Each of the three cores carries a flight computer, parachute deployment system, and thrust vector control assembly. The upper stage flies with thrust vector control as well, and it carries a 3d printed sports car – no Falcon Heavy model would be complete without one.
The Falcon Heavy flight computers are upgraded versions of Signal, a computer designed and built specifically for thrust vector control in model rockets. Each computer has a set of MEMS gyroscopes and accelerometers for sensing movement and orientation on the rocket, the same kind of sensors found in most smartphones. A barometric pressure sensor is used to determine the rocket’s altitude above ground level.
On each flight computer, a 48mhz Cortex M0 processor reads the flight sensors at 400hz and logs 31 channels of data to a flash chip, 40 times per second. Every second that Falcon Heavy is in the air, 4,960 points of data are recorded onboard – never too much data.
The flight computers don’t communicate with each other during flight, the sensors and software are accurate enough that it’s not required for short flights. None of the flight settings are hard-coded; depending on the flight profile, they can all be changed using the Signal iOS/Android app.
The flight software is written in C++, the iOS app in Swift, and the Android app in Java. Flight guidance is computed using quaternions, which are more computationally efficient, and are used for guidance in some space launch vehicles. More details about the various BPS.space flight computers can be found via the button below. You can also click here for footage of Z axis gyroscope testing.
The attachment points at the top of each side core slide down a ramp on the center core, giving them a bit of clearance during stage separation. This passive setup keeps things very simple during flight, but the cores are usually bolted together while the vehicle is on the ground. All three cores are also connected at the base of the vehicle using a slightly simpler thrust plate.
The side cores remain attached to vehicle by maintaining a slightly higher net thrust force than the center core/upper stage. As soon as the CC/US produces more net thrust than the side cores, the stages will separate. For flexibility, each side core also has a slot for a small separation motor. When used, the sep motor fires at a slight angle through the side core to ensure a clean separation away from the center core. This setup has not been needed on flights so far, but may be helpful later down the road.
The center core of the Falcon Heavy model goes through two boost phases during flight. During the first phase, the side cores are attached. Right around side core burnout, the center core lights a second motor, and the second boost phase begins. These two rocket motors are mounted on top of each other in the center core’s thrust vectoring mount. When the second motor ignites, the lower, spent motor, is ejected. This same technique used to control ascent and propulsive landing motors in other BPS.space rockets.
The rocket motors in each stage of the Falcon Heavy model can be gimbaled ±5 degrees in any direction. Because the side cores are not firing directly through the vehicle’s center of mass, they can be used not just for pitch and yaw, but roll control as well. Each multi-core flight runs a roll program which usually targets 20 degrees of positive roll, executed at 30-40 degrees per second.
Launching the Falcon Heavy model requires a bit of forethought. The center core motor has a slight thrust spike at ignition, after which the amount of force produced slowly tapers off. At liftoff, the side cores must have a greater net thrust than the center core. Because of this, during a full FH flight, the center core is lit at T-1 second while the vehicle is still held on the pad by the launch clamps. At T-0 the side cores are lit, and at T+0.25 the beast is released. Click below for more details on the launch pad.
In February of 2018, the full Falcon Heavy rocket was held down on the launch pad for a static fire test. Some space launch vehicles will fire their engines for a few seconds on the launch pad to make sure everything is working properly before launch day. The purpose of this static fire was instead to make sure the new launch pad was working correctly, and frankly, to get some cool footage for project promotion.
Each individual stage of the rocket was tested on its own before the first full FH launch. Many of these early tests also served as testbeds for the Signal R2 thrust vectoring kit, which was still in development at the time. First, the center core was tested by itself. The flight was a success; the core was recovered and flight tested several more times.
After several more flights of the center core, the side cores were attached for a full booster test flight. The center core had a software bug which caused instability after stage separation. However, the most complex parts of the flight were successfully tested, including proper ignition timing, a roll program, and a mostly clean stage separation(right booster was damaged by the center core at stage sep).
Before attaching it to the center core, the upper stage was tested on its own as well. The first test flight was unsuccessful, which is why it’s important to test! The flight failed for several reasons, mostly having to do with the stability and tuning of the vehicle. The failure and analysis is discussed in detail here.
The second test flight was a partial success – stability greatly improved with an increase in both PID gains and PID sample rate. Several other changes were made as well. However, the parachute(and sports car) still failed to deploy, and the vehicle had another hard impact. The fairing wasn’t separating properly, so another round of fairing separation tests was needed.
The final major test before the first launch of the full Falcon Heavy model, these fairing separation tests served to ensure the upper stage could be safely recovered after flight. Several small modifications were made to the fairing design in the process, and the tests resulted in a much more reliable recovery system.
Flight 1 of the Falcon Heavy model was a partial success. The more complex ignition sequence worked as intended, letting the center core burn a bit on the pad. The first boost phase of flight was picture perfect. Both side cores hit the roll program target dead on, rolling to +20 degrees at 40 degrees per second beginning at T+0.9 seconds. The vehicle remained stable during boost phase one. After a clean separation of both side cores, the center core lost control.
A heat induced structural failure in the center core’s motor mount resulted in a loss of thrust vector control. After sensing a loss of control, the upper stage’s flight computer called an in-flight abort, blowing the fairing apart to deploy its parachute. As the rocket began falling, the center core deployed the upper stage, and its own chutes, bringing both stages safely to the ground. Both side cores had reached the ground softly under their own parachutes by that point. The rocket remains in excellent condition, thanks to the in-flight abort system.
Flight 2 of the Falcon Heavy model will take place in early 2019. Primary goals will be resolving the mechanical issues with the center core, reducing overall mass, and streamlining vehicle flight prep procedures.
Also, do I even have to say it? Of course those boosters are gonna land! The propulsive landing test program is isolated from the Falcon Heavy program right now, but the two will merge as the success rate for both programs increases.