JetStudio — Micro Turbojet Design Tool

Cross-section (to scale)

Design box — constraint checks

Brayton cycle — T-s diagram

The cycle's fingerprint: temperature vs entropy through the engine. Up-and-right legs cost you (compression heat, combustion), the down leg 4→5→8 pays you back. The rightward lean of the compression line is inefficiency (ideal compression would be vertical); the long orange leg is fuel heat; whatever temperature survives to leg 5→8 becomes jet velocity. A taller, wider loop = more work per kg of air.

Station conditions

The gas state at each numbered station (see the bubbles on the Design cross-section). Practical anchors: 3 is what your combustor and its o-rings must survive, 4 is the turbine material limit, 5 is what your EGT probe reads, and 8−0 pressure tells you whether the nozzle chokes.

Station	Where	T [K]	T [°C]	P [kPa]

Pressure-ratio sweep (at your TIT — dot = your design)

The fundamental trade study: raising πc buys specific thrust (blue, less air needed per newton) and fuel economy (orange TSFC falling). So why not max it out? Because compressor work rises with πc, and work = tip speed² — the RPM and wheel stress climb until the aluminum gives up. The curve also flattens: past ~3.5 you pay a lot of stress for little thrust.

TIT sweep (at your pressure ratio)

Hotter gas = more energy left after the turbine = more thrust per kg of air (blue, nearly linear). But EGT (red) tracks TIT almost 1:1 — and uncooled Inconel quits around 1050 K TIT / ~750 °C EGT. This chart is why turbine material, not aerodynamics, is the real thrust limit of a micro engine.

Throttle curve (first-order estimate, no compressor map)

Thrust vs spool speed. Note how steep it is: at 70% RPM you have only ~¼ of full thrust — this is why turbine models need momentum management on landing. Real engines also show an EGT spike during fast accels that this steady-state estimate can't capture; a proper prediction needs the compressor map, which is why it's labeled an estimate.

Sensitivity — which knob matters most?

Each parameter nudged ±3% while the rest hold still; bars show the resulting change in specific thrust. Long bars = high leverage — this is your build-effort priority list. Note how much the "boring" loss terms (intake recovery, nozzle Cv) matter: polishing a bellmouth is free thrust, while the faint mirror-image bars confirm the effect works both ways.

Energy accounting

Where the fuel's chemical power actually goes. Only a few percent becomes jet kinetic energy — the rest leaves as hot exhaust. This is normal at micro scale (low πc, small parts, big relative clearances); compare the TSFC line against a full-size engine and appreciate why real aircraft engines are big.

3D preview (drag to orbit · scroll to zoom · double-click to reset — cutaway shows the gas path)

The STL is the full (non-cutaway) engine at your exact dimensions in mm — open it in any CAD package, slicer, or Windows 3D Viewer. It's a visualization mesh of the major components, not a machining model.

Build notes / bill of materials

Part	Specification	~ cost

Rough 2026 hobby prices for bought parts and raw stock — excludes tools, fuel, and the inevitable second turbine wheel.

Export

The share link encodes your entire design in the URL — send it to a friend and they'll open JetStudio with your exact parameters loaded.

How a micro turbojet works

Intake → Compressor → Combustor → Turbine → Nozzle (0→2) (2→3) (3→4) (4→5) (5→8)

A single-spool turbojet is the Brayton cycle cast in metal: squeeze air (centrifugal compressor, pressure ratio ~1.8–4), burn kerosene in it at constant pressure (up to the turbine inlet temperature, TIT), expand it through a turbine that extracts exactly the work the compressor needs, then let the leftover pressure accelerate the gas out of a converging nozzle. Thrust = mass flow × exit velocity (plus a pressure term if the nozzle chokes).

Micro engines are thermodynamically bad on purpose: low pressure ratio and small size give ~5–10% thermal efficiency and roughly 10× the specific fuel consumption of a full-size engine. They trade efficiency for simplicity, cost, and robustness — the right trade at this scale.

The five core components

1 · Compressor — a turbocharger wheel, spun by you

The hobby standard is an automotive turbocharger compressor wheel (KKK/BorgWarner, Garrett), 54–76 mm tip diameter, cast or billet aluminum. Work scales with tip speed squared (w ≈ μ·U², μ ≈ 0.75–0.80): pressure ratio 2.2 needs ~360–400 m/s of tip speed → ~110,000 rpm on a 66 mm wheel. Behind the wheel a diffuser (wedge vanes or a drilled ring) converts swirl into pressure — the #1 home-build efficiency killer. Expect only 70–78% stage efficiency.

2 · Combustor — annular, with vaporizer sticks

An annular flame tube of 0.5 mm stainless with rows of carefully sized holes: primary zone (recirculating, near-stoichiometric flame), secondary (burnout), dilution (mix down to a TIT the turbine survives). Hobby engines don't atomize fuel — they boil it inside hooked vaporizer tubes sitting in the flame. Rules of thumb: reference velocity 15–25 m/s, residence time 4–6 ms (sets length ~80–100 mm), pressure loss 5–8%.

3 · Turbine — the part that wants to die

A ring of nozzle guide vanes accelerates the gas onto a single axial rotor of cast Inconel 713C (usable to ~900–950 °C metal temperature). There is no blade cooling at this scale, so TIT above ~1050 K means creep: blades stretch until they rub. Buy the cast wheel — it is one of two parts you should never improvise (the other is the compressor wheel).

4 · Shaft & bearings

Two angular-contact hybrid bearings (ceramic Si₃N₄ balls) with light spring preload, lubricated by ~5% turbine oil mixed into the fuel and misted through the shaft tunnel. Speed metric: DN = bore(mm) × rpm — keep under ~2.0–2.5 million. The shaft must stay below its first bending critical speed: short and stiff wins.

5 · Nozzle

A simple converging cone, often over a center bullet. At hobby pressure ratios it is just barely choked or unchoked (exit 300–450 m/s). Exit area is a live tuning knob: too big → cold and weak; too small → hot and surgy.

The design box — what limits you

Constraint	Hobby limit	If exceeded
Compressor tip speed	~450 m/s cast · ~520 billet	wheel burst
TIT (uncooled Inconel 713)	~1000–1050 K	blade creep, rub, burst
EGT continuous	600–750 °C	ECU cuts fuel; nozzle suffers
Bearing DN (fuel-mist lube)	~2.0–2.5 M mm·rpm	bearing cooks, shaft whip
π_c, one centrifugal stage	2–3 realistic, 4.5 max	tip speed the alloy can't take
Combustor loading	15–25 m/s, ≥4 ms	blowout, flame in the turbine
Shaft speed	< 1st bending mode	destructive whirl

The design loop this app automates: pick thrust → cycle gives mass flow & compressor work → wheel diameter sets rpm → check every limit → iterate.

Reference engines

Engine	Thrust	RPM	π_c	Mass flow	Claim to fame
Schreckling FD3/67	~25 N	~85k	~1.9	~0.10 kg/s	1992 original; plywood compressor
KJ66	75–92 N	110–128k	~2.2	~0.23 kg/s	THE homebuild reference, plans available
P100 class	~100 N	~150k	~2.6	~0.26 kg/s	modern commercial sport engine
P160/P180 class	160–180 N	~125k	~3+	~0.4 kg/s	2 m sport jets

Fuel, starting, control

Fuel: kerosene / Jet-A1 (43.1 MJ/kg, ~0.80 kg/L) + 4–5% turbine oil (Mobil Jet II, AeroShell 500). Budget 150–400 ml/min at full thrust for KJ66-class.
Starting: electric starter spins the rotor → ignite propane/butane pre-heat gas (or a kerostart glow vaporizer) → ramp to self-sustain (~45–60 krpm idle) → transition to kerosene. Failed starts that pool fuel cause the classic tailpipe fireball — purge before retrying.
ECU/FADEC: monitors rpm + EGT, drives the fuel pump, enforces limits (max EGT, max rpm, flameout detection, auto-cooldown). Use an off-the-shelf ECU (Xicoy, JetCat-style, Orbit); do not run open-loop needle-valve control.

Safety — non-negotiable

A turbine wheel failure at 120,000 rpm releases several kilojoules of shrapnel (see your design's rotor energy on the Build Sheet). First runs: engine bolted to a test stand, bystanders out of the rotor plane, barrier between you and the engine.
Kerosene pools burn. CO₂ or dry-powder extinguisher within arm's reach; never fuel near the hot section.
Hearing protection — >120 dB close up. The exhaust is ~700 °C for a meter behind the nozzle.
Follow your national model-flying body's turbine code of practice (e.g. the GTBA's). Check insurance.

A sensible build path

See one run first. Attend a jet meet or find a local turbine flyer.
Build from proven plans — KJ66 or a GTBA-published design. Buy the compressor wheel and the cast Inconel turbine wheel; machine the housings, shaft, diffuser, and combustor yourself.
Instrument from day one: EGT + rpm minimum, ECU strongly recommended.
Only after a proven runner, iterate your own aero: diffuser, combustor holes, NGV angles, nozzle area. Change one thing at a time; log everything.

Sources and further reading are in RESEARCH.md next to this app — including Kurt Schreckling's book (the classic), GTBA design pages, KJ66 documentation, and papers on wheel tip speeds, Inconel 713 limits, and kerosene-lubricated hybrid bearings.