Researching Technologies
Once you've invested some KCT upgrade points in your R&D rate, and you've earned some science points, what nodes should you research? And in what order? While there's not one “right” answer, there are some general principles to follow which should help get you started in a reasonable fashion.
While the order is up to you, these two nodes should be some of the first ones you unlock. Supersonic Plane Development is mainly about one part – the X-1 cockpit. This is the first manned part which will let your astronauts survive above 16 km altitude, and can even be used (some might say abused) for brief suborbital hops. The contracts and milestone rewards for these types of flights pay well, and it will take time to unlock the part and train your astronauts. Note that proficiency training can begin the moment you start researching a node with the desired cockpit or capsule, so make sure your pilots begin training sooner rather than later. Post-War Rocketry Testing will unlock the RD-101 upgrade of the RD-100 and the XASR version of the Aerobee engine. Both of these engines are significant improvements over the base versions you start with, and will be useful for both manned and unmanned rockets.
The line of nodes in the middle of the tree which begins with Post-War Rocketry Testing will eventually start referring to specific years (e.g., 1956-57 Orbital Rocketry, 1958 Orbital Rocketry, etc.) and contain many of the most common and useful engines, at least until mid-career. If you want to recreate an A-4, a Thor, an Atlas, a Titan, or one of the many R-7 variants, you'll find the engines in this line of nodes. 1956-57 Orbital Rocketry is a key node, as it provides some of the first truly capable kerolox engines, including the LR79, LR89, LR105, RD107, and RD108; achieving orbit becomes much easier once this node is unlocked.
Similar to the stock tech tree, most of the science experiments are found in the lower portions of the tech tree. This area also has upgraded probe cores and avionics, batteries, communications, solar panels, and RTGs. Unlocking additional science experiments in particular will help keep your career moving along, both for future tech node purchases and for earning additional KCT upgrade points. Unlocking the avionics line of nodes will not only make your avionics smaller, lighter, and use less power, but it will start unlocking more capable probe cores and upper stage avionics which will make exploring outside LEO possible. Purchasing communications nodes is necessary to enable some Tracking Station upgrades to be purchased, while also providing antennas which will reach the moon and beyond.
Unlocking Improved Hydrolox Engines won't do much good if you still have heavy fuel tanks and large, power-hungry avionics. Try not to let one line of research get too far ahead of the others, especially if the parts are going to be used in the same rocket.
In direct contrast to the above, feel free to ignore entire lines of tech nodes if you won't use them. Many will ignore the majority of the airplane nodes along the top of the tree. A portion of players will strongly favor liquid fuel engines, and will pay little attention to the SRB nodes. Some may decide to “go Soviet” and unlock the line of staged combustion nodes while ignoring the hydrolox nodes. Others may choose to “go American” and unlock hydrolox engines, and possibly large SRBs, while ignoring staged combustion. And then there are “hybrid” players who will mix and match parts without much regard for historical rocketry development. Since many of these lines of tech nodes require all previous nodes as prerequisites, it's worth giving some thought as to what sort of rockets you want to launch. This also gives RP-1 some great replayability since you can choose a Soviet path in a new career if you previously went American, and vice versa.
Many significant technological advances are either contained within or gated behind the two lines of Blue Sky nodes, one for materials science and the other for electronics. The materials science nodes are especially important, as they unlock lighter fuel tanks and service modules. Make sure you're researching these in a timely manner and purchasing any needed R&D Complex upgrades to keep moving your career forward. It can be a real let-down when you plan to research a specific node so you can unlock a particular capsule by a certain time, then belatedly realize you need to unlock two Blue Sky nodes first... and those nodes can't be purchased before you drop seven figures on an R&D Complex upgrade.
The nodes in between the capsules and the orbital rocketry lines contain some critical technologies, including heat shields, docking ports, landing legs, and various upgrades to RCS and thruster technology. Your space program will shift into a higher gear when hydrazine is unlocked, and really get going once hypergolic bi-propellants (e.g., Aerozine-50 and NTO) become available. And you won't be offered a manned orbital contract until you unlock heat shields in the first EDL (Entry, Descent and Landing) node.