TWR on the Mün.

[DMSP]

Hi all, I would like to know what is the Kerbin TWR you need to safely land on the Mün. On another note, what is the equation for a TWR?

Short post-But thanks anyways.

[Plusck]

Weight from mass: 1kg = 1N x local gravity. So on Kerbin, 1kg mass = 9.81 newtons weight (gravity on Kerbin at sea level is 9.81 m/s²)

Therefore on Kerbin, 1 tonne ≈ 10 kN. If you have a 6t craft and a 60kN engine, your TWR is essentially 1.

Mun gravity is (roughly) a sixth of Kerbin's, so that same craft would have a TWR of 6 on the Mun. You can get the exact gravity in m/s² for any moon or planet from the wiki.

And for landing safely on the Mun, you can go as low as a TWR of 1.5. To do it easily, probably more like a TWR of 4. 

However, it may be easier to consider how long your landing burn has to be. If you want your burn to be a maximum of, say, 40 seconds, and knowing that you have to slow down by about 600 m/s, that means you want an acceleration of 15 m/s. That means a TWR of approximately 1.5 on Kerbin, or, for the Mun (with local gravity being 1.63 m/s) = 15 / 1.63 ≈ 9. So a TWR of 9 on the Mun would let you start your suicide burn slightly less than 40 seconds before impact.

[Empiro]

TWR (i.e. "thrust-to-weight ratio" is literally just that: thrust / weight. Thrust is computed by adding up all the engines' thrust. Weight = mass * local gravity. For the Mun, local gravity is 1.63 m/s/s.

KSP conveniently expresses thrust in kN, and mass is in tons (1000kg), so those two cancel out. (e.g. an engine with 20 thrust and a craft with 10 mass would have a TWR of 20 / (10 * 1.63) = 1.227 on the Mun.

[Adelaar]

The lazy way is to install Kerbal Engineering Redux, with which you can check the TWR of your craft's stages (fully fueled and empty), on different bodies, from within the VAB/SPH. If you want to feel more like a rocketscientist, use the equations as mentioned by Empiro and Plusck.

[Superfluous J]

6 minutes ago, Adelaar said:

The efficient and not boring way is to install Kerbal Engineering Redux

fixed that for you

[DMSP]

@Empiro and @Plusck, thank you. I'll keep those equations in mind as I continue onward. Before when I landed on the Mün, I just tried to guess what the TWR would be, but that ended up in my most recent lander hitting the surface at 230 m/s.

@Adelaar, I had it once, but for some reason barley used it. Uninstalled in 1.1.

Also, sorry, I couldn't choose which was better, so I just did Heads or Tails.

Tails won.