Charge

Black powder is sized so its combustion gas reaches a target pressure inside the pressurized volume, using the ideal-gas relation rearranged for mass:

m = (P · V) / (R · T)

P

Target pressure inside the section. Either entered directly, or derived from the separation force as P = F / A over the bore area A = π/4 · ID².

V

Pressurized volume, V = π/4 · ID² · length.

R = 22.16

Specific gas constant of black-powder combustion gas, 22.16 ft·lbf/(lbm·°R) — the value used across HPR references.

T = 3307

Combustion (flame) temperature, 3307 °R (≈ 1837 K).

·144

psi → lbf/ft², so pressure and volume share units.

·453.59237

pounds-mass → grams, the unit you actually weigh on a scale.

The same arithmetic the calculator just ran, with your current inputs (canonical units shown):

Volume

150.8 in³ = 0.08727 ft³

Pressure

7.64 psi × 144 = 1,100.1 lbf/ft² (from 96 lbf over 12.57 in²)

Mass

(1,100.1 × 0.08727) / (22.16 × 3307) = 0.00131 lbm

In grams

0.00131 × 453.59237 = 0.59 g

The ideal-gas method is a model, and a generous one. It assumes black powder burns completely and instantly, that all the heat goes into the gas (no loss to the bulkheads, wadding, or airframe walls), and that nothing leaks past the bulkhead, the shear path, or vent holes. Real wells violate every one of those: they lose heat and vent gas, so the pressure actually reached can be lower than the model predicts.

One loss it leaves out entirely matters most for a parachute: the chute protector and packed recovery gear soak up a large share of the combustion energy before it can build pressure. That's the dominant real-world shortfall — it's why a packed deployment can need several times this number, and why the Fetter mode sizes higher. So treat this figure as a floor to test up from, not a conservative ceiling.

It also doesn't know your particular powder, granulation, ignition, wadding, or how free your airframe really is to slide. Shear-pin forces vary by screw, supplier, and fit; friction is a guess until you feel it.

Two things the number does assume: real black powder, and that you weigh it on a scale. FFFFg (4F) is the usual ejection granulation and FFFg is fine; substitutes like Pyrodex light slower and peak lower, so they don't deploy the way this figure expects. And weigh each charge rather than dipping it by volume — density shifts with granulation, so a volume scoop drifts.

So ground-test before you fly. Build the charge and fire it remotely — from behind cover, with the airframe restrained and pointed somewhere safe, wearing eye protection, and everyone clear: it's a live pyrotechnic charge. Confirm it cleanly separates the airframe and throws the recovery gear, then adjust from what you observe — the tested charge is the real answer, and the log below is where to keep it.

A fair question, since the method is a simplification: shouldn't there be an efficiency or “derating” knob? Deliberately, no.

The constants already carry the real chemistry. R and T here come from black powder's own combustion, not from an idealized pure gas — they're the values the high-power community uses for exactly this calculation. A separate efficiency multiplier on top would double-count and imply a precision the model doesn't have.

And the real-world error runs one way. Heat lost to the bulkheads and airframe, gas that leaks past seals and vent holes, and powder that doesn't fully burn all push the same direction: real wells often need a little morethan the formula says, not less — and black powder gets less efficient at high altitude. So the only thing a “derate” dial would invite is the one dangerous move, trimming the charge down until it doesn't separate.

The honest levers are the ones already here, and they only add margin: the safety margin (it sizes the charge above your target pressure, or above the bare separation force), rounding up rather than down, and letting the ground test set the final number.

Most high-power flyers run two altimeters — a primary and a backup — each wired to its own ejection charge and its own e-match. The two fire independently: the backup is set to go a moment after the primary (apogee plus a short delay on the drogue, a lower altitude on the main), so if the primary altimeter, battery, or match fails, the backup still gets the airframe open.

The backup charge is sized a little largerthan the primary, not equal to it. The reason is the failure it's there for: if the primary already fired but didn't separate the airframe — a charge that was a touch light, a tight joint, shear pins that bound — the backup has to break free a section the first charge may have strained against. The widely-used convention, including NASA's Student Launch handbook, is to make the backup about 20% larger (or at least ~0.5 g more, whichever is greater). Some flyers go larger; that's the dial in the controls above.

Both charges still get ground-tested. The primary has to separate the airframe on its own, and so does the backup — fire each one on the bench and confirm a clean, energetic separation before you fly. Redundancy is a second chance, not a reason to skip the test.

Every value here comes from the high-power community's established references, not from anything invented for this tool. The primary ones:

The ideal-gas method, R and T

The m = (P·V)/(R·T) method and the constants (22.16 ft·lbf/(lbm·°R), 3307°R) are the values used across HPR ejection references — Ted Apke's ejection-charge method (ROL INFOcentral), and guides like HARA's How to size ejection charges.

Backup charge (+20% or 0.5 g)

The “20% larger, or at least 0.5 g, whichever is greater” backup convention follows NASA's Student Launch handbook and common club practice.

Altimeter vent ports

The one-¼″-port-per-100-in³ rule (and its area form) comes from widely-used guidance such as Vern Knowles' port-sizing write-up and the broader community. Your altimeter's own manual takes precedence.

Shear-pin forces

The nylon-screw presets are widely-cited single-shear approximations that vary by supplier and fit — starting points to verify, not authority.

These are references, not guarantees. The method is a model; the ground test is the measurement. Where a source and your own bench disagree, the bench wins.