Temperature sensor in rocket testing and aerospace

During engine tests, temperatures exceed 1,500 degrees Celsius, combined with extreme vibrations and pressure conditions. This article explains which sensors measure reliably under these conditions and what engineers should consider when selecting them.

Which Temperature Sensors Are Suitable for Rocket Testing

A temperature sensor in rocket testing must withstand conditions that far exceed standard industrial applications. Combustion chamber temperatures above 1,500 degrees Celsius, transient temperature spikes within milliseconds and mechanical stresses from combustion oscillations place the highest demands on the measurement technology used. Thermocouples have established themselves as the preferred sensor solution in this environment because they deliver the necessary combination of temperature range, response speed and robustness.

Therma has been manufacturing thermocouples by hand at its site in Germany for over 30 years, supplying projects ranging from industrial process measurement to space applications. This article explains what requirements rocket tests impose on temperature sensors, which thermocouple types are suitable for extreme conditions and how Therma sensors are used in a European rocket project.

Why Precise Temperature Measurement Is Critical in Rocket Testing

During a rocket engine test, temperatures in the combustion chamber reach between 1,500 and 3,500 degrees Celsius. The nozzle walls, cooling channels and injector plates must withstand these thermal loads without structural failure. Temperature sensors provide the measurement data on which engineers base their assessment of the thermal integrity of the entire propulsion system.

Without reliable temperature measurement in rocket testing, neither cooling concepts can be validated nor material fatigue detected early. A sensor that reacts too slowly or drifts at high temperatures delivers distorted data. In the worst case, this leads to misinterpretation of the thermal load and consequently to engine failure during the next test run.

Particularly with regeneratively cooled engines, which are increasingly used in both student and commercial rocketry, temperature monitoring at multiple measurement points simultaneously is essential. The sensors must precisely capture both the hot gas side and the coolant temperature, often in locations with limited space and high mechanical stress.

What Requirements Rocket Tests Impose on Temperature Sensors

Temperature Ranges Exceeding 1,500 Degrees Celsius

The thermal requirements in rocket testing significantly exceed most industrial applications. While a standard Type K thermocouple reliably measures up to approximately 1,260 degrees Celsius, measurements directly at the combustion chamber or nozzle throat require sensors with measurement ranges up to 1,700 or 1,800 degrees Celsius. Noble metal alloy thermocouples are used for such positions, capable of continuous operation at these temperatures.

Outside the combustion chamber, requirements remain high. Exhaust gas temperatures, injector temperatures and the temperature of regenerative cooling channels typically range between 400 and 1,200 degrees Celsius. Mineral-insulated thermocouples with stainless steel or Inconel sheaths have proven effective here, combining mechanical protection with chemical resistance.

Response Times in the Millisecond Range

A rocket engine reaches its operating temperature within fractions of a second after ignition. Sensors with long response times miss these transient temperature peaks and deliver smoothed values that underestimate the actual thermal load. For the analysis of ignition and shutdown events, thermocouples with response times in the single-digit millisecond range are therefore required.

Coaxial thermocouples and thin-film thermocouples achieve response times down to the microsecond range. In practice, temperature changes of over 200 degrees have been measured in less than 10 microseconds. The smaller the sheath diameter of the thermocouple, the faster the reaction to temperature spikes.

Resistance to Vibration and Corrosion

Combustion oscillations in rocket engines generate mechanical stresses that can break thin sensor wires. The mineral-insulated sheath design protects the internal thermocouple wires through a closed metal sheath filled with mineral-insulated powder. This makes mineral-insulated thermocouples largely resistant to vibration, bending and impact.

Additionally, combustion products such as soot particles, nitric acid or metal oxides attack the sensor surface. Sheath materials made of Inconel or high-temperature stainless steels reliably withstand these chemical effects over the duration of a test campaign.

Thermocouple Types for Extreme Conditions Compared

Type K and Type N for the Mid-Range High Temperature Spectrum

Type K (NiCr-Ni) is the most widely used thermocouple type in the world and covers a measurement range from -200 to +1,260 degrees Celsius. Its strengths lie in universal applicability, good availability and comparatively low cost. In rocket testing, Type K is particularly suitable for monitoring cooling circuits, structural temperatures and exhaust gas measurements outside the combustion chamber.

Type N (NiCrSi-NiSi) offers a similar measurement range but surpasses Type K in long-term stability and drift resistance. For test series where sensors must deliver consistent readings across multiple ignition cycles, Type N is the better choice. Both types are available in various configurations in the Therma thermocouple range.

Noble Metal Thermocouples: Type R, S and B

For measurement points directly at or inside the combustion chamber, base metal thermocouples are insufficient. Noble metal thermocouples made from platinum-rhodium alloys enable continuous measurements at significantly higher temperatures. Types R and S reach up to 1,600 degrees Celsius, while Type B reaches up to 1,800 degrees Celsius in continuous operation.

The disadvantage of these types lies in their higher acquisition costs and lower mechanical robustness. For rocket tests, where individual sensors are replaced after a test campaign anyway, the cost factor is put into perspective considerably.

The following table summarises the relevant thermocouple types for aerospace applications:

Type	Material Pairing	Measurement Range	Strength	Typical Application
K	NiCr-Ni	-200 to +1,260 °C	Robust, versatile	Exhaust temperature, cooling
N	NiCrSi-NiSi	-200 to +1,260 °C	High long-term stability	Monitoring, test series
R	Pt-PtRh13	0 to +1,600 °C	High precision	Combustion chamber, near-field
S	Pt-PtRh10	0 to +1,550 °C	Reference class	Calibration measurements
B	PtRh30-PtRh6	+200 to +1,800 °C	Highest continuous range	Near-nozzle measurement points

For most measurement positions in a rocket test, Type K and Type N offer the best balance of performance and cost-effectiveness. Noble metal types are deployed specifically where the temperature continuously exceeds the 1,260 degrees Celsius threshold.

Thermocouple or RTD in the Aerospace Environment

Advantages of Thermocouples in Rocket Testing

In a direct comparison with resistance temperature detectors (RTDs), thermocouples offer three decisive advantages for rocket testing. First, they cover a significantly higher temperature range — exceeding 1,800 degrees Celsius compared to a maximum of 850 degrees Celsius for Pt100 sensors. Second, they respond faster to temperature spikes, which is indispensable during transient processes in the engine. Third, they can be manufactured in smaller form factors, which becomes relevant in confined installation situations on engine components.

Additionally, thermocouples are less sensitive to mechanical shock. The mineral-insulated sheath design with its insulated interior forms a compact, flexible unit that reliably withstands vibration loads on the engine test stand.

When RTDs Are the Better Choice

In cryogenic propellant systems using liquid oxygen or liquid hydrogen, temperatures operate in the range of -200 to -50 degrees Celsius. Here, resistance temperature detectors (Pt100, Pt1000) clearly surpass thermocouples in terms of precision and long-term stability. They are also the better choice for permanent structural monitoring in the moderate temperature range, such as on tank walls or piping systems.

A detailed comparison of both sensor types with all technical details can be found in our separate technical article.

Case Study: Propulse NTNU and Project Heimdall

About Propulse NTNU and the Student Rocket Programme

Propulse NTNU is Norway’s first student rocket team, founded in 2018 at the Norwegian University of Science and Technology in Trondheim. The organisation consists of over 70 active members from various engineering disciplines, including propulsion engineering, avionics, mechanical design and software development. As part of Project Heimdall, the team developed a 5.8-metre hybrid rocket with a launch mass of 150 kilograms.

The launch took place in 2025 from Tarva in Norway and reached an official apogee of 3,318 metres. The mission was also the first fully self-organised launch campaign by Propulse, with the team independently managing logistics, flight safety and recovery. Over 85,000 working hours were invested in the project.

Therma Sensors in the Engine Testing of Project Heimdall

For the engine test campaign of Project Heimdall, Therma supplied thermocouples used in engine testing, thermal validation and flight data analysis. The sensors had to deliver reliable measurement data under the demanding thermal conditions of a hybrid rocket engine, where temperature gradients occur within milliseconds.

Propulse NTNU stated in a public announcement: “Accurate temperature measurements are critical in rocket development.” The partnership between Therma and the student team demonstrates that high-quality temperature sensors handcrafted in Germany have a rightful place in aerospace engineering. For Therma, the collaboration with Propulse NTNU confirms that its thermocouples perform reliably not only in industry and motorsport, but also under the extreme conditions of a rocket test.

What to Consider When Selecting Sensors for Extreme Applications

Matching Temperature Range and Sensor Type

The most important decision in sensor selection is matching the expected temperature range with the appropriate thermocouple type. If the maximum measurement temperature is below 1,260 degrees Celsius, Type K or Type N will suffice. For measurement points above 1,260 degrees Celsius, noble metal thermocouples are essential. For cryogenic applications below -200 degrees Celsius, RTDs or specialised cryogenic sensors are the right choice.

Beyond the temperature range, the question of standard or custom manufacturing plays a role. Many rocket projects require individual sensor lengths, sheath diameters or connection configurations. In addition to its standard range, Therma also offers custom manufacturing based on drawings or STEP files, with production lead times of 1 to 2 weeks.

Installation and Protection Under Extreme Conditions

The installation situation determines the form factor. For measurement points on pipelines and vessels, threaded thermocouples are suitable. For surface measurement on structural components, surface thermocouples with adhesive or magnetic mounts are an option. On test stands where sensors are frequently replaced, stainless steel compression fittings offer a secure and reusable mounting solution.

The choice of extension wire deserves particular attention. Thermocouple extension wires with fibreglass or PTFE insulation withstand higher ambient temperatures than standard cables. For individual advice on the optimal sensor configuration, the Therma team with over 30 years of experience in temperature measurement technology is at your disposal. Contact us for a free technical consultation.

Conclusion

Temperature sensors in rocket testing must meet three core requirements: high-temperature capability exceeding 1,500 degrees Celsius, response times in the millisecond range, and resistance to vibration and corrosion. Thermocouples are the first choice for this combination. The Propulse NTNU case study demonstrates that Therma sensors, 100% handcrafted and Made in Germany, perform reliably in aerospace engineering. All thermocouples are manufactured to ISO 9001 standards, many are available from stock with a standard delivery time of just one week. For custom configurations in the aerospace sector, we are happy to advise you personally.