Experimental Rocket Motor Safety & Standards Manual

Experimental Rocket Motor Safety & Standards Manual Compiled by Glen Meyerowitz, YPL Director, Yale University Version 1.2 June 28, 2013

YPL

Experimental Rocket Motor Safety & Standards Manual

Table of Contents

Section 1 – Overview

3

Section 2 – General Safety Procedures

4

Section 3 – Static Test Stand Design and Considerations

6

Section 4 – System Plumbing Design and Considerations

8

Section 5 – Rocket Motor Design and Considerations

10

Section 6 – Motor Testing Location

13

Section 7 – Motor Testing and Certification

14

Appendix A – NAR High Power Rocket Safety Code Minimum Distance Table

15

Appendix B – Motor Test Safety Checklist

16

References

18

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Section 1 – Overview 1.1 Manual Purpose

This manual serves to outline the procedure and methods used by members of the YPL in designing, creating, and testing experimental rocket motors. All methods outlined here delineate the operating procedures of the YPL and shall be adhered to at all times. Additions to this manual will be made when shortcomings are found, or new projects are adopted by YPL that require additional safety precautions. This manual supplements the requirements of the National Fire Protection Association (NFPA) Code 1125, Code for the Manufacture of Model Rocket and High Power Rocket Motors. In addition, this manual supplements the Standards & Testing Committee Motor Testing Manual from the National Association of Rocketry (NAR). 1.2 YPL Organization

The YPL is working on rocket propulsion with the support of the Yale School of Engineering & Applied Science and the Center for Engineering, Innovation, and Design (CEID). Team members have considerable experience working with commercially purchased rocket motors, both solid and hybrid fuel, and YPL is planning to make use of this experience in designing, testing, and creating new rocket motors. The motors to be tested are hybrid fuel motors. These make use of a liquid oxidizer and a solid fuel grain. The liquid oxidizer is nitrous oxide (N2O) and the solid fuel will be chosen from different types of plastics such as polypropylene, polyethylene, PVC, and HTPB. The test stand is constructed from steel unistrut and is designed to handle a much larger load than will be expected for these motor tests. Designs for the static test stand are available in Section 3.

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Section 2 - General Safety Procedures 2.1 General Precautions

We will follow all procedures outlined in the National Fire Protection Association (NFPA) Code 1125 regarding testing of rocket motors. We are verifying function, safety, and performance as indicated in Chapter 8 of NFPA Code 1125. 2.2 Experimental Rockets

The motors are classified as experimental rocket motors. This classification means that they have not yet been certified by the National Association of Rocketry (NAR), Canadian Association of Rocketry (CAR), or Tripoli Rocketry Association (TRA). 2.3 Storage of Materials

The various materials being used for fuel are all separate from each other prior to combustion. Thus they have no way of interacting and safety is increased. The nitrous oxide is being stored in the lab of Professor Juan Fernandez de la Mora. The solid fuel sources are being stored in the basement of the Becton Center in the IEEE room. Igniters and preheater grains are also being stored in the IEEE room with the approval and guidance of Yale Environmental Health and Safety. 2.4 Minimum Distance from Test

During testing, all observers shall be at a distance equal to or greater than the launch separation distance in the NAR Safety Code (Appendix A). The static test stand has been designed to withstand and contain unexpected blasts created by the rocket motor. In the unlikely event of the failure of the test stand to contain event, this minimum distance will help to ensure that all observers are safe. 2.5 Weather Conditions

Weather must be suitable for test. If there is signiﬁcant rain, snow, or other precipitation, the test will be called off. If there are any other conditions that might affect the test, the test will also be called off.

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2.6 Alert Yale Security and Yale Police

Yale Security and Yale Police will both be contacted 24 hours prior to a test as well as 15 minutes prior to a test to ensure that they are aware of all activities and ready to assist in case of emergency. They will also be contacted after a test to update them on the status. 2.7 Safety Equipment

A class ABC dry chemical fire extinguisher will be on site at all times during testing. Everyone observing the launch will be required to wear appropriate safety glasses. Work gloves will be worn by team members when appropriate, such as when dealing with nitrous oxide, sharp edges, or dangerous materials.

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Section 3 - Static Test Stand Design and Considerations 3.1 Static Test Stand Design

The static test stand for these motors is constructed out of galvanized steel unistrut channel pieces. The stand is 18”x15”x48”, and will hold the motor in place during testing. Motors will be vertical in the stand to increase reliability and safety. The test stand will be able to collect data such as thrust, pressure measurements, and temperature readings to increase awareness and safety.

Figure 1 - YPL static test stand design

There are two internal layers that have locking mechanisms to hold the motor in place during test, but allow it to still press against the load cell. Impact resistant polycarbonate will be placed around the exterior of the test stand to contain any unexpected event during testing. There is a blast deflection shield constructed from a layer of steel and aluminum to diffuse the flame and protect the stand. The top of the stand will be left open to allow for venting during testing. 3.2 Stability of Static Test Stand

The test stand, when fully instrumented and installed, weighs approximately 45kg. In order to increase the stability of the test stand, unistrut channel will be added along the base to increase the area of the base to 18”x60”. This additional area also serves as a storage space for additional weight that can be added depending on the needs of the motor. Version 1.2, 6/28/2013

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3.3 Explosion of the Motor

All observers shall be at a distance equal to or greater than the launch separation distance in the NAR High Power Rocket Safety Code Minimum Distance Table (Appendix A) during testing. In the event of an explosion, this will help to ensure that all individuals remain safe. The static test stand has four pieces of ¼” polycarbonate around the outside of the static test stand. This helps to ensure that fragments of the motor cannot blow in the direction of the observers if a casing failure were to occur. Numerous ballistic tests have been performed on polycarbonate to analyze the strength and ability of the material to absorb energy. One particular study by Wright et al. demonstrates the 3 impact strength of polycarbonate. Wright shot 7mm silver steel balls (density of 7.80 g/cm ) at a 7 2mm thick piece of polycarbonate and did not perforate the plate. The polycarbonate we are using for our test stand is more than three times as thick as that used by Wright, and the total kinetic energy of the rocket motor will be approximately equal to the kinetic energy of the projectile used in that analysis, which can be estimated off the mass and velocity of the motor. Therefore, we believe it is safe to assume that our blast shielding will be able to absorb any debris created by an event. 3.4 Motor Exhaust and Blast Deflection

A blast deflection shield will be placed at an angle along the base of the test stand to deflect the blast away from the test stand, and observers, during testing. This blast deflection shield is made of two separate layers. The top layer is 1/16” 1018 mild steel, and the bottom layer is 1/8” T6 temper 6061 aluminum. The top layer of steel provides a strong way to absorb the blast strength, as this material has an ultimate tensile strength of 63,800 PSI, nearly 50% greater than that of 3 aluminum 6061-T6. The second layer of aluminum serves to provide both additional strength and also to help dissipate heat caused by the motor test. 1

Aluminum 6061-T6 has a thermal conductivity of 167 W/m-K, and this allows for a very rapid dissipation of heat from the material. With the use of Fourier’s law of heat conduction, shown below, the expected temperature of the blast deflection shield can be calculated depending on the expected burn time of the motor. In the question, is the local heat flux, k is the thermal conductivity and T is the temperature gradient.

∇

⃗ ⃗ ∇

= o The melting point for this material is also between 562-652 C, and this is a greater temperature than we are expecting from the exhaust of the rocket motor.

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Section 4 - System Plumbing Design and Considerations 4.1 Plumbing Design for Test Stand

Hybrid rockets are complex pieces of technology and the design of an appropriate system for the transfer of oxidizer to the rocket motor is crucial to performance. YPL has designed an injection system for the testing of rocket motors that is both robust in its performance and contains built-in safety measures to help to ensure that tests are conducted as safely as possible.

Figure 2 - YPL Hybrid Rocket Plumbing Diagram

Our system contains a method for transferring nitrous oxide from the canister to the rocket motor itself. There is a 50 foot fill line separating the manual purge valve from the motor, and then additional tubing between the valves and the N2O tank to help to ensure there is no flashback. The flow is controlled by a solenoid valve which must be activated for the N2O to flow. There is a manual purge valve which will be opened prior to approaching the rocket motor. In addition to the nitrous oxide, there will be a separate nitrogen tank attached to the test system. Nitrogen (N2) is an inert gas that will not combust. Thus, the system can be flooded with nitrogen gas after a test to cease combustion inside the motor chamber. The system will be flooded with nitrogen before any individual is allowed to approach. Again, the nitrogen will be controlled by a solenoid valve and there will be a manual purge valve to clear the line.

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4.2 Nitrous Oxide Injection

The N2O will be injected into the solid fuel grain to begin combustion. There will be a 50 foot tube connecting the N2O tank to the motor, thus decreasing the likelihood of any interaction between the motor and the tank itself. The N2O is denatured, which means there is 50 ppm SO2. The N2O will be stored in a 10 pound tank. The use of denatured N2O is to help ensure that this substance is not abused by members. 4.3 Measuring Pressure in the System

Pressure can be measured at several points throughout the system. There will be pressure gauges connected to the nitrous oxide tank and the nitrogen tank to measure the internal pressures. In addition to this, several pressure transducers will be connected to the combustion chamber to help ensure that the internal pressure does not rise above critical design features, determined by the material properties of the motor casing. In the case of an unexpected rise in pressure at any point in the system, the test will be terminated.

4.4 Electronics for Plumbing System

The solenoid valves and pressure sensors will be connected to an electronics system that will allow YPL members to collect data and run the test from a safe distance away. Ed Jackson has been contacted about the design of the electronics system used in this system and his comments have been incorporated. Failsafe systems exist to terminate the test if there are unexpected readings on any of the sensors. Failsafe systems will be both electrical and manual. YPL members will be able to actively disconnect power to the system and terminate the flow of nitrous oxide to the combustion chamber.

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Section 5 - Rocket Motor Design and Considerations 5.1 Motor Design

The motor that we plan to test has been completely designed and machined by YPL student members to meet very precise specifications and design requirements. Motor parts will experience high pressure and temperatures and will be designed to withstand these conditions. The current design is shown to the right. The figure contains two end caps, a top bulkhead, a solid fuel grain, a motor casing, and a nozzle.

Figure 3 - YPL rocket motor design

5.2 Solid Fuel

The rocket motor is a hybrid motor, which makes use of both solid fuel and a liquid oxidizer. The fuel grain is a plastic such as solid polypropylene, polyethylene, PVC, and HTPB. The oxidizer is nitrous oxide (N2O). Prior to ignition, the solid and gaseous fuels are separated at all times, which makes hybrid rocket motors much safer than traditional rocket motors. 5.3 Motor Casing

Our motor is designed to use T6 temper 6061 aluminum for the outer motor casing. This material 1 has a tensile yield strength of 40,000 PSI. Barlow’s formula, below, can be used to relate the internal pressure of the motor casing to its dimensions and strength of material. 2 =

 

In this equation, P is the pressure, S is the yield strength, t is the wall thickness and D is the material outside diameter. For instance, a 1” OD aluminum 6061-T6 tube with 0.0625” wall thickness can withstand pressures up to 5,000 P SI. Inside a hybrid rocket motor, we are expecting pressures to remain around 300 PSI. The example above has a safety factor greater than 16. All motors will be designed with a safety factor of at least 3. 5.4 Threading Design

The motor casing will be threaded on each side, and an appropriate cap will be made to screw onto the motor casing and seal it. The threads between the cap and the motor casing are crucial to the performance of the motor and must be designed to withstand the force that the rocket motor Version 1.2, 6/28/2013

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is expected to experience. If the thread fails, the motor will no longer be contained within the 4 casing. The following diagram demonstrates key p arts of threading.

Figure 4 - Diagram of screw, with threading

Usually a fine thread is used on tubing so that tensile fracture of the tube will not occur. The shear strength for the tube can be calculated by obtaining the shear area and multiplying by the shear strength of the material. Shear area c an be calculated with the following formula: 1 = + 0.57735( ) 2 Here, n is the number of threads per inch, D pitch is the pitch diameter of the external thread, and Dmin is the minimum diameter of the internal thread. As is the resulting shear-stress area.

   � 

ℎ  

Using the tube in the previous example, a 1” OD aluminum 6061-T6 tube with 0.0625” wall thickness, with 28 TPI, it can be shown that this has a shear strength of well over 100,000 N. This is a considerable greater force than our test stand is designed to withstand. All threaded material will be subject to physical tests to help ensure it will stand expected pressures with a safety factor of at least 3. 5.5 Nozzle Design

Rocket nozzles will be machined out of graphite. Graphite is able to withstand very high temperatures without melting, and thus is the perfect material to make a standard convergentdivergent nozzle. There are numerous methods to calculate the size and characteristics of rocket nozzles. The following equation requires knowledge of the Mach number at several points in the flow and the heat capacity ratio, γ. The heat capacity ratio is the ratio of the heat capacity at 7 constant pressure to the heat capacity at constant volume.

     γ  22 +1 −1 γ  =

1+[(

1)/2]

1+[(

1)/2]

(

)/(

)

In this way, by knowing the Mach numbers and the heat capacity ratio for the motor we are designing it is possible to create a nozzle with the appropriate dimensions to allow for best performance.

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5.6 Hydrostatic Testing

In order to help ensure that the motor can withstand expected pressures, the motor casing will be subject to hydrostatic tests. These tests will be conducted in a specialized pressure vessel designed by students and will allow us to experimentally verify the strength of the parts that we have designed and machined. We are in the process of securing a specialized hydrostatic pump that will be able to pressurize the casing to at least 300 PSI. Standard procedure used in hydrostatic testing will be used to ensure that the motor casing is able to support the internal pressure and that the threading has been done in such a way as to support the force created by the test. The location for the test has yet to be determined, and will depend on the availability and location of the pump.

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Section 6 - Motor Testing Location

YPL plans to test experimental rocket motors from Yale West Campus. West Campus is the only location that offers us the distance that allows for increased safety, and the remoteness required for testing experimental rocket motors. YPL is seeking a permanent location to establish a test site. A permanent location will increase safety and reliability of our tests. The static test stand described in Section 3 is the key safety feature that allows YPL to consistently perform tests of rocket motors. As noted, the test stand weighs 42 kilograms, which makes transportation of the stand difficult and cumbersome. In order to successfully transport the stand, it will need to be disassembled and then reassembled onsite. With a permanent location on West Campus, we will not need to disassemble and reassemble the test stand whenever we plan to test. This will greatly increase safety and reliability and decrease risk of human error.

Figure 5 - YPL West Campus testin g locatio n

If we are able to have a permanent test facility at West Campus, this would also allow us the freedom to more securely plant our static test stand at the site. The static test stand can be securely placed in a foundation and thus greatly increase strength, stability, and durability. YPL is waiting for confirmation from Yale West Campus Administration to proceed with establishing a test facility. In Section 4, various plumbing designs were discussed to increase the safety and reliability of the test stand. With a permanent location, we will be able to have a nitrogen tank delivered to the site and not need to worry about transporting a large canister of pressurized gas.

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Section 7 – Motor Testing and Certification 7.1 Test Personnel

The YPL Director will be responsible for overseeing all aspects of the test program, and will decide on issues related to rocket safety. All test sessions will require a minimum of three persons to be present and able to deal with emergencies as they arise. If the Director cannot be present, a different member will be designated to take on those responsibilities. The individual with these roles will be referred to as the “test sup ervisor.” 7.2 Motor Test Procedure

All tests will be in accordance with Chapter 8 of NFPA Code 1125. As noted in the NAR High Power Safety Code, all observers will be required to stand a minimum distance from the rocket. The motor test procedure is outlined in the Motor Test Safety Checklist found in Appendix B. 7.3 Certification of Rocket Motors

In order for a rocket motor to gain YPL certification, it will need to be successfully tested a minimum of three (3) times. YPL also accepts NAR, TRA, and CAR certification. Data and information relating to the test will be kept on hand for all certified motors. Only motors that have received YPL certification will be used in YPL rockets for launches.

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Experimental Rocket Motor Safety & Standards Manual 4

Appendix A - NAR High Power Rocket Safety Code Minimum Distance Table

Installed Total Impulse (Newton-Seconds)

Minimum Personnel Distance (ft.)

0 -- 320.00

100

320.01 -- 640.00

100

640.01 -- 1,280.00

100

1,280.01 -- 2,560.00

200

2,560.01 -- 5,120.00

300

5,120.01 -- 10,240.00

500

10,240.01 -- 20,480.00

1000

20,480.01 -- 40,960.00

1500

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Appendix B – Motor Test Safety Checklist

•

•

Alert police to a test o

24 hours prior to test, contact Yale Police and Yale Security to alert them of test

o

15 minutes prior to test, contact Yale Police and Yale Security to alert them of test

o

After test, contact Yale Police and Yale Security to update them on test status

Check test stand for signs of wear or dama ge o

•

•

•

Check polycarbonate for signs of scratching or damage. o

No dents may be deeper than 3 mm or longer than 1 inch.

o

No visible cracks on the shield.

Calibrate all test equipment in accordance with the procedures associated with the hardware and the data acquisition system. o

Thermocouples will be used to measure the external casing temperature during testing. Measurements are taken to determine that this temperature does not o exceed 200 C during or after firing, as required by NFPA 1125. If temperature exceeds limit, cease testing.

o

Pressure sensors will be used to measure the pressure at several points alon g the system. Sensors will be placed in tanks, along tubing, and inside the motor casing. If there are any unexpected changes in pressure, cease testing.

o

Check to see that all valves are securely connected

o

Each static test firing will be videotaped and pho tographs will be taken.

Assemble motor appropriately o

Install igniters appropriately.

o

Motors, fuel grains, oxidizer tanks etc. shall be kept within the environmental conditions specified by NFPA 1125 for testing.

o

o

•

Check hardware on test stand for appropriate strength and stability

Testing will be conducted in a manner as consistent as possible with actual launch conditions Procedures will duplicate operational conditions and maintain storage environmental conditions as closely as possible.

Motors will be weighed to the nearest gram or better before and after firing. o

o

In the case of hybrid motors or other situations where this is not entirely practical, subcomponents (such as hybrid fuel grains) will be weighed before firing, and post-firing weights will be made by calculation or reference. In all cases, test personnel will tabulate pre-firing and post-firing weights as accurately as possible.

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•

•

•

•


Clear audible countdowns will be given, and all participants and observers must be situated at safe distances and locations. o

Safe distances and locations are determined b y NAR safety codes

o

All personnel and observers must wear safety glasses

Electronics o

Check all electrical connections

o

Check to make sure that safety interlock systems on the control box are working

o

Turn on safety strobe light before test

Keep fire extinguisher accessible NAR Safety Code procedures, where applicable, shall be observed during test sessions. In the event of a catastrophic failure on the test stand, personnel are to remain situated at a safe distance until all possibility of further action, (propellant after burn, debris landing) has ended. o

Then, test personnel shall assess the test equipment and motor hardware for damages.

o

The test crew will make a complete photographic record of the test hardware and effects of any catastrophic failure.

o

If test equipment is damaged to the point where further testing is not possible or, in the judgment of the test supervisor, further testing may be questionable, the test session will be suspended. 

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Further testing will not be resumed until repairs have been effected.

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References

1. ASM Aerospace Specifications Metals Inc., http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6 2. Dennet, Mike. Motor Certification Committee Manual. Canadian Association of Rocketry (CAR), January 25, 2002. 3. Eagle Steel. Carbon Steel Grades. http://www.eaglesteel.com/download/techdocs/Carbon_Steel_Grades.pdf 4. Fastenal Engineering & Design Support. Screw Thread Design. 5. Kreck, Bob. Standards & Testing Committee Motor Testing Manual. National Association of Rocketry (NAR), July 1, 2011. 6. National Fire Protection Association (NFPA) Code 1125 7. Sutton, George, and Oscar Biblarz. Rocket Propulsion Elements. New York, NY: WileyInterscience, 2000. 8. Wright, C. S., Fleck, N. A., and Stronge, W. J. Ballistic Impact of Polycarbonate – An Experimental Investigation. Int. J. Impact Engng Vol. 13, No. 1, pp. 1-20, 1993.

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Experimental Rocket Motor Safety & Standards Manual

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