A black-powder ejection-charge calculator for high-power rocketry, at charge.fusionspace.co.
Tell it your tube inner diameter, the length of the pressurized section, and how hard your airframe is to separate (target pressure, or shear pins plus friction), and it gives you a starting black-powder mass in grams. It shows the full formula and every constant it uses, and it keeps a log of your ground tests — because the charge that actually separated your airframe on the ground is the only number that counts.
This is a starting estimate, not gospel. Always ground-test a charge before you fly it. Black powder is an explosive; sizing, handling, and flying ejection charges is your responsibility.
Part of Fusion Space — free, polished tools for high-power rocketry. See also the HPR Motor Finder.
The page is built around one loop — size → ground-test → validate → take it to the field — and reads top to bottom in that order. A collapsible "What's in here" overview near the top lays out every feature grouped by what it's for, with a jump-link to each section, so the whole tool is discoverable at a glance.
- Size by target pressure, or by separation force — shear pins (with editable presets for common nylon screws) plus friction.
- Or by the Fetter model — Tom Fetter's research-backed method for parachute deployment, which accounts for the energy a chute protector / recovery blanket absorbs (the reason the traditional model under-predicts). The Fetter charge is shown alongside the traditional result with the ratio, so you can see which model produced which number and why they differ. Its own safety factor is built in — no separate margin is applied — and the model's envelope (chute protector assumed, no piston, sea level) is enforced in the UI. It works with single or dual deploy and redundant altimeters like the other modes: each parachute- deployment event is its own Fetter compartment, with a per-bay altitude-envelope check (a drogue fires at apogee, a main down low), and the redundant backup follows the same +20% / +0.5 g convention — as redundancy, not extra model margin.
- A safety margin in either mode: it sizes the charge above the bare separation force, or above your target pressure so a leaky real well still reaches it.
- Single and dual-deploy: separate drogue and main wells.
- Redundant altimeters: size a backup charge alongside the primary — a little larger, following the same +20% / +0.5 g convention — for the second altimeter most flyers carry.
- Altimeter vent holes: size the static sampling ports for your electronics bay by the standard one-¼″-port-per-100-in³ rule, split across the ports you plan to drill.
- Units that convert in place — diameter and length in mm or in, pressure in psi, kPa, or bar, force in lbf or N. Black powder is always reported in grams.
- The full formula, every constant, and a worked example using your own inputs, so the number is never a black box — with a references section citing where each value comes from.
- Sanity hints that flag a likely input error — a unit mix-up, the outside diameter entered as the bore, a pressure outside the usual band — without ever changing the math.
- A ground-test log kept in your browser, because the charge that actually separated the airframe on the bench is the one worth flying.
- Closes the loop: once a saved airframe has a clean ground test logged, the calculator surfaces that proven charge above the estimate — so you fly what you validated, and it warns if the setup has drifted from what you tested.
- Learns your calibration: across the clean tests you log, it shows how much bigger your real charges run than the formula — your own correction, never an auto-applied trim.
- Coaches the bench loop: after each test it suggests what to pack next (step up after a failed or partial separation, repeat to confirm after a clean one), explains the likely causes when a test doesn't separate, and marks a charge validated once it's separated cleanly twice.
- Build & ground-test card — a one-page sheet with each well's charges and a grid to record results at the bench or the field. Export as HTML, PDF, or plain text.
- Recovery report — a write-up of the configuration, the sizing rationale with the formula, and your logged ground-test results, for a cert package or a build thread. Export as HTML or PDF. Both exports are self-contained (no dependencies) and work offline.
- A dual-deploy sequence diagram so beginners can see what each charge actually does in flight, common tube-ID presets, and a field-elevation advisory for thin-air launches.
- Inputs live in the URL, so a configured calculation is a link you can share or bookmark — via the OS share sheet on mobile, or a copied link anywhere.
- Back up & restore your whole setup — saved rockets, ground-test log, and theme — as one file, so a cleared cache or a new device doesn't lose a season's work.
- Private by default: everything stays in your browser. Nothing is uploaded, there's no account or server, and no analytics or tracking of any kind.
- Installable, and works offline once loaded — launches happen where there's no signal.
- Bench mode: a full-screen, high-contrast, big-number view of the charges and test ladder for using your phone at the bench or the pad — bright sun, greasy hands. Tap a charge to start logging that test.
It's the standard ideal-gas ejection-charge method, m = (P·V)/(R·T), with the gas
constant and flame temperature every HPR reference uses. The result is a theoretical
starting point, not a guarantee — the full derivation and its assumptions are laid out in
the app under "Where the numbers come from". The physics lives in lib/charge.ts as pure
functions with tests in lib/charge.test.ts.
The Fetter model is a separate method in lib/fetter.ts, a clean-room reimplementation of
the published equations (not a copy of the spreadsheet). lib/fetter.test.ts is a fixture
that asserts the implementation reproduces Fetter's published numbers — the deployment-test
results (paper Table 16-1), the shear-pin table (Table 17-2), and the reference
spreadsheet's worked example — within a stated tolerance.
The parachute-deployment model is Tom Fetter's (NAR 15551). Only the math is reimplemented here; his paper, spreadsheet, prose, and layout are not redistributed. His research found the traditional model under-predicts the black powder a parachute needs — by 1–4× for small-to-medium high-power rockets — because a Nomex chute protector / recovery blanket absorbs much of the combustion energy. His work:
- Paper — "Using Black Powder for Parachute Deployment" (Rev 1.2), on speedmotionrockets.com.
- Talk — his NARCON-2025 presentation on the @SpeedmotionRockets channel.
As with the ideal-gas modes, the Fetter number is a starting recommendation. Ground-test in full flight configuration — chute, recovery blanket, and shock cord — and fly what you prove.
Static site built with Next.js and Tailwind, exported to plain HTML/CSS/JS. Everything runs in the browser; there is no backend.
npm install
npm run dev # local dev server
npm run build # static export to ./out
npm test # run the unit tests
npm run lint # lint
Hosted on Cloudflare Pages as a fully static site. Build command npm run build, output
directory out. No Functions, no server-side code.
Personal, non-commercial project — not affiliated with any rocketry vendor or manufacturer. Built for the hobby rocketry community.
Released under the MIT License — fork it, modify it, deploy your own copy, no attribution required.