When to ignite Main engine

[Kerbal007]

Hello all,

Could someone please advise me or point me to a post / document where I might learn when to fire the main engine? Presently I use boosters first then fire the main and I am undecided if this is best practice.

 

Kind regards,

Kerbal007

[Blaarkies]

Every engine on your rocket that is not active, is dead weight you are lugging around(this means the bottom stage engines needs to be a bit bigger to help carry those around). In most situations it is more beneficial to run all your engines at max thrust:
-at launch, you want to minimize the amount of time that you are hovering or not-yet-in-orbit...those contribute to gravity drag.
-at landing(i.e Mun) because hovering or "slowly coming to a stop" costs more fuel
-burn/eject to Jool, because you only have a few minutes of burn time near Kerbin(the Oberth effect's advantages decay as you get away from a gravity well)
*Note, this disregards mixed Isp engine setups(burning only your nukes might get you further than even though it takes longer...because nukes are super efficient)

Activate all possible engines simultaneously. Exceptions to this is:
-high isp vacuum engines(poodle, terriers, nuke) have very poor efficiency in low atmosphere, they waste lots of fuel to provide very little thrust below 10km altitude at Kerbin
-when using jet engines, you want to get as much power from them before switching to rocket engines

If you have to down throttle your main engine to less than 80%, your engine is too big. Remember, every ton of extra weight is less DV, and an engine running at 80% is already being 20% dead weight.

Unless of course if you are role-playing a real-life space launch.

[Spricigo]

Test it.  First launch with only SRBs in first stage,  main engine secound stage. Then launch the same rocket with SRBs + main engine in the first stage. If the later make the rocket overheat or wobble trust down your main engine or tweak SRBs in VA.  See wich one works better for you.

Personally I usually design my rockets with  a single LFO engine or 1 main LFO + 2 auxiliare SRBs in the first stage but I'm not that worried if  they are super-optimized. 

 

[GoSlash27]

Kerbal007,

 If your main engine has a higher Isp than the solid boosters, you should fire it. If it has a lower Isp, you should leave it off.

Best,
-Slashy

[Ohm is Futile]

Normally, it's not even a question for me, everything fires from the start unless:

- My TWR ratio is above 1.5 (atmospheric drag/heat issues at low altitude, but if that is a problem, I normally adjust thrust limiters first)

- My SRBs can lift my rocket off on their own with a non-crazy TWR (which seldom happens)

...but I usually use SRBs to help with TWR and provide a bit of dV. Something I would highly recommend if you are using liquid fuel "boosters" is to have a fuel line feeding the main engine to have it fire from the start. It's more efficient that way.