Warning Probably requires a math degree to answer

[Sathurn]

Delta-V is a function of the Fuel, which means more engines equals less Delta-v.

But the most efficient use of Delta-V is instantaneous and in lower orbit. Both of these thing need more thrust.

So assuming assuming one burn, at what point, if it even exists, is it better to add another engine?

Also of interest is how do you calculate for multiple burns on consecutive orbits without missing the optimal window?

[musicpenguin]

0-10x5.2xamountofburns)= Answer.

[Sma]

I can't really answer the question but I was under the impression deltav is dependant on fuel, engine output and mass, not just fuel. As for adding another engine, I'd imagine depending on the size of your ship, one engine may not be enough, unless you're talking about already having 2 engines and maybe adding a 3rd.

All that being said, I haven't even payed attention to delta-v yet, maybe I would do better if I did LOL. Though I don't generally have a problem with getting things to orbit/other destinations. I usually run into partlag before that happens.

[Neomatt]

A few points:

- delta-V is (simplified) thrust over mass in a period of time (integrals, aaaah, I forgot those). And no, more engines but same fuel means shorter burns with a higher deltaV.

It (in its most simple form) doesn't include the gravitional pull, so you have to "remove" it from the thrust in the equation.

- when to add another engine? Depends on a lot of criteria: ISP of the engine, its weight, how you stage those (asparagus is a prime example of this: your rocket as-is, or your rocket + some fuel + some engines, then as-is: you keep the deltaV for the central part after staging, but have a different one for the whole). So, no easy answer.

I also think you're thinking of the TWR (thrust-to-weight ratio), which also varies over time (supposing your rocket is going up ).

Edited June 6, 2013 by Neomatt

[Brenok]

I don't know exactly the answer but I'm quite sure it just need some simple calculus and the rocket equation, not a math degree. I could always be wrong, though.

I will try to write an answer as soon as I find the information, and if nobody answers until then.

[Sathurn]

I not referring to launch, but once in low orbit. Adding additional engines with the same isp, will increase weight of the craft and lower the over all delta-v, if you don't add more fuel. But it will also come close to halving the required burn times (assuming going from 1 to 2 engines). Making the use of Delta-v more effective. My question is how much delta-v is wasted by the longer burn?

[Atanar]

I not referring to launch, but once in low orbit. Adding additional engines with the same isp, will increase weight of the craft and lower the over all delta-v, if you don't add more fuel. But it will also come close to halving the required burn times (assuming going from 1 to 2 engines). Making the use of Delta-v more effective. My question is how much delta-v is wasted by the longer burn?

I am pretty sure that adding another engine to make the altitude change to catch the Mun in 2 instead of 4 minutes is way less efficient than doing the longer burn. I mean, if you do not burn exactly at the periapsis you still put that energy in the (almost) right direction.

Missing your burn is of course another case, you you should have at least enough trust to not miss your target delta-v.

[RoboRay]

Also of interest is how do you calculate for multiple burns on consecutive orbits without missing the optimal window?

Depending on the destination, a near-optimal launch window will be many (6-hour Kerbin) days wide. If your final Pe Kick raises your Ap to the Mun, that's still an orbital period of just six hours or so (or is it 12? I forget). Your earlier kicks will result in orbits of no more than an hour or so. As long as you start your kicks a day or two before reaching the ideal phase angle, you're good.

It's only if you push your Ap out to Minmus or beyond on the final kick that you really have to plan ahead, because your orbital period becomes many days long. And the delta-v difference between pushing Ap to Minmus and stopping at Kerbin is too little to really worry about, unless your craft is ridiculously marginal for the mission.

As the efficiency costs of adding engines, a second engine is almost always worth it. You're doubling your thrust, as you note. It's when you start adding more engines that diminishing returns really start to kick in... You need four for the next doubling of thrust, then eight, then sixteen... it rapidly becomes counterproductive, especially when you consider all the extra mass in support structures to hold those engines.

Edited June 6, 2013 by RoboRay

[Neomatt]

Okay, so in this case, it's the Rocket Equation (from Tsiokovsky): deltaV = Isp of your rocket x 9.8m/s² x ln (mass at the beginning/mass at the end).

[AfailingHORSE]

Im pretty sure you just really need to go through physics 1 and remember it for this question (I only remember that F=M*A), not a math degree