Estimating time required to burn

[jarmund]

I've found that the ingame time for burn is often inacurate, especially when dealing with complex engine setups or a newly staged vessel. As I'm a big fan of skimping on thrust to allow for a high-efficiency setup, this sometimes have dire consequences: For example, when I am arriving at some foregn planet, the first thing I normally do is to lower apoapsis to a decent orbital insertion. I then place a maneuver node at the apoapsis for insertion. Too many times this has resulted in the burn time jumping just when I think I'm starting the burn at the right moment, and with very little thrust applied way too late, this normally results in either a very inefficient insertion, or no insertion at all (flying by, leaving the SOI before I manage to turn around).

So today I simply don't trust the maneuver node burn time estimation.

My question then is: Seeing as I know the TWR of my ship, and I know the deltaV that I need to burn, is there a quick and dirty way of calculating how long the burn will be? It doesn't have to be acurate and account for increased TWR during, due to fuel being burnt. Just acurate enough to give a ballpark estimation so that I know if the ingame number is correct.

[Taki117]

Check out this post for an in-depth explanation.

Basically you calculate the total burn time of your engine for the amount of dV you need then using the time to periapsis as your zero point divide your total burn time in half, at that gives you the amount time prior to your node in which you should start your burn, in this case your Periapsis. When your time to periapsis is that number execute your burn and you should be good to go.

Another option is to perform a "test burn" where, when you get close to your maneuver node (But still far enough away for it not to matter) you go full throttle (Or whatever throttle you will be using) for a moment then cut your engines to reset the burn time.

[Red Iron Crown]

You should be able to calculate both the mass flow rate of propellant (given in engine stats in tons/sec) and the amount of propellant needed to produce the required dV (rocket equation). Divide the latter by the former and you get exact burn time. If you want to get really fancy you can do it for 1/2 the required dV, that will give you the ideal "advance" before node time, as ideally you want to split the dV around the node rather than just 1/2 the burn time.

KER and MechJeb can both calculate such things automagically for you, even accounting for staging events during the burn.

[Streetwind]

Umm. Can't you simply do like, "TWR 1.0 equals 10 m/s², so my TWR of 0.25 equals 2.5 m/s². I need to expend 500 m/s dV, so my burn time is no higher than 500 / 2.5 = 200 seconds"?

All he's asking for is to estimate his total burn time based on his TWR. No need to overcomplicate.

@ jarmund: the reason the game shows the wrong value is because for the correct value to be calculated, the engines need to have run at 100%at least once. So every staging action or engine toggling while the engines are off causes your ship to no longer have a correct estimate (until you throttle up to full the next time).

Edited August 19, 2015 by Streetwind

[Sharpy]

@Streetwind: If you can easily obtain TWR...

Sum of thrust of active engines can be easily pulled from the wiki. Weight is trickier. You can get the *current* mass of the ship but as you burn fuel, your mass is dropping, so your TWR grows. Just look at stats of the Kickback SRB: Full - TWR 2.85. Empty: TWR 15.18. If you assume the burn doesn't use up any significant amount of fuel you can go with current mass. This won't be true for any large burns like orbital insertion though.

Personally, I'm just going with KER. It usually doesn't lie about node burns.

OTOH lengthy burns on low orbits are always calculated for average altitude being at the node. If you begin the burn 1/4 the orbit away from the node your altitude will be waaay off by the time you reach the node time. (actually, with nukes I prefer to burn at higher orbits simply not to drop myself into the atmosphere by chance!)

[Streetwind]

@Streetwind: If you can easily obtain TWR...

Indeed. Conveniently, the OP specifically states that he knows his TWR.

[cantab]

Sure. Force = mass x acceleration. Change in velocity = acceleration x time. Solving for time,

Time = (Mass x Change in velocity) / Force

This simple equation is what the game uses, but the game as you've found doesn't always pick up on the value for force.

Of course it's inaccurate because the mass isn't constant. Kerbal Engineer will give you accurate burn times taking changing mass, and changing thrust from staging, into account. They are displayed under the Orbit page.

[monstah]

Umm. Can't you simply do like, "TWR 1.0 equals 10 m/s², so my TWR of 0.25 equals 2.5 m/s². I need to expend 500 m/s dV, so my burn time is no higher than 500 / 2.5 = 200 seconds"?

All he's asking for is to estimate his total burn time based on his TWR. No need to overcomplicate.

@ jarmund: the reason the game shows the wrong value is because for the correct value to be calculated, the engines need to have run at 100%at least once. So every staging action or engine toggling while the engines are off causes your ship to no longer have a correct estimate (until you throttle up to full the next time).

Indeed. Conveniently, the OP specifically states that he knows his TWR.

To me, it's this. I agree with RIC's dV method, but as a simple approximation this one is the best IMO.

Sure. Force = mass x acceleration. Change in velocity = acceleration x time. Solving for time,

Time = (Mass x Change in velocity) / Force

This simple equation is what the game uses, but the game as you've found doesn't always pick up on the value for force.

Of course it's inaccurate because the mass isn't constant. Kerbal Engineer will give you accurate burn times taking changing mass, and changing thrust from staging, into account. They are displayed under the Orbit page.

but then, (Mass / Force) = (1 / g * TWR), which he has already (approximating g by 10m/s²)