When to throttle up while initiating a maneuver?

[Zerro]

Greetings,

I've seen videos of people setting maneuver points for burns. My question is: When should I fire my engines?

I've heard and seen different methods and I would just like to have more clarity on the subject:

* I've seen people take the the estimated burn time, divide by 2 (ie: 20/2 =10secs), and fire engines 10secs before reaching the maneuver point.

* I've also seen people throttle up right as they arrived at the maneuver point.

I have a few other side questions about maneuvers as well:

1. How accurate is the estimated time? Is it like within +/-2secs? Sometimes I've seen the time say 20secs and when I throttle up, the time would jump, for example: jump to 40secs, jump to +1min, etc

2. What happens if you start throttling up after passing the maneuver point?

3. Are there any specific engines that are better for burns?

Thanks for better explaining how to efficiently use maneuver points.

Zerro

Brian D.

[SuperWeegee4000]

I find it best to start burning around T-2 seconds.

[longhornchris]

Assuming the system calculated your burn time correctly, I start my burns ~1/3 the burn duration ahead of the burn. So for a 15 sec burn I start at T-5. Using KES or MechJeb this results in long burns delta-V consumption match the originally predicted burn.

As to what engines to use, this is a loaded question. Technically the delta-V calculated assumes an instantaneous burn - or infinite thrust to weight ratio. The lower your TWR, the longer the burn and potentially less efficient burn - especially when in a highly elliptic orbit. The tradeoff is that the low thrust engines tend to have the best ISP which more than offsets the losses due to long duration burns.

One way to minimize losses due to long-duration burns is to break your maneuvers up. As an example, when attempting to get to Mun rather than performing a single burn to get away from Kerbin, perform multiple burns over a few orbits at periapsis to raise your apoapsis.

[Mazen]

I'm using the first method. So Est.Time/2 = Start Time.

1.: It's actually pretty acurrate.

The thing is this: If you burn a stage the system "knows" how much deltaV the engine can provide. But if you, for example, decouple a previous stage shortly before the manouver and don't fire the "new stage" your system still calculates with the last know deltaV, which is the one of the previous stage. (Hope this is right and comprehensible)

2.: You won't get the tageted orbit.

3.: Some are more efficent (ISP) or provide more deltaV than others so they will need less/more time to perform a manouver. But at last all of them will need to provide the same amount of deltaV.

Hope this helps you out

[Eric S]

I've seen videos of people setting maneuver points for burns. My question is: When should I fire my engines?

It depends. What I tend to do is cut the time of the maneuver in half, add two seconds (for throttle up time, since it's not instant in most cases), and burn that amount of time before the maneuver node.

The idea is you want half of the burn before the mark, and half after it. Splitting it in half is a good enough estimate for most cases. However, if the maneuver is changing a large portion of your velocity, or if you're burning enough fuel that your TWR will change significantly, you might want to shift the timing.

1. How accurate is the estimated time? Is it like within +/-2secs? Sometimes I've seen the time say 20secs and when I throttle up

It tends to be very accurate, provided that you've run the engine since you last staged/switched to this vehicle and the burn won't significantly alter your TWR (it might even take that into account if the change in TWR is strictly from fuel consumption, I've never checked).