hybrid rocket propulsion for future aircraft

Hybrid Rocket Propulsion for

Future Space Launch

May 09, 2008

Aero/Astro 50th Year Anniversary

Arif Karabeyoglu

President and Chief Technology Officer, SPG Inc.

Consulting Professor, Department of Aeronautics and Astronautics, Stanford University

Aero/Astro 50th Year AnniversaryHybrid Rocket Configuration

Most Hybrids:Oxidizer: Liquid

Fuel: Solid

2

Fuel and oxidizer are physically separated

One of the two is in solid phase

Reverse Hybrids:Oxidizer: Solid

Fuel: Liquid

Aero/Astro 50th Year AnniversaryHybrid Rocket System

Solid Fuel

• Polymers: Thermoplastics,(Polyethylene, Plexiglas),Rubbers (HTPB)

• Wood, Trash, Wax

Liquid Oxidizer

• Cryogenic: LO2

• Storable: H2O2, N2O, N2O4,IRFNA

3

Aero/Astro 50th Year AnniversaryAdvantages of Hybrids

4

- Reduced development costs are expected

- Reduced recurring costs are expected

Cost

- Reduced number and

mass of liquids

- Reduced environmental

impact

Other

- Higher fuel density

- Easy inclusion of solid

performance additives (Al,

Be)

- Better Isp performance

- Throttling/restart

capability

Performance Related

- Reduced fire hazard

- Less prone to hard starts

- Reduced chemical

explosion hazard

- Thrust termination and

abort possibility

Safety

- Mechanically simpler

- Tolerant to fabrication

errors

- Chemically simpler

- Tolerant to processing

errors

Simplicity

LiquidsSolidsCompared to

Aero/Astro 50th Year AnniversaryHybrid Rocket History

Early History (1932-1960)

• 1932-1933: GIRD-9 (Soviet)– LO2/Gellified gasoline 60 lbf thrust motor– Firsts

• Hybrid rocket• Soviet rocket using a liquid propellant• First fast burning liquefying fuel

– Tikhonravov and Korolev are designers– Maximum altitude: 1,500 m

• 1937: Coal/Gaseous N2O hybrid motor 2,500 lbf thrust(Germany)

• 1938-1939: LOX/Graphite by H. Oberth (Germany)• 1938-1941: Coal/GOX by California Rocket Society

(US).• 1947: Douglas Fir/LOX by Pacific Rocket Society (US)• 1951-1956: GE initiated the investigations in

hybrids. H2O2/Polyethylene. (US)

5

GIRD-9

Aero/Astro 50th Year AnniversaryHybrid Rocket History

Era of Enlightenment (1960-1980)

• 1960's: Extensive research at variouscompanies.– Chemical Systems Division of UTC

• Modeling (Altman, Marxman, Ordahl,Wooldridge, Muzzy etc…)

• Motor testing (up to 40,000 lb thrustlevel)

– LPC: Lockheed Propulsion Company,SRI: Stanford Research Institute,ONERA (France)

• 1964-1984: Flight System Development– Target drone programs by Chemical

Systems Division of UTC• Sandpiper, HAST, Firebolt

– LEX Sounding Rocket (ONERA, France)– FLGMOTOR Sounding Rocket

(Sweeden)

6

CSD’s Li/LiH/PBAN-F2/O2

Hybrid

Measured Isp=480 sec

Firebolt Target Drone

Aero/Astro 50th Year AnniversaryHybrid History Recent History (1981-Present)

• 1981-1985: Starstruck company developed and sea launched the Dolphinsounding rocket (35 klb thrust)

• 1985-1995: AMROC continuation of Starstruck

– Tested 10, 33, 75 klb thrust subscale motors.

– Developed and tested the H-1800, a 250 klb LO2/HTPB motor.

• 1990’s: Hybrid Propulsion Development Program (HPDP)

– Successfully launched a small sounding rocket.

– Developed and tested 250 klb thrust LO2/HTPB motors.

• 2002: Lockheed developed and flight tested a 24 inch LO2/HTPB hybridsounding rocket (HYSR). (60 klb thrust)

• 2003: Scaled Composites and SpaceDev have developed a N2O/HTPBhybrid for the sub-orbital vehicle SpaceShipOne. (20 klb thrust)

7

SpaceShipOne

Dolphin

AMROC Motor Test


8

Small Launch Vehicle Data

4/716,66715.0900PSLV

22/2214,37523.01,600Long March 2

Others

1/128,57120.0700Strela

6/653,8929.0167Start

422/44815,48412.0775Kosmos

9/1033,33310.0300Dnepr

Russian Launchers

0/014,33720.01,395Vega

EU Launchers

6/754,54636.0660Taurus

7/759,93619.0317Minotaur

34/39105,26320.0190Pegasus XL

US Launchers

ReliabilityCost/Payload, $/kgCost#, M$Payload*, kgLauncher

*Sunsynchronous Orbit: 800 km, 98.7o #FY02 Values


9

PegasusXL Launch Vehicle

• ORBITAL Sciences

• Air Launched (L1011): Dropped at 39 kft

• Propulsion System:

– Stage 1: 50SXL (Solid – AlliantTechsystems)

– Stage 2: 50XL (Solid – AlliantTechsystems)

– Stage 3: 38 (Solid – Alliant Techsystems)

– Stage 4: HAPS (Hydrazine monoprop. –Aerojet)

Reasons for high recurring cost:

– Expensive propulsion system

– Air platform/low launchfrequency


10

PegasusXL Launch Vehicle Dilemma of Launch Business

– High launch costs limit thedemand

– Low launch frequencyincrease the cost

• This cycle is hard to break withcurrent propulsiontechnologies (improvementshave been gradual since1970’s)

• Disruptive technologies areneeded: Hybrids

• Number of launches decreased in time

• Presently average is one launch a year


11

Hybrid Propulsion – Non-Technical Challenges

Non-Technical Challenges

• Lack of technological maturity

• Hard to compete against established solid and liquid technologies

• Established propulsion industry is fine with the status quo

• Smaller group of rocket professionals relative to solid and liquid rockets

Approach

• Keep educating young engineers on the virtues of hybrid propulsion

– Growing number of young professionals interested in hybrid propulsion

• Understand that hybrids will NOT eliminate the solid and liquid technologies

– Hybrids are complementary to other chemical rockets

– Initially concentrate on the niche and easy applications that clearly benefit from thehybrid approach

• Suborbital Applications: Sub-orbital space tourism (SpaceShipTwo)

– Performance is secondary to safety and cost

• Small launch vehicle propulsion


12

Hybrid Propulsion –Technical ChallengesTechnical Challenges

• Low regression rates for classical hybrid fuels– Results in complicated fuel grain design

• Low frequency instabilities– Instabilities are common to all chemical rockets

– They need to be eliminated

– Expensive and long process

• Lack of benign, high performance, cost effectiveoxidizers (common to all chemical rockets)

Approach

• Solutions to these technical issues should be such thatthey do NOT compromise the simplicity, safety and costadvantages of hybrids.

Aero/Astro 50th Year AnniversaryRegression Rate Versus Fuel Port Designs

Single Circular

13

Decreasing Regression Rate

Increasing System Size

Fuel Grain

Port

Case

rcw

Fuel Grain

Port Port

Case

rc

w

2 wCruciform

Double-D

4+1 Port

6+1 Port WagonWheel

Aero/Astro 50th Year AnniversaryDisadvantage of Multiport Designs

Issues with multi-port design

• Excessive unburned mass fraction (i.e. typically in the 5% to 10% range)

• Complex design/fabrication, requirement for a web support structure

• Compromised grain structural integrity, especially towards the end of the burn

• Uneven burning of individual ports.

• Requirement for a substantial pre-combustion chamber or individual injectors for each port

14

CSD (1967)13 ports

AMROC (1994)15 ports

LockheedMartin(2006)

43 ports

Aero/Astro 50th Year AnniversaryApproaches for High Regression Rate

All based on increasing heat transfer to fuel surface

Technique Fundamental

Principle

Shortcoming

Add oxidizing

agents self -

decomposing

materials

Increase heat

transfer by

introducing surface

reactions

• Reduced safety

• Pressure

dependency

Add metal particles

(micron-sized)

Increased radiative

heat transfer • Limited

improvement

• Pressure

dependency

Add metal particles

(nano-sized)

Increased radiative

heat transfer • High cost

• Tricky

processing

Use Swirl Injection Increased local

mass flux • Increased

complexity

• Scaling?

15

Aero/Astro 50th Year AnniversaryEntrainment Mass Transfer Mechanism

Regression Rate = Entrainment + Vaporization

16

• A new transfer mechanism:

– Certain fuels form a liquidlayer

– If the conditions are right,mechanical entrainment ofliquid droplets occur

• Liquid Layer HybridCombustion Theory(Stanford - 1997)

• Most important scaling:

– The entrainment masstransfer increases withdecreasing viscosity of theliquid layer

Aero/Astro 50th Year AnniversaryRegression Rate Law for Paraffin-Based Fuel, SP-1a

62.0 488.0 oxGr =&

Three fold

improvement

over HTPB

is confirmed

17

Aero/Astro 50th Year AnniversaryParaffin-Based Fuels Technology Progress

Motor testing experience (SPG/Stanford/NASA Ames)

– Small Scale(i.e. 50-100 lbf): >500 tests

– Scale-up (i.e. 900-7000 lbf): >80 tests

– Oxidizers: Liquid Oxygen, Gaseous Oxygen, Nitrous Oxide

18

SPG work on paraffin-based fuel technology

– Formulation (Keep cost < 1 $/lb)

– Processing (24 inch OD fuel grains – 800 kg)

– Structural testing and modeling

– Internal ballistic design of single circular port hybrids

– Scale up motor testing (in 2009 25,000 lbf class motors)

Large single circular port hybrids are feasible


HPDP 250k lbf Motor 2 Test 2

Low Frequency Instabilities

8.4 inch LO2/Paraffin

• Many mechanisms

– Feed system coupling

– Intrinsic hybrid combustion

– Chuffing at low fluxes

– Oxidizer vaporization delay

(common to LO2 systems)

• Hybrids are prone tolow frequencyinstabilities (2-100Hz)

• Limit cycle nature

19

P,

psi

Time, sec

Aero/Astro 50th Year AnniversaryLow Frequency Instabilities - Remedies

• We believe that a LO2 motor can bemade stable

– Without the use of heaters or TEAinjection

– By advanced injector and combustionchamber design

• Demonstrated in 7,000 lbf thrustclass LO2/Paraffin motor

• Solutions used in the field

– Lockheed Martin –Michoudand HPDP used hybridheaters to vaporize LO2

– AMROC injected TEA(triethylaluminum) tovaporize LO2

• Both solutions introducecomplexity minimizing thesimplicity advantage of hybrids

– Heaters- extra plumbing

– TEA – extra liquid,hazardous material

20

Aero/Astro 50th Year AnniversaryOxidizers Overall Picture

21

Red: Toxic or sensitiveBlue: Low performance

Relatively benign, low cost and readily available oxidizers: LOX, N2O

Aero/Astro 50th Year AnniversaryNitrous Oxide - Introduction

Nitrous Oxide Uses

• Oxidizer: Rocket propulsion, motor racing

• Anesthetic Agent: Medicine, dentistry

• Solvent

Physical

• A saturated liquid at room temperature. Self pressurizing liquid (744 psi @ 20 C)

• Two phase flow in the feed system (complicated injector design)

• Highly effective green house gas (Global Warming Potential: 310 x CO2)

Chemical

• Oxidizer

• Monopropellant. Positive heat of formation. Decomposes into N2 and O2 byreleasing significant amount of heat

• Highly effective solvent for hydrocarbons

Biological

• Mildly toxic. Anesthetic and analgesic agent still used in medicine, “Laughing gas”

molekcalONON /61.192

1222 ++!

• Aerosol propellant: Culinary use(in whip cream dispensers)

• Etchant :Semiconductor industry

22

Aero/Astro 50th Year AnniversaryNitrous Oxide – SpaceShipTwo

Explosion at Scaled Composites facility in

Mojave Airport on July 26, 2007 as they were

conducting a cold flow test with N2O

Sub-orbital Space Tourism

• Virgin Galactic has contracted ScaledComposites to build SpaceShipTwo

• SpaceShipTwo design uses a N2Obased hybrid rocket

• Testing of the propulsion systemstarted in summer 2007

23

Aero/Astro 50th Year AnniversaryNitrous Oxide – Explosion Hazard

Medical Accidents

• Many medical explosions reported inoperating theater– Found 10 cases (3 fatal)

• Intestinal/colonic explosions during diathermy– High content of H2 and CH4 in the intestines

and colon– The concentration of N2O increases

significantly in the body cavities following itsapplication as an anesthetic

Car Exploded in Garage

24

SPG Experience

• Small N2O/paraffin motor

• First N2O explosion in February2006

• Many small explosions in the feedsystem – minor damage tohardware

Industrial Accidents

• N2O used as solvent forhydrocarbons

• Welding full N2O tanks

• Heating source tanks with openflames

Aero/Astro 50th Year AnniversaryN2O Decomposition Physics

25

• N2O decomposition follows theelementary unimolecular reaction

• This reaction is considered“abnormal” since it requires achange in multiplicity from a singletstate to a triplet state.

• This change in multiplicity isforbidden by the quantum mechanics

• The transmission can only takeplace through “tunneling” resulting ina reduced transmission rate

• The reaction rate for N2O is morethan 12 times lower than the reactionrate predicted for a “normal”unimolecular reaction (such as thedecomposition of H2O2)

• This quantum mechanical effect isthe root cause for the relative safetyof N2O

( ) ( )PONON31

22 +!"

Ref.:Stearn and Eyring (1935)

Resonant structure


• The decomposition of N2O is believed to follow the elementaryreactions:

• Steady-state assumption for [O] results in the following kineticequation for the decomposition of N2O

• Note that m=2 and it comes from stoichiometry

2223 ONOON

k+!"!+

NONOOONk

+!"!+ 2

2

MONMONk

++!"!+ 221

26

N2O Decomposition Kinetics

][][][

212

MONkmdt

ONd=!


• Largest hazard is in theoxidizer tank during vaporphase combustion

• An ignition source (hot injectorplate) could start a combustionwave which would result insignificant pressure increase

27

N2O Decomposition Hazard

Oxidizer Tank

• Deflagration in tank

• Tank Length: 4.0 m

• Initial pressure: 750 psiRisk Mitigation

• Respect the propellant (set andfollow strict procedures)

• Supercharge with inert gas (He)

• Incorporate a burst disk

N2O is a widely used and fairly safe

material

• Max pressure: 9,100 psi

• Time scale is seconds

NFPA Rating


• Hybrid concept has been around since the start of the modern rocketry

• Hybrids lack the intense development cycle that the liquid and solid systems had since1940’s (primarily in the 1940-1970)

• The liquid and solid rocket technologies are fairly mature and the progress is slow ornonexistent. Hybrids could provide the disruptive propulsion technology needed toenergize the space launch industry by

– Providing a safe and affordable option

– Breaking the present oligopoly in the rocket propulsion industry by allowing relatively smallcompanies to enter the business

• Hybrids will not eliminate the liquid and solid systems. It is critical to find the nichemarkets for hybrids

• The emerging sub-orbital space tourism market is ideal since

– It could end up being a lucrative private market

– Performance is secondary to safety and cost (an easy start for the hybrid technology)

– The suborbital rocket ca be the basis for a much needed cost effective, reliable orbital system

• Solutions to the technical challenges should NOT eliminate the safety and simplicityadvantages of hybrids.

• We believe that viable solutions exist for these technical problems, assuming that thefollowing conditions prevail

– Creative and competent technical team

– Adequate funding for technology development

28

Concluding Remarks


29

Spares

Aero/Astro 50th Year AnniversaryRocket Propulsion Fundamentals

RocketPropulsion

Electric Nuclear Chemical

Liquid Solid Hybrid

Cold Gas

Propulsive Force

=

Mass Ejected per Unit Time x Effective Exhaust Velocity

Mass Energy

30


Hybrid Combustion Scheme

• Diffusion limited combustion– Burning Rate Law: independent of pressure (flux dependent)

• Flame zone away from surface and blocking effect– Low regression rate

Fuel Grain

Flame Zone

Oxidizer

+

Products

Fuel + Products

Concentration

Profi lesTemperature

Profi leVelocity

Profi le

z x

TeU

e

Tb

Ts

Ta

Yo

= 1

31


32

Small Launch Vehicle Data

Aero/Astro 50th Year AnniversaryLiquid Layer Hybrid Combustion Theory

• Scaling for entrainment mass transfer

!"

#$

µ%l

dent

hPm &&

Operational Parameters: (Pressure, Oxidizer Flux)

Material Properties: (Viscosity, Surface Tension)

33

• Modification on the classical Hybrid Combustion

Theory

– Reduced heating requirement for the entrained

mass

– Reduced “Blocking Effect” due to two phase flow

– Increased heat transfer due to the increased

surface roughness

Aero/Astro 50th Year AnniversaryEntrainment for CnH2n+2 Series

C:

Mw:

1 5 25 45 14,000

16 72 352 632 200,000

Cryogenic Non-cryogenic

Methane(Tested) Paraffin Waxes

65 80

PE WaxesHDPE Polymer

(Tested)

912 1262

(g/mol)

Pentane(Tested)

Gas Liquid Solid Polymer

Mw

Entr

ain

ment

Entraiment

Boundary

34

Aero/Astro 50th Year AnniversaryLiquefying Hybrid Fuels

• These hybrid fuelsburn by forming aliquid layer on theirburning surfaces

• Possibility ofentrainment masstransfer from theliquid layer

• Solid cryogenic and paraffin-based hybrids: Tested by– Air Force (Pentane and several other hydrocarbons)

– ORBITEC (Methane, SOX, CO etc..)

– Stanford/SPG (Paraffin waxes)

• Very high regression rates (Factors of 3-5)

Aero/Astro 50th Year AnniversaryHomologous Series of n-Alkanes (CnH2n+2)

Methane (CH4):C

Ethane (C2H6): C-C..

Pentane (C5H12):C-C-C-C-C..

“Wax” (C32H66):C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C

A number of practical fuels (pure form or mixtures):Methane, Kerosene (n~10), Paraffin Waxes (n=16-45),PE waxes (n=45-90), HDPE Polymer (n in thousands)

• Normal Alkanes: Fully saturated, straight-chain hydrocarbons• Examples:

36


Theory Prediction and Motor Test Data for CnH2n+2

Regression rateincrease over the

classical value is ashigh as 6.1

Theory prediction isfairly accurate

Paraffin waxes burn5-5.5 times fasterthan the HDPE

polymer

37

Aero/Astro 50th Year AnniversaryMelt Layer Temperatures for CnH2n+2 Series

38


• Physical steps of the first reaction are

• Steady-state assumption for the excited complex [N2O*] results inthe following kinetic equation

• At high pressures the reaction becomes first order

• At low pressures the reaction is second order

• For N2O

ONON bk

+!"! 22 *

MONMON ak

+!"!+ #

22 *

MONMON ak

+!"!+ *22

39

Lindemann’s Unimolecular Theory

][

][][][ 22

Mkk

MONkkm

dt

ONd

ab

ba

!+=!

][][][

2122

ONkmONk

kkm

dt

ONd

a

ba !

"

=="

][][][][][

2122

MONkmMONkmdt

ONd o

a ==!

( ) 1000,30111 103.1 !!"

= seTkT

Karabeyoglu


• Data follows theunimoleculartheory in general

• For pressureslarger than 40atm (~600 psi) thereaction is shownto be first order

• Note that for thefirst order reactionthe collisionpartner [M] doesNOT play a rolegreatly simplifyingthe analysis

40

Reaction Order Data

Ref.:Lewis and Hinshellwood (1938)

Karabeyoglu

hybrid rocket propulsion for future aircraft

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