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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
Aero/Astro 50th Year Anniversary
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
Aero/Astro 50th Year Anniversary
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
Aero/Astro 50th Year Anniversary
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
Aero/Astro 50th Year Anniversary
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
Aero/Astro 50th Year Anniversary
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
Aero/Astro 50th Year Anniversary
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
Aero/Astro 50th Year Anniversary
• 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=!
Aero/Astro 50th Year Anniversary
• 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
Aero/Astro 50th Year Anniversary
• 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
Aero/Astro 50th Year Anniversary
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
Aero/Astro 50th Year Anniversary
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
Aero/Astro 50th Year Anniversary
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
Aero/Astro 50th Year Anniversary
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
Aero/Astro 50th Year Anniversary
• 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
Aero/Astro 50th Year Anniversary
• 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