Procedural Avionics Basics
Any vessel in RP-1 needs a control part: a probe or capsule that represents the computers or telemetry units responsible for determining the (space)craft position, velocity and attitude, as well as controlling experiments, staging and action groups, among other functions.
Additionally, if this control part is avionics-capable, it can directly control the craft, allowing it to change attitude, execute maneuvers, etc.
In RP-1,these functions are, usually, done by a procedural avionics part, which is a flexible procedural part that allows the player to define its capabilities and tweak it to fit the craft being designed. In this tutorial, you will learn: 1) What is possible to do with, and without avionics control; 2) The three types of procedural avionics, and when to use each; 3) Tooling and reuse; 4) Built-in experiments and antennas.
This distinction between having enough avionics in a craft to control it or not is one of the most important mechanics we will discuss, and one of the main functions of the avionics system.
The first thing to understand is that not all rockets/spacecraft need to have avionics control. If you have an aerodynamically stable sounding rocket, or a spin-stabilized satellite, it can send telemetry/experiment results back even without avionics control. It will still be able to run experiments and communicate, and you can even send staging as well as basic thrust changing commands (basically, in-game, you can stage, and use z/x to activate and deactivate thrust).
For anything else, you need to have enough avionics control in the craft to be able to control it. This involves changing attitude, executing maneuvers, using ascent guidance and most MechJeb functions. This mechanics represents the need for computers, actuators and hydraulics systems to be able to control a craft and change its attitude, and in the game is represented in a mass-based system. Basically, the heavier your craft, the more of these components for control you need, and that translates into a higher mass for the avionics. Additionally, as your avionics technology improves, the relationship between avionics mass and controllable mass improves, allowing you to control a craft with higher mass using a lighter avionics package.
When you right click in a procedural avionics part, you can click in configure avionics, and then choose its tech level and type. There are three types of procedural avionics parts: Science core, Near-Earth and Deep-Space.
Fig. 1 - The Avionics config screen
Each one of these gives different capabilities, and has different cost and mass:
The science core avionics is usually the first avionics type you will use, especially useful early game when you have unguided sounding rockets, and later on unguided spin-stabilized kick stages and probes/satellites. This type provides no avionics control, so it is only useful for unguided rockets/stages/probes. Still, given it is much lighter and consumes much less power than an avionics-capable part of the same tech level, it is incredibly useful early game.
This part is very easy to understand and get going. It has a fixed mass, cost and power consumption for the avionics, dependent on tech level, and a brief breakdown on how to use it is:
Fig. 2 (probably composite): Configuring a Science Core
Change its size until the utilization is below 100%. As this part doesn't give avionics control, you don't need to worry about the controllable mass slider. Once the utilization is below 100% (which means all the telemetry equipment actually fits inside the part), you will have a Service Module tank occupying the remaining space. This is very useful to add a battery (open the tank UI and select electric charge).
Fig. 3: Adding a battery
You can choose the amount of the batteries based on the expected duration of the mission, either using kerbalism's planner, or simple Math. One useful way of always having some space for batteries, is to put the desired utilization slider lower than 100%, and when you click Resize to Utilization it will adjust the length of the part to fit that utilization.
The main use cases are detailed below:
- Unguided sounding rockets: They will be the only avionics part in the rocket, and need enough battery and a built-in antenna, along with the experiments you will use.
- Spin-stabilized kick stages: The last stage in an early orbital vehicle is often unguided. While the rest of the rocket is guided and uses other types of avionics, the last stage will be pointed in the direction it needs to be burned, and spun up using either RCS or a solid motor, and then orbital insertion is done by a high TWR stage that has only the payload and a Science Core.
- Unguided probes: Similar to the kick stages, these are probes often used for lunar exploration (either impactors or orbiters). The rocket is guided, as before, but the last burn towards the moon can be guided or not. Regardless, after that the science core separates from the guided stage, so that the low power consumption of the Science core allows for a much longer life of the probe.
Near-Earth avionics is also available from the start of the game along with Science cores. This part allows you to have full control of the craft, in contrast to the limited control from the Science core. All the avionics mechanics apply to it, so that the mass and volume of avionics necessary to control a craft depends on the mass of the whole craft, as well as the tech level. Its limitation (and the reason you will unlock Deep space avionics later) is that it is functional only when close to Earth (up to 2x GEO distance, roughly 72Mm), and it cannot be put on hibernation. That is not an issue for LV ascent guidance or upper stages that operate around the Earth. However, for other uses you may need the more advanced Deep-Space avionics.
Configuring this part is slightly more involved than the science core, but still quite simple. While the Science core has a fixed volume, cost, power usage and mass for a given tech level, in a Near-Earth Avionics part these will depend on how much mass the part is configured to control. This is defined by the controllable mass slider, and as you change the amount of controllable mass, you can see how much of the part volume is being used for avionics, its cost, mass and power usage. As you increase controllable mass, you can see that the part mass and power usage does not increase linearly, but rather in a quadratic way. This is important to define optimal designs, as we will discuss further below.
Fig. 4: Configuring Near-Earth Avionics
To be able to control a rocket/craft, you need a total controllable mass of at least the total mass of your craft. This is true for any stage you want to control, and it is not necessary for the avionics to be in a single part - you can have avionics in different stages. Note that avionics units do not stack. This means that to control a rocket of 20 tons, at least one avionics unit must have a controllable mass of 20+ tons. You can still have separate lower and upper stage avionics, they just don't combine their controllable masses. You can check whether a craft has enough avionics in the RP-1 UI in the VAB, but keep in mind that the number shown there is for the whole craft, and you want to check whether some upper stage has enough avionics you need to have it isolated to show its numbers in the UI.
Finally, when designing a LV or probe, you need to keep in mind the non-linear relationship between controllable mass and Avionics part mass, cost and power usage, it is not optimal to have separate avionics parts in all stages, as that increases the cost of the craft much more than having fewer separate avionics parts. The optimal balance between cost and mass fraction optimization will depend on the situation, but especially for more advanced tech levels, it's usually worth having a single avionics part for an LV, and then a separate one for a long-term upper stage or just the payload.
Deep space avionics is the most advanced procedural avionics, and it only becomes available at the Basic Avionics and Probes tech node. It has two differences compared to to Near-Earth avionics: it works and offers control even when the probe is in deep space, and it can be put on hibernation mode to save power. Once this part is unlocked, it will be used for all lunar and interplanetary probes. By this point, unguided science core probes will be very niche, with really limited use). Additionally, deep space avionics can be used even for Earth satellites, when you want the hibernation feature.
Configuring a Deep space avionics part is very similar to a Near-Earth one. You will note that under power usage, both the active and hibernating values will be shown. Additionally, deep-space avionics is much more expensive than Near-Earth, so only use it when you need it - namely, for long-term satellites that need to be shutdown and deep space probes. For any other use, including upper stages that will have a long coast, Near-Earth avionics with bigger batteries is more efficient.
Procedural avionics need to be tooled. That means that each time you change the tech level, controllable mass or dimensions of a procedural avionics part, you will need to pay the tooling cost. Additionally, if you change the battery tank level of the part, you will need to retool that tank, which may be quite expensive.
Given the above, it is not useful to tweak the Near-Earth avionics in your LVs too much - set it to a value that fits your LV with maximum payload, tool it, and use it for as long as possible.
If you are playing with Kerbalism and Real Antennas, Procedural avionics parts include, in addition to the avionics and the service module tank, a built-in omnidirectional antenna, and the option to add up to four additional experiments.
Regarding the antenna, you don't need to add one to every avionics part in a rocket. It is usually optimal to disable the antenna in lower stages, keeping only the antenna in the LV upper stage avionics enabled. That antenna can be used in LEO for maneuvering even if your payload/probe has an antenna that needs to be activated, like the communotron-16, or a dish that may not work when in a lower orbit. Also, if a given probe has external antennas for all its needs, it is safe to disable the built-in antenna. That is especially true for Lunar probes that will have a Communotron-16 or something similar and do not need the additional built-in antenna.
About the experiments, if you click configure experiments, an UI will open where you can select up to four experiments to be added. These can be almost any of the experiments that generate data (not samples) and were already unlocked.