Need help with rocket info

[Dimetime35c]

Ok so I've played for bout a month now. Gotten the hang of the basics for the most part. Now I'm moving into more advance stuff and I'm not understanding what the info mechjeb is telling me when I use vessel info. What does the max thrust in KN mean? I'm not understanding why when I make a rocket that's only 100 ton or whatever the measure is and my max thrust is over 5000 why it's so slow lifting off. What does the TWR mean? When it says 1.45 what does that really mean??? Any advice would really help. Please try and keep the answers in simple terms. Not that good at the math part of rocketry.

[Sapphire]

TWR is your thrust-to-weight ratio. If it's a really low number, your rocket will lift slowly. If it's below 1.00, then your rocket won't take off at all.

[Morokth]

TWR is thrust to weight ratio. In total vacuum this is (from my own observations at least) not so important, but as a rule you pretty much won't move / lift if you have less than 1.0 TWR. That is, a rocket on the launchpad with more weight than what the thrust can move isn't going to get lifted. So you need at least 1.0 TWR from launch.

I'm sure someone can give a far better explanation of this than I can.

As for thrust and kN (which I understand to be kilo-newtons, and newton being a measurement of thrust), there is an equation that can be done to "manually" determine your TWR. Of course you would also need to know your rockets weight as well but I don't know tha formula. Again there are more well informed / well spoken people that can give you a better grasp than my simplistic layman's terms I just used.

[leax256]

10KN can lift 1 ton on kerbin. divide that my the relitive gravity of the planet/moon to find TWR. BTW your mass can be found in the map view on the right side.

[DMagic]

To find your TWR (that's weight, not mass, which is an important distinction) you need to multiply the mass of your vessel in tons (from MJ, or the map view, or however else you want to add it up) by the surface gravity. For Kerbin that's 9.8m/s^2, but usually 10 is close enough, so leax256's estimate works fine. Of course, you also have air pressure to contend with, which is why most people try for a launch TWR of somewhere around 2. But anything over 1 will be enough to at least get you moving (just really inefficiently).

[VirtualCLD]

To find your TWR (that's weight, not mass, which is an important distinction) you need to multiply the mass of your vessel in tons (from MJ, or the map view, or however else you want to add it up) by the surface gravity. For Kerbin that's 9.8m/s^2, but usually 10 is close enough, so leax256's estimate works fine. Of course, you also have air pressure to contend with, which is why most people try for a launch TWR of somewhere around 2. But anything over 1 will be enough to at least get you moving (just really inefficiently).

For a while I didn't understand the units involved, but after some digging I realized I was confusing metric tonne with Imperial Ton. The Wiki shows a specific example for lifting off of Kerbin in their article: http://wiki.kerbalspaceprogram.com/wiki/Thrust-to-weight_ratio

Metric tonnes are denoted by lowercase t, which is equal to 1000kg. Since engine thrust is given in kN = 1000N, you don't have to worry about the order of magnitude, you just divide engine thrust by the quantity (mass in metric tonnes times the acceleration due to gravity).

[aeronaut]

To find your space craft's weight use newton's second law:

F=ma

m=1 ton=1000kg

a=9.81 m/s^2

Where F is the weight of your spacecraft, m is mass of your spacecraft and a is the acceleration due to gravity.

F=1000kg*9.81 m/s^2

F=9810kg*m/s^2 = 9.81kN

Thrust to weight ratio is your available thrust divided by your spacecraft's weight:

TWR=T/W

So if your spacecraft produces 20kN of thrust your TWR is:

TWR=20kN/9.81kN = 2.04