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Nozzle Erosion in Long Burn Duration RocketsPavan Narsai, Brian Cantwell

Department of Aeronautcs & Astronautics, Stanfrord University

Evans, B. (2008). Nozzle Erosion Characterization in a NonMetallized Solid Propellant Rocket Motor Simulator.Internation Journal of Energetic Materials

and Chemical Propulsion , 90.Coy, E. S. (2010).Film Cooling of Liquid Hydrocarbon Engines for Operationally-Responsive Space Access.Edwards AFB, CA: Air Force Reserach Laboratory (AFMC).

Sutton, G. B. (2010).Rocket Propulsion Elements.Hoboken, New Jersey, USA: John Wiley & Sons, Inc.

Thakre, P. A. (2009). Chemical Erosion of Refractory-Metal Nozzle Inserts in Solid-Propellant Rocket Motors.Journal of Propulsion and Power, 25(1).

A major obstacle ahead of the use of long-burning rocket motors forupper stage systems is the nozzle erosion problem. Nozzle throat

regression can lead to large performance losses due to reducedexpansion ratios and possibly reduced efficiencies. Although the nozzle

erosion problem has been extensively studied for liquid engines andsolid rocket motors, relatively little work has been done for hybrid

rockets.

Introduction

Nozzle erosion is a problem t hat has been relatively untouched forhybrid rockets. Regression rates for common materials are not readily

available for hybrid rocket systems, and only simple mitigatingtechniques have been explored to reduce erosion. However, these

simple solutions may not be satisfactory for long burning systems, which

can experience significant erosion.

Purpose

Nozzles are simple devices that convert thermal energy of gas intokinetic energy. Nozzle performance is characterized by thrust generation

from high pressure fluids. Parameters include the expansion area ratio,Ae/AT, chamber to ambient pressure ratio PT/Pa, exit to ambient pressure

ratio Pe/Paand the ratio of specific heats !.

The throat sections of nozzles typically regress significantly, causing a

reduction of the expansion area during burn. Furthermore, asignificantly larger throat area can cause losses in c* efficiencies.

Nozzle erosion can be attributed to three different phenomena:

Erosion due to ablation and heat transfer Erosion due to particle collisions Erosion due to surface oxidation

Nozzle Performance and Erosion

One method of modeling the regression of the throat section is treatingit as a hybrid system. The regression rate is then related to a

regression rate coefficient and a propellant flux term.

For simplicity, the regression rate coefficient is assumed to be constant.

The differential equation can then be integrated.

This model predicts increasing throat regression rates as chamberpressure is increased. It also predicts large losses in expansion area

ratio for long burn times.

Modelling Nozzle Erosion

Nozzle erosion issues for hybrid rockets needs to be further studied.Currently, most nozzle protection schemes mimic those of seen in solid

rocket motors. However, the hot gases flowing through the nozzle are

more similar to t hose found in liquid rocket systems, which typicallyemploy more complex cooling schemes. Some possible nozzle

protection schemes can include: Mimicking film cooling by careful placement of fuel grain ahead of

nozzle throat. Nozzle cooling by using the pressurant fluid that is usually already

present in hybrid systems to maintain oxidizer tank pressure, an inert

gas, or the oxidizer.To fully understand nozzle erosion in hybrid systems, regression rates

for common materials need to be measured, along with the testing andmodeling of various nozzle cooling and protecting schemes.

Future Work

Bibliography

Financial support for this research is provided by the Department ofAeronautics & Astronautics at Stanford University.

Acknowledgments

For further information, please contact Pavan Narsai.Email: pnarsai@stanford.edu

Further Information

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Techniques for minimizing nozzle erosion fall into three classes: LiquidEngines, Solid Rocket Motors, and Hybrid Rocket Motors.

Liquid Rocket Engines Film cooling Nozzle cooling through liquid fuel and oxidizerSolid Rocket Motors Additives used to reduce oxidizing speciesHybrid Rocket Motors Additives can be used to reduce oxidizing species, as well as reduce

the optimal oxidizer to fuel mass (O/F) ratio.

At Stanford, zirconium oxide coating of nozzle surfaces has beenused to protect nozzles during short burns.

Minimizing Nozzle Erosion

Above: Simulations for nozzle erosionvs. chamber pressure with a 3600 second

burn time. An conservative regressionrate constant is used.

Below: Thrust coefficient curves forvarious pressure and area ratios. Note

the maximum point of each curverepresents perfect expansion, where exit

pressure matches ambient pressure.Note: Regression rates listed are typical for solid rocket systems.

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Pt/Pa = 5

Pt/Pa = 10

Pt/Pa = 20

Pt/Pa = 50

Pt/Pa = 100

Pt/Pa = 200

Pt/Pa = 500

Pt/Pa = 1000

Pt/Pa = 2000

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