Mounting

This page is where the claim that oApogee attaches to any model rocket either becomes true or does not. It covers two form factors, the vent holes without which the barometer is useless, and the stability check that keeps the rocket flying straight with a payload on it.

Two form factors

Form A: the internal sled

A printed sled that fits inside a payload bay, sized for a 24 mm coupler or a BT-50 tube.

Use this if your rocket already has a payload section. It is the lighter and lower-drag option, it does not change the outside of the airframe at all, and it puts the payload on the vehicle centreline where its mass has the least awkward effect.

Its limitation is that it only works on rockets that have somewhere to put it.

Form B: the external pod

A streamlined printed fairing that straps or tapes to the outside of the body tube, in the manner of a rail button pod.

This is the one that makes the "any rocket" claim true. It requires no modification to the airframe, it works on a rocket you already own and like, and it comes off again afterwards. Nothing gets drilled, cut, or glued to a rocket you care about.

It also costs you something, and this page is the wrong place to be shy about it. A pod on the outside adds drag, adds asymmetric drag, and moves both the centre of gravity and the centre of pressure in ways that depend entirely on where you put it. On a small light rocket that is not a rounding error. "No modification to the airframe" is not the same as "no effect on the rocket."

Unverifiedmeasure the altitude penalty of the pod against the same rocket and motor flown without it, several times each, and publish both the mean and the spread. This is the number that decides whether the pod is worth it and it should come from flights rather than from simulation alone.

Printed files

Needs hardwarethe models do not exist yet. When they do, publish the parametric source rather than only exported STLs, in a format that can be opened and edited without a commercial licence. The pod is parametric on tube diameter, so a builder with an unusual airframe changes one number rather than asking for a new file.

The parameters that must be exposed: body tube outside diameter, board outline, cell dimensions, static port positions, and strap slot width.

Static ports

A barometric sensor inside a sealed tube measures the tube. It produces a smooth, confident, entirely fictional altitude curve, which is worse than an obvious failure because nothing about it looks wrong.

Static ports are the holes that let the sensor see outside air. Getting them right is the difference between a flight profile and a fiction.

The rules

1. More than one hole, evenly spaced around the circumference. A single hole makes the reading depend on which way that hole faces relative to the airflow. Three or four spaced evenly average out yaw and roll.
2. Away from discontinuities. Keep them clear of the nose cone shoulder, fin roots, and any step in the airframe. Air near those places is disturbed by the airframe, and disturbed air is not ambient.
3. Beside the sensor, not far from it. The volume the sensor shares with the ports should be small and directly connected.
4. Deburr them. A raised lip around a drilled hole trips the airflow over it, which is exactly the thing the port exists to avoid. Take the burr off with a knife or a larger drill bit turned by hand.
5. Check them on every flight day. Paint, glue, tape and swarf all block ports, and a blocked port is on the preflight checklist for that reason.

Sizing

Unverifiedpublish the port diameter and count oApogee recommends, along with the reasoning that produced them. The rule of thumb in hobby rocketry relates total vent area to the enclosed volume, and this site should state the specific rule it is using, cite where it comes from, and show the arithmetic for the standard sled and pod volumes rather than quoting a number with no derivation. Then confirm it by flying the same rocket with two different port configurations and comparing.

Until that is measured, this page will not print a number. A confident wrong port size produces confident wrong altitudes for everybody who follows it.

Stability, which is a safety issue

Re-simulate your rocket in OpenRocket with the payload mass and position included, before you fly it.

Not after. Not on the second flight. A rocket that has flown well ten times tells you nothing about how it flies with a payload in a new place, because the thing that makes a rocket stable is the relationship between two points that you have just moved.

A rocket is stable when its centre of pressure sits behind its centre of gravity. Adding mass moves the centre of gravity, and which way depends entirely on where the payload goes relative to the existing balance point: forward of it moves the centre of gravity forward, aft of it moves it backwards. Too far back and the rocket is unstable, which does not mean it flies badly, it means it flies sideways.

Too far forward has a cost too. A large stability margin makes the rocket weathercock hard into the wind, which loses altitude and can send it downrange over people.

The external pod makes this easier to get wrong than the sled, because the pod can be strapped anywhere along the tube and the convenient place is not always the right one.

What to aim for

Unverifiedstate the caliber margin oApogee recommends, quoted from NAR or Tripoli guidance with a citation rather than asserted from memory. This is a safety number and it does not get a rule of thumb from an anonymous website.

A worked example

Unverifiedwork a complete example. Take a named, commonly available kit, state its published stability margin unloaded on a specific motor, then show the same rocket in OpenRocket with an oApogee pod at two different positions, one sensible and one badly chosen, with the resulting margins and what each means. Include the OpenRocket file so a reader can open it rather than trusting a screenshot.

Mass budget

Every gram you add is altitude you do not get. How much depends on the motor: the smaller the motor, the more a fixed payload mass costs you as a fraction.

Unverifiedpublish a table of apogee against payload mass for A, B, C, D and E motors on a representative airframe, generated from OpenRocket and checked against at least one real flight per motor class. A reader deciding between Solo and Track deserves to see what the extra mass costs them before they order.

The qualitative shape, which is safe to state without measuring: a payload that is a small fraction of the total liftoff mass costs little; one that is a significant fraction costs a lot; and on the smallest motors, a payload can be a large enough fraction that the flight is not worth doing. That last case is real and it is why Solo exists.

Why the hardware is bright yellow

The board soldermask and the printed pod are hi-vis yellow or orange. This is a recovery decision, not a styling one.

A payload that comes off the rocket, or a rocket that lands in tall grass, is found by eye. A bright object in a green or brown field is visible from many times the distance a black or white one is, and the walk to recover it is shorter and more likely to succeed. Every recovery aid on this board exists because a payload you cannot find is a payload whose data you do not have.

Print your pod in the brightest filament you own. Do not print it black because it looks better on the rocket.

Securing the payload

Two things must not move: the board, and the cell.

The board moving means joints flex under boost, and a flexing joint fails at the worst moment. The cell moving is worse: a lithium cell that shifts under acceleration and lands on a solder joint is a short circuit inside a sealed plastic box.

Route the wires so that closing the enclosure cannot pinch them. Then shake the closed assembly hard and listen. Anything you can hear moving in your hand moves far harder under boost, where the acceleration is many times what your arm can produce.

Fitting to the rail

Check that the rocket still slides freely on the launch rail with the pod fitted. An external pod is the most common thing to catch on a rail button or a launch lug, and discovering it at the pad with a queue behind you is a bad time to find out.