How to be accurate while making a burn with different engines?

[Nivee~]

The Nav Ball shows how much time it will take to complete a burn by an engine. But what if I have 2 stages, first has a very high thrust, second has a lower thrust? Does the burn time take that into account? If yes, ok.... If no, then how do I compensate for that?

[FireKerb]

The burn time does not account for the two stages. And the extra calculations needed to figure out when to burn would probably not be worth the minuscule delta-V difference (Provided it isn't something like rendezvous which may make the calculations less useless, but still useless). Just burn somewhat earlier, or dump the first stage.

[Xd the great]

Well, that IS some serious rocket science. Do you have KER/mechjeb? It helps with the delta V /TWR calculation.

Lets say that your 1st stage has 400 delta v left, and you need to do a burn of 1000 delta v. I use mechjeb myself, so i can see the estimated burn time left.  Also, lets assume your second stage has 2400 delta V, and a burn time of 4 minutes. You divide the stage delta V over stage burn time, and that gives you a rough estimation of delta v per second, i.e. 10 delta v per sec. Now, divide the remaining delta v (1000-400) by the delta v per second of the second stage, and that gives you the burning time of the second stage. The same can be done for the first stage.

This method only gives you a rough estimation of burning time required, and has excess time for you to execute manouvers accurately by reducing thrust.

Edited June 2, 2018 by Xd the great
For lazy people, use mechjeb. Or use corrective burns midway.

[Superfluous J]

The easy way to do it is to F5 first with plenty of leeway before the node, do the first half of the burn, stage it, find out what the second half will cost in time, and then reload your quicksave to do the actual burn.

[Bakkerbaard]

18 hours ago, Xd the great said:

You divide the stage delta V over stage burn time, and that gives you a rough estimation of delta v per second, i.e. 10 delta v per sec. Now, divide the remaining delta v (1000-400) by the delta v per second of the second stage, and that gives you the burning time of the second stage.

Node ETA: 20s. I can't even add up my groceries in 20 seconds and now I have to divide stuff involving Greek symbols? ;o)

[Tyko]

Another low tech way to solve this is to right click on the lower stage engine and reduce its power level to match the TWR of the next stage. This simplifies your calculations. Here's the process I've used:

• Before you launch, note the TWR of the second stage involved in the burn.
• Lower the fuel level of the first burn stage to where it will be when you start the burn.
• Right click and adjust the power level of the first stage engine until it has the same TWR as the second stage. Note the power level that works.
• Reset everything back to normal for the launch.
• When you're ready to execute your maneuver, right click on the first stage engine and dial the power down to the level you worked out in the VAB.

Edited June 4, 2018 by Tyko

[Zhetaan]

@Xd the great has the right idea, at the expense of calculation time.  For a faster approximation, remember that you generally want to split the burn in half around the node.  This means expending half of the dV before the node, no matter the number of stages it takes to do so, and expending the other half after the node.  If, to borrow the example, you have about 400 m/s left in a stage and need to execute a 1,000 m/s burn, then that means burning 500 m/s before the node and that, in turn, means that you will need to stage before you reach the node.

If, on the other hand, you have enough fuel in your first stage to make it past the node, then you don't need to do the calculation at all; just make certain that you perform the first half of the burn before the node, and then stage with impunity to finish the burn.

Depending on the difference in thrust between stages and the amount of fuel left in the first stage, this can result in some extremely lopsided burns.  However, unless you're trying for fine adjustment of the orbit where 1 m/s can make a significant difference or you're changing engine thrust from the level of a Mammoth to that of a Spark, this effect doesn't often wreck your orbital plans.  If it does have a detrimental result, then it's time to calculate everything out.

Alternatively, use Kerbal Engineer, which has a burn time readout that does take later stages into account.  Don't pay any attention to the version in the title; while the original author seems to have suspended development, @jrbudda is maintaining an active fork that is up-to-date with KSP.