgeon, you can just leave all your engines on.

[SteveD80]

Thanks, I will run them all most of the time, but it lives at Minmus and running more than 2 terriers there is difficult to control.

[RexKramer]

When you're conducting burns to transfer from one body to another, you want to have maximum thrust.  You lose efficiency with long, slow burns because less burn time is spent at the ideal burn location.  As your 5 engines have nearly identical ISP, I would certainly run all 5 for transfer burns.

Also, during landing a similar principle applies.  A 'suicide' burn is usually more efficient than a long, slow burn down to the surface (although also more risky..).  Since you said control is difficult with all 5 engines going, perhaps you could use all 5 engines during most of the descent, turning off 3 engines for the final touchdown phase.

When I send a huge mining/refining operation out to Jool, it's normally very under-powered, with 2-4 LV-Ns.  The nukes are very efficient during long burns.  However, the burn to escape Kerbin is very inefficient with the low TWR.  So, for the Kerbin escape burn I bring a Mainsail.  The Mainsail allows a more efficient escape from Kerbin, and makes it easy to get a boost from the Oberth effect.  After I've left Kerbin and am coasting to Jool, I jettison the Mainsail, as it's no longer needed.