ARC Avionics and Altimeters: Recording Altitude and Hitting Your Target Score

The American Rocketry Challenge is a precision event. Your rocket does not simply have to fly — it has to hit a specific target altitude and land within a specific flight duration window. Miss either number by enough and your score drops regardless of how clean the flight looked. That means altitude measurement is not a secondary concern; it is central to everything your team does from the first test flight through the final qualifying attempt.

The target altitude and duration window change each season, and the official altimeter used for scoring is defined by the rules. Before you build anything, download the current year's ruleset from the official ARC website and confirm both numbers. Everything in this guide describes principles that are stable across seasons — the physics of how altimeters work, how to mount them correctly, and how to use flight data to improve. If you are earlier in the process, getting started with the American Rocketry Challenge and how to prepare for ARC season are good places to begin.

What the Altimeter Measures and Why It Matters

The official ARC altimeter records two values: apogee (the highest altitude the rocket reached above the launch pad) and total flight time. These two numbers are what scorers use. Apogee accuracy drives the altitude portion of your score, and flight duration drives the time portion. A rocket that reaches the right altitude but falls outside the duration window still loses points. Both must be correct simultaneously.

ARC designates which altimeter is official for scoring each season, and the rules specify whether you may carry an additional altimeter of your own for supplementary data. Read that section carefully. Some seasons allow a secondary altimeter; others restrict what additional electronics you can fly. Do not assume — check the current rules. What this guide calls the "altimeter" is whichever device is officially recording the scored flight.

How Barometric Altimeters Work

Barometric altimeters measure altitude by measuring air pressure. Air pressure decreases predictably as altitude increases — at sea level, standard atmospheric pressure is 101,325 pascals, and it falls by roughly 12 pascals for every meter of altitude gained near sea level (about 1,200 pascals per 100 meters). The altimeter contains a small pressure sensor that samples this continuously during flight.

The device zeroes itself on the launch pad before ignition. Every pressure reading after launch is compared to that baseline. The sensor converts the pressure difference into an altitude change using the barometric formula, and the highest altitude recorded becomes the logged apogee.

Total flight time is typically measured from the first detected motion event (launch) to the moment the altimeter detects landing — usually defined as when altitude stops changing and settles back near zero.

There are a few sources of error worth understanding:

Temperature. The pressure-to-altitude conversion assumes a standard atmospheric temperature lapse rate. On unusually hot or cold days, the actual lapse rate differs, introducing small errors. For most ARC flights these errors are minor but worth keeping in mind if you are chasing a tight altitude window.

Weather. Ambient pressure changes between the time you zero the altimeter and the time you fly. If a weather front moves through or the field elevation differs from where you calibrated, readings shift. Zero the altimeter at the launch site, as close to flight time as practical.

The static port. This is the most operationally significant source of error and has its own section below. If pressure is not sampled correctly, the reading is wrong regardless of how accurate the sensor itself is.

Static Ports and the Payload Bay

The altimeter must sense the ambient air pressure outside the rocket in real time. It cannot do that sealed inside an airtight tube. You achieve this by drilling small holes — static ports — into the altimeter bay so that inside pressure tracks ambient pressure as the rocket climbs.

If you get this wrong, you get bad data. Here is what goes wrong in each failure mode:

• No static ports or ports that are too small. The bay acts like a sealed chamber. As the rocket climbs rapidly, outside pressure drops faster than inside pressure can equalize. The altimeter sees higher-than-ambient pressure and reports a lower-than-actual altitude.
• Ports placed where airflow disturbs them. A static port must sense still ambient pressure. A hole facing into the airstream registers ram (stagnation) pressure — higher than true static pressure — so the altimeter computes a lower-than-actual altitude. Conversely, fast airflow across a poorly placed hole can create suction that lowers the sensed pressure and inflates the altitude reading. Either way the static reading is corrupted, which is why placement matters as much as hole size.

The standard guidance is to drill two to four small holes (typically 1–3 mm diameter, but verify against your altimeter's documentation and test results) spaced evenly around the circumference of the airframe. Place them in the middle of the altimeter bay section, away from fins, launch lugs, or any surface feature that disrupts smooth airflow. The holes should be perpendicular to the airframe axis.

You also need to seal the altimeter bay from the ejection charge gases. When the recovery system fires, hot pressurized gas travels through the body tube. If that gas reaches the altimeter bay, it can overwhelm the static pressure reading and damage the sensor. Use a bulkhead or coupler to physically isolate the altimeter bay from the motor-and-recovery section. The static port holes are small enough that a brief pressure spike from the ejection charge does not equalize through them quickly — but a direct path will.

Test your static port setup before you need competition data. Fly with the altimeter and compare the logged apogee to your simulation. Consistent under-reading by a fixed amount points to undersized ports or poor bay sealing. Erratic over-reads point to dynamic pressure intrusion.

Mounting and Wiring

The altimeter needs to survive a flight that involves high-G acceleration at launch, vibration throughout the burn, and a sharp ejection event. Loose mounting means the sensor samples vibration artifacts instead of clean pressure data. In the worst case, a loose altimeter slides and blocks the static ports entirely.

Mount the altimeter to a sled or bulkhead using standoffs or foam-backed tape rated for vibration. The device should not rock or slide when you shake the rocket firmly by hand. If your altimeter uses a removable battery, verify that the battery connection is secure under vibration — a momentary power interruption mid-flight corrupts the log.

The arming switch (if the altimeter has one) should be reachable from outside the airframe so you can arm the device after the rocket is on the rail without disassembling anything. Most teams drill a small hole in the airframe aligned to the switch and use a rod or straightened paper clip to actuate it. Plan this access point before you fiberglass or paint.

Keep wiring short and routed away from the altimeter bay's static ports. A wire running across a port hole partially blocks it and changes its effective area. Zip-tie or tape wiring flat against the sled.

After any flight that involved an ejection charge, inspect the altimeter bay for black powder residue. Even well-isolated bays can accumulate residue over a season. A contaminated pressure port reads erratically.

Reading and Interpreting Flight Data

After each flight, download the log and write down three numbers before you do anything else: apogee, time to apogee, and total flight time. These are your primary diagnostics.

Apogee vs. the target. If you are 50 feet low, you need more energy or less mass. If you are 50 feet high, you need less energy or more mass. The direction is straightforward; calibrating the magnitude takes a few flights.

Time to apogee. This tells you how the motor burn and coast phase are behaving. A rocket that reaches apogee in 4 seconds on a 1-second motor is behaving differently than one that takes 8 seconds. If time to apogee is shorter than your simulation predicted, the rocket is lighter than modeled or the motor burned hotter. If it is longer, check whether the rocket weathercocked or flew a non-vertical trajectory.

Total flight time. This is the number that must fall inside the target window. Subtract time to apogee from total flight time and you get descent time. If descent time is too short, your parachute is too small or deployed late. If descent time is too long, your parachute is too large and you are drifting.

Log every flight in a table: date, motor, mass, apogee, time to apogee, total duration, weather conditions. Three flights of data reveal trends that a single flight cannot.

Using Data to Tune Toward the Target

Once you have a few flights logged, you can start iterating deliberately rather than guessing.

Mass adjustments. Adding ballast (typically small weights in the nose cone) increases the total mass of the rocket, which lowers the coast-phase acceleration and reduces apogee. Removing mass does the reverse. A useful rule of thumb: for a typical ARC rocket, adding roughly 10–15 grams shifts apogee by 20–40 feet, depending on the specific design and motor. Your simulation will give a more precise number — use it. Run simulate the flight in OpenRocket with the exact mass you flew and compare the predicted apogee to the measured one. If they match, you can trust the simulation for future mass adjustments. If they diverge consistently, the simulation has a wrong input (drag coefficient, motor burn curve, or mass) that you need to correct before relying on it.

Motor and delay adjustments. Different motors of the same total impulse class produce different thrust profiles. A faster-burning motor reaches a higher peak velocity and a higher apogee for the same total energy; a longer-burning motor spreads the impulse and stays lower. See motor selection for ARC for a full treatment of how to match motor characteristics to your design. The ejection delay stamped on the motor also affects flight time — a longer delay lets the rocket coast further past apogee before the chute deploys, which can shift total duration.

Iterating a flight series. A disciplined tuning sequence looks like this: fly, log, simulate with measured conditions, identify the delta, adjust one variable at a time, fly again. Do not change motor and mass simultaneously — you will not know which change produced the result. Change one thing, verify the effect, then change the next.

Pre-Flight Avionics Checklist

• Altimeter battery is fresh and connection is secure
• Altimeter is armed and the status indicator (LED or beep sequence) confirms it is active
• Static ports are open, unobstructed, and free of debris or tape
• Altimeter bay is sealed from the recovery section
• Mounting sled is secure — no movement when shaken
• Switch access hole is clear and the switch state is confirmed
• Altimeter log from the previous flight has been downloaded and saved before the flight that would overwrite it
• Current flight conditions (temperature, barometric pressure) have been noted for post-flight comparison

Common Mistakes to Avoid

• Drilling static ports that are too small and then attributing persistent low altitude readings to motor variation.
• Mounting the altimeter directly against the body tube wall with no standoff, so vibration couples directly into the sensor.
• Placing a static port on the same circumferential line as a launch lug or fin root, where airflow separates and creates local pressure disturbances.
• Leaving the altimeter in the rocket with a loaded battery between sessions — batteries drain slowly and a dead battery at the range is a wasted qualification attempt.
• Not downloading flight data after each flight and overwriting logs. You need the full season's record to diagnose trends.
• Assuming a good simulation means you do not need to verify static port behavior empirically. Simulation does not model static port error — only flights do.
• Not re-zeroing the altimeter at launch site conditions. If you armed it in the parking lot and the pad is at a different elevation, your apogee reading is offset by that elevation difference.

Where to Go From Here

Getting altimeter readings you can trust, and then using those readings systematically to close the gap to the target, is what separates teams that qualify consistently from teams that have good days and bad days. The physics is manageable — barometric pressure measurement is well-understood — but the execution details (port sizing, bay isolation, systematic logging) require deliberate practice across a full test flight series.

If you want structured guidance on avionics setup, flight data analysis, and tuning your rocket toward the ARC target across a coached season, explore our ARC coaching classes.