Reddit reviews LSC Understanding Space: An Introduction to Astronautics + Website (Space Technology)

Understanding Space is really easy to understand. http://www.amazon.com/LSC-Understanding-Space-Introduction-Astronautics/dp/0077230302

Try "Understanding Space: An Introduction to Astronautics" by Jerry Jon Sellers. Great explanations of everything from orbital mechanics to rocket propulsion.

If you want to get super serious about this game and really know what you're doing, then I'd suggest getting this book and reading up on actual rocket science. Taking a class on astronautics in college was when I personally really started to understand the concepts required by KSP. However, in the interest of saving forty bucks, there's a lot out there on the internet that can teach you this stuff just as easily. Scott Manley's videos are pretty good. I'm also sure that there's some written tutorials out there.

By far the most important concept is that of deltav, which is actually formally written as Δv. Mathematically, this literally translates to change in velocity. When Kerbal Engineering Redux tells you how much Δv one of your stages contains, it's telling you that if there were no gravity, and your ship was floating in a pure vacuum motionless, and you pointed in one direction, and fired your rockets and emptied that entire stage's worth of fuel, that Δv number is how fast your rocket would be going at the end of the burn. Δv is a measure of how much "effort" your stage can put out and how much of a change in velocity it can impart on your rocket as a whole.

Δv is one of the most important concepts in navigating space, because in order to change from one orbit to another, there is a very specific, easily calculable change in velocity that must take place. You probably know that in order to orbit at a certain height above Kerbin, there is a very specific speed that your ship must have in order to maintain a perfectly circular orbit(this is assuming your orbit is perfectly spherical and not elliptical like most orbits are). Similarly, there is a very specific velocity that your ship must have when it leaves that orbit to head to the Mun in order to get that smooth elliptical transfer to the Mun. Therefore the difference in velocity between the circular orbit and the transfer orbit is the Δv that must be imparted onto the vehicle in order to transfer to the Mun. This is approximated for you on those Δv maps like this one.

So, by using Δv maps and maneuver nodes, you can figure out how much Δv you need to make your maneuvers, but now you need to figure out how much fuel you need to perform those maneuvers. That all depends on how much fuel is burned, how efficiently it is burned, how much structural weight is present in the rocket, and the weight ratio between fuel and structure. Another point to consider is that rocket acceleration is not constant, for as the rocket burns fuel, it will constantly get lighter and experience stronger and stronger acceleration, assuming that it is experiencing constant throttling. This has all been simplified by the Tsiolkovsky rocket equation. For all non-air breathing engines, Δv = (g0)(Isp)ln(Mi/Mf), where g0 is the gravitational acceleration at the surface of Kerbin(this is constant everywhere in the game, it's simply a unit conversion constant), Isp is the efficiency of the fuel and the engine burning it, Mi is the initial total mass of the vehicle before the burn, and Mf is the mass of the vehicle after the burn. You can calculate this yourself but since Kerbal Engineering Redux does this for you, why bother? However, it is important to understand the main criteria for adjusting Δv in your designs. g0 and Isp are mainly fixed values. The main variable to adjust in your designs is the mass ratio Mi/Mf. The less dead weight on your vehicle(this includes upper stages that haven't burned yet), and the more fuel burned, the more Δv you'll achieve in your designs. This is also why staging is so important. By staging and dropping your dead weight, you're decreasing structural mass hindering your upper stages and gaining more Δv. If you want to get more serious about your designs, you can add up the masses of the parts you want on your ship in a spreadsheet and calculate optimal staging sizes for your ship using the limited parts in the KSP inventory.

Other considerations are TWR, or Thrust to Weight ratio, which is simply the thrust of that stage matched to the weight of the whole ship. Changing the reference body in Kerbal Engineering simply adjusts the weight for each body. A lander on Minmus doesn't have to be super powerful, and might have a really small TWR on Kerbin, but that doesn't matter because all it needs to take off from Minmus is a TWR greater than 1 on Minmus, whose gravity is way weaker than Kerbin's.

I'll also throw out another tip that I hardly ever see mentioned here. Before you launch, check your center of mass and center of lift. In a rocket, your center of lift should always be below the center of mass. If it isn't, then you need to add stabilizing fins at the bottom of your rocket. A rocket with the center of lift above the center of mass is very likely to flip over backwards during its launch. Also, if you're attaching control surfaces, they're more effective the farther away they are from the center of mass, so it's always important to know where your center of mass is.