Hybrid motors · version 2.0
Hybrid rocket motor design
The app sizes the fuel grain, nozzle and oxidizer feed from an average thrust, burn time and chamber pressure, then solves the burn with a published regression law. With a nitrous oxide tank it follows the tank pressure through the burn, including after the liquid is gone.
Windows 11 (64-bit) and Apple silicon Macs (macOS 14 or later), version 2.0.1. The Mac app is signed and notarized; the Windows installer is not code-signed.
What it computes
- Sizing
- From average thrust, burn time and chamber pressure at ignition (and O/F, if you set one), the app proposes the grain, throat, expansion ratio and, for a tank feed, the tank and injector, then solves the motor.
- Oxidizer feed
- A constant oxidizer flow; a self-pressurizing nitrous oxide tank in blowdown, including the vapour phase; a regulated gaseous oxygen supply; or liquid nitrous oxide held up by a pressurant.
- Regression
- Port-wall regression r = a·Goxn against oxidizer mass flux, integrated through the burn as the port opens. Each law keeps the oxidizer and fuel it was measured with, its tested flux range, and its source.
- Chamber
- Chamber pressure from the total flow and the throat, with c*, flame temperature, ratio of specific heats and molecular mass from thermochemistry tables against O/F and chamber pressure. The O/F shift through the burn is shown.
- Ignition and tail-off
- 0-D filling at ignition and emptying after the valve closes.
What you enter
- Targets. Average thrust, burn time, chamber pressure at ignition, an optional O/F averaged over the burn, and the grain outer diameter.
- Oxidizer and fuel. The pair selects the thermochemistry table. Fuel density carries its source, or use your own.
- Regression law. A built-in published law or your own: a, n, the units of a, the tested oxidizer flux range, the source and the test conditions. The page plots each published law for the pair over the flux it was tested at.
- Grain, feed and nozzle. Grain outer diameter, port diameter and length; the feed with its tank, regulator or flow; throat, expansion ratio and ambient pressure.
What you get
The Review page shows thrust, chamber pressure and tank pressure through the burn, with O/F, regression rate and port diameter; a burn summary (motor class, burn time, ignition time, tail-off after the valve closes, total impulse); the instant of peak chamber pressure; and every warning. The PDF report carries the same values with the inputs and their sources.
Files
.eng- Thrust curve in the RASP format that flight simulators read.
.csv- Thrust, pressures, O/F and the other burn variables against time.
.dxf- Motor section drawing for CAD.
.pdf- Report: inputs with their sources, the solved motor against its targets, curves and warnings.
.png- The page's charts and drawings.
- Imports
- Projects (
.rplproj).
Limits
- Regression laws are burn-averaged fits, and their authors advise verifying the regression rate for your own fuel and motor by test. Laws measured by different groups for the same pair can differ by tens of percent at the same oxidizer flux.
- One circular port; the fuel grain's end faces do not burn.
- Chamber thermochemistry comes from tables against O/F and chamber pressure; ignition and tail-off are 0-D.
- Not modelled: combustion instability, injector atomization, and heat transfer to the case and nozzle.
Trial and licenses
The 30-day trial runs hybrid motors, with a watermarked PDF report. Saving projects, DXF export, and the flow solutions beyond the steady nozzle preview need a license. Personal ($10/month or $99/year, noncommercial work) and Business ($29/month or $290/year, commercial work) both include the whole application: liquid engines, solid and hybrid motors, vehicle and flight, and all of the CFD, with no plan-tier mesh cap. See pricing.