Field day / ref 24-003
Wind limits, range distances and how a launch day actually runs
A launch day at our field is mostly waiting, walking and writing. The flying takes about four seconds a time. The procedure below is the one we have run since 2019, adjusted twice, and it exists so that nobody has to make a judgement call under social pressure with fourteen people watching.
01
The field, and why the geometry matters
The ground we use is roughly four hundred metres by two hundred and sixty, bounded by a farm track on the south side, a shallow drainage ditch on the west, open ploughed land to the north, and a pine plantation along the north-east corner. The trees are about eighteen metres tall and they are the reason half our rules exist. Everything about the day is arranged to put distance between the rockets and those trees, because a rocket in a pine plantation is a rocket that takes two hours to find or is never found at all.
Pads go near the south-west corner and we move them as the wind changes rather than leaving them where they were set up at nine in the morning. The rule is simple: the rockets drift downwind, so the pads go upwind, and if that means carrying the pad box a hundred and fifty metres after lunch then that is what the afternoon starts with. In four seasons of doing this we have lost two airframes to the plantation. In the two seasons before it we lost five.
02
Distances we measure rather than estimate
The club code we fly under sets a minimum distance between the pad and anybody not actively launching, and for the motor classes we use that comes out at fifteen metres for the smallest motors and thirty for the larger low-power ones. We do not estimate these. There is a length of builder's line in the pad box with knots tied at fifteen and thirty metres, and the spectator line gets laid out with it at the start of every day, marked with four plastic cones.
The other distances are about the surroundings rather than the people. The pads sit at least sixty metres from the plantation edge, the firing panel sits behind the spectator line with the person operating it facing the pad, and the safety key that arms the panel lives in the range officer's pocket and goes into the panel only when the range is called closed. There is nothing clever in any of this. It is entirely a matter of doing it the same way every time so that no individual has to decide in the moment.
Checklist as it stands
- Spectator line laid with a knotted line, not paced out
- Minimum sixty metres between any pad and the plantation edge
- Firing panel behind the line, operator facing the pad
- Arming key carried by the range officer, fitted only at the count
- A single loud call to close the range, and a single call to open it
03
The wind limit, and the day we tested it
Our limit is six metres per second sustained at head height, measured with a hand anemometer at the pad, and we do not fly above it. Below that there is a second rule: above four metres per second, nothing lighter than 90 grams goes up, because light airframes weathercock badly and then drift a long way. The measurement is taken at the start of the day, again after lunch, and any time the range officer thinks it has changed.
In May 2022 we drove out to the field on a forecast of four and found seven gusting nine. Eleven people had given up a Saturday, three of them had driven from the far side of the city, and two children had built their first rockets that week. We stood about for an hour, took three more readings, and packed up without flying. It was a thoroughly miserable morning and it is the single most useful thing the club has ever done, because since then nobody has argued with the anemometer. A limit you have kept once under pressure is a limit; a limit you have never tested is a preference.
04
How a flight actually goes
Preparation happens at a table well away from the pads: motor fitted, wadding in, recovery gear packed, card filled in and signed. The card carries the airframe name, its mass, the motor code, the delay, the recovery device, the wind reading and the name of the person who prepped it. The rocket and its card are then carried together to the range officer, who looks the airframe over, checks the balance point mark, and either accepts it or sends it back.
From there it is procedural. The rocket goes on the rod, the leads are clipped on with the panel unarmed, everyone walks back behind the line, the range is called closed, the key goes in, and the count runs down out loud from five. After the flight the range stays closed until the rocket is on the ground and visibly still, and the range officer calls it open before anyone walks. Then the card gets the apogee estimate, the altimeter reading if it was carrying one, the recovery result, and anything unusual. The card is finished before the next rocket goes on the pad.
05
What the log is worth afterwards
We have 640 cards from 2017 onwards, in four ring binders, and they answer arguments that would otherwise go round the room forever. Whether a particular kit really does fly higher on one motor type than another. Whether our recovery failure rate is going up or down. Which member has flown the most flights, which is a question that comes up at the autumn meeting every year without fail.
The most useful thing in there is the weather column. Nine launch days a season is not many, and after a few years you can see exactly which weekends in the year are reliably flyable in Pirkanmaa. Late April is usually too wet underfoot to park. The first half of June has given us our best conditions five years running. August is thundery more often than we expect it to be. We now set the season calendar off the log rather than off optimism, and we have cancelled two fewer days a year since we started doing that.
Altitudes are barometric readings off the airframe unless the entry says they were estimated from the ground. Masses are measured empty on the club scale. Where a flight failed, the failure is recorded as it happened and the card number stays with it.