Interplanetary guide question

[seyss]

Hello,

I am using the "Interactive Illustrated Interplanetary Guide and Calculator for KSP" to help me get to far planets.

The website says I should start my burn at 112.04º from Prograde, however how does it know how long will my burn take?

I am asking this because I recently switched from "Kerbodyne KR-2L Advanced Engine" (low burn time, high fuel consumption) to "LV-N Atomic Rocket Motor" (very long burn time, very low fuel consumption).

Thanks.

Edited April 13, 2015 by seyss

[Mister Dilsby]

You can do the calculation by hand pretty easily. One newton of thrust will change the velocity of a one kilogram object by one m/s in one second. One kilonewton (kN) of thrust changes the velocity of a one ton object by one m/s in one second.

So: if you have a 300t ship with a single LV-N engine (60kN thrust), it takes 5 seconds (300t/60kN) to add 1 m/s to your velocity. If your burn requires a change in velocity (delta V) of about 900 m/s, then (ignoring the change in mass as fuel depletes), this burn will take 900m/s * 5s/(m/s) = 4500s, or 75 minutes.

So-- how much delta V do you need for your burn, and how much does your ship mass?

[seyss]

Info from Kerbal Flight Engineer:

Delta-V needed: 2069.86 m/s

Ship mass: 38,708 / 74,683 kg (why there are 2 values?)

Edit: I have 2 LV-N engines.

I'm sure the calculation will give me a very long burn, so it means I'll have to start my burn earlier than 112.04º ?

Edited March 17, 2015 by seyss

[Kryxal]

I'd suggest at least dropping a node to plan the burn, and likely splitting the burn. I'd expect the two values are current-stage and remaining-stages.

[Vanamonde]

To make the math manageable, Olex' calculator assumes you instantaneously accelerate to the specified speed at the point in time indicated by the calculator. What most players do is estimate the burn time and start before the specified moment at T-(burn time/2), doing half the burn before and half the burn after the ideal moment. Very low thrust ships might be wise to do it in "kicks," or more than one burn as you pass around the planet to the ejection point on successive orbits.

[Mister Dilsby]

So your ship has a mass of about 75 tons and you have 120kN of thrust. You need 2100m/s of dV

Roughly, your burn will take (2100m/s)*(75,000kg)/(120,000kg*m/s2)=1312 seconds, or just shy of 22 minutes. You might indeed want to do it in 'kicks', expanding your periapsis with a burn of a few minutes per orbit, until you can burn one last time and escape right at the departure angle... or pick a closer target if you don't want to do all that

[tsotha]

Ship mass: 38,708 / 74,683 kg (why there are 2 values?)

The first one is this ship's mass without fuel.

[cicatrix]

A side note - LVNs have low TWR so the burn will take a long time. Keep in mind, that once you determine the burn time t, you should divide it by 2 (t2) and activate your engines t2 seconds (or minutes) before you actually get to the maneuver node.

[Brainlord Mesomorph]

Why not just plot the node and see?

As for how to long burn, see the tutorial in my sig.

[seyss]

The first one is this ship's mass without fuel.

Don't think so

[Superfluous J]

Don't think so

You have more information than we do, but I have no idea what that could be if it's not wet vs dry mass.

Load your craft in the VAB and take all the booster stuff off so you have exactly the ship you have in orbit, in the VAB. Then empty all the tanks of fuel. See if your dry mass is that number and then you won't have to think so or not, you'll know so or not.

[seyss]

I'd expect the two values are current-stage and remaining-stages.

Yep, just tested this.