Altimeter readings, and the airframe we left in the plantation

01

Everybody guesses high

Before the altimeters, every flight card carried an apogee estimate written by the range officer, based on watching it go up. Those estimates are still in the binders, and for the 2024 season we have 31 flights where an estimate and a measured reading sit side by side on the same card. The estimates are higher than the readings on 27 of them. The mean estimate was 186 metres. The mean measurement was 131. Two people out of the fourteen were consistently close, and neither of them could explain why.

The over-estimate gets worse with height, which makes sense once you think about what the eye is actually doing. A rocket at 80 metres is still a recognisable object with a visible shape. At 150 it is a speck, and the eye has nothing to judge distance with except how small it has become, which is not information the brain handles well against a plain sky. On a day with broken cloud the estimates were better, because there was something at a known sort of distance to compare against.

02

What the altimeter is actually measuring

A barometric altimeter records air pressure and converts the drop in pressure into a height above the pad. It is not measuring the rocket at all; it is measuring the air inside the body tube, which means where the vent holes are and how big they are matters enormously. Our first four flights with one of the units gave nonsense, including a reading of 402 metres on a motor that cannot physically manage half that, because the bay had a single oversized hole and the airflow past it was pulling pressure down as the rocket accelerated.

Three small holes spaced evenly around the tube, sized roughly as the instructions suggested for the bay volume, fixed it. After that the units agreed with each other to within about four metres when we flew both in the same airframe, which is as much confirmation as we are equipped to get. We also learned to log the ground pressure at the start of the day, because two flights an hour apart under a moving front will disagree with each other for reasons that have nothing to do with the rockets.

03

Two seasons of numbers

The club kit we use for beginners, at 74 grams empty, averaged 91 metres over nine flights on the smallest motor class we fly, 158 metres over fourteen flights on the middle class, and 243 metres over six flights on the largest low-power motors we use. The scatter within each group was about fifteen per cent, and most of that correlates with mass rather than with wind, which surprised us. Adding paint to an airframe costs real altitude: two identical kits, one bare and one with three coats and a decal sheet, differed by eleven grams and by 23 metres on the same motor type.

The other thing that emerged was the cost of a crooked flight, which we had already suspected from the fin work but could now put numbers on. Flights the range officer logged as visibly spiralling came in an average of 19 per cent below the group mean. That is a big enough penalty that it is now worth someone spending an extra twenty minutes with the jig at home rather than getting a rocket onto the pad half an hour sooner.

04

The flight on the seventeenth of May

It was the second launch day of the 2026 season, wind at 3.2 metres per second from the south-west at the morning reading, which is well inside our limit and was drifting recovery gently towards the north-east corner. That is the plantation corner, and the pads had already been moved once. The airframe was one of the older ones, 96 grams, carrying an altimeter, on a middle-class motor with a five-second delay.

The boost was straight and the ejection fired late, well past apogee, with the rocket already nose-down and moving. The parachute came out, opened, and took it downwind fast while it was still high. It cleared the sixty metres of margin we keep, cleared the first row of trees, and settled somewhere in the plantation about thirty metres in. Four of us searched for an hour and twenty minutes. We found it eventually by standing still and listening for the altimeter beeping out its reading, which it does on a loop after landing, and it was about eighteen metres up a pine with the parachute wrapped round two branches.

It is still there. Nobody climbs an eighteen-metre pine for a cardboard rocket, and we did not ask the landowner to fell a tree for us. The altimeter was beeping 171 metres, which is the highest we have recorded on that motor type, because a late deployment means the rocket is not slowed at apogee. We wrote the flight up as a loss and left it.

05

What we changed afterwards

The delay was the fault, and it was a fault we had already fixed once. That airframe had gained mass over four seasons of repairs and repaints, and it was still being flown on the delay that suited it when it was new. We now reweigh every airframe at the start of the season and write the new figure on the airframe itself in pencil, not just on the build note, because the build note stays in the box and the airframe goes to the prep table.

The second change is about the plantation. Our margin was sixty metres from the pad to the treeline and that was calculated for a normal deployment at apogee. A late deployment puts the parachute out lower and much faster, which is a completely different drift problem. The margin is now a hundred metres on any day with wind from the south-west quarter, which on some days means we simply have less useful field to work with and fly fewer flights. We have lost one airframe since, to a ditch rather than a tree, and we got that one back.

The third change is that the altimeter goes in everything now, not just in the flights where somebody is curious. It is the only part of the kit that tells you where a rocket is once it is out of sight, and on the seventeenth of May it was the only reason we knew for certain where our rocket had ended up rather than wondering all summer.