Small-Turbojet Design Brief Checklist
The decisions, inputs and tolerances that make a small-turbojet design brief buildable, and the questions that can safely stay open.
A design brief defines what "good" means, written down while the definition can still hurt: the thrust number, the fuel number, the mass ceiling, and which of the three is allowed to move. Everything a propulsion designer hands back to you (the design point, the trade-offs, the evidence) is bounded by what you freeze in that document. A vague brief does not produce a vague engine. It produces a precise engine aimed at the wrong target, with numbers that are all correct and none of them useful.
This checklist is for the founder, UAV team lead or integrator about to commission a small-turbojet concept design. It is ordered the way a designer will read your brief: mission first, numbers second, tolerances third.
Decide the mission before the engine
A turbojet is designed at one point and flown everywhere else. The single most consequential line in your brief is which flight condition the cycle is closed around, and the most common mistake is choosing sea-level static, the easiest number to quote in a pitch deck.
Sea-level static thrust is a bench figure. If the aircraft cruises at 8 km and Mach 0.6, an engine tuned to look good on the bench may be the wrong engine everywhere it actually flies. Thrust depends on mass flow, flight speed and exhaust conditions, and all three move with altitude and Mach, NASA Glenn's turbojet thrust reference walks through the relations in plain terms. Your brief should name:
- The design point: one altitude, Mach and throttle condition the cycle is closed at.
- Two or three off-design points that matter: typically cruise, a dash or climb condition, and sea-level static for ground operations and comparison.
- Expected throttle usage: how much of the mission runs at full, part and idle power. This shapes where fuel burn actually accrues.
A worked example of how short this can be: the brief behind the Orion engine was one sentence: beat a supplied baseline's fuel burn of 125.3 kg/(kN·h) without giving up its 12.35 kN of thrust. Every downstream decision, including the rejected screening point, was checkable against that sentence. That is the standard your brief should meet.
Bring numbers, not adjectives
"Efficient," "lightweight" and "high-performance" are hopes rather than inputs. Each row below is something a designer must either read from your brief or guess, and a guess you did not review is a decision you made by accident.
| Input | Why it gates the design | If you leave it open |
|---|---|---|
| Thrust target, at a declared condition | Sizes mass flow, which sizes everything downstream | The designer picks the condition; you get a bench number |
| Fuel-burn goal, with units and basis | TSFC trades directly against pressure ratio and turbine temperature | "Efficient" is untestable; any result passes |
| Operating envelope points (altitude × Mach) | Defines the off-design deck you can hold the designer to | Performance away from the design point is fiction |
| Mass ceiling | Bounds materials, architecture and stage count | The engine grows silently into your aircraft's weight budget |
| Length and diameter bounds | Sets stage count and annulus geometry | The engine closes thermodynamically and fits nothing |
| Fuel type | Drives combustor and fuel-system assumptions | Kerosene is assumed; your logistics chain says otherwise |
| Baseline engine, if you are replacing one | Sets the no-sacrifice floor for every metric | "Better" cannot be shown, only asserted |
| Installation constraints | Duct routing, bay volume, nozzle station | Integration becomes a surprise invoice later |
Two more inputs are administrative but real: export or IP constraints that shape what can be delivered, and a decision date. A brief is frozen to a schedule; without one, the thresholds drift every time someone new joins the call.
Freeze thresholds, not wishes
Keep the difference between a target and a requirement explicit, and write both down. A target is where you hope to land. A requirement is a line with a side: TSFC at or below 100 kg/(kN·h), dry mass at or below 170 kg. When the brief freezes requirements, every result becomes a check with a published margin, and a thin published margin is worth more than a fat vague one, because it proves the requirement was real. Orion closed its fuel target at 99.2591 against a declared ceiling of 100. A 0.74% margin, published, tells you the threshold had teeth; "exceeds expectations" tells you nothing.
Also declare the trade hierarchy in advance: which number may move if another improves. "Mass may grow 10% if the TSFC target holds" is a usable instruction to a designer. "Light and efficient" is a coin flip conducted later, by someone else, without you.
What can legitimately stay open
A brief is not a specification tree, and over-specifying at concept stage is its own failure mode. These can stay open, provided they are declared open and someone carries an allowance for them:
- Manufacturing process and supplier selection: concept geometry should be producible, not production-planned.
- Detailed material selection beyond the declared basis class (for example, "nickel superalloy turbine" rather than a specific alloy and heat treatment).
- Control-system architecture, as long as throttle response requirements are stated.
- Maintenance concept, though turnaround expectations are worth one line if they matter to your operation.
The rule is simple: open is fine if it is labeled open, priced into the mass and schedule allowances, and assigned an owner. Open and unmentioned is a hidden assumption, and hidden assumptions surface as change orders.
Brief failure modes
Four patterns recur in bad briefs. First, the single-point spec: one thrust number, no envelope, no fuel goal. The designer optimizes the bench and the mission is someone else's problem. Second, contradictory corners: maximum thrust at minimum fuel burn at minimum mass, with no priority among them; thermodynamics will choose for you, and you will not like the choice. Third, hidden priorities: the metric the board actually cares about (usually cost per flight hour or a specific mission radius) never makes it into the document. Fourth, precision theater: thrust specified to the newton with zero tolerance at concept stage. Real precision is earned at verification; demanding it in the brief just teaches everyone to round dishonestly.
The one-page skeleton
If your brief answers these eleven lines, a serious designer can start:
- Mission paragraph: what the aircraft does, in plain language.
- Design point: altitude, Mach, throttle.
- Off-design points: two or three, with why each matters.
- Thrust target, at the declared design condition.
- Fuel-burn goal, with units and basis.
- Mass ceiling and envelope bounds.
- Fuel type.
- Baseline, if replacing an existing engine.
- Trade hierarchy: what flexes, what does not.
- Open items, each with an owner and an allowance.
- Decision date.
The packages page lists the same inputs in the form we ask for them, and the method page explains why the freeze comes before the solve: thresholds written after the numbers exist are not thresholds. When you are comparing what comes back against this brief, the companion guide to concept-design package deliverables defines what a serious response contains, and the framework for comparing competing proposals covers the case where two designers answer the same brief differently.
Once the skeleton above is filled in, a design intake takes under five minutes, but the brief is the work, and it is yours to do.