Calculating wrong or what

[Spaceman83]

I buy this xbox one and i really like learning new things with this game:-) one is delta v! Xbox you can't add mods and so i have to calculate delta v my own i also find android app for that too but some reason delta v count doesn't match up? Number is usually couple of hundred m/s more of actual speed of rocket (sometimes even more)! So why is this is it gravity or air pressure what effects this even vaccuum calculation is little wrong! How much this effect if i built a rocket and want to go mun and numbers are wrong hunreds or even thousands i calculate like smarter people say ln(m1/m2)×Isp×9.18=delta v! I am not very good of math by the way:-D but have look many youtube videos learn this calculations.

[LN400]

First, I assume that 9.18 was a typo and that you meant 9.81.

Second, for some reason was it (I don't know if that has been corrected) 9.82 that would give a better result. Try 9.82 and see if that gives you better results.

Third: If you talk about going through the atmosphere then yes, delta v increases with altitude. If you look at the engine data in the VAB you will see 2 numbers for thrust, one in vaccuum, the other at sea level (on Kerbin that is). Same for Isp. Calculating accurately for delta v through the atmosphere and against gravity takes a deep dive into a lot of nasty maths, but with experience can you tell how much each stage of the rocket needs.

Example:

I know that with around 2000 dv, will the first stage take me to between 20,000 and 30,000 meters, up beyond 40,000 if the thrust is just right and the gravity turn hasnt been fubar-ed totally. I use 20,000 as a rule of thumb that I break and set to 25,000 to give me the ballpark number I need. However, the speed will not be anywhere near 2000 at that altitude, more like 700-800 m/s.

As I get higher, the burn will be increasingly horizontal meaning more and more of the thrust goes to give me orbital velocity and less goes to climbing, and the dv on paper gets closer to what I actually spend.

Edited August 4, 2016 by LN400

[Hupf]

When launching from ground to orbit, the delta-V required is not compareable to the resulting orbital velocity (i.e. display on the speedometer once in space). This would only be applicable when already in orbit and then doing a maneuver (you can try this out by calculating dV for a small craft, then attaching it wit a decoupler to a large booster stage to get it into orbit with all the fuel still there and then detach and perform a maneuver with the calculated amount of delta-v. You should then have enough fuel to finish it).

During launch, much of the delta-v is "spent" for fighting against gravity and against atmospheric drag.

[foamyesque]

You've got the right formula (bar what I assume is a typo), but there's a few things that impact it in practice you might've not accounted for:

 

1. In atmospheres, there's both drag and changes to your rocket's Isp. Rockets operating in air have lower Isps, sometimes very significantly, than ones working in a vacuum. This means that the actual deltaV it provides during atmospheric flight depends on the flight path, and so it is not exactly calculable. Likewise, the amount of that deltaV you lose to drag also depends on your flight path, as well as your rocket's streamlining and size; it's not possible to exactly calculate in advance.

2. Vertical velocity will, in typical ascents, be lost to gravity for the vast majority of the flight path. That's why guides to getting to orbit recommend turning sideways as early as possible; only drag can remove horizontal velocity and drag has much less impact than Kerbin's gravity does. Even once in orbit, vertical thrusting is typically much more inefficient than lateral thrusting for changing your rocket's energy (i.e. what deltaV ultimately represents). How much you lose to gravity depends on your flight path and can't be exactly calculated in advance.

However, for both of these, you can get some approximate estimates with practice. In general, in a 'typical' Kerbin ascent, gravity losses are on the order of 800m/s and drag losses around 200m/s; they can be much higher or lower depending on the rocket characteristics, though.

3. If you're running multiple kinds of rockets at the same time, you need to use an averaged Isp; the KSP wiki has the formulas here: http://wiki.kerbalspaceprogram.com/wiki/Specific_impulse. In effect what you do is thrust-weight the Isps, so for example if you have 2x solids burning at 250sec Isp at 200kN and 1x liquid burning at 280sec Isp for 200kN, you'd have an average Isp of (400kN/600kN * 250sec) + (200kN/600kN * 280sec) = 260sec.

Edited August 5, 2016 by foamyesque

[Spaceman83]

Thanks for the answers!!! Cleared a lot! never been do so much calculating any game! Damn when i was a kid this kind of games/simulators could have possible make me a rocket scientist!? Now old and stupid take so much time to understant this kind of math:-)

[purpletarget]

"Any fool can calculate...." -Sylvanus P. Thompson

Don't let math intimidate! When playing KSP, Math can be your greatest ally! True story.

[Spricigo]

59 minutes ago, purpletarget said:

"Any fool can calculate...." -Sylvanus P. Thompson

Don't let math intimidate! When playing KSP, Math can be your greatest ally! True story.

Someone give more likes this post,  I can only give one

[Red Iron Crown]

17 hours ago, LN400 said:

Second, for some reason was it (I don't know if that has been corrected) 9.82 that would give a better result. Try 9.82 and see if that gives you better results.

That's been fixed, g₀ is now the correct 9.80665 N/kg in game.