Introduction to Propulsion: Rocket Engines

Propulsion:Safety, Handling, and Test Procedures

By Mac Seidel

General Propellant Safety*Evaluate Safety and Handling Before

Finalizing Propellant Selection

Some Considerations• Toxicity

• harmful to the environment or human

health when coming in contact

• Exothermal Decomposition• rapid reactions can be dangerous by

releasing lots of heat

• Explosive• decomposes or rearranges with extreme

rapidity, yielding much gas and heat

• Volatility• how readily a substance vaporizes

• Flammability• ability to ignite and combust

• Cryogenic• boiling points below -150°C

• Hypergolic• fuels that ignite spontaneously upon

contact

Hyperbolic Fuels

Hazard Symbol Ex.

Nitrous Oxide Properties• Non-cryogenic, liquid oxidizer

• Density and vapor pressure are highly

dependent on temperature

• Colorless, odorless

• Can displace air and cause asphyxiation

• Can get cold and cause frostbite

• Possibility of rapid exothermic

decomposition

• Avoid direct contact with skin

• Open cylinder valve slowly

• Thermodynamic Properties of N2O• Tables of properties for range of

temperatures

• Chemical Properties and Hazards• Data sheet from PRAXAIR

• Instructions for danger relief

Linde N2O Vapor Pressure

N2O Density

Nitrous Material CompatibilityIncompatible• Organic compounds

• Materials not rated for oxygen use

• Copper, nickel, other catalysts

• Viton, FKM, FPM are chemical compatible but

are swelling significantly when exposed to

N2O (spl.ch)

• Certain grades of elastomers such as Viton®

or Neoprene® are known to swell (AGIA)

Compatible• 304 and 316 Stainless Steel, Aluminum, PTFE

and other Teflon's, Kalrez/FFKM

• PTFE, PCTFE, FEP, PEEKTM, and EPDM

(AGIA)

Elastomer O-rings

Stainless Steels

Nitrous Pros/ConsPros

• Decent availability

• Nontoxic

• Relatively “safe”, only if handled with

caution and safe practices by knowledgeable

personnel

• Easy storage in cylinders

Cons

• Density (therefore performance) highly

dependent on temperature

• Need to have high system pressures to

stay above vapor pressure

• Greenhouse gas

• Susceptible to two phase flows

Two Phase Flow Regimes

Common for Performance Cars

Nitrous Dangers• Decomposition can occur at elevated temperatures

or in contact with incompatible materials

• Explosions have occurred• Industrial and rocketry projects

• Sources like contaminations, local heating,

adiabatic compression/water hammer,

combustion instabilities, electrostatic discharge,

reverse flow of hot chamber gas into tank vapors

• Freezing of valves and/or vents• Temperature decreases with venting

• Good solubility in oil, grease, plastics, and other

hydrocarbons• Creates highly explosive material

• Be very carful of seals, Viton has been used, if

swelling is observed then try Kalrez

(FFKM/FFPM)

• Asphyxiation from vapors

• More on dangers and suggestions• SPL, Is nitrous oxide safe?

• Air Force, Nitrous Oxide Explosive Hazards

• Aspirespace, Hybrid Safety

• More in google drive Scaled Composites

Hybrid Motor

Linde N2O Storage Facility

Nitrous Safety and HandlingRecommendations

• Be extremely attentive when ox cleaning lines

• Use nitrous and pressurant filters • 500 to 150 micron for gases (AIGA)

• Max pressurizing rate of 20 psi/s (SpaceDev)

• Keep nitrous temperatures low

• Electrically ground tanks

• Remote testing operations

• Decent injector pressure drop to avoid

combustion instabilities

• Never run cold flows with nitrous• Use water or CO2

• Avoid nitrous vapors going through engine

• Avoid two phase flow by further pressurizing

with inert gas• N2 or He

• Note that N2 is soluble in N2O, but should be

fine if pressurizing for short periods

• AIGA Safe Practices for N2O• Additional Info

AIGA Oxidizer Pipe Velocity

Vapor Lock and Temperature Effects on Filling• Vapor lock

• When the flow of liquids stops due to

vaporization of the liquid

• Flow is driven by pressure differential

• NO2 Filling• Sometimes use a pump, but not always

needed

• As vapor in NO2 tanks build, the pressure

differential decreases

• To keep pressure difference use small tank

vent, or maintain temperature differential • Temperature drives NO2 vapor pressure

• Can use ice around run tank and cool

down by venting

• Could use warm water over fill tank, not

hot though

NOS Fill Station Ex.

Ethanol Safety and HandlingProperties and Hazards• Different concentrations available

• Lower concentrations with water yield lower

combustion temperatures

• Highly flammable

• Avoid direct contact with skin

• Low volatility but vapors still present, don’t inhale

• Data Sheet

Incompatible Materials (degradation over time)• brass, lead, zinc and lead-based solder

• natural rubber, polyurethane, cork gasket material,

leather, polyvinyl chloride (PVC) polyamides, and

certain thermoplastic or thermoset polymers

Compatible Materials (resistance to degradation)• unplated steel, nickel-plated steel, stainless steel,

black iron and bronze

• reinforced fiberglass, Buna-N, Neoprene rubber,

polypropylene, nitrile rubber, Viton and Teflon

Ethanol Material Compatibility

Chart (google doc)

Personal Protective Equipment (PPE)When to use?

• Leak testing

• Pressure testing

• Hot fires

• Manufacturing

• Anytime when handling hazardous

chemicals, pressurized systems, or

dangerous equipment

• Check for proper PPE before testing!

Typical PPE

• Safety goggles

• Face shield

• Nitrile gloves

• Ear plugs

• Long pants

• Closed toe shoesVarious PPE

Oxidizer Cleaning• Oxygen rich environments need to be clean

of dirt, rust, organics, etc.• If not, then there’s a large fire hazard

• Use different solvent chemicals for

different materials or situations

• Ex: isopropyl alcohol, acetone,

trichloroethylene, simple green

• Basic Procedures• Pre-cleaning

• Deep cleaning

• Rinse in DI water

• Dry with nitrogen gas or in lab

convection oven

• Inspect visually with UV light and lint-

free wipes

• Bag in plastic zip-locks and label

• Standards• ASTM G93

• NASA Safety Standards for Oxygen

• Other team’s procedures

• Specifically for Nitrous Oxide• SpaceDev Report

• Youngblood Paper

Ultrasonic Cleaner

Lint-free Wipes

Pressure and Leak TestingLeak Testing• Ensure leak tight connections before using

propellants

Pressure Testing• Proof test tanks, engines, or other plumbing for

pressurized use

• Typically to 1.2-1.5 FOS, different standards and

requirements out there, so check before doing

• Pneumatic pressurization• Inert gas like nitrogen or helium, helium

molecules are small so it is very good at

finding cracks

• Compressible, so there can be danger of rapid

expansion of gases throwing loose fittings,

hoses, or fragmentation

• Depending on pressure range, can test in

person or far away by monitoring sensors

• Hydrostatic pressurization• Need hydrostatic pump and water

• Less hazardous since lower expansion

potential, incompressible

• At high pressures, possibility of water jet

cutting hazardHydrostatic Pumps

Hose Whip Arrestors

Pneumatic Leak Testing Procedures• Not bad idea to do hydrostatic proof test first

• Could also do hybrid test with nitrogen pressing on

water

• Connect nitrogen cylinder with pressure regulator to

pressurize system• Set regulator pressure before allowing gas to

flow through rest of plumbing

• Check each fitting by applying drops of leak detection

fluid, like Snoop (soapy water)

• Pressurize section, check fittings/monitor decaying

pressures, depressurize, tighten fittings where

needed, and repeat• Slowly go up to about 110% of nominal system

pressures, (not above MAWP)

• Use increments of 50-100 psi

• Only increase pressures when no leaks are

detected

• Never adjust fittings while pressurized

• Progressively test feed system in smaller sections to

isolate leaks

• Can torque stripe fittings after all leaks fixed

• This allows to keep track of fittings already

tested and easily show if any become un-

torqued

Snoop Bottle

Torque Stripping

Bubbles Indicate Leak

Cold Flows and Ignition TimingCold Flow Testing• Flow fluids through feed system and injector

without combustion• Use safe fluids with similar properties, like

water or CO2

• For calibrating system and evaluating

performance• Determining pressure drops, flow rates, flow

coefficients, injector atomization, any issues,

etc.

• Determine actual set pressure of regulators,

can use needle valves in place of engine to

simulate pressure drop

• Determine Cv or Cd coefficients for injector,

make sure to use desired pressure drop

Ignition Timing• Test timing of ignitor in relation to propellant flow

• Good ignition timing is needed to ensure

propellant ignition and avoid hard starts • Light ignitor before opening propellant valves

Fluid Impulse

Purdue Pintle Injector

BURPG Impinging Injector

Developing Testing Procedures• Plan out operating and contingency procedures in clear

lists to read off during test day• SOP – instructions to carry out testing operations

• COP – instructions to follow in case of off nominal

conditions, if stuff goes wrong

• Walk through step by step, and write down line by line

specific, concise operation instructions• Think through each step and what states each part

of the system will be in

• Safety should be most important priority

• Include the action, the component name, target values,

and expected results

• Make sure to include verification checks of valve

positioning, data collection, power supply, etc.

• Revisit and revise procedures many times to catch

errors and constantly improve• Make sure to keep track of edits and reasoning

• Practice running them in person without propellants to

nail down all steps and ingrain into memory• Make sure all test technicians and control operators

know these

• Get them reviewed by a professional if possible

Draft of NETS Operating Procedures

Risk and Hazard Analysis• Create a document or chart of the potential

hazards that could occur and how to mitigate them• Ex. Leaking propellant, ignition failure, etc.

• Could include danger level, likelihood,

preventative measures, possible causes, etc.

• MASA has a good example in their Spaceport

doc

• HAZOP – Hazard and Operability Study• Go through each plumbing component and

operating process

• Determine what the expected result would be if

each part fails

• If it poses a serious hazard then try to find a

design solution to fix it

• Very tedious but can easily help to find flaws Rocket Engine Diagram

Basic Hazard Analysis

TNT Equivalence and Safe Testing Distance• TNT Equivalence is a common measure of energy released

from an explosion• It compares the energy in a system to the energy per

weight of TNT• TNT Energy = 4850 kJ/kg

• Different models/equations for calculating stored gas

energy release, isentropic model is historically common

• Include all compressed gas cylinders, in case of rupture

• Include decomposition energy of nitrous present

• Safe Stand-off Distances• The TNT equivalence can then be used to determine safe

distances for testing

• Different for enclosed or open space, can lessen affected

area by enclosing test or using shielding

• Distances vary for dangers• Blast wave vs. Fragment throw

• Overpressure: Glass break (0.2 psig), Ear drum rupture

(2.4 psig), Lung hemorrhage (14.5 psig)

• Some Resources• DOE Stored Energy Risk Analysis

• Understanding Explosives• See Appendix B

• ASME Pneumatic Testing Distances• Safe distances for TNT weights

• ASME PCC-2 Mandatory Appendix 501-II, pg. 253-255

• Piping-World online summary

Compressed Gas Energy Release

(Isentropic Method)

Nitrous Decomposition

TNT Equivalent

Weight

Scaled Blast Radius

Additional Resources• Google Drive: Nitrous Safety and Handling Folder

• Doc with lists of safety and handling info and links to helpful sources!!!

• Material compatibility pdf’s

• Ox line cleaning and safety

• TNT equivalence calculations

• Aspirespace• Hybrid Safety

• Has info on nitrous as well as other oxidizers

• Purdue Video• Safety Procedures and Best Practices

• Mach 5 Low-Down • Rocket Test Stand and Testing Tips

• Gas Cylinders• Penn State Policies

***Please feel free to add information/slides for future presentations!!!