J-Class motor altitude record
A J motor gives you 640.01 to 1280 Ns of total impulse. This is high power rocketry: you need UKRA, Tripoli or NAR Level 2 certification to fly one. This challenge is simple: fly the highest rocket you can on a J motor, logged with an Altimeter Cloud flight computer. The first flight past the height shown on this page wins. After that, every flight that beats the last winning height by 12% wins again, so the bar keeps climbing.
About this challenge
A J motor gives you 640.01 to 1280 Ns of total impulse. This is high power rocketry: you need UKRA, Tripoli or NAR Level 2 certification to fly one. This challenge is simple: fly the highest rocket you can on a J motor, logged with an Altimeter Cloud flight computer.
The first flight past the height shown on this page wins. After that, every flight that beats the last winning height by 12% wins again, so the bar keeps climbing.
Which motors count as a J?
Any motor with 640.01 to 1280 Ns of total rated impulse, such as the 38 mm Aerotech J350 and Cesaroni Pro38 J reloads. The class goes by the total for the whole flight, so a cluster or a second stage that takes it past 1280 Ns makes it a K and won't count.
Tips for flying high on a J
- Check the clearance first. Many UK sites have a height limit well under what a J can reach, so plan the flight to the site.
- Simulate the flight with your real weights and pick a motor that keeps you under that ceiling.
- Use dual deployment: drogue at apogee and main low down, so it lands near the pad.
- Fly a GPS tracker. A J flight can land a long way off, and tracking makes recovery far easier.
- Use glass or carbon fins with strong fillets. Flutter at J motor speeds can strip fins.
- Weigh the rocket ready to fly, motor and all, and enter the lift-off weight on the flight page.
Entering your flight
Upload the flight as normal, then on its flight page add the motor and the lift-off weight, and set the launch site, date and time. Then open the Add my flight tab here and pick it. Your flight computer's accelerometer measures the burn, and it has to be within 10% of the rated motor, which proves the motor really was a J.
Heights are corrected for the day's weather, so a cold day doesn't give anyone a head start.
Fly under your certification and the launch's range safety officer, and stay within the site's clearance.
Your altimeter works out height from air pressure using a standard atmosphere: the sea-level pressure and temperature in its settings, usually 1013.25 hPa and 15 °C. The real day is rarely standard. Cold air is denser, so an altimeter set for 15 °C reads high on a cold day and low on a warm one, by roughly 1% for every 3 °C, which is about 2 m at 200 m.
So every flight is judged on the height it really reached, we take the weather added to your flight for its launch site, date and time, and recalculate the whole log with it. It's the same adjusted apogee the flight page shows, and it's the one that scores.
How it pays
- First win
- Reach 3250.0 m
- After that
- Beat the last winning result by 12%
- Your own result
- Beating it pays again
- Points paid
- Once we have checked the entry
- Dates
- From 5 Nov 2026
Required altimeters
The flight is checked against the altimeter that recorded it
- Hardware
- Rev 4 or later
- Firmware
- 1.65 or later
- Hardware
- Rev 1 or later
- Firmware
- 2.50 or later
- Hardware
- Any revision
- Firmware
- 1.02 or later
- Hardware
- Any revision
- Firmware
- 1.00 or later
Required flight details
Everything is checked when you add a flight- Your own flightFrom an altimeter on your account, and a real flight rather than a test.
- A J motorAdded on the flight page. The class is the total rated impulse of the flight's motors: J is 640.01 to 1280 Ns.
- Single motorOne motor on the whole flight: no cluster and no staging.
- Single stageCounted from the stages of the motors added on the flight page.
- Motor proven by its burnThe total impulse measured from the altimeter's accelerometer and the lift-off weight has to be within 10% of the rated motor, as NFPA 1125 allows.
- Lift-off weightEntered on the flight page.
- Launch siteSet on the flight page.
- Launch date and timeSet on the flight page.
- Launch weatherAdded to the flight once its site, date and time are set. Altitudes here are corrected for it.
- Flown during the challengeOn or after 5 Nov 2026.
Your altimeter works out height from air pressure using a standard atmosphere: the sea-level pressure and temperature in its settings, usually 1013.25 hPa and 15 °C. The real day is rarely standard. Cold air is denser, so an altimeter set for 15 °C reads high on a cold day and low on a warm one, by roughly 1% for every 3 °C, which is about 2 m at 200 m.
So every flight is judged on the height it really reached, we take the weather added to your flight for its launch site, date and time, and recalculate the whole log with it. It's the same adjusted apogee the flight page shows, and it's the one that scores.