Orbital Maneuver Thrusters

[KatzOhki]

I have a craft design (SSTO) that maintains an orbit using a LVT-30. Would I save fuel doing orbital maneuvers by using some LV-1Rs or 24-77s? I'm accepting the loss of dV / ISP.

[Dave Kerbin]

No, both of those engines have much lower ISP, and when it comes to orbital maneuvers ISP is what will determine your fuel use. The LV-T30 already has an ISP of 370 in a vacuum.

Unless you are willing to eject the previous engine (treat it as a stage) or the ship is very heavy you probably wouldn't get any improvement from fitting the LV-909 (390 ISP) since the weight of carrying both engines will reduce the efficency. Generally you would drop the LV-T engine and fuel tanks as a booster and use an LV-909 or the Poodle as an orbital engine. If your SSTO is intended to land without discarding parts it probably doesn't make sense to add another engine just for orbital changes.

[Nurph]

You would most definatly not save fuel. More engines means more mass and both the LV-1R and 24-77 have lower ISP, which means you'll need even more fuel to accelerate the same amount. You're much better off sticking with your main engine or RCS if you have it.

Saying you want better fuel efficency but are accepting dv losses is kind of an oxymoron, especially when your talking orbital mechanics where your velocity (speed and direction) is what defines the shape of your orbit.

[Seret]

If you're going to be doing a lot of orbital manuevers why bring your orbital engine with you every time? Leave it up there. Send a good vacuum engine like an LV-N (with a docking port) up to orbit. Whenever your SSTO makes orbit, dock with the orbital engine package and use that while up there.