Researching Technologies
Once you've hired some researchers for your R&D, 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.
Each starting node has the potential to improve your rockets or let you complete new contracts. Check and see which ones will interest you and don't discount the value of lighter tanks or avionics.
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 increasing research efficiency. 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, service modules, and fairings. They will also increase the efficiency of your launch complexes. 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.
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 once hypergolic bi-propellants (e.g., Aerozine-50 and NTO) become available. And crewed orbit contracts won't be available until you unlock heat shields in the first EDL (Entry, Descent and Landing) node.