Quote:
Originally Posted by Red October1984
I haven't quite got the gravity turn downpat yet. I don't know how to figure my thrust to weight ratio. I'm not familiar with any of these equations mainly because my high school dropped Physics in favor of Anatomy and Physiology.
I know. Terrible.  MIT offers physics courses for free online...i may read through some of that stuff. |
The TWR is relatively simple to figure out. It's just the ratio between the rocket's thrust and the force acted on it by gravity. You can easily find out your thrust at sea level by hovering over the engine type you're using, while the weight of your ship is your ship's mass (it's in the Engineer's Report in the VAB) multiplied by the gravitational acceleration, which is 9.81 for all intents and purposes when lifting off from Kerbin. Like so:
TWR = (Thrust) / (Mass * 9.81)
If you're under a TWR of 1 then your rocket cannot counteract the gravity pulling it down and it'll stay on the launch pad forever (or topple over and explode), so more boosters are clearly needed. As long as you're above 1 you should be able to achieve liftoff. If you're just barely above that then your acceleration might be too low and you'll lose energy because your ship spends more time pushing against gravity, but if you're much above it then you will accelerate too fast and lose energy on atmospheric drag. IIRC the point where extra gravitational drag because of a slow ascent and atmospheric drag balance out is your terminal velocity, but that's a bit hard to figure out. Anyway, you can throttle down liquid fuel rockets or limit the thrust on SRBs until you've found the optimal ascent profile.
The other thing that you should know if you want to plan out missions is Konstantin Tsiolkovsky's famous rocket equation, which is something like this:
Delta-V = (Exhaust Velocity) * In[Mass Ratio]
Your exhaust velocity is the engine's specific impulse times 9.81 and is the basic measure of efficiency when it comes to rockets (a higher exhaust velocity will mean you can accelerate while consuming less propellant). Your mass ratio is the mass of the ship with fuel tanks full divided by the mass of the ship while empty (you can set the amount of fuel in the tanks by right-clicking on them in the VAB, at which point the Engineer's Report should reflect the new mass).
Delta-V is the measure of your rocket's total ability to change velocity, which basically amounts to your fuel budget for the mission (unless you re-fuel at a station or something). If your rocket has enough delta-V for a specific mission then under ideal circumstances it would be able to perform it. For example, I believe getting to Kerbin orbit requires a delta-V of 3,500m/s nowadays, so if your rocket is capable of that (and has TWR > 1) then it should be able to make orbit. If it had less than that it would never make orbit, and if it has more to spare then it can go on to do other missions or de-orbit.
Calculating the delta-V of multi-stage rockets is unfortunately a bit of a pain, but if you want to do it then you basically need to calculate the delta-V of the entire rocket until the first stage's fuel tanks are dry, then calculate the delta-V of the rocket with the first stage jettisoned until the second stage's fuel is out and so forth until you reach the last stage, at which point you add the results together. Like this:
Δv = (Ve1 * In[M / M1e]) + (Ve2 * In[(M - M1) / M2e]) + ...
Or something...
Anyway, there are probably places that explain all this much better than I do, and naturally Kerbal Engineer would also calculate all this stuff for you as well. But if you don't want to mod the game then knowing how to calculate your TWR and delta-V along with how to execute basic orbital maneuvers should allow you to complete most types of missions without too much trouble.