Conservation of energy experiment on the Mun

[SneekyKerbalDesu]

Hi, I'm new to the forums, I think my question could in this section since it deals with physics and experimentation.

I'm trying to do some test on the Mun about conservation of energy, TLDR at the bottom

So I'm applying a problem from a dynamics workbook, which is that supposing that a lunar module can safely land on the Moon if the magnitude of it's vertical velocity at impact is no greater than 5 m/s, at what height can the LM land if it has a vertical velocity of 2 m/s downward or vertical, I already have the answer for the workbook problem (6.4814 m )but when trying it out on KSP I'm having problem on the execution.

For example I can't always get to the desired height or get the velocity of 2 m/s, I've installed Kerbal Engineer Redux to make this easier since the UI doesn't give an exact measurement of the height and vertical velocity, I have also tried to use MechJeb's ascent guidance to get the desired height but since it's for orbits it always flips the lander side ways. From seeing the mod section for command and control I guess I could install Time Control 2 and/or kOS to have greater control over the lander.

Any help with solving my problem would be greatly appreciated

TLDR: Is there a mod or a way in vanilla KSP to set a target altitude and vertical velocity?

[James Kerman]

Welcome to the forum @SneekyKerbalDesu, your question has been moved from science and spaceflight as you are requesting gameplay mods.

[Clamp-o-Tron]

I don't think that there's a way to set altitude and velocity, but there is a way around that. Just use the alt+F12 menu, go to cheats-set position, and edit the altitude. It's not exactly the scenario you want, but it's the best way to get precise altitude and velocity numbers.

I'll see if you can do the math for that

Spoiler: hint

Spoiler

Just use some standard kinematics math to extrapolate backwards the starting velocity.

 

Spoiler: solution

Spoiler

v = 2 m/s , final velocity at 6.4814 meters above surface level

i = 0 m/s , initial velocity of the lander

a = 1.63 m/s^2, Mun's surface gravity

d = what we're solving for (in meters), how much higher than 6.4814 you must set the lander

 

Now one of the standard kinematics equations:

                           v^2 = i^2 + 2ad

For which we add our values:

                        (2 m/s)^2 = (0 m/s)^2 + 2 * (1.63 m/s^2) * (d)

Simplifying ,,,

                      4 m/s = 3.26 m/s^2 * (d)

And finally...

                    (4 m/s)/(3.26 m/s^2) = d

                   d ~= 1.227 meters

... so you should set your craft 1.227+6.4814 = 7.7084 meters above the Mun's surface.

 

Please check my math, it's been a while and I needed to look up the kinematics equations.

 

 

[SneekyKerbalDesu]

Thanks for your answer, I was also trying to do something similar, but fortunately I found an outdated mod (I know it's not the best practice) that still works, that sets the upwards and downwards vertical velocities.

I think I should use the extra height from your equation and the mod, since it gives me more breathing space if something needs correction when performing the tests.

TBH I didn't even know that KSP had a cheat menu, I guess I should use to not waste fuel getting the required height for the tests.

h= (v_f²-v_vrt²) / (2g) = (5m/s)²-(2m/s)² / (2*(0.166*9.81m/s²)) = 6.4814m

v_f²=v_i²+2ah; (v_f²-v_i²)/(2a) = h = (2m/s)²-(0m/s)²/(2(0.166*9.81m/s²)) = 1.2281m

We pretty much used the same equations, but I started with K₂+U₁ = K₂+U₂ both equations had the same form since in my case the mass gets removed when solving for the height