Overview and status

Coming soon. RustCase is in development and not available yet. This page describes what it is designed to do; details may change before release.

What RustCase is

RustCase designs the hardware a solid rocket motor is built in: the case tube, the forward closure, the aft closure that carries the nozzle, the retention that holds the ends in, and the O-rings that seal them. It checks every part against the pressure and the heat of the burn, gives the case a rated maximum operating pressure, writes the hydrostatic proof-test procedure, and exports the parts for a machine shop with a bill of materials.

It is part of the RocketSalvo family, and it is the fourth corner of the same motor: RustPEP is the propellant, RustCore is the grain and its burn, RustStand is what happened on the test stand, and RustCase is what the motor is fired in.

RustCase gives engineering estimates for amateur and research motors. It is not a pressure-vessel code, it does not replace a hydrostatic proof test, and its results must be checked against your rocketry organization’s safety code — NFPA 1125 and 1127, and Tripoli and NAR rules. Proof test every case with water before you fly it.

Where it stands

Version 0.4.0 runs on the developer’s machine. The desktop app opens its own window and shows the Design page with the case’s rating and its checks, and the Design, Closures, Heat, Proof, Parts and BOM, Link and Help pages are all on screen. The same interface has been driven end to end in the test harness, all 61 tests pass, and FreeCAD opens every STEP part it exports.

Recent builds added a 3D view of the assembly that you can rotate and cut away, with RustCore’s grains and cores drawn inside the case, and 0.4.0 fixed a hang and about thirty bugs found in a code review.

It has not been released or installed anywhere else. There is no released installer, so nothing described here is available to anyone but its developer yet.

What it designs

A case is not a tube with ends on it. The parts set each other’s dimensions — a snap-ring groove removes wall that the burst check needs, and the screws retaining a closure have to sit where there is metal to carry them — so RustCase works the assembly as a whole.

• Sizes: 29 mm to 12 in, from presets or typed in.
• Tube: machined metal or filament-wound composite. In metal, stock sizes from an OnlineMetals catalog in 6061-T6 (the default), 7075-T6, 7075-T73 or 7050-T7451. For composite, see filament-wound composite tubes below.
• Closures: a forward closure, and an aft closure carrying the nozzle.
• Retention: radial screws, snap rings or threads in metal; radial pins or a bonded joint in composite.
• Liner: runs over a spigot on the bulkhead and on the nozzle — by default 0.25 in, 0.5 in from 75 mm, and 1 in from 6 in.
• Nozzle washer: a 304 stainless washer between a graphite nozzle and its snap ring, with its own drawing and bill-of-materials line.
• Seals: O-rings, worked from the Parker handbook.
• Units: imperial or SI.

The checks

These are the checks for a machined metal case. A filament-wound composite tube fails in a different way and is checked differently; it has its own section below.

Around forty of them, each reporting a margin rather than a pass stamp, so you can see which part is closest to the edge and by how much:

• Tube: burst and yield.
• Screws: shear, bearing, tear-out and net section. Screws can be sized automatically, in alloy steel or 18-8 stainless, imperial or metric.
• Snap rings: ratings from the Rotor Clip HO and KG series.
• Threads: thread shear.
• Closures: bending in the bulkhead, and bending at the nozzle lip.
• O-rings: squeeze, fill, stretch and extrusion gap.

Design factors default to 2.0 on ultimate and 1.5 on yield, and are yours to change.

Heat: a metal case

Heat is usually what decides a case, not pressure alone. Aluminium loses strength long before it melts, so a case that passes on pressure at room temperature can still fail late in a burn. (A composite wall is limited by its resin instead, which is covered in its own section.)

RustCase works the wall temperature through the liner as a transient, including the soak after burnout — the motor has stopped producing thrust, but the heat already in the liner is still arriving at the wall. Strength against temperature comes from the half-hour curves in MIL-HDBK-5J. The liner is then sized to keep the wall inside what the alloy can still hold.

Liner ablation and charring are not modelled. That makes the estimate err hot: it predicts a hotter wall than you will actually get, not a cooler one.

Rating and proof test

The output is a rated maximum operating pressure together with the basis it came from — which check governed, at what temperature, under what factor — rather than a bare number.

RustCase then writes the hydrostatic proof-test procedure, by default 1.5 × MEOP with water, and keeps a record of the tests a case has passed. The procedure is the point. A calculation says a case of this design should hold; a proof test is what establishes that this particular piece of metal, with its own machining and its own flaws, actually does.

Filament-wound composite tubes

Under Tube, setting Construction to Filament-wound composite designs a wound tube instead of a machined one. This is a different material failing in a different way, not the metal case with different numbers, so it has its own checks and its own limits.

Pick a fibre system — carbon/epoxy wet-wound, or E-glass/epoxy — then set the helical angle, the number of ±α helical layers and 90° hoop layers with their thickness, and the resin with its Tg. Or let RustCase choose the layup. Tick Sealed liner if a rubber bladder or a metal liner lines the composite, because that changes which failure governs.

Closures are retained by radial pins through the wall, or bonded in with a structural adhesive — EA 9394, EA 9430, DP460 or DP420. Snap rings and threads are refused for a wound tube, because cutting a groove or a thread cuts the fibres that carry the load.

What it checks

• Fibre burst, by netting theory, with fibre strength knocked down to 0.75 × measured, after Tew (1995). The test suite checks the method against two published burst-test series — Yurdakul (2022) and Karpuz (2005), both METU — and lands either on the safe side or within 15%.
• Weep, which is first-ply failure, by laminate theory with the Tsai-Wu criterion. This is the one to understand: a thin wound wall cracks in the matrix and leaks at around 40% of its burst pressure, long before the fibres break. Without a sealed liner, weep is usually what sets the rating — so a wound case is often limited by the resin letting gas through, not by the fibre strength that makes composite attractive in the first place.
• Temperature, from the resin. The wall limit is the resin’s wet Tg less 28 °C, after MIL-HDBK-17. Matrix strength falls off following Chamis’s model, and fibre strength loses up to 15% near Tg.
• Joints. Pin bearing in the composite, with at least 3 hole diameters to the end of the tube and 4 between holes, plus the net section of the fibres. Bonded joints are checked in lap shear against temperature over a Hart-Smith effective length, with a factor of 2.

Limits specific to a wound case

• Room-cure laminating epoxies are unsuitable. West System and its relatives have a Tg around 55 °C, which leaves a wall limit near 27 °C. The app says so rather than letting you design with one.
• The composite pin-bearing strength is a derived estimate, not a published allowable — 360 MPa for carbon, 200 MPa for glass — and the app labels it UNVERIFIED. If a pinned joint governs your design, that is the number to go and establish for yourself.
• Domes are not modelled. The closures are metal plugs in a straight tube.
• Also not modelled: fibre slip and thickness build-up at the tube ends, impact damage, moisture, creep and bending.
• Hand wet winding scatters. The strength coefficient of variation runs as high as 17%, against a few per cent for machined aluminium stock.

Wound cases vary from build to build far more than machined aluminium. Proof every case, and burst-test samples of each winding process.

Exports

• Geometry: STEP with the exact geometry, plus DXF for a waterjet or laser, and STL.
• Drawings: PDF, in Rocket Salvo colours.
• Bill of materials: CSV, and a web page with links to McMaster-Carr, OnlineMetals, Rocket Motor Parts and Rotor Clip.

Supplier part numbers appear only where they have been checked against that supplier’s own listing. Most McMaster lines are search links rather than a specific part, so expect to confirm what you are ordering.

What it will exchange with the other programs

RustCore tells RustCase the pressure the motor will reach and when the burn exposes the liner. RustCase tells RustCore the case bore and the pressure rating it has to design inside. RustStand records which case a firing was fired in, so a case accumulates a history of what it has been through.

RustCore 0.23 and later already do their part: they read RustCase’s cases and write case loads for it. RustCase itself is not released yet, so no one can use the exchange end to end today. The files, all under Documents\Rocket Salvo\, are:

Direction	Where	Format
RustCase writes	Cases\<id>.rcase	rocketsalvo.rustcase.design v1
RustCase reads	CaseLoads\<id>.json	rocketsalvo.case-loads v1, written by RustCore
RustCase writes	Inbox\RustCore	rocketsalvo.case-suggestion v1
RustCase reads	replies to those suggestions	rocketsalvo.case-reply v1
RustCase reads	RustStand’s firing evidence	samples.evidence.json
RustCase writes	Apps\RustCase.json	rocketsalvo.app presence

Every pressure in the design file is gauge.

What is not modelled

• Fatigue over many firings of the same case.
• Stress corrosion.
• Flight and buckling loads. The case is checked against the motor’s pressure and heat, not against what the airframe does to it.
• Liner ablation and charring, so the heat estimate errs hot.
• Stress concentrations beyond the net-section checks.

A wound composite tube has further limits of its own, listed under filament-wound composite tubes.

Price, licensing and the release date are not decided yet.