Based in Westcott Venture Park in Buckinghamshire, AMPAC In-Space Propulsion (ISP) is the UK’s leading organisation involved in the design, manufacture, and test of liquid rocket engines for satellite propulsion. AMPAC-ISP was established by the UK government in 1945 and holds a long and prestigious heritage in the aerospace and defence industries. One unique aspect of AMPAC-ISP is our facilities for hot-firing small monopropellant and bipropellant rocket engines of up to 20 N of thrust in a vacuum environment.
Hot-Fire Rocket Test Facility Upgrade Required
We have been performing hot-fire tests for our international customer base at Westcott for many years, so it was time to modernize the testing facilities. Due to the increasing expectations of customers, we also needed to improve the systems to continue successfully competing in the rocket engine market. Higher accuracy, greater reliability, and faster data turnaround were key development requirements.
To improve the outcome of the hot-fire test campaigns, the digital data acquisition system was one of the fundamental areas requiring an upgrade, leading us to contact National Instruments for advice and assistance. We chose NI for this data system upgrade because the company has a reputation for reliable hardware and continued customer support and its products have been successfully used throughout the aerospace industry. It was also important to us to deal with a company with longevity, as this had been one of the reasons for not being able to maintain the old acquisition system.
Instrument Control and Data Acquisition
Hot-fire testing, as the name suggests, is when rocket engines are run, or ‘fired,’ using propellants. We typically perform these tests inside a vacuum cell with a 2 m diameter using a two-stage steam ejector vacuum engine. The test cell itself can maintain a simulated altitude of >160,000 ft (>48,768 m) to imitate the conditions of a near-space environment. Each firing can last anywhere between one one-hundredth of a second during pulsing modes to a few hours in steady state (continuous) modes.
During the test campaigns, we must record a large amount of data. This includes taking temperature readings using thermocouples, pressure readings using pressure transducers, and thrust forces from the load cells. We interpret the collected data to determine the engine performance and to monitor test equipment and testing environment conditions
During the firing, we must also control the amount of propellant entering the engine and maintain inlet pressures in the region of 5 to 24 bar (73 to 348 psi). With relatively short engine reaction times, data acquisition accuracy and speed is very important and real-time data acquisition is essential to monitor and control such potentially hazardous tests. Typically, data acquisition rates are in the order of 4 kHz for pulse-mode firing and 1 kHz for steady state firing. Depending on the test being performed, we use 20 to 30 data channels.
To fulfil the data acquisition requirements, we selected an NI CompactRIO system as the central piece of hardware. It is compatible with all of the existing measuring instruments, so we are using it to receive, handle, and temporarily store all of this raw information before sending it to the test engineer. Then we use LabVIEW software to interpret the data and display it numerically and graphically on the computer screens in the remotely located control room.
Much of the testing we conduct at Westcott uses the same equipment setup, but with an embedded field-programmable gate array (FPGA) integrated into the CompactRIO system, we have the flexibility to develop the hardware to meet changing requirements. This also applies to the hot-swappable modules, which we can use to make hardware and equipment changes throughout test campaigns if necessary. Because the test equipment is located next to the vacuum facility in an uninsulated building, it must also be capable of operating in harsh environments. The CompactRIO system satisfies this requirement with its wide operating temperature range.
Instant Visual Temperature Monitoring
As part of the hot-fire testing, we must carefully monitor rocket engine temperature to ensure that maximum material temperature limits are not reached. This is particularly important in the combustion chamber because temperatures exceed 1,200 °C. The chamber walls could melt and the chamber could potentially explode. We also monitor the temperature distribution on the rocket nozzle to check that the exhausting gasses are relatively central because this avoids creating hotspots and maintains thrust efficiency.
As part of continued system development, we hope to incorporate optical temperature imaging as part of the customer data set. We will use digital temperature sensing cameras in conjunction with a Compact Vision System to closely and instantly monitor these temperatures and more accurately record data.
Following a number of successful test campaigns, the upgraded control and data acquisition system has proven to be reliable and accurate, as well as capable of instantly providing useful data sets. This has led to improved customer satisfaction and has enabled AMPAC-ISP to continue to successfully compete for business. Because of the success at the high-altitude facility and the reliability of the data acquisition system hardware, we have replicated the instrument set up at our sea-level test site where we recently tested and collected data for an engine with 11,100 N (1.1 tons) of thrust.