Rocketry Calculator

Dual Deployment Altimeter Delay Calculator for HPR Rockets

The only free tool that solves for your optimal main deployment altitude based on landing zone size, wind speed, and drift physics. Calculate drogue timing, main setpoint, phase descent times, and FAA waiver compliance in one shot.

⏰ Optimal Main Altitude Solver 📐 Descent Profile Chart ✈ FAA Waiver Check ✅ Drift Calculation 📲 Mobile Friendly 🏳 NAR / TRA Compatible

Dual deployment is a two-stage recovery system in which a barometric altimeter fires a drogue ejection charge at or near apogee, then fires a main ejection charge at a user-programmed altitude above ground level. The two settings every HPR flyer must configure before launch are: the drogue deployment delay in seconds after apogee detection (typically 0 to 1 second), and the main deployment altitude in feet AGL (typically 400 to 800 feet for open-field sport launches). This calculator uses the drift equation to solve for the main altitude that keeps your rocket within your landing zone at your specific wind speed, then outputs the full descent timeline and a visual descent profile. No other free online tool does this.

The Altimeter Programming Equations: Drogue Timing, Main Altitude, and Total Drift

Enter flight parameters and landing zone size. The calculator solves for the optimal main altitude, or you can override with your planned setting to see the resulting drift.
▲ Flight Parameters
ft AGL
From apogee estimator or OpenRocket
ft MSL
For FAA waiver ceiling check
ft MSL
Your club’s waiver (check with RSO)
sec
Most altimeters: 0 to 1 sec
🎖 Recovery System
fps
Safe: 70-90 fps (from chute calc)
fps
Target: 15-20 fps (from chute calc)
🏄 Launch Site and Field
mph
Forecast wind at launch time
ft
Half-width of your safe landing area
ft AGL
Enter your planned setting to check drift instead of auto-optimizing
⏰

Enter your flight parameters and landing zone. The calculator will solve for the optimal main deployment altitude, generate your descent profile chart, and verify FAA waiver compliance.

Tip: use drogue and main rates from the Parachute Descent Rate Calculator for the most accurate drift estimate.

▸ Program These Settings Into Your Altimeter
Drogue fire delay 0 sec after apogee
Main deployment altitude 600 ft AGL
Optimization note 600 ft AGL optimal
Drogue Phase Time
0 sec
apogee to main fire
Main Phase Time
0 sec
main fire to landing
Total Drift Distance
0 ft
at planned wind speed
FAA Waiver Status
OK
vs waiver ceiling
Descent Profile: Altitude AGL vs Time from Apogee

How Dual Deployment Altimeters Detect Apogee and Execute Two-Stage Recovery

A barometric altimeter continuously reads atmospheric pressure through a set of vent holes in the avionics bay. As the rocket climbs, pressure drops. As it descends, pressure rises. The altimeter samples this pressure several hundred times per second during flight and converts it to an altitude reading using a known pressure-altitude relationship. Apogee is detected when the altitude reading stops increasing and begins to decrease. Most modern altimeters, including the Perfectflite Stratologger CF, Missileworks RRC3, and Altus Metrum EasyMini, use a moving average over multiple samples to confirm apogee rather than reacting to a single reading, which prevents false firings from momentary pressure fluctuations caused by motor ejection gas or airframe flexing at burnout.

Once apogee is detected, the altimeter waits for the user-programmed apogee delay in seconds (typically 0 to 1 second) before firing the drogue ejection channel. The delay ensures the rocket has definitively passed apogee before the drogue charge fires, since pressure sensing can detect a brief false apogee from the sudden pressure spike of motor burnout. After the drogue fires and the rocket begins descending under the drogue, the altimeter continues monitoring altitude. When altitude drops to the user-programmed main deployment altitude in feet AGL, the main ejection channel fires. The entire system requires only two user settings: the apogee delay in seconds and the main deployment altitude in feet AGL.

The Two Settings You Actually Program Before Every HPR Flight

Every dual-deployment altimeter from any manufacturer ultimately requires the same two numbers: how long to wait after apogee before firing the drogue, and at what altitude to fire the main. Every other configuration option (arm altitude, landing detect, redundancy settings) is secondary to these two. The apogee delay is straightforward: 0 seconds is perfectly safe for most flights and is what most experienced HPR flyers use. Some choose 1 second as a conservative buffer against false apogee detection, at the cost of slightly more freefall before the drogue opens. The main deployment altitude is far more consequential because it directly determines how much your rocket drifts before landing. Getting this number right requires understanding the drift physics, which is exactly what this calculator solves.

Why “Default to 500 ft AGL” Is the Wrong Answer for Most Flights

Most altimeter datasheets and many introductory tutorials suggest setting main deployment to 500 feet AGL as a sensible default. For a large open-desert site with unlimited landing zone, 500 ft is fine. For a 500-foot-radius club field in a 15 mph wind, a rocket descending at 17 fps for 29 seconds under the main chute will drift more than 600 feet at 15 mph, landing outside the field boundary. The correct main altitude depends on four variables: landing zone radius, wind speed, drogue descent rate (determines how long you drift at altitude), and main descent rate (determines how long you drift at low altitude). Our calculator solves for the altitude that balances all four, then verifies the result against your FAA waiver ceiling.

Wind Speed Apogee 2,000 ft AGL Apogee 5,000 ft AGL Apogee 10,000 ft AGL Optimal Main Altitude (for 1,000 ft zone)
5 mph290 ft drift730 ft drift1,460 ft drift700 ft AGL
10 mph580 ft drift1,460 ft drift2,920 ft drift600 ft AGL
15 mph870 ft drift2,190 ft drift4,380 ft drift450 ft AGL
20 mph1,160 ft drift2,920 ft drift5,840 ft drift350 ft AGL

Drift computed at 80 fps drogue, 17 fps main. Optimal main altitude computed for a 1,000 ft landing zone radius. At 15+ mph, apogee flights above 3,000 ft AGL will often exceed a standard club field landing zone no matter what main altitude you choose: the drogue drift alone exceeds the zone radius.

Setting the Main Deployment Altitude for Your Landing Zone and Site

Total drift equals wind speed in feet per second times total descent time. Total descent time is the drogue phase time plus the main phase time. Drogue phase time equals (apogee altitude minus main deployment altitude) divided by drogue descent rate. Main phase time equals main deployment altitude divided by main descent rate. Substituting and simplifying, total drift equals wind speed times (apogee divided by drogue rate plus main altitude times (1 divided by main rate minus 1 divided by drogue rate)). Because drogue rate is much faster than main rate, the term (1/main rate minus 1/drogue rate) is positive, meaning higher main altitude always means more total drift. The optimal strategy is therefore to set main altitude as low as is safely possible: just high enough for the main chute to fully inflate before landing, but no higher.

Solving for the Optimal Main Altitude

Setting total drift equal to your landing zone radius and solving for main altitude gives the critical equation our calculator uses: h_main = (zone_radius divided by wind_fps minus apogee divided by v_drogue) divided by (1 divided by v_main minus 1 divided by v_drogue). This is the maximum main deployment altitude that keeps total drift within your landing zone radius. If the solution produces a value below about 300 feet AGL, the drift problem is unsolvable at that wind speed with those descent rates: the drogue phase alone causes more drift than the landing zone allows, regardless of how low you set the main. The fix in that situation is to increase drogue descent rate (smaller drogue chute), reduce wind exposure by moving the pad, or accept a longer walk to recovery.

The FAA Waiver Ceiling Constraint

Your club’s FAA waiver specifies a ceiling altitude in feet MSL, not feet AGL. To verify compliance, add your site elevation MSL to your estimated apogee AGL to get apogee MSL, then compare against the waiver ceiling. An apogee of 3,000 feet AGL at a 3,000-foot MSL site like Lucerne Dry Lake puts the rocket at 5,900 feet MSL. If your waiver ceiling is 5,000 feet MSL, that flight exceeds the waiver. Most club waivers run from 7,000 to 14,000 feet MSL for sport launches, but verify with your RSO before flight day. Flying above the waiver is a federal violation under 14 CFR Part 101 regardless of the actual altitude achieved.

Altimeter Model Manufacturer How to Set Main Altitude How to Set Drogue Delay Data Download
Stratologger CFPerfectfliteUSB via Stratologger software (Windows)Fixed at apogee detectionUSB / software
MAWDPerfectfliteRotary switches on board (100 ft increments)Fixed at apogeeBeep codes
RRC3 / RRC3+MissileworksPush-button programming or mDACS softwareProgrammable 0-10 secUSB / mDACS
EasyMini / EasyTimerAltus MetrumUSB via AltosUI software (Mac/Win/Linux)ProgrammableUSB / AltosUI
Raven 3FeatherweightMagnetic switches + softwareProgrammableBluetooth / software
Quark / QuantumEggtimerWiFi browser interfaceProgrammableWiFi download

Three Real Dual-Deployment Programming Scenarios at US Club Launch Sites

These three scenarios show how the optimal main altitude calculation changes with field size, wind conditions, and site elevation. Every number is computed from the drift equation and verified against the landing zone constraint.

📍 Stockton, CA
CAL-40 Club L1 Cert, 4-Foot Rod, Large Field

Large agricultural field near the San Joaquin Delta. Site at 13 ft MSL, FAA waiver 7,000 ft MSL. Calm 5 mph winds typical on summer morning launches. 1-mile landing zone radius. H128W motor, estimated apogee 2,740 ft AGL.

Apogee AGL2,740 ft
Drogue rate80 fps
Main rate17 fps
Wind5 mph
Zone radius5,280 ft (1 mile)
Optimal main alt600 ft AGL
Total drift584 ft
FAA check2,753 ft MSL (OK)
600 ft main is ideal. Drift well within the large field boundary.
📍 Hartsel, CO
CRASH Club L2 Flight at 8,878 ft MSL

South Park valley site. Typical afternoon winds 15 mph. FAA waiver 18,000 ft MSL. J350W motor, estimated apogee 9,200 ft AGL (18,078 ft MSL, within waiver). 1,500 ft field radius. Drogue 80 fps, main 17 fps.

Apogee AGL9,200 ft
Wind15 mph
Zone radius1,500 ft
Optimal main alt350 ft AGL
Drogue phase time111 sec (1 min 51 sec)
Total drift1,494 ft
FAA check18,078 ft MSL (OK)
15 mph pushes main alt to 350 ft AGL: very low. Confirm chute inflates fully at this altitude. Long drogue time means distant drogue drift too.
📍 Millbrook, AL
NAHR Club Flight in Variable Crosswind

Club field at 225 ft MSL near Montgomery. FAA waiver 10,000 ft MSL. I287W motor, estimated apogee 5,800 ft AGL (6,025 ft MSL, OK). Afternoon winds 12 mph. 800 ft field radius.

Apogee AGL5,800 ft
Wind12 mph
Zone radius800 ft
Optimal main alt400 ft AGL
Drift at 600 ft main1,140 ft (exceeds zone)
Drift at 400 ft main799 ft (just within)
FAA check6,025 ft MSL (OK)
Default 500-600 ft main exceeds zone. Dropping to 400 ft AGL keeps rocket just inside the 800 ft radius field.

The Millbrook example is the most common real-world situation: a default 500 to 600 foot main altitude that feels safe produces drift that exceeds the landing zone in a moderate wind. The calculator shows this immediately and gives the correct lower setting. Most flyers who set 500 ft by default and then walk 1,200 feet to recover their rocket have never done this calculation.

Six Expert Altimeter Programming Tips From Experienced HPR Flyers

1

Always Bench Test Your Altimeter After Programming

Programming an altimeter incorrectly happens. After entering your main altitude and drogue delay settings, use the altimeter’s bench test mode (if available) to confirm the outputs fire at the correct simulated altitudes. Perfectflite’s software includes a bench test function. Missileworks RRC3 allows manual trigger testing. Altus Metrum altimeters can simulate a flight in test mode. If your altimeter has no bench test, use a vacuum pump to create a simulated altitude and observe the LED or buzzer output. Never trust programming that has not been verified on the bench before the altimeter goes into the rocket.

2

Size Your Vent Holes Correctly for the Altimeter Baro Sensor

Barometric altimeters require properly sized vent holes in the avionics bay to read ambient pressure accurately. Too small and the pressure inside the bay is buffered from the outside atmosphere, causing delayed or inaccurate apogee detection. The standard calculation: vent hole area (square inches) equals 0.001 times the internal volume of the avionics bay in cubic inches. For a 4-inch diameter bay 10 inches long, internal volume is about 125 cubic inches, so vent hole area should be at least 0.125 square inches. That is two 5/16-inch holes or four 1/4-inch holes. Altus Metrum and many club resources provide detailed vent hole sizing guides, or use AltimeterCloud’s dedicated vent hole calculator.

3

Use Redundant Altimeters for Any Flight Over G Motor Class

A single altimeter failure at apogee on an H or I motor flight means your main chute never deploys. The rocket descends under the drogue all the way to the ground at 80 fps, which is fast enough to destroy a fiberglass airframe and anything it lands near. For any HPR flight, two independent altimeters wired to independent ejection circuits is strongly recommended. Wire them in parallel so either one can trigger the charge independently. Many experienced HPR flyers run two altimeters from different manufacturers (for example, a Perfectflite MAWD and an Altus Metrum EasyMini) to eliminate any shared firmware failure mode. Two altimeters add less than two ounces to a typical HPR avionics bay.

4

Recalculate Main Altitude on Launch Day With Actual Wind Data

Wind conditions on launch day are never exactly what the forecast said. If you pre-calculated your main altitude for 10 mph and you arrive to find 18 mph, your drift calculation is wrong and your pre-set main altitude may be too high. Check the actual conditions with an anemometer at the launch site, plug the real wind speed into this calculator, and confirm your pre-programmed main altitude still keeps drift within the landing zone. If it does not, reprogram before loading the altimeter into the rocket. This takes two minutes and prevents a long recovery walk or a landing outside the safe zone.

5

Set a Minimum Main Altitude of 350 ft for Any Airframe Over 1 Pound

The main chute needs enough altitude to fully inflate before the rocket reaches the ground. For a large elliptical chute packed into a body tube, full inflation after ejection gas clears typically takes 100 to 200 feet of fall time. At 17 fps descent, 200 feet of inflation distance takes about 12 seconds. Setting main deployment at 200 feet AGL leaves almost no margin for partial inflation or shock cord stretch before landing. For any rocket over about 1 pound, set main deployment no lower than 300 to 350 feet AGL. For large chutes above 48 inches diameter packed into tight bays, consider 400 to 450 feet as the practical minimum.

6

Account for GPS Drift When Planning Your Landing Zone Walk

This calculator estimates total linear drift distance from the launch pad in a single constant wind direction. Real drift is cumulative: drogue drift in one direction plus main drift in the (possibly different) wind direction. On days with shifting winds, the actual landing point can be anywhere in a circle of radius equal to total drift distance from the pad, not necessarily downwind. When flying at a site with variable wind direction, plan for recovery anywhere within that drift radius, not just in the current downwind direction. This is why launching from the downwind edge of your landing zone (so drift carries the rocket into the center of the zone rather than over the boundary) is standard practice at tight club fields.

Dual Deployment Quick Reference: Common Settings and Altimeter Ranges

Parameter Typical Safe Range Common Default Notes
Drogue deployment delay0 to 1 second0 seconds0 sec is safe for most flights; 1 sec adds a buffer against false apogee detection
Main deployment altitude300 to 1,200 ft AGL500 to 600 ft AGLOptimize with this calculator for your specific wind and field size
Minimum main altitude300 ft AGL (1-2 lb rocket)400 ft AGLLarger chutes need more altitude to fully inflate
Altimeter arm altitude100 to 200 ft AGL100 to 150 ft AGLPrevents arming on the ground from pressure fluctuations; set in altimeter settings
Drogue descent rate70 to 90 fps80 fpsFaster is better for minimizing altitude drift; too fast increases shock cord load
Main descent rate15 to 20 fps17 fpsSlower means less drift under main but larger chute required
Ejection charge BP amount0.5 to 2.0 grams FFFFg1g per 100 cu inAlways ground test before flight; double-redundant charges recommended
Battery voltage check9V alkaline or 11.1V LiPo9V fresh alkalineCheck voltage day before launch; low battery is the leading cause of altimeter failure

Your Dual Deployment Altimeter Questions Answered

How does a barometric altimeter detect apogee?+
A barometric altimeter samples atmospheric pressure hundreds of times per second during flight. As the rocket climbs, pressure decreases. As it descends, pressure increases. The altimeter converts each pressure reading to an altitude using a stored pressure-altitude table, then compares successive readings. Apogee is confirmed when altitude readings consistently show a decrease over multiple samples (typically 4 to 10 consecutive readings showing descent to filter out noise). Most modern altimeters use a moving average rather than a single sample to avoid false apogee detection from motor burnout pressure spikes, ejection gas, or airframe dynamics at apogee. The number of samples required for confirmation is usually configurable; the default is appropriate for most sport HPR flights.
What main deployment altitude should I use as a starting point?+
A common starting point of 500 to 600 feet AGL works well for large open-field launches in calm to light winds. For any launch site with a constrained landing zone or in winds above 10 mph, use this calculator to find the specific optimal altitude for your conditions. The calculation is fast: enter your apogee estimate, descent rates, wind speed, and landing zone radius, and the calculator returns the mathematically correct main altitude. Using 500 feet by default at all sites is like using the same tire pressure for all vehicles: it works often enough that people do it, but it is not optimized for the specific situation.
What is the drogue deployment delay and how long should I set it?+
The drogue deployment delay is the number of seconds the altimeter waits after detecting apogee before firing the drogue ejection channel. It is set in the altimeter’s configuration and typically ranges from 0 to 3 seconds. A 0-second delay fires the drogue immediately upon apogee detection, which is what most experienced HPR flyers use. A 1-second delay provides a buffer against false apogee detection from motor ejection gas or burnout pressure transients, at the cost of slightly more freefall before the drogue opens. For most APCP composite motor flights, 0 seconds is safe and appropriate. For flights on smoky motors with active ejection gas at burnout, consider 0.5 to 1 second. For pyrotechnic motor flights, check the motor’s ejection delay specification.
Can I use a single altimeter for dual deployment?+
Yes, technically. A single altimeter with two independent output channels (drogue and main) can control a full dual deployment system. However, a single altimeter means a single point of failure: if it malfunctions, neither the drogue nor the main deploys. For this reason, most experienced HPR flyers use two independent altimeters, each capable of triggering both recovery events independently. With redundant altimeters, either one can save the rocket if the other fails. NAR and TRA certification flights (Level 1 and above) do not technically require redundant altimeters, but many RSOs recommend them for Level 2 and above. For Level 3 flights, the certification committee typically requires documented redundant electronics.
Why does my altimeter sometimes show a different apogee than my GPS tracker?+
Barometric altimeters and GPS trackers measure altitude by completely different methods and have different error characteristics. GPS altitude is based on satellite triangulation and is accurate to about 30 to 50 feet vertically but updates only once per second, which may miss the actual peak altitude on a fast HPR flight. Barometric altimeters update hundreds of times per second and are very accurate for changes in altitude relative to the launch pad, but can be offset by weather pressure changes during flight. The altimeter’s displayed apogee altitude reflects peak AGL altitude based on the pressure reading, while GPS reports absolute elevation above sea level (or an ellipsoidal reference) with lower vertical accuracy. Small discrepancies of 50 to 200 feet between the two are normal and expected.
Does elevation affect how my barometric altimeter reads altitude?+
Modern barometric altimeters are not affected by launch site elevation in any operationally meaningful way because they measure relative altitude change from the launch pad pressure (which they sample and record at power-up on the pad), not absolute altitude from sea level. An altimeter at a 5,000-foot elevation site will read pressure at arm time, record that as its zero reference (ground), and then accurately track altitude changes above that ground reference throughout the flight. The main altitude setting in feet AGL works correctly regardless of site elevation. Where elevation matters is the FAA waiver check (which is in MSL) and the parachute sizing calculation (which uses absolute air density at site elevation). The altimeter programming itself requires no site elevation correction.
What is a master arm switch and why is it required?+
A master arm switch (also called a safety switch) physically disconnects power from the altimeter and all ejection charge circuits while the rocket is on the ground, being loaded, or being transported. Without a master arm switch, a battery connection or altimeter power-up could theoretically fire an ejection charge while someone is working on the rocket, which is an extremely dangerous situation. NAR and TRA safety codes require a means to safe all pyrotechnic devices on a rocket at the pad. The master arm switch is the standard implementation: it is left in the “off” or “safe” position until the rocket is on the launch rod with all spectators and crew cleared to a safe distance, then switched to “arm” immediately before pressing the launch button. The RSO will verify the arm switch position as part of the pad safety check.
How do I calculate the black powder charge size for dual deployment?+
The standard starting point for ejection charge sizing is 1 gram of FFFFg (4F) black powder per 100 cubic inches of tube volume that the charge must pressurize. Compute the internal volume of the bay as pi times the inner radius squared times the bay length in inches. For a 4-inch inner-diameter bay 12 inches long: volume = 3.14159 x (2)^2 x 12 = 150.8 cubic inches. Starting charge = 1.51 grams. Always ground test the charge at least twice before flight: once to confirm it reliably ejects the section, and once at the coldest temperature you expect to fly in, since cold reduces black powder burn rate. Adjust the charge weight in 0.1-gram increments until the minimum reliable charge is found, then fly 10 to 15 percent above that minimum as a reliability buffer.
What is the difference between a drogue ejection charge and a main ejection charge?+
In a typical dual-deployment HPR design, the drogue ejection charge is fired by the altimeter’s “drogue” output channel at or near apogee. It pressurizes the aft section of the airframe, separating the forward body tube from the aft body tube and deploying the drogue chute packed between them. The main ejection charge is fired by the altimeter’s “main” output channel at the programmed low altitude. It pressurizes the forward section of the airframe, separating the nose cone from the forward body tube and deploying the main chute packed in the nose cone bay. Both charges typically use FFFFg black powder ignited by an electric match (e-match) wired to the altimeter output. The charge sizes may differ because the volumes they must pressurize are different.
How does the optimal main altitude change with wind speed?+
Higher wind speed requires a lower main deployment altitude to keep total drift within the landing zone. The relationship is approximately linear: doubling the wind speed roughly requires halving the main deployment altitude for the same drift result. In practice, very high winds (above 15 mph for a typical club field) often produce optimal main altitudes so low (300 to 350 feet AGL) that the main chute barely has time to fully inflate before landing. At that point, the correct response is not to lower the main altitude further but to accept a longer recovery walk, move the launch pad into the center of the field, increase the drogue descent rate (smaller drogue, less altitude drift), or wait for calmer conditions. The calculator shows you when the math produces an unsafe answer so you can make an informed decision before launch day rather than discovering the problem during recovery.
What altimeters are most commonly used by US HPR flyers?+
The most widely used dual-deployment altimeters in the US HPR community include the Perfectflite Stratologger CF (good all-rounder, USB programmable, widely supported by RSOs), the Perfectflite MAWD (simple rotary-switch programming, no computer required), the Missileworks RRC3 and RRC3+ (programmable via push-button or software, third programmable output for staging or airsatarts), the Altus Metrum EasyMini (open-source hardware and software, AltosUI works on Mac/Win/Linux), the Featherweight Raven 3 (data logging and Bluetooth), and the Eggtimer Quark/Quantum (WiFi-configured, extremely low cost). For GPS tracking combined with altimeter duties, the Altus Metrum TeleMega and Eggtimer Quasar are popular. At Level 3 flights, redundant electronics typically use two different altimeter models to eliminate shared failure modes.
Does the drogue descent rate change my optimal main altitude significantly?+
Yes, significantly for high apogee flights. A faster drogue (higher fps) means the rocket spends less time at high altitude under the drogue, reducing drogue-phase drift. This allows a higher main deployment altitude while still keeping total drift within the landing zone. For a 5,000-foot apogee in 10 mph wind with a 1,000-foot landing zone: at 80 fps drogue, optimal main is about 600 feet AGL and total drift is about 1,000 feet. At 60 fps drogue (larger, slower drogue chute), the drogue phase takes longer, accumulating more drift, and optimal main drops to about 400 feet AGL to compensate. For high-apogee flights in any wind, using a relatively fast drogue (80 to 90 fps) significantly expands your options for main altitude.
What is an apogee event versus a timed event in a dual deployment altimeter?+
An apogee event fires the drogue channel when the altimeter detects apogee barometrically. A timed event fires a channel a fixed number of seconds after motor ignition (detected by acceleration), without any apogee sensing. Timed events are used as backup deployments or in situations where barometric apogee detection is unreliable, such as very fast flights (supersonic) where pressure readings can be disrupted by shock waves, or flights where the avionics bay venting may be suboptimal. Most sport HPR flights use barometric apogee events exclusively. Some sophisticated avionics bays (like the Altus Metrum TeleMega) can use both: primary barometric apogee deployment plus a timed backup that fires if the barometric event does not occur within a programmed window after ignition.
How do I verify my altimeter settings are correct before the flight?+
Most altimeters use a continuity check (audible beeps or LED flashes) to confirm ejection circuit continuity when powered up. Listen for the continuity indication on both drogue and main channels before arming. Many altimeters also beep out or display the programmed settings (main altitude, apogee delay) during power-up as a confirmation. For altimeters with USB connectivity (Perfectflite, Altus Metrum, Eggtimer), read back the programmed settings from the device using the manufacturer’s software before the flight. Never assume programming persisted correctly after reprogramming in the field without reading back the confirmed settings from the device. If your altimeter cannot beep out its programmed settings and has no USB readback, use a dedicated bench test to trigger each channel at the simulated programmed altitude before final installation.
Why might my rocket land farther away than the drift calculation predicted?+
Several real-world factors cause actual drift to exceed the calculated prediction. Wind speed varies with altitude: winds are typically much stronger at altitude than at ground level, so the drogue phase (at high altitude) often experiences significantly more drift than the surface wind reading suggests. The calculator uses a single constant wind speed for the entire flight, which is conservative at ground level but may underestimate drift at altitude. Wind direction can also shift between the drogue phase and the main phase, adding vector drift rather than simple additive drift. Additionally, the main chute inflates over a few seconds rather than instantly, during which the rocket is descending faster than the calculated terminal velocity. Finally, the rocket may have been drifting slightly off-vertical during the powered phase due to weathercocking, placing the apogee point away from directly above the launch pad.
Is dual deployment required for NAR Level 1 certification?+
No. NAR Level 1 certification requires flying an H or I motor successfully with a rocket that recovers safely and is undamaged. The recovery system can be single deployment (one parachute, no electronics required) as long as the rocket lands safely within the launch area at an acceptable landing velocity. Many Level 1 cert flights use a single main chute with a timed motor ejection delay. However, most experienced HPR flyers recommend using dual deployment even for Level 1 flights because it dramatically reduces drift: a 5,000-foot H-motor flight with a 30-foot main chute opens at apogee can drift a mile or more from the launch pad in moderate winds, while dual deployment brings it back to within a few hundred feet. The Perfectflite MAWD or Missileworks RRC3 are popular entry-level altimeters for first-time dual-deploy setups.

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Accuracy, Limitations, and Editorial Transparency

This calculator uses linear descent rate assumptions throughout: constant drogue descent rate from apogee to main deployment altitude, and constant main descent rate from main deployment to landing. Actual descent rates vary during chute inflation, are affected by wind turbulence, and change slightly as altitude changes air density. Drift estimates assume a constant single-direction wind at the surface wind speed throughout the entire descent, which underestimates actual drift on days with stronger upper-level winds. The optimal main altitude solver uses the drift equation and clamps output between 300 and 1,200 feet AGL. If the math produces a value below 300 feet, the calculator defaults to 300 feet and flags the situation in the fix recommendations. FAA waiver checks add site elevation MSL plus apogee AGL; actual apogee may differ from estimates by 5 to 15 percent. Always confirm altimeter settings with a bench test and verify with your RSO on flight day. Consult the NAR Safety Code, 14 CFR Part 101, and your club’s site-specific rules before any HPR flight. Last reviewed August 2026.