Xi is my newest rocket project, which began in the late summer of 2017. After many successful flights of both the Zeta and DS rockets, it was felt that a rocket with a larger payload capacity was needed. The design of the Xi rocket was based on both the positive and negative experiences gained from the Zeta and DS series of flights. The objective was to incorporate the better features of the two predecessors in a larger size rocket that would allow for greater payload and for accommodating larger rocket motors.
The Xi rocket is largely of aluminum construction. The airframe is made of thin-walled (0.035 inch/0.89mm) aerospace grade aluminum tubing, of 3 inch (76mm) diameter for the forward section, and of 2.5 inch (64mm) for the aft section that houses the motor. A tapered coupler, machined from 6061 aluminum, joins the two body sections of different diameter. The coupler additionally serves to transfer thrust load from the motor to the rocket airframe. The four fins are fabricated of high-strength 2024-T3 sheet aluminum, bonded in place through slots cut into the body tube. The original conical nosecone (which houses the GPS transmitter) was machined from a single piece of Delrin thermoplastic. This was later replaced with a lightweight conical nosecone 3D printed using PLA plastic. In anticipation of higher speed flights, a 3D printed tangent-ogive nosecone was made and was first flown on Flight Xi-27. The avionics (recovery electronics) are housed within an Av-bay compartment which also serves as a coupler between aft and forward body sections. Additional payload can be accommodated in the Aft body section, which also houses an aft-facing video camera (not used for all flights).
In addition to the nosecone, a number of Xi rocket parts are 3D printed, such as parachute piston (replacing earlier versions made of delrin or aluminum) and boattail (used on some flights). In all cases, PLA filament is used to produce the parts. Load testing has demonstrated that 3D printed PLA parts have appreciable strength, combined with excellent stiffness and surface hardness, well suited to rocketry applications. STL files for some of the 3D printed parts are given below.
Based on overall positive experience with the Raven, this unit was chosen as the primary flight computer. The Raven measures and records flight data based on accelerometer and barometric readings throughout the entire flight. The Raven also activates main and backup pyro charges at apogee, and fires a pyro charge at a pre-determined altitude to release the parachute. Post-flight downloading of the Raven data provides altitude, axial and lateral acceleration, and velocity, as well as details regarding pyro charge activation. To ensure high reliability of safe recovery, triple redundancy is utilized. In addition to the Raven as the primary system for both apogee separation and parachute deployment, two additional flight computers are used as backup. The Eggtimer (Classic) and the BREO flight computer. For initial Xi flights the same backup Timer used on both Zeta and DS was used as the sole backup.
The Raven, EggTimer and BREO are each separately powered by a 9V lithium primary cell battery. The main reason for choosing this battery is its excellent cold-weather performance. Recovery of the rocket after landing is greatly facilitated by use of a Big Red Bee (BRB) BRB900 GPS telemetry system. This system, also flown in the Zeta and DS rockets, consists of an on-board 250mw transmitter operating at 900 Mhz spectrum (no license required), which transmits GPS coordinates of the rocket throughout the flight and after landing. The signal is picked up by a matching receiver. The receiver has an LCD display showing the GPS coordinates. After landing, the GPS coordinates are entered into my Garmin hand-held GPS unit. The Garmin displays the distance to the downed rocket and direction, which makes tracking and recovery of the rocket a breeze.
The BRB unit is mounted inside the nosecone, housed in a protective styrofoam shell. This shell additionally provides thermal insulation, a requirement for winter launches.
Av-bay avionics, showing Raven and Eggtimer
Dual-deployment is used for recovery in order to minimize down-range drift of the rocket.The method is very similar to that of the DS rocket. At apogee, a pyro charge is fired which separates the rocket into two sections. The two sections, connected by a tether, then free-fall in a tumbling fashion, at a relatively slow velocity, typically around 70 feet/sec. (21 m/s). When a predetermined altitude is reached (typically 700 feet, or 215m.), a pyro charge is triggered by the flight computer which blows off the Av-bay compartment. The momentum pulls the recovery parachute out of the forward airframe. The pyro charges typically employ between 1 and 2 grams of Crimson Powder, contained within a sealed polyethylene tube.
The Av-bay is secured to the airframe with two joints featuring nylon shear screws. The aft joint, which separates at the apogee event, is secured with three #4-40 nylon screws. The forward joint, which separates at the parachute deployment event is secured with six #6-32 flush-head nylon screws. The parachute originally used for recovery is the same as that used for Zeta flights, a 36" Fruity-Chutes semi-ellipsoidal chute. This was later replaced, starting at Xi-27, with a larger 40" chute. Descent rate under parachute is typically 20 feet/sec. (6 m/s).
As mentioned, a primary and dual-backup system is employed for both the critical primary recovery event (separation at apogee) and parachute deployment event. The Raven flight computer fires a pyro charge at apogee. The Eggtimer and BREO likewise each fire a separate pyro charge at apogee. Parachute deployment has a single backup, the Eggtimer, which fires a separate pyro charge at an altitude of 500 feet (150m).
It is expected that the Impulser-X motor will be used for most initial flights. Plans are in the works for flights powered by the new Helios-XX "I-class" motor powered by ANCP, which has a similar total impulse as Impulser-X. Another motor that is slated to loft the Xi rocket is the SSJ-F motor, a "J-class" motor that is the flight version of the SSJ motor that was used for static testing back in 2006-2007 timeframe. As a longer term goal, the Xi rocket will be utilized for flight testing of future APCP based rocket motors.
Drawing ofXi rocket
STL files (3D printed parts)