Recovery · ejection charge sizing
Sizes the FFFFg (4F) ejection charge for a parachute bay using a force‑first model — it solves for the force needed to shear pins and eject the recovery laundry with authority, then derives the pressure and powder mass. This corrects the main weakness of fixed‑pressure calculators, which under‑charge small airframes and over‑charge large ones. Always treat the result as a ground‑test starting point.
Offline — showing the copy this device saved. The numbers are the model as of the version stamped above; reconnect to be sure it is current.
Inputs
Enter the bay you are pressurizing. Measure length from the rear bulkhead to the point of separation.
These are typical inner diameters — the nominal name is not the ID. A “4-inch” airframe is about 3.9″ inside, a “54 mm” tube about 2.15″. Wall thickness varies by maker, so confirm against your actual tube. Picking one also auto-fills the rocket and nose-cone masses below with typical averages for that size. Full table in the Reference tab.
Part of your rocket’s physical characteristics, alongside diameter and length. Picking an airframe from the dropdown above auto-fills these with typical averages for that size — so a forgotten field won’t skew the velocity; edit them to your actual build. They do not change the charge — per Fetter, mass sets how fast the two halves separate, not whether they do — but they drive the ejection velocity shown in the safety-factor panel below, and Auto-tune uses them. The same 11.67″ nose can be <5 lb in plastic or >25 lb in filament-wound glass, so don’t guess low.
How far the piston travels before the sections separate — usually the shoulder or coupler overlap inside the airframe. The default assumes 1 caliber (your airframe diameter), so this tracks the diameter automatically; measured a different travel? Just type it in. This is the distance the ejection force acts over, so it sets the ejection velocity shown in the result (a longer stroke gives the payload more room to accelerate). It does not change the recommended charge — that is sized on the bay volume.
This is the single biggest variable in Fetter's testing. The chute and chute‑protector soak up most of the charge's heat — a tube stuffed full can need 10× the powder of an empty one. 0 = empty tube (piston/no laundry), 1 = packed solid. The charge fires into the laundry here (worst case); a chute parked at the far end needs less.
Per‑pin shear force = π/4 × minor-dia² × nylon shear strength (Fetter §6, confirmed by Damerau lab tests). Editable. Use at least 2 pins. Friction‑fit, no pins? Set the count to 0 and rely on the nose‑cone friction below — the charge can then be very small. Never torque pins (clamping friction can double the charge needed).
The force needed to overcome the grip/friction holding the nose or coupler in — a slip fit is ~1–2 lbf, a snug fit more. Added to the pin force. This is separate from packing: packing factor accounts for heat absorption, not pull-out friction (per Fetter). Default ~2 lbf.
The black powder generates this much force above the shear‑plus‑friction minimum. It covers black‑powder variability and sets how energetic the deployment is — a higher safety factor raises the ejection velocity. Fetter recommends starting at 40% and increasing until the velocity is in the 20–50 ft/s energetic band. Auto-tune does this for you (it never drops below Fetter’s 40%); drag the slider any time to override, and Auto-tune will re-apply when you change pins, friction or masses. Changing the shoulder length updates the velocity reading but never the charge.
Classic rule of thumb: piston 8–10 · friction‑fit 10–13 · shear pins / tight 14–20 psi. Larger airframes can use the low end because force = pressure × bulkhead area.
The safety factor — the main deployment-velocity control — lives in the main panel above, next to the velocity readout.
Charge at the rear bulkhead firing forward into the bay — the usual arrangement.
Real high-altitude performance is worse than even this predicts — use a long (3″+) charge canister to burn the powder completely. Fetter’s own calc is not intended above ~20k ft.
Air temperature at deployment. Colder air is denser (slightly more to heat); minor effect.
A piston isolates the chute from the hot gas, so less energy is lost — but the shock cord coiled inside still absorbs some. Beyond Fetter’s published calc (piston not yet included there). Lower = piston needs less powder.
Optional additional cushion on top of the safety factor, shown as a second “work up to” figure. Default 0, because the 40% safety factor already covers black-powder variability. Raise only if you want a deliberately hot deployment.
Computes the pressure and bulkhead force this charge produces in the bay above, and how many shear pins it could reliably break.
How to use this calculator
A two-minute orientation to the workflow, every setting, and how to read the result.
It sizes the FFFFg black-powder ejection charge for one parachute bay. Instead of guessing a pressure like most calculators, it works from the force the charge must deliver — enough to shear any pins and overcome the nose-cone friction, scaled by a safety factor — then derives the pressure and powder mass for your tube diameter and packing. It also reports the resulting separation velocity so you can confirm the deploy is energetic. The output is a starting point for ground testing, never a guaranteed flight charge.
Recommended charge (big orange figure) — the powder mass that produces Fetter’s required force, (pin shear + nose friction) × (1 + safety factor), using his conservative packed-against-the-charge heat absorption for your bay and packing. It already includes the 40% safety factor, which covers black-powder variability and ensures an energetic deploy. It is still a bench starting point — ground test in the flight configuration before you fly.
Ejection velocity (teal figure) — how fast the two halves separate, computed from the required force acting over the nose-cone shoulder length against the rocket and nose-cone masses. Fetter calls 20–50 ft/s energetic for a medium rocket. This is a diagnostic: it does not change the charge. A heavier rocket separates slower but carries more momentum, so a lower velocity is usually fine; if it reads low, raise the safety factor. Leave the masses blank and the charge still computes — you just won’t get the velocity check.
The stat grid shows the working numbers (bulkhead area, bay volume, peak pressure, required force, heat absorbed, total pin force, nose friction, and ambient pressure). The cross-check bars compare this tool’s Fetter-enhanced result against the classic fixed-15-psi m=PV/RT formula and the D²·L·0.006 rule of thumb. Warnings flag a very small charge (friction-fit, no pins), an ejection velocity outside the energetic 20–50 ft/s range, the piston pre-move spike, head-end and altitude caveats, unusually high pressure, a charge large enough to threaten an unreinforced phenolic airframe, and inputs that sit outside the sizes Fetter actually tested.
Ground test before you fly. No calculator can know your exact friction, leakage, packing, or powder lot. Treat every number here as a smart starting estimate, confirm it on the bench, and fly with margin. The Ground-test protocol tab walks through it step by step.
(pin shear + nose-cone friction) × (1 + safety factor) — nose-cone friction is a small explicit input, separate from packing (which is energy absorption only). The old g × nose-weight ejection authority is removed: rocket mass now drives a reported separation velocity (energetic = 20–50 ft/s), not the charge. The pressure floor, free-volume subtraction and the v2.0.1 area-scaling pull term are gone (none are in Fetter’s model). Fixes the small-airframe over-charge: a 24 mm tube now reads a sane ~0.03 g friction-fit instead of ~3.9 g. Charge varies gently down with diameter at fixed pins, matching Fetter’s Section 17.2026-06-22How to dial in the real number
The calculated charge gets you close. Ground testing finds the true minimum for your build; you then fly above it.
| Pin | Fetter calc | Damerau meas. |
|---|---|---|
| #2 / M2 nylon | ~27 lbf | ~21–25 |
| #2-56 nylon | ~31 lbf | — |
| #4-40 nylon | ~50 lbf | ~38–40 |
| #6-32 nylon | ~75 lbf | — |
| #8-32 nylon | ~119 lbf | — |
| 1/16″ styrene rod | ~12 lbf | ~½ of #2 |
| 1/8″ styrene rod | ~45 lbf | ≈ #4-40 |
Fetter (§6) computes per-pin force as π/4 × minor-dia² × shear strength, nylon ≈ 9,600 psi. Damerau’s lab tests run a touch lower for #4 (nylon creeps; measured strength drops slightly as you add pins, 2→4). Values scatter with lot and material — fiberglass shears cleaner than phenolic, round rod more repeatably than threaded screws. Use as defaults, then confirm by ground test. Min 2 pins (one pin can cock and jam); never torque them (clamping friction can double the charge needed).
| Packing | Loose/far | Against charge |
|---|---|---|
| 0.25 — loose | ~39% | ~64% |
| 0.50 — half | ~65% | ~85% |
| 0.70 — snug | ~79% | ~91% |
| 1.00 — stuffed | ~94% | ~94% |
The chute and chute-protector soak up most of the charge’s heat — this is the single biggest driver of how much powder you need (Fetter measured up to a 10:1 range from empty to stuffed). The calculator recommends the conservative “against charge” column; the “loose/far” column is the optimistic edge of the band.
| Setup | Typical psi |
|---|---|
| Piston (sealed) | 8–10 |
| Friction fit, snug | 10–13 |
| Shear pins / tight | 14–20 |
| Pressure | C factor |
|---|---|
| 5 psi | 0.002 |
| 10 psi | 0.004 |
| 15 psi | 0.006 |
| 18 psi | 0.0072 |
| 20 psi | 0.008 |
D = diameter (in), L = bay length (in). This is just m = PV/RT with the cylinder volume folded in.
| Airframe | ID (in) | ID (mm) | OD (in) | OD (mm) |
|---|---|---|---|---|
| 24 mm | 0.945 | 24.0 | 1.045 | 26.5 |
| 29 mm | 1.145 | 29.1 | 1.255 | 31.9 |
| 38 mm | 1.520 | 38.6 | 1.645 | 41.8 |
| 54 mm | 2.152 | 54.7 | 2.232–2.277 | 56.7–57.8 |
| 2.5 inch | 2.560 | 65.0 | 2.685 | 68.2 |
| 3 inch | 3.000 | 76.2 | 3.125 | 79.4 |
| 3.5 inch | 3.400 | 86.4 | 3.525 | 89.5 |
| 4 inch | 3.900 | 99.1 | 4.025 | 102.2 |
| 5 inch | 5.000 | 127.0 | 5.150 | 130.8 |
| 6 inch | 6.000 | 152.4 | 6.170 | 156.7 |
| 7 inch | 6.810 | 173.0 | 7.000 | 177.8 |
| 7.5 inch | 7.518 | 191.0 | 7.708 | 195.8 |
| 8 inch | 7.815 | 198.5 | 8.005 | 203.3 |
| 9 inch | 8.780 | 223.0 | 9.005 | 228.7 |
| 11.67 inch | 11.410 | 289.8 | 11.670 | 296.4 |
Representative values — wall thickness (and therefore ID) varies by manufacturer and layup, and 54 mm comes in thin- and standard-wall versions with the same ID. Always measure your own tube. The diameter dropdown in the calculator uses the ID column.
Older calculators (this one included, through v1.8) used the ideal-gas law on the combustion gas alone: m = P×V/(R×T). That correctly predicts the final, cooled pressure — but it under-predicts the peak pressure that actually does the ejecting by roughly 4×. The reason, in Fetter’s words: for a mostly empty bay, most of the pressure comes from the combustion heat warming the air already in the tube, not from the extra moles of gas the reaction adds. That thermal spike is most of your deployment pressure, and the old calculators miss it.
The charge gas is generated by Chevreuil’s reaction:
2 KNO₃ + S + 3 C → K₂S + 3 CO₂ + N₂
The peak gauge pressure is P = (n_gas + n_air)·R·T_mix / V − P_atm, where T_mix rises by the combustion heat not absorbed by the recovery laundry. This tool reproduces Fetter’s calculator spreadsheet exactly: his default case (3″ × 15″, 2× 2-56 pins, 2 lbf friction, packing 1.0, 40% safety factor) returns 2.08 g at 12.7 psi with a 54.8 ft/s ejection velocity — matching his sheet to three figures. The combustion physics also validates against his bench data: 0.5 g in an empty 150 in³ chamber → 23.8 psi peak (his figure 23.75).
Fetter’s key finding: the chute and chute-protector absorb most of the combustion heat — up to ~94% when packed against the charge — so a stuffed tube can need many times the powder of an empty one. The packing factor models exactly this energy absorption, using his empirical curve A = 0.951·(1 − e−4.491·Pf). Per Fetter, this is only about energy absorption — it is not the force needed to pull the chute out against friction. That pull-out friction is a separate, explicit input (nosecone friction, below). An optional sensitivity band in Advanced shows the smaller charge if the chute sits loose and far from the charge; the headline figure uses Fetter’s conservative “packed against the charge” absorption.
What has to happen is a force on the bulkhead, F = P × (π/4) D². Fetter’s required force is simply:
F_required = (F_shear + F_friction) × (1 + safety factor)
— the pin shear force plus the nosecone pull-out friction, scaled up by the safety factor (default 0.4). The tool sets that force, divides by bulkhead area for the target pressure, and inverts the enhanced model for the charge. There is no nose-weight term and no pressure floor in the charge calculation: per Fetter, the rocket’s mass does not decide whether it separates, only how fast.
Because force scales with diameter², the same shear/friction force on a bigger bulkhead is a lower pressure — so for identical pins and friction, a wider tube actually needs the same or slightly less powder, not more (Fetter’s Section 17). Bigger rockets need bigger charges in practice because they use more and larger pins and pack more laundry, not because diameter alone demands it.
The safety factor does double duty: it covers the large run-to-run variability of black powder, and it ensures an energetic deployment. The tool reports the resulting ejection velocity — how fast the two halves separate — from the required force acting over the nosecone shoulder length against the nosecone and body masses (Fetter’s spreadsheet formula; not in the paper). Fetter calls 20–50 ft/s energetic for a medium rocket. A heavier rocket deploys slower but carries more momentum, so a lower velocity is usually fine; if the velocity reads low, raise the safety factor. This figure is a diagnostic — it does not change the charge.
These three are this tool’s own extensions; Fetter’s spreadsheet does not yet cover them. A piston isolates the chute from the gas, modeled as the direct case with reduced absorption (0.75 by default; Fetter’s own fit to his piston tests was 0.70, so this runs deliberately a little rich — the slider in Advanced will take 0.70 if you want to match his measurements). Note the pre-move pressure spike is far higher than the working figure. Head-end deploy fires down the full body column with laundry in the gas path; the charge maths is the same as aft, since the worst-case absorption curve is already the default — what differs is the need for a long canister.
Altitude is the extension that changed most recently, and it is worth understanding why. Thin air carries less of the combustion heat into pressure, and the enhanced model does not capture how sharply that collapses: at 3.3 psia (roughly 39,000 ft) Fetter measured 10.21 psi where the undamped model predicts about twice that, and with a parachute filling the tube he measured barely more than the pressure the added gas alone would make. Sizing from the undamped model therefore under-charges at altitude. This tool now credits only part of the heat term as ambient pressure falls, calibrated to that 10.21 psi measurement, and above ~20,000 ft — where Fetter states his calculator is no longer intended to be used — blends toward sizing on the added gas alone, which is his own stated prescription for high flights. Sea-level results are unchanged. All of this rests on two data points: treat it as a floor to ground test against, not a number to trust, and use a long (3″+) charge canister so the powder burns completely. Above ~45,000 ft black powder struggles to sustain a burn at all and a compressed-gas system is the usual answer.
Real combustion efficiency varies with confinement, grain size and lot; gas leaks past couplers; friction and binding are build-specific; nylon pins creep. That irreducible uncertainty is exactly why every experienced flier ground tests and flies with margin — and why the headline number here is labeled a starting point, not an answer.
Primary source for the enhanced model: Thomas B. Fetter (NAR 15551), “Using Black Powder for Parachute Deployment,” presented at NARCON 2025 (paper Rev 1.2), and his Black Powder Calculator spreadsheet, Rev 1.3 — available at Speedmotionrockets.com or the NAR Archive. This tool reproduces that spreadsheet’s charge, pressure and ejection-velocity outputs, and uses its Chevreuil-equation combustion model, the empirical packing/absorption curve, and the shear-pin shear-strength tables (cross-checked against Damerau’s lab measurements). The classic m = PV/RT cross-check matches Chuck Pierce’s 2001 spreadsheet.