The Historic Rocket Engine Test Facility (RETF)
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Rocket Engine Testing 101

The next sections describe different stages of a sample test-run at the Rocket Engine Test Facility (RETF). This theoretical engine test uses liquid hydrogen as the fuel and liquid oxygen as the oxidizer. The stages that are examined include:

• Setting up the test site
• Running the test
• Scrubbing the rocket exhaust
• Handling problems during the test

Setting Up the Test Site

The researchers at RETF were performing complex experiments that required specially designed equipment and sensitive instruments, and a great deal of attention went into preparing the test site to achieve the best results. Precision and accuracy were crucial, and researchers followed standard procedures when conducting a test run.

To prepare the fuel source, pressurized liquid hydrogen flowed from a mobile tanker-trailer into the cylindrical tanks that were housed in the fuel pit. These tanks were mounted in a special framework that contained sensors called “load cells” in the suspension. The load cells measured the weight of the tanks, which enabled the researchers to measure the amount of fuel in them. By comparing the weight of a full tank with the weight of the tank after a test, researchers could accurately determine the amount of fuel burned during the test. They could then calculate the rate of fuel use by plotting the weight of the tank against the time length of the test.

When liquid oxygen was used as the oxidizer, it was transferred from a mobile tanker-trailer through a pipe into a tank in the oxidizer pit. Special equipment was needed to keep the oxygen in a liquid state as it traveled through the pipe and was stored in the tank. Each tank, called a “dewar” was vacuum-insulated and double-walled. A liquid-nitrogen bath was circulated between the two walls of the tank to keep the oxygen cool and in its liquid form. The pipe to the tank also had an outer covering that contained a liquid-nitrogen bath.

To gather data from the experiments, researchers used numerous sensors on the engines, such as load cells, strain gages, thermocouples, and pressure gages. The wires from the sensors were connected to terminal blocks and amplifiers in the terminal room of Building 202. Because the magnitude of some of the electrical signals was quite small, the signals needed to be amplified before they were transmitted to the control room. Once amplified, the electrical signals were transmitted over data-transmission cables to the readout devices in the control room in Building 100, some 1,600 feet away.

Great care was taken to make sure that the test was completed with the utmost safety, and RETF used a number of different systems to help warn people to stay away from the site during a test. They positioned signs and barriers around the perimeter of the test facility, and they flew flags over the test area, using green, yellow, and red flags to indicate different alert levels. The facility had a public address system that could be used to warn workers in the surrounding areas, and, most important, they turned on a siren thirty seconds before they fired a rocket and continued the siren until the test was complete.

Running the Test

Once the test site was prepared and safety precautions completed, it was time to run the test. Procedures were divided into carefully choreographed periods called “zones” for accomplishing specific tasks, with the test itself running for between 2 and 3 seconds.

At the pre-test zone, engineers prepared the rocket equipment:

• Pressurized and test-fired the igniter system
• Used liquid helium to chill the liquid hydrogen line leading into the injector
• Pressurized the propellant tanks

After the pre-test, during the first 15-second period, the engine was readied for firing:

• The test engineer-in-charge pushed the start button.
• Data systems automatically calibrated the instruments and started recording pressure and temperature data on strip charts.
• Gaseous nitrogen, used to purge the propellant lines, and water started to flow into the scrubber.
• After 11 seconds, a 2-second flow of oxygen began in order to cool the engine.
• At 13 seconds, gaseous nitrogen purges cleared the engine of liquid oxygen.

The most critical time was this next 3- to 5-second period, when these events occurred:

• The test engineer-in-charge turned on the igniter and checked the pressure of the combustion chamber.
• Two propellant valves automatically opened, allowing the fuel (liquid hydrogen) and the oxidant (liquid oxygen) to flow into the rocket engine.
• After 450 milliseconds, propellant and combustion chamber pressures were checked. If they were within limits, the test continued, though the test conductor kept his hand near the abort button.
• After 2.5 seconds had lapsed, the fire valves were closed and the engine was shut down. The propellants were flushed from the lines beyond the fire valves, the purges were restarted, and finally, test shutdown initiated.

In the final 120 seconds of the test, the engineers:

• Completed the engine shutdown.
• Rendered the area safe for inspection.
• Took calibrations again.
• Turned off the scrubber.
• Turned off all data systems.

At any time during a test, if the engineers monitoring data in the control room noted abnormal propellant and chamber pressures, the engineer-in-charge could immediately abort the test by pressing the ABORT button. However, it was more likely that the computers would sense a problem and automatically shut down the test. During an aborted procedure, propellant fire valves automatically slammed shut. The shut-off valves on the two propellant tanks also automatically closed, and the prime vent valves, which were located in the line between the tank shut-off valves and the fire valves, opened to vent any propellants trapped in the line. Slamming the tank valves shut prevented propellants from escaping into the test facility itself, a potentially dangerous event that could allow unburned propellants to explode. When explosions did occur, a full investigation was carried out to determine the causes before researchers resumed any testing.

Rocket Exhaust Scrubbing

The exhaust from a rocket-engine test needed to be cleaned before it enters the atmosphere. Exhaust was scrubbed in this manner:

• Hot gases from the rocket engine nozzle entered the scrubber through a narrow opening in the test cell. These gases were traveling at velocities of 9,000 to 12,000 feet per second (fps) (1.7 miles per second to 2.3 miles per second) and at temperatures up to 6,000°F.
• The exhaust stream entered the large scrubber tank (25’ wide by 100’ long) at a speed of 25 fps.
• The gases contacted the water spray coming from the nozzles.
• Much of the exhaust was converted to steam and escaped through the vertical portion of the scrubber at a velocity of approximately 20 fps and a temperature of 160°F. Several burners on top of the scrubber ignited any hydrogen or other fuel not consumed by the engine. These burners prevented unburned fuel from building to explosive levels inside the scrubber.
• Combustion by-products, water, and condensed steam were trapped by the scrubber and drained into a 20,000-gallon detention tank. Wastewater was retained in this tank until the day’s test program was completed, when the wastewater was pumped to a neutralizing tank.
• Chemical technicians analyzed the wastewater and determined the quantity and type of additives they needed to neutralize either acidity or alkalinity.
• Chemicals were added so that the pH value would meet municipal wastewater standards
• The wastewater was pumped to a municipal wastewater treatment plant.

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