Turbojet Airframe Integration Checklist
Inlet path, engine-face conditions, bay clearance, mass and balance, fuel, controls and access: the installation questions concept design must answer.
A small turbojet that closes beautifully on the cycle deck can still fail inside an airframe: starved by its inlet, cooked by its own bay, or bolted to a station the balance cannot tolerate. Installation is where propulsion meets aircraft, and it repays early paranoia. This checklist walks the installation in airflow order, then separates what concept design can settle from what legitimately waits for detailed engineering.
The running example is the Orion-T1, because it is the case where we did this to ourselves: a 165.2 kg turbojet closed first, then an airframe closed around it. The build story tells that program; this article is the checklist version.
The inlet path
Size the capture area to the engine's mass flow across the envelope, not just at static conditions. Orion swallows 16.65 kg/s at sea level and 7.08 kg/s at 10 km and Mach 0.70; the duct has to serve both ends of that range without choking the engine at one or separating at the other. Every bend in the path costs total pressure and adds distortion, and the boundary layer on the fuselage ahead of the inlet is either diverted, bled, or ingested. Ignoring it is a decision to ingest it.
The T1 answer was a blended chin inlet feeding a hollow, area-scheduled S-duct up and over the internal volume to an aft-mounted engine. S-ducts earn their reputation: each offset is another chance to hand the compressor a flow field it cannot swallow. If your layout has bends, the duct is a design problem in its own right, not plumbing, which is why it appears in the propulsion trade study long before it appears in a drawing.
Engine-face conditions
What the compressor face actually sees (the pressure field at the aerodynamic interface plane) sets surge margin and stability, and it is set by the airframe as much as by the engine. The industry's framework for this is SAE's AIR1419, which treats inlet total-pressure distortion at the interface plane as the primary inlet/engine compatibility problem.
At concept stage you can and should declare two things: a compressor-face Mach target (Orion's published points solve at a face Mach of 0.450 to 0.501) and the duct architecture that delivers it. Quantifying distortion under maneuver (high angle of attack, sideslip, gust response) belongs to CFD and eventually test. What you must not do is leave the interface plane undefined until both sides have built to different assumptions.
Bay clearance and thermal environment
Mechanical clearance is checkable in CAD at concept stage, and it should be checked, not estimated: the T1 installation closed at 15.55 mm of radial engine-bay clearance, verified against the sealed geometry of both the engine and the airframe. That number exists because both sides of the interface came out of the same pipeline.
Thermal is the harder half. A turbojet bay absorbs heat soak after shutdown, radiates through the casing during the run, and shares volume with structure, fuel and wiring that all have temperature limits. Concept design allocates bay volume and ventilation airflow; the detailed thermal model comes later. What kills programs is discovering at detailed design that the clearance that fit mechanically cannot survive thermally.
Mass and balance
A 165 kg engine in a five-metre airframe sets the architecture. Mounting station sets the empty CG, and fuel burn then drags the CG through its flight-long travel. The T1 closes at 696.5 kg MTOW balancing at 49.0% of length with a 0.130 MAC static margin, with 155.5 kg of fuel aboard, all of it forward or aft of somewhere that matters.
Check balance at both ends of the fuel state, with the engine in and out, before you fall in love with a layout. An installation that balances at gross weight and runs out of trim authority dry is a layout error discovered late.
Fuel system
Declare the fuel type in the brief and let the system follow: kerosene-class fuels dominate this engine class, and the logistics argument for them is made in the propulsion comparison. The installation questions are pump-inlet head pressure across the attitude and altitude envelope, negative-g tolerance, vapor behavior at altitude, and feed for relight. None of these are exotic; all of them are unforgiving if nobody owned them early.
Controls and starting
A small single-spool turbojet has its own transient character: thrust lags the lever while the rotor accelerates, and the autopilot or the mission profile has to be designed around the lag rather than surprised by it. Define the control interface (what the airframe commands, what the engine limits) and the starting concept. If the aircraft is air-launched, the windmilling or assisted relight envelope is a requirement with corners, not a feature to be demonstrated someday.
Hot end and nozzle
The exhaust path decides what the tail of the aircraft is made of. Nozzle exit station relative to tail surfaces, jet effects on stability and structure, and the thermal environment aft of the engine are layout questions with long shadows. The T1 publishes its nozzle exit station at x = 4,280 mm, an example of the discipline itself: the exhaust station is stated explicitly, so thermal, structural and stability checks all have a fixed interface to work against, and it was fixed at concept stage, when moving it was still cheap. Actuation, cooling and detailed materials downstream of the turbine belong to detailed design; where the hot gas goes does not.
Access and turnaround
How does the engine come out, what is inspected in place, and how many people and hours stand between sorties? Unglamorous, and routinely the difference between a demonstrator and a fleet. A bay that requires half the airframe to come apart for a borescope inspection is a maintenance concept failure wearing an engineering costume.
What concept design settles, and what waits
| Question | Concept design answers | Later engineering answers |
|---|---|---|
| Duct architecture and inlet placement | Yes: layout, capture sizing, face-Mach target | Detailed duct CFD and distortion test |
| Engine-face conditions | Target and interface definition | Distortion quantification under maneuver |
| Bay clearance | Yes: verified in CAD against sealed geometry | Thermal soak and ventilation analysis |
| CG and static margin | Yes: across fuel states | Loads, flutter, structural sizing |
| Fuel type and capacity | Yes | Pump, valve and component selection |
| Nozzle exit station | Yes | Actuation, cooling, local materials |
| Mass budget | Yes: component-level with declared bases | Weighed article and growth tracking |
| Access concept | Yes: removal path and inspection provisions | Panel design and ground-support equipment |
The pattern in the table is the division of labor: concept design fixes geometry, stations, targets and budgets (the things that are expensive to change later) and defers analysis depth, not decisions. That is the scope of the engine-airframe work described under propulsion and integration, and of the airframe side under airframe development.
Next step: read the installation as it actually closed (clearances, duct, balance and the six flight tests) on the Orion-T1 page, and carry the left-hand column of the table into your own requirements.