21.1 TYPES OF TESTS

Before any rocket propulsion systems are put into operational use, they are subjected to several different types of tests, many of which are outlined below in the approximate sequence in which they are normally performed.

1. Manufacturing inspection and fabrication tests on individual parts (dimensional inspection, pressure tests, X‐raying, leak checks, electric continuity tests, electromechanical checks, etc.). These tests are usually done at the factory.
2. Component tests (functional and operational on igniters, nozzles, insulation, valves, controls, injectors, structures, tanks, motor cases, thrust chambers, turbopumps, thrust‐vector control, etc.). May need special equipment/facilities.
3. Static rocket propulsion system tests (with complete propulsion system) on a test stand: (a) complete propulsion system tests (under rated conditions, off‐design conditions, with intentional variations in environment or calibration); (b) with liquid propellants a partial or simulated rocket operation (for proper function, calibration, ignition, operation—often without establishing full thrust or operating for the full duration); for reusable or restartable rocket propulsion systems this can include several starts, long‐duration endurance tests, and postoperational inspections and reconditioning; (c) some tests on chemical propulsion systems and nearly all tests on electrical propulsion systems are performed in large vacuum facilities that simulate the high‐altitude rarified atmosphere conditions.
4. Static vehicle tests (when rocket propulsion system is installed in a restrained, nonflying vehicle or stage) on a vehicle test stand.
5. Flight tests: (a) with a specially instrumented or new propulsion system in a developmental flight test vehicle; (b) with a production vehicle.

Each of these five types of tests can be performed on at least three basic program types:

1. Research on and development or improvement of a new (or modified) rocket propulsion system, its propellants, materials, or components.
2. Evaluation of the suitability of a new (or modified) rocket engine or novel rocket motor for an alternate specified application or for flight readiness.
3. Production proof tests and quality assurance of existing operational rocket propulsion systems.

The first two program types are usually concerned with novel devices and often involve the testing and measurement of new concepts or conditions using experimental rocket propulsion systems. Here examples are the testing of a new solid propellant grain, the development of a novel control valve assembly, and the measurement of the thermal expansion of a nozzle exhaust cone during firing operation.

Production tests are concerned with measurements of a few basic parameters on propulsion systems to assure that their performance, reliability, and operation are within specified tolerance limits. If the number of units is large, test equipment and instrumentation used for these tests are usually partly or fully automated and designed to permit the testing, measurement, recording, and evaluation in a minimum amount of time.

During early development phases of a program, many special or unusual tests are performed on components and on complete rocket systems to prove specific design features and performance characteristics. Special facilities and instrumentation or modification of existing test equipment are used. Examples are large centrifuges to operate propulsion systems under acceleration, or variable frequency vibration equipment for shaking complete systems and/or components. During the second type of program, special tests are usually conducted to establish statistically the performance and reliability of a rocket system by operating a number of units of the same design. During this phase, tests are also made to demonstrate the ability of the rocket system to withstand extreme operating conditions limits, such as high and low ambient temperatures, variations in fuel composition, changes in the vibration environment, or exposure to moisture, rain, vacuum, or rough handling during storage or transport. To demonstrate safety, sometimes intentional malfunctions, spurious signals, or known manufacturing flaws are introduced into the propulsion system to determine the capability of its control system or the safety devices to handle and prevent potential failures.

Before an experimental rocket system can be flown in a vehicle it has to pass a set of preliminary flight rating tests aimed at demonstrating its safety, reliability, and performance. It is not a single test, but a series of tests under various specified conditions, operating limits, and performance tolerances, simulated environments, and intentional malfunctions. Only thereafter may the rocket system be used in experimental flights. However, before they can be put into production, rocket systems have to pass another a series of tests under a variety of rigorous specified conditions, known as the qualification test or preproduction test. Once a particular propulsion system has been qualified, or passed such a qualification test, it is usually forbidden to make any changes in design, fabrication, or materials without going through a careful review, extensive documentation, and often also a partial or full set of requalification tests.

The amount and expense of component and complete propulsion systems testing has greatly decreased in the last few decades. The reasons here are more experience with prior similar systems, less hardware development, better sensors, better analytical model simulations, better prediction of system parameters, use of health monitoring systems to identify incipient damage (which saves hardware), and more confidence in predicting certain failure modes and their location. Validated computer software have removed many uncertainties and obviated needs for some tests. In some applications the number of firing tests has decreased by a factor of 10 or more.

21.2 TEST FACILITIES AND SAFEGUARDS

For chemical rocket propulsion systems, each test facility usually has the following major systems or components:

1. A test cell or test bay where the article (a rocket propulsion system or a thrust chamber) to be tested is mounted, usually in a special test fixture. If the test is hazardous, the test facility must have provisions to protect operating personnel and to limit damage in case of accidents or explosions.
2. An instrumentation system with associated computers for sensing, maintaining, measuring, analyzing, correcting, and recording the various physical and chemical parameters. It usually includes calibration systems and timers to accurately synchronize the measurements.
3. A control system for starting, stopping, and changing the operating conditions during tests.
4. Systems for handling and lifting heavy or awkward assemblies, for supplying (or removing) liquid propellant, and for providing maintenance, security, and safety.
5. For highly toxic propellants and toxic plume gases the capture of hazardous gases or vapors has been required (by firing inside a closed duct system) to remove most or all of the hazardous ingredients (e.g., by wet scrubbing and/or chemical treatment), allowing the release of the nontoxic portion of the exhaust gases and the safe disposal of the toxic solid or liquid residues. With exhaust gases containing fluorine, for example, the toxic gas removal can be achieved by scrubbers with water that contains dissolved calcium; this will then form calcium fluoride, which is essentially insoluble and can be precipitated and removed.
6. In some tests specialized equipment and unique facilities are needed to conduct static testing under various environmental conditions or under simulated emergency conditions. For example, high and low ambient temperature tests of large rocket motors may require a temperature‐controlled enclosure around the motor; a rugged explosion‐resistant facility is needed for bullet impact tests of propellant‐loaded missile systems and also for cook‐off tests, where gasoline or rocket fuel is burned with air below a stored missile. Similarly, special equipment is needed for vibration testing, measuring thrust vector side forces and moments in three dimensions, and/or determining total impulse for very short pulse durations at low thrust.

Most rocket propulsion testing is accomplished in sophisticated facilities under closely controlled conditions. Modern rocket propulsion test facilities are frequently located several miles away from the nearest community to prevent or minimize effects of excessive noise, vibration, explosions, and toxic exhaust clouds. Figure 21–1 shows one type of an open‐air test stand for vertically down‐firing large liquid propellant thrust chambers, medium‐sized SPRMs or LPREs (100,000 to 2 million pounds thrust). It is best to fire the propulsion system in a direction (vertical or horizontal) similar to the actual flight condition. Figure 21–2 shows a simulated altitude test facility for rockets of about 10.5 metric tons thrust force (46,000 lbf). It requires a vacuum chamber in which to mount the engine, a set of steam ejectors to create the vacuum, water sprays to reduce the gas temperature, and a cooled diffuser. With flow magnitudes of chemical rocket propellant combustion gases, however, it is impossible to maintain a high vacuum in these kinds of facilities; typically, between 15 to 4 torr (equivalent to 20 to 35 km altitude) can be maintained. This type of test facility allows the operation of rocket propulsion systems with high‐nozzle‐area ratios that would normally experience flow separation at sea‐level ambient pressures.

Prior to performing any test, it is common practice to educate the test crew by going through repeated dry runs to familiarize each person with his or her responsibilities and procedures, including all emergency procedures.

Typical personnel and plant security and/or safety provisions in a modern test facility include the following:

1. Concrete‐walled blockhouses or control stations for the protection of personnel, instruments and ground‐based computer terminals (see Fig. 21–3), located remotely from the actual rocket propulsion test stand.
2. Remote controls, indications, and recordings of all hazardous operations and measurements; isolation of propellants and exhaust gases from all instrumentation and control rooms.
3. Automatic or manual emergency water deluge and fire‐extinguishing systems, where applicable.
4. Closed circuit television systems for remotely viewing the test.
5. Warning signals (sirens, bells, horns, lights, speakers) to notify personnel to clear the test area prior to testing, and an all‐clear signal when the conditions are no longer hazardous.
6. Quantity and distance restrictions on the storage of liquid propellants, some solid propellant ingredients, and solid propellants to minimize damage in the event of explosions; robust separation of fuels and oxidizers.
7. Barricades around hazardous test articles to reduce shrapnel damage in the event of a blast.
8. Explosion‐proof electrical systems, spark‐proof shoes, and nonspark hand tools to prevent ignition of flammable materials, such as certain SPRM ingredients or vapors.
9. For certain hazardous propellants also safety clothing (see Fig. 21–4), including propellant‐ and fire‐resistant suits, face masks and shields, gloves, special shoes, and hard hats.
10. Rigid enforcement of rules governing area access, smoking, safety inspections, and so forth.
11. Limitations on the number and type of personnel that may be in a hazardous area at any time.

Monitoring the Environment and Controlling Toxic Materials

Open‐air testing of chemical rocket propulsion systems frequently requires measurement and control of exhaust cloud concentrations and gas movement in the surrounding areas for safeguarding personnel and the environment. Most test and launch facilities have several stations (both inside and outside the facilities) for collecting and measuring air samples before, during, and after testing. Toxic clouds of exhaust gases (some with particulates) can result from normal rocket operation or from vapors or reaction gases from unintentional propellant spills, or from fire borne gases, explosions, or from the intentional destruction of vehicles in flight or rockets on the launch stand. Environmental government regulations usually limit the maximum local concentration or the total quantity of toxic gas or particulates that can be released to the atmosphere. The toxic nature of some of these liquids, vapors, and gases has been mentioned in Chapters 7 and 13. One method of control is for tests with discharges of moderately toxic gases or products to be postponed until favorable weather and wind conditions are present.

Extensive analytical studies and measurements of the environmental exposure from explosions, industrial smoke, and exhausts from missile and space vehicle launchings provide useful background for predicting the atmospheric diffusion and downwind concentrations of rocket exhaust clouds. In ground‐test analyses, the toxic cloud source is modelled as a point source and in flight tests as a ribbon source. Diffusion rates of exhaust clouds are influenced by many propulsion system variables, including propellant types, vehicle size, exhaust gas temperatures, and thrust duration; also by many atmospheric variables, including wind velocity, direction, turbulence, humidity, vertical stability or lapse rate (see definition below), and by the surrounding terrain. Reference 21–2 describes hazards related to toxic gas cloud concentrations and dispersals. Reference 21–3 evaluates the environmental impact of rocket exhausts from large units on the ozone layer in the stratosphere as well as on the ground weather near the test site (it concludes that the impacts are generally small and temporary). Reference 21–4 describes a test area atmospheric measuring network.

A widely used relationship for predicting atmospheric diffusion of gas clouds has been formulated by O. G. Sutton (Ref. 21–5). Many modern equations and models relating to downwind concentrations of toxic clouds are extensions of O. G. Sutton's theory. Given below are equations from Ref. 21–5 of primary interest to rocket and missile operators.

For instantaneous ground‐level point‐source nonisotropic conditions,

21–1

For continuous ground‐level point‐source nonisotropic conditions,

21–2

where χ is concentration in grams per cubic meter, Q is source strength (grams for instantaneous, grams per second for continuous); C_{x, y, z} are diffusion coefficients in the x, y, z planes, respectively; ū is average wind velocity in meters per second, t is time in seconds, and the coordinates x, y, z are in meters measured from the center of the moving cloud in the instantaneous case and from a ground point beneath the plume axis in the continuous case. The exponent n is a stability or turbulence coefficient, ranging from almost zero for highly turbulent conditions to 1 as a limit for extremely stable conditions, and usually falling between 0.10 and 0.50.

A few basic definitions relevant to the study of atmospheric diffusion of exhaust clouds are as follows:

1. Micrometeorology. The study and forecasting of atmospheric phenomena restricted to a region approximately 300 m above the Earth's surface and to a horizontal distance of approximately 5 miles.
2. Lapse rate. The rate of decrease in temperature with increasing height above the Earth's surface. The standard atmosphere (Appendix 2) has a lapse rate of about 6.4 °C per 1000 m. Lapse rates are also affected by altitude, wind, and humidity.
3. Inversion or Inversion Layer. Condition of negative lapse rates (temperature increases with increasing height). Usually forming near the ground at night.

The following are some general rules and observations derived from experience with atmospheric diffusion of rocket exhaust clouds:

1. Inversion presents a very stable layer and greatly reduces any vertical dispersion (the higher the lapse rate, the greater the vertical dispersion).
2. A highly stable atmospheric condition tends to keep the exhaust plume or cloud intact and away from the Earth's surface, except when the exhaust products are much heavier than the surrounding air.
3. High winds increase the rate of diffusion and reduce the thermal effects.
4. For short firings (<500 sec) approximate dosages downwind are about the same as from an instantaneous point source.
5. When the plume reaches about one‐fourth the distance to a given point before emission is stopped, peak concentrations will be about three‐fourths of those from a continuous source of equal strength.
6. The presence of an inversion layer significantly restricts the mixing or diffusion capacity of the propulsion system exhaust gases with the atmosphere; the effective air mass is that mass existing between the Earth's surface and the inversion layer.
7. Penetrations of the inversion layer arising from buoyant forces in the hot exhaust cloud seldom occur.
8. Earth surface cloud dosages drop rapidly when missiles or space launch vehicles are destroyed in flight above a height of 1500 m when compared to lower altitudes of 600 to 1000 m.

Interpretation of the hazards that exist once the concentration of the toxic agent is known also requires knowledge of their effects on the human body, and the environment. Government tolerance limits for humans are given in Chapter 7 and in Ref. 7–5. There are usually three limits of interest: one for the short‐time exposure of the general public, one for an 8‐hr exposure limit of workers, and one for concentrations needing evacuation. Depending on the toxic chemical, the 8‐hr limit may vary from 5000 ppm for a gas such as carbon dioxide, to less than 1 ppm for more toxic substances such as fluorine. Human poisoning by rocket exhaust products usually occurs from gas and fine solid particle inhalation, but also any solid residuals that may remain around a test facility for weeks or months following a test firing can enter the body through cuts and other avenues. Moreover, certain liquid propellants may cause burns and skin rashes or are poisonous when ingested, as explained in Chapter 7.

21.3 INSTRUMENTATION AND DATA MANAGEMENT

In the last few decades, considerable progress has been made in instrumentation and data management. For further study the reader is referred to standard textbooks on instruments and computers used in testing, such as Ref. 21–6. Some of the physical quantities commonly measured during rocket propulsion testing are:

1. Forces (thrust, thrust vector control side forces, short thrusting pulses).
2. Flows (hot and cold gases, liquid fuels, liquid oxidizers, leaks).
3. Pressures (chamber, propellant, pump inlet/outlet, tank, etc.).
4. Temperatures (chamber or case walls, propellant, structure, nozzle).
5. Timing and command sequencing of valves, switches, igniters, full pressure attainments, and others.
6. Stresses, strains, and vibrations (combustion chamber, structures, solid propellants, liquid propellant lines, local accelerations of vibrating parts) (Ref. 21–7). Also, thermal growths.
7. Movements and positions of parts (valve stems, gimbal position, deflection of parts under load or heat).
8. Voltages, frequencies, and currents in electrical or control subsystems.
9. Visual observations (flame configuration and color, test article failures, explosions) using high‐speed cameras or video cameras.
10. Special quantities such as propellant strains, turbopump shaft speeds, liquid levels in propellant tanks, burning rates, flame luminosities, sound pressures and/or exhaust gas composition.
11. Ambient air conditions (pressure, temperature wind speed and direction, toxic gas content) at stations in the test facility area and also at stations downwind of the test area.

During any one test of a production RPS, only some of the 11 measurements listed above are made and then automatically compared (by computer) with the same measurements on a prior identical rocket propulsion system, which performed properly in previous tests. Any deviation between the two that is identified by the computer unit is usually investigated; if appropriate, an adjustments or hardware modifications are then made to bring the rocket propulsion system being tested to the desired performance.

Reference 2–8 describes some specialized diagnostic techniques used in propulsion systems, such as using nonintrusive optical methods, microwaves, and ultrasound for measurements of temperatures, velocities, particle sizes, or burn rates in solid propellant grains. Many of these sensors incorporate specialized technologies and, often, unique software. Each measured parameter can be obtained with different types of instruments, sensors, and analyzers, as indicated in Ref. 21–9.

21.4 FLIGHT TESTING

Flight testing of the larger rocket propulsion systems is always conducted in conjunction with flight tests of their vehicles and other subsystems such as guidance, vehicle controls, or ground support. These flights usually occur along missile and space launch ranges, often over an ocean. If a flight test vehicle deviates from its intended path and appears to be headed for a populated area, a range safety official (or a computer) will either have to destroy the vehicle, abort the flight or cause it to correct its course. Many propulsion systems therefore include devices that will either terminate operation (shut off the rocket engine or open thrust termination openings into rocket motor cases as described in Section 14.3) or trigger explosive devices that will cause the vehicle (and therefore also the propulsion system) to disintegrate in flight.

Flight testing requires special launch test range support equipment together with means for observing, monitoring, and recording data (cameras, radar, telemetering, etc.), equipment for assuring range safety and for reducing data and evaluating flight test performance, and specially trained personnel. Different equipment is needed for different kinds of vehicles. This may include launch tubes for shoulder‐held infantry support missile launchers, movable turret‐type mounted multiple launchers installed on an army truck or a navy ship, transporters for larger missiles, and track‐propelled launch platforms or fixed complex launch pads for spacecraft launch vehicles. Launch equipment has to have provisions for loading or placing vehicles into a launch position, for allowing access of various parts and connections to launch support equipment (checkout, monitoring, fueling, etc.), for aligning or aiming vehicles, or for withstanding the exposure to hot rocket plumes at launch.

During experimental flights extensive measurements need to be made on the behavior of the various vehicle subsystems; for example, rocket propulsion parameters, such as chamber pressure, feed pressures, temperatures, and so on, are measured and these data are telemetered and transmitted to a ground receiving station for recording, monitoring and analyzing. Some flight tests also rely on salvaging and examining the test vehicle after the flight.

21.5 POSTACCIDENT PROCEDURES

In the testing of any rocket propulsion system there will invariably be failures, particularly when some of the operating parameters are close to their limit. With each failure comes an opportunity to learn more about the design, materials, propulsion performance, fabrication methods, and/or the test procedures. A careful and thorough investigation of each failure is needed to learn the likely causes in order to identify remedies or fixes to prevent similar failures in the future. The lessons to be learned from these failures are perhaps some of the most important benefits of development testing. A formalized postaccident approach is often used, particularly if the failure had a major impact, such as high cost, major damage, or personnel injury. Any major failure (e.g., the loss of a space launch vehicle or severe damage to a test facility) often causes the program to be stopped and further testing or flights put on hold until failure causes are determined and remedial actions have been taken to prevent a recurrence.

Of utmost concern immediately after a major failure are the needed steps to respond to the emergency. These may include giving first aid to injured personnel, bringing the propulsion system and/or the test facilities to a safe/stable condition, limiting further damage from chemical hazards to the facility or the environment, working with local fire departments, medical or emergency maintenance staff or ambulance personnel, and debris clearing crews, and quickly providing factual statements to the management, the employees, the news media, and the public. It also includes controlling access to the facility where the failure has occurred and preserving evidence for the subsequent investigation. All test personnel, particularly the supervisory people, need to be trained not only in preventing accidents and minimizing any impact of a potential failure, but also on how to best respond to the emergency. Reference 21–15 suggests postaccident procedures involving rocket propellants.

REFERENCES

1. 21–1. K. Yanagawa, T. Fujita, H. Miyajima, and K. Kishimoto, “High Altitude Simulation Tests of LOX‐LH2 Engine LE‐5,” Journal of Propulsion and Power, Vol. 1, No. 3, May–Jun. 1985, pp. 180–186.
2. 21–2. Handbook for Estimating Toxic Fuel Hazards, NASA Report CR‐61326, Apr. 1970.
3. 21–3. R. R. Bennett and A. J. McDonald, “Recent Activities and Studies on the Environmental Impact of Rocket Effluents,” AIAA Paper 98‐3850, Jul. 1998.
4. 21–4. R. J. Grosch, “Micro‐Meteorological System,” Report TR‐68‐37, Air Force Rocket Propulsion Laboratory, Nov. 1968 (AD 678856).
5. 21–5. O. G. Sutton, Micrometeorology, Chapter 8, McGraw‐Hill, New York, 1973.
6. 21–6. D. Ramsey, Principles of Engineering Instrumentation, University of Michigan, Ann Arbor, Ml, 1996.
7. 21–7. K. G. McConnell and P. S. Varoto, Vibration Testing: Theory and Practice, 2nd ed., John Wiley & Sons, Hoboken, NJ, 2008.
8. 21–8. Y. M. Timnat, “Diagnostic Techniques for Propulsion Systems,” Progress in Aerospace Sciences (Series), Vol. 26, No. 2, 1989, pp. 153–168.
9. 21–9. R. S. Figliola and D. B. Beasley, Theory and Design for Mechanical Measurements, 4th ed., John Wiley & Sons, Hoboken, NJ, 2005.
10. 21–10. R. Cerri, Sources of Measurement Error in Instrumentation Systems, Preprint 19‐LA‐61, Instrument Society of America, Research Triangle Park, NC.
11. 21–11. P. M. J. Hughes and E. Cerny, “Measurement and Analysis of High‐Frequency Pressure Oscillations in Solid Rocket Motors,” Journal of Spacecraft and Rockets, Vol. 21, No. 3, May–Jun. 1984, pp. 261–265.
12. 21–12. Handbook for Estimating the Uncertainty in Measurements Made with Liquid Propellant Rocket Engine Systems, Handbook 180, Chemical Propulsion Information Agency, Apr. 30, 1969 (AD 855130).
13. 21–13. “Grounding Techniques for the Minimization of Instrumentation Noise Problems,” Report TR‐65‐8, Air Force Rocket Propulsion Laboratory, Jan. 1965 (AD 458129).
14. 21–14. S. Schmalzel, F. Figuero, J. Morris, S. Mandayam, and R. Polikar, “An Architecture for Intelligent Systems Based on Smart Sensors,” IEEE Transactions on Instrumentation and Measurement, Vol. 34, No. 4, Aug. 2005.
15. 21–15. D. K. Shaver and R. L. Berkowitz, Post‐accident Procedures for Chemicals and Propellants, Noyes Publications, Park Ridge, NJ, 1984.