The Department of Defense, specifically the United States Air Force, is seeking innovative solutions for the development of robust instrumentation tailored for Rotating Detonation Engine (RDE) applications through the Small Business Innovation Research (SBIR) program. The primary objective is to create sensors capable of accurately measuring time-resolved channel pressures and instantaneous temperature/wall heat fluxes in the challenging RDE environment, where detonation waves present significant measurement difficulties. This initiative is critical for advancing the understanding and practical application of RDE technology, which has implications for enhanced propulsion systems. Interested parties should note that the solicitation is currently open, with proposals due by November 6, 2024, and further details can be found at the provided source link: https://www.dodsbirsttr.mil/topics-app/.

## Description

The RDE environment is particularly challenging due to the presence of detonation waves. A detonation wave is a shock wave closely coupled to an intense heat release zone. In an RDE, detonation waves continuously travel around an annular geometry and are sustained by the injection of fresh reactants ahead of the wave. Products from the detonation wave are expanded to generate work or thrust. Detonation velocities in an RDE are typically on the order of 1-2 km/s leading to operating frequencies between 1 and 10 kHz [1]. Understanding of these devices is limited due to the challenge of getting high quality data in this environment.Detailed CFD models and some cycle deck models require time-resolved pressures within the annulus for validation and tuning purposes. This information provides insights into injector/detonation interaction regarding inlet blockage and inlet recovery. It is difficult to obtain accurate, time-resolved pressures in the annulus. The passing detonation wave and resultant expansion occurs over a short period of time compared to the cycle frequency. This means that the Nyquist criteria can capture the frequency of the passing detonation wave, but a much faster sampling rate is required to resolve the detonation pulse. Typical sampling rates of 500 kHz to 1 MHz are required to capture the waveform [2]. Absolute pressure sensors exist that can sample at rates up to 1 MHz; however, temperature limitations mean that the sensors cannot be directly exposed to the RDE environment. A common method to capture the passing of a detonation wave is to use an Infinite Tube Pressure (ITP) measurement where the sensor is separated from the main channel by a long tube with a small diameter that is opened at the end. The transducer is perpendicular to the tube at some distance from the RDE to capture the pressure pulse as it propagates down the tube [2]. This allows the sensor to be isolated from the harsh environment but reduces the fidelity of the measurement and introduces an unknown transfer function that could interfere with the measurement. An alternative method is to flush mount a sensor that is water-cooled but these typically have reduced natural frequencies which distort the detonation waveform. Water-cooled dynamic pressure transducers have also been used but cannot give the absolute pressure profile for model comparisons.Another important aspect of making RDEs practical is understanding the thermal environment and what cooling/material solutions are required. Instantaneous wall temperature/heat fluxes are needed as boundary conditions for thermal modeling. High frequency heat flux measurements have been made in an RDE environment using thin-film heat flux gauges [3]. These can resolve the passage of a detonation wave but are rapidly destroyed within several hundred milliseconds.KEY SENSOR PARAMETERS<ul><li>&emsp;Must integrate into the RDE like a standard piece of instrumentation. In essence, the sensor should only require a hole in the rig to make the measurement.</li><li>&emsp;The natural frequency of sensor should be sufficient to correctly resolve high frequency content contained in the detonation waveform.</li><li>&emsp;If directly exposed to combustion gases, highest temperature rating possible is desired. Water or air cooling is allowed if the sensor maintains a high enough sample rate and natural frequency.</li><li>&emsp;If directly exposed to combustion gases, the sensor must be capable of surviving impulsive loading due to strong shock waves. A detonation in an air breathing RDE can generate peak pressures up to 4-5x the initial pressure in the channel. For example, if the pre-detonation pressure is 50 psia, the sensor could see instantaneous pressures up to 250 psia with a rapid expansion back to 50 psia. </li><li>&emsp;Sampling rate of at least 200 kHz for time-resolved measurements; 500-1000 kHz preferred.</li><li>&emsp;Desired uncertainty of +/- 1 psia for pressure and +/- 50K for temperature.&bull;&emsp;</li><li>&emsp;Compatible with standard DAQ hardware. Voltage outputs preferred for analog signals.</li><li>&emsp;Capable of surviving at least 100 short duration tests. Each test would be 1-2 seconds in length. Sensor would experience 2000 to 20000 detonation events each test.</li><li>&emsp;Smaller the form factors the better. Spatial resolution of &lt; 1mm for wall measurements. Path-integrated measurements are acceptable.</li><li>&emsp;Low-cost desirable. Single sensor cost &lt;$5K and associated systems &lt;$20K.</li></ul>

## Objective

Develop sensors capable of operating and collecting useful information in the Rotating Detonation Engine (RDE) environment. Measurements of interest include time-resolved channel pressures and instantaneous temperature/wall heat fluxes.

## Phase I

Develop concept and prototype for a proposed measurement device. Prototype may be tested with a torch to test temperature rating and heat flux if required. A shock tube may be used to test impulsive shock loading if required. Testing in an air-breathing RDE is preferred. If time-resolved, must show ability to resolve properties on time-scales associated with the detonation waveform. Must show how the sensor would integrate into an RDE. Natural frequency, response time, and uncertainty needs to be determined. Criteria for success is to demonstrate confidence that the sensor or measurement port will survive in the RDE environment. If time-resolved, measurement frequency must be least 200 kHz.

## Phase II

This phase will improve upon sensor robustness. Sensors must be integrated and tested in an air breathing RDE. Criteria for success is surviving at least 100 one to two second tests in an RDE while accurately measuring detonation properties. There are numerous RDE facilities in universities, at AFRL, and at the Naval Postgraduate School that can be utilized for demonstrations, contacts can be provided upon request.

## Phase III: Dual Use Applications

Phase III will still seek improvements to sensor performance and robustness but will introduce a focus on the development of manufacturing methods to improve production time and component cost. Criterion for sensor performance in this phase is still dependent on the parameters of the previous phase. The new criterion for this phase includes the time and cost to produce the sensor.

Robust Instrumentation Development for Rotating Detonation Engine Applications

STTR Opportunity Analysis

SBIR Documents

Related SBIR/STTR Opportunities

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