Early Orbital Rocket Tutorial
Note: This tutorial was largely written by Norcalplanner in March 2021 based on RP-1 and RO masters from mid-February, 2021 running on KSP 1.8.1. and is somewhat outdated, you may encounter issues.
After you've completed your sounding rocket development, the next big step is to launch a satellite into orbit. To demonstrate one way of accomplishing this important milestone, this tutorial will show you how to build an analog of a early American rocket, the Thor-Able. Please note that this is simply one way of progressing in an RP-1 career. There are many paths to success!
This will be a two-stage rocket, using an AJ10 (Early) upper stage and an LR79 lower stage. Both of these engines are unlocked in the 1956-57 Orbital Rocketry tech node. The rocket will also use isogrid fuel tanks, which are lighter and more capacious compared to earlier conventional (separate structure) fuel tanks. Continuing the visual theme established in the Early Career Tutorial, the example rocket will use the legacy Redstone texture for fuel tanks, and shiny metal avionics to make it easier to understand how the pieces fit together. Because the finished rocket will mass around 45 tons, make sure to order a 60-ton launch complex early enough that it will be ready in time for your first orbital launch attempts.
Let's design the rocket from the top down. First, create a satellite using a procedural avionics part. This extremely boring satellite uses the TL3 science core configuration. This tech level reduces power draw dramatically, while also continuing to reduce mass compared to earlier versions. This particular science core has a radiation detector, a thermometer, and a barometer all integrated into the core (select the "Configure Experiments" button to enter this menu), and is therefore capable of completing the First Science Satellite contract. It also has 6,000 EC, which will allow it to power the experiments in orbit and broadcast the results back for quite some time. Feel free to customize your own satellite with whatever you may need to accomplish a particular mission, such as antennas, solar panels, or experiments that don't fit inside the probe core.
Next, we're going to add a fairing base and two fairing halves. You can see that this rocket uses a 1.25m diameter for the payload fairing and upper stage, which is wider than the 0.84m diameter of the historical Able second stage. There are a couple reasons for doing this, including reutilizing existing 1.25m tooling that you'll already have if you've been flying X-planes. However, the main reason is aesthetic - the original Thor-Able looks like an electric toothbrush, which is suboptimal. This rocket has an appearance closer to a later Thor-Ablestar or Thor-Agena.
Once you've placed the fairing base and fairing halves, it's time for avionics which can actually control the upper (Able) stage. Select another procedural avionics part and attach it beneath the fairing, adjusting it to 1.25m diameter. Configure the avionics to use the Near-Earth TL2 "Early Avionics" configuration, and set the controllable mass to be around 2.5 tons. Add a few hundred EC to power the avionics, and leave the RealAntenna stuff alone - the defaults are fine for this early rocket.
Next, select a procedural isogrid fuel tank, and place it beneath the avionics. Increase the diameter to 1.25m and the length to around 1m, and select the HP (high pressure) variant in the tank's PAW (aka the right-click window). Be sure to increase utilization to the maximum of 95 percent to take full advantage of the tank's capacity.
We'll need a way to provide ullage and attitude control, so select the smallest RCS thruster part (the 28/45N one) and place four of them near the bottom of the fuel tank. Configure the RCS thrusters to use HTP, which is a nice compromise between cost, power, efficiency, and fuel volume for early rockets. Click on the "Show Tank UI" button, then add just 2 or 3 units of HTP to the tank.
Note: You may have unlocked Nitrous Oxide as an RCS fuel. On paper it may seem like a better RCS fuel than HTP, but it's important to note that NO is a gas, not a liquid. This means it's storage density is very low and it needs much larger, and thus heavier, tanks. (The liters listed in fuel for NO is the gaseous volume at 1 atm, but it's stored under pressure, so that's why the tank seems to hold so many more liters than HTP) In this situation, you probably want to use HTP. Fuel density will come up again later in your RP-1 career, probably in the form of Hydrogen. The important thing to remember from this is that the efficiency of your fuel is a combination of the fuel's performance and its density (due to the tank's mass.)
Select the AJ10 Series (Early) engine to power this stage, which will have the beginning AJ10-37 configuration. Place it on the bottom of the fuel tank, then fill the remainder of the fuel tank with UDMH and IWFNA using the button in the PAW. Fine-tune the length of the tank so you have somewhere between 1m 50s and 2m of burn time. Please note that the AJ10-37 configuration is one of the least reliable engines in the game, and will fail early and often if you're using Test Flight or TestLite. Upgrade this engine to the next configuration (AJ10-42) as soon as you unlock the 1958 Orbital Rocketry node.
The engine will need to be covered for proper aerodynamics, so select an interstage fairing base. Place it so that the top "floating" node is the one that connects to the bottom of the engine bell. Using the PAW, reduce the height and increase the extra height until the bottom of the engine is near the interstage, and the ghostly blue lines are approximately reaching the bottom of the tank. Add a fairing in whatever symmetry the interstage is configured for, which is 4x by default. You'll probably need to turn auto-shape off on the fairing base and tweak things manually to achieve the desired look and size. You can also reduce the strength of the fairing slightly to save a little mass. (Don't reduce the nose fairing below 0.04, or you may have problems with it burning up.) The upper stage is now complete.
Start the lower (Thor) stage with a second procedural avionics unit, also using the TL-2 Near-Earth "Early Avionics" configuration. Increase the controllable mass to around 43 tons, and give it a few hundred EC so it won't run out of power during the launch.
Place two more isogrid fuel tanks beneath the avionics, both with 95 percent utilization in standard (not high pressure) configuration. Make them 2.4m in diameter, with the top tank being a cone which tapers down to the 1.25m diameter of the upper stage. Increase the length of each tank to approximately 5.8m.
Select an LR79 engine and place it on the bottom of the rocket stack. Use the PAW of each tank to fill them with the correct mix of kerosene and liquid oxygen. Note that this engine will have the starting S-3 configuration, which is less powerful and less reliable than subsequent configurations. Make sure that the total burn time is less than three minutes, and that the initial TWR is somewhere in the range of 1.2 to 1.35 or so. As with the AJ10-37, upgrade the LR79 as soon as you unlock the 1958 Orbital Rocketry node. The second configuration ("S-3D") is more reliable and can be used for a year or two before upgrading again.
Adjust your staging, and see what sort of delta V this rocket has. It should have close to 10 km/s on tap with this small payload. We could launch it now, but it wouldn't be ideal - the bottom of the rocket isn't very aerodynamic, and there's no provision for roll control on the first stage. Let's address these and finish the rocket.
Remove the engine, and place a 2.4m boattail fairing base on the bottom of the rocket. Be sure to turn it over before attaching it, so the ghostly blue lines are heading down towards the ground. Re-attach the engine, then attach fairings to the side, and adjust their size and shape using the PAW, similar to the interstage. You can keep the fairing straight, or you may be able to taper it a bit down to 2.3m or 2.2m, depending on the LR79 engine model you're using.
We're almost done. Choose a small vernier engine for roll control (either the LR101 or S-3D exhaust) and place two of them down near the LR79. If you want a more Thor-alike appearance, select the LR101 variant which is straight up and down (not angled), and tuck them up beneath the boattail fairing. You may need to use the offset tool to place them exactly where you want, and may need to rotate them so the single-axis gimbal on each engine is oriented correctly. Finish by adding some launch clamps to provide stability, power, and fuel while the rocket is on the pad. You can add small fins if you wish for an appearance that's closer to a historical Thor, but they're not really needed. Here's a picture of the completed rocket.
Note: If you have ROTanks installed, you can also use a single 2.5m diameter integral tank with a 2:1 conic nose and an integrated engine fairing in lieu of the two tanks and boattail fairing described above. While the rocket will appear a bit more portly (it may end up resembling a Juno II more than a Thor) going this route it will reduce part count and provide some different appearance options.
Enter KCTSim to test the rocket. You can use MechJeb PVG with this design, but be sure to deselect the "Omit Coast" box. You may also wish to deselect the "Autostage" box if your second stage ignites prior to leaving the atmosphere, and stage your rocket manually. A booster pitch start around 25 or 30 m/s using 0.5 deg/s should work well. Note that the latter half of the first stage burn will have very high TWR, so don't flatten out too much if it looks like the nose fairings are going to burn off.
This demonstration rocket is launching into a 310 x 1550 km orbit, which should complete the First Science Satellite contract with delta V to spare. Once the first stage has burned out, let the rocket coast upward until it's above 140 km. Then ditch the fairing, separate from the lower stage, and activate the RCS and AJ10-37; or alternately, tick the Autostage box to turn it on. PVG will take it from there, delivering your satellite to the requested orbit. Detach the satellite from the upper stage (otherwise the upper stage avionics will drain the batteries quickly), and you're done! You can also fly this manually if you wish - should you choose this route, make sure you tip over enough so that the majority of the speed gained is horizontal rather than vertical. The sweet spot is somewhere between seeing a little bit of aerodynamic heating flames (good) and burning off the nose fairings (bad). If you're now in orbit for the first time (even just in a simulation), be sure to savor the moment.
Once you've figured out a launch profile that works, go back into the VAB. Tool everything, then place two or more rockets into the KCT construction queue. Hopefully the RNG will behave when it's time to launch for real, and your engines will function all the way to orbit. If they don't, simply keep launching more rockets (or perhaps fly some smaller test rockets to gain more du on the engines) until you succeed.
After you've successfully launched your rocket to orbit and spent a few moments basking in your achievement, you may wonder what comes next. The answer, in a word, is "Evolution". Go back into the VAB, and start tweaking your rocket design. See how you can improve it. Try to fly it more efficiently or bring up a heavier payload than you thought it could handle. See if you can bring up an unguided payload with 3150 m/s or more of delta V to orbit (hint, hint...), and see just how far this rocket can take your space program.
This evolution is one of the reasons the tutorial uses the Thor-Able. Both engines have many upgraded variants which become available as you progress down the tech tree, so this design is a great starting point. You'll begin to see the utility of changes that were made historically, such as stretching fuel tanks, swapping out engines, adding SRBs to the first stage, and increasing the diameter of the upper stage. Thor-Able eventually led to Thor-Delta, which eventually became just "Delta". Kerolox-burning Delta rockets flew for decades and lofted many noteworthy payloads, including GPS satellites and interplanetary probes, such as Mars Pathfinder.
But enough about real history. Take the skills and concepts you've learned, fire up your RP-1 career, and go make your own history!