Glossary

Every term the rest of the site assumes you know. Defined once, here, so that no page has to choose between explaining itself and getting to the point.

draftupdated 2026-08-1135 terms

apogeethe highest point of the flight: The highest point a rocket reaches, and the instant at which its vertical velocity passes through zero. It is the single number most fliers want, and it is what the project is named after. Note that apogee and the moment of parachute deployment are not the same event: a motor's delay grain is chosen to fire the ejection charge near apogee, and how near is a measure of how well the flight was planned.; delay grain coast
barometric altitudealtitude inferred from air pressure: Altitude calculated from the drop in atmospheric pressure with height, relative to a pressure reference taken on the ground before launch. It is the standard method for model rocket altimeters because it needs one small cheap sensor. Its weaknesses: it is noisy, it is disturbed by airflow around the airframe, and because the ground reference is taken once, a weather system moving through during a long pad wait shifts the zero.; static port pad calibration sensor fusion
burnoutthe moment the motor stops producing thrust: The end of the propellant burn. On an acceleration plot it is a sharp change from strong positive acceleration to deceleration, and it is usually the most visually obvious feature of a flight graph after apogee.; coast delay grain
caliberone body tube diameter, used as the unit for stability margin: The diameter of the rocket's body tube, used as the unit for measuring stability margin. A rocket with a 24 mm tube and a 24 mm gap between its centre of gravity and centre of pressure has a one caliber margin.; stability margin
centre of gravitythe balance point of the loaded rocket: The point the rocket balances at, with the motor installed and the payload fitted. Adding a payload moves it, and which way depends entirely on where the payload sits: forward of the existing balance point it moves the centre of gravity forward, aft of it, backwards. This is why a payload cannot be added without re-checking stability.; centre of pressure stability margin
centre of pressurethe point aerodynamic forces effectively act through: The point at which the total aerodynamic force on the airframe can be treated as acting. A rocket is stable when the centre of pressure is behind the centre of gravity, so that any deviation from straight flight produces a force that corrects it rather than amplifies it.; centre of gravity stability margin
coastthe unpowered climb between motor burnout and apogee: The phase after the motor stops thrusting and before apogee, during which the rocket climbs on momentum alone while drag and gravity slow it. Most of the altitude on a well-flown low power rocket is gained during coast.; apogee burnout
delay grainthe slow-burning section of a motor that times the ejection charge: The section of a model rocket motor that burns after the propellant is spent and before the ejection charge fires, chosen to put deployment near apogee. The number after the dash in a motor designation is the delay in seconds, so a C6-5 has a five second delay.; ejection charge apogee
dynamic platform modela GNSS receiver setting for the kind of motion it should expect: A configuration setting on u-blox receivers telling the receiver what sort of motion to expect, which it uses to filter position solutions. The default assumes ground vehicle dynamics and will discard solutions under rocket acceleration, losing lock at exactly the moment tracking matters. The airborne model must be set explicitly, and this is the single most common reason a GNSS-equipped model rocket payload returns with no track.; GNSS
ejection chargethe pyrotechnic charge that pushes the parachute out: A small pyrotechnic charge that pressurises the body tube and pushes out the nose cone and recovery system. On a model rocket it is part of the motor. oApogee does not fire, arm, control, or interact with an ejection charge in any way.; passive instrumentation payload delay grain
FAA Part 101 Subpart Cthe US federal rules for amateur rockets: The subpart of the US Federal Aviation Regulations covering amateur rockets, which defines the classes of amateur rocket and the operating limits for each. Class 1, the model rocket class, is where low power model rocketry sits and carries the fewest requirements. The class boundaries are set by propellant mass and total vehicle mass, so adding a payload is a thing that can move a rocket across a boundary.; high power rocketry primary source
FCC Part 15the US rules for unlicensed low power transmitters: The part of the US Federal Communications Commission rules covering unlicensed radio devices. Operating within Part 15 limits in the 902 to 928 MHz band requires no licence and no callsign, which is why it is the oApogee default. Part 15 devices must accept interference and must not cause harmful interference, and the equipment authorisation obligations fall on whoever builds and operates the transmitter.; FCC Part 97 LoRa primary source
FCC Part 97the US amateur radio service rules, which require a licence: The rules governing the amateur radio service in the US. Operating under Part 97 requires a licence, requires station identification, and prohibits messages encoded to obscure their meaning, which rules out encrypting a telemetry link. In exchange it allows far higher transmit power and other bands. Part 97 and Part 15 are separate legal regimes and oApogee documentation never blends them into one procedure.; FCC Part 15 primary source
GNSSsatellite positioning, the general term that includes GPS: Global Navigation Satellite System. The general term for satellite positioning, of which GPS is the US system and Galileo, GLONASS, and BeiDou are others. Modern receivers use several at once, which is why the site says GNSS rather than GPS.; dynamic platform model
high power rocketryrocketry above the model rocket limits, requiring certification: Rocketry beyond the model rocket thresholds, requiring certification through NAR or Tripoli, and subject to additional FAA requirements. Electronic deployment control lives here. oApogee is designed for the low power side of that line and does not attempt to serve the other side.; FAA Part 101 Subpart C NAR Tripoli passive instrumentation payload
IMUan inertial measurement unit, accelerometer plus gyroscope: Inertial Measurement Unit. A chip combining an accelerometer, which measures linear acceleration, and a gyroscope, which measures rotation rate. Together they give orientation and the acceleration channel the sensor fusion uses.; sensor fusion saturation
LiPoa lithium polymer rechargeable cell: Lithium polymer battery. High energy density for its mass, which is why oApogee uses one, and correspondingly unforgiving when damaged, overcharged, or shorted. A punctured or crushed cell can ignite, sometimes minutes after the damage rather than immediately. Handling rules are on the Safety and rules page and are not optional.; protection circuit
LittleFSa filesystem designed to survive power loss on flash: A small filesystem for microcontroller flash, designed to be resilient to power loss and to wear-level. On oApogee it holds the flight logs. The power loss property matters because a payload's power can be interrupted by an impact.; UF2
LoRaa long range, low data rate radio modulation: A spread spectrum modulation that trades data rate for link budget, which makes it well suited to telemetry: the packets are small and infrequent, and the requirement is that they arrive from far away through an airframe at an unfavourable angle. LoRa is the modulation. It is not itself a licence, a band, or a network.; FCC Part 15 spreading factor
NARthe National Association of Rocketry, a US hobby organisation: The National Association of Rocketry. It publishes the Model Rocket Safety Code that most US clubs fly under, and it runs a certification programme for high power rocketry.; Tripoli FAA Part 101 Subpart C primary source
oApogeethis project, an open source rocket telemetry payload: An open source telemetry payload for model rockets, and the documentation that teaches you to build one. Written oApogee, one word, lowercase o, capital A. Never shortened to "Apogee", which in this hobby means Apogee Components, an unrelated retailer.
pad calibrationaveraging ground pressure before launch to set the altitude zero: The procedure of sampling atmospheric pressure while the rocket sits on the pad and averaging it over a settling window, to establish the zero reference that every later altitude is measured against. Without it, the altimeter reports height above sea level rather than height above the launch site.; barometric altitude
passive instrumentation payloada payload that only measures, and never commands anything: A payload that records and transmits but never actuates. oApogee does not fire ejection charges, control deployment, ignite motors, or command any pyrotechnic device. This is a scope boundary, not a limitation to be removed later. A payload that can fire a charge has a completely different failure consequence profile and belongs to certified high power rocketry.; ejection charge high power rocketry
payloadanything the rocket carries that is not part of flying it: Whatever the rocket carries beyond what it needs to fly. On a model rocket this is usually an altimeter, a camera, or an egg. oApogee is a payload, specifically a passive instrumentation payload, meaning it observes and records but does not act on the rocket.; passive instrumentation payload
protection circuitthe small board on a cell that stops over-discharge and shorts: A small circuit built into the cell's packaging that disconnects it on over-discharge, overcharge, and short circuit. Bare cells without one are sold and are cheaper. oApogee does not recommend them and the build guide says how to tell the difference.; LiPo
recovery ethicsthe rules about where you may go to retrieve a rocket: A landed rocket does not grant permission to go and get it. Property lines, crops, livestock, and dry grass fire risk all outrank recovering hardware. A payload is worth less than a relationship with a landowner and far less than a field fire.; walkaway
saturationa sensor pinned at its maximum, reporting that instead of the truth: What happens when the real quantity exceeds a sensor's full-scale range. The dangerous property is that the sensor does not report an error, it reports its maximum, so a saturated boost phase looks like a flat plateau rather than like missing data. A typical 6-axis IMU tops out around 16 g, which anything above a C motor will exceed, and that is why a separate high-g accelerometer exists.; IMU
sensor fusioncombining two imperfect sensors into one better estimate: Combining measurements from sensors with different weaknesses so that each covers for the other. On oApogee, barometric altitude is noisy and is actively wrong during the pressure disturbance around burnout, while acceleration integrated to get altitude drifts steadily because integrating any sensor's bias accumulates it. Fused, the barometer anchors the long term and the accelerometer carries the short term.; barometric altitude IMU
spreading factorthe LoRa setting that trades data rate against range: The parameter that sets how much a LoRa symbol is spread in time. A higher spreading factor gives more range and more resistance to noise, and costs data rate and airtime. Because a rocket generates its most interesting data in the few seconds where update rate matters most, this is a real trade and the site states the setting alongside every range figure it publishes.; LoRa
stability marginthe gap between centre of gravity and centre of pressure, in calibers: The distance between the centre of gravity and the centre of pressure, expressed in calibers, where one caliber is the body tube diameter. Too little and the rocket is unstable. Too much and it weathercocks hard into the wind, which loses altitude and can send it downrange over people.; centre of gravity centre of pressure caliber
static porta vent hole that lets the barometer see outside air pressure: A hole in the airframe, positioned so the pressure sensor inside reads ambient static pressure rather than the pressure of a sealed volume or the dynamic pressure of moving air. A barometric altimeter inside a sealed tube measures the tube, not the sky, and produces confident nonsense. Sizing, count, placement relative to the sensor, and deburring all matter.; barometric altitude
telemetrymeasurements taken on the vehicle and sent to the ground: Measurement at a distance. On oApogee it means the flight data recorded on the payload, and on Link and Track, the live stream of that data sent down to a ground station while the rocket is still flying.
Tripolithe Tripoli Rocketry Association, focused on high power rocketry: The Tripoli Rocketry Association. Publishes its own safety code and runs a high power certification programme.; NAR high power rocketry primary source
UF2a firmware file you flash by dragging it onto a USB drive: A firmware file format that a board in bootloader mode accepts by appearing as a USB mass storage device, so flashing is copying a file. It is the reason oApogee uses the RP2040 and RP2350 family: a first-time builder installs no toolchain to get a working payload.; LittleFS
walkawayhow far you have to walk to get the rocket back: The distance from the pad to where the rocket lands, and by extension the recovery walk itself. Long walkaways are what GNSS tracking and audible beacons exist for, and are also where landowner permission stops being a formality.; recovery ethics