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Aero/Astro 50th Year Anniversary
Hybrid Rocket Propulsion for
Future Space Launch
Arif Karabeyoglu
President and Chief Technology Officer, SPG Inc.
Consulting Professor, Department of Aeronautics and Astronautics, Stanford University
May 09, 2008
Aero/Astro 50th Year Anniversary
Hybrid Rocket Configuration
Fuel and oxidizer are physically separated
One of the two is in solid phase
Most Hybrids: Reverse Hybrids:
Oxidizer: Liquid Oxidizer: Solid
Fuel: Solid Fuel: Liquid
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Aero/Astro 50th Year Anniversary
Hybrid Rocket System
Solid Fuel Liquid Oxidizer
• Polymers: Thermoplastics, • Cryogenic: LO2
(Polyethylene, Plexiglas),
Rubbers (HTPB) • Storable: H2O2, N2O, N2O4,
IRFNA
• Wood, Trash, Wax
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Aero/Astro 50th Year Anniversary
Advantages of Hybrids
Compared to Solids Liquids
Simplicity - Chemically simpler - Mechanically simpler
- Tolerant to processing - Tolerant to fabrication
errors errors
Safety - Reduced chemical - Reduced fire hazard
explosion hazard - Less prone to hard starts
- Thrust termination and
abort possibility
Performance Related - Better Isp performance - Higher fuel density
- Throttling/restart - Easy inclusion of solid
capability performance additives (Al,
Be)
Other - Reduced environmental - Reduced number and
impact mass of liquids
Cost - Reduced development costs are expected
- Reduced recurring costs are expected
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Hybrid 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)
GIRD-9
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Aero/Astro 50th Year Anniversary
Hybrid Rocket History
Era of Enlightenment (1960-1980)
• 1960's: Extensive research at various
companies.
– Chemical Systems Division of UTC
• Modeling (Altman, Marxman, Ordahl,
Wooldridge, Muzzy etc…)
• Motor testing (up to 40,000 lb thrust
level)
– LPC: Lockheed Propulsion Company,
SRI: Stanford Research Institute,
ONERA (France)
• 1964-1984: Flight System Development CSD’s Li/LiH/PBAN-F2/O2
– Target drone programs by Chemical Hybrid
Systems Division of UTC Measured Isp=480 sec
• Sandpiper, HAST, Firebolt
– LEX Sounding Rocket (ONERA, France)
– FLGMOTOR Sounding Rocket
(Sweeden)
Firebolt Target Drone
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Aero/Astro 50th Year Anniversary
Hybrid History Recent History (1981-Present)
• 1981-1985: Starstruck company developed and sea launched the Dolphin
sounding 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 hybrid
sounding rocket (HYSR). (60 klb thrust)
• 2003: Scaled Composites and SpaceDev have developed a N2O/HTPB Dolphin
hybrid for the sub-orbital vehicle SpaceShipOne. (20 klb thrust)
AMROC Motor Test
SpaceShipOne
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Aero/Astro 50th Year Anniversary
Small Launch Vehicle Data
Launcher Payload*, kg Cost#, M$ Cost/Payload, $/kg Reliability
US Launchers
Pegasus XL 190 20.0 105,263 34/39
Minotaur 317 19.0 59,936 7/7
Taurus 660 36.0 54,546 6/7
EU Launchers
Vega 1,395 20.0 14,337 0/0
Russian Launchers
Dnepr 300 10.0 33,333 9/10
Kosmos 775 12.0 15,484 422/448
Start 167 9.0 53,892 6/6
Strela 700 20.0 28,571 1/1
Others
Long March 2 1,600 23.0 14,375 22/22
PSLV 900 15.0 16,667 4/7
#FY02 Values
*Sunsynchronous Orbit: 800 km, 98.7o
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Aero/Astro 50th Year Anniversary
PegasusXL Launch Vehicle
• ORBITAL Sciences
• Air Launched (L1011): Dropped at 39 kft
• Propulsion System:
– Stage 1: 50SXL (Solid – Alliant
Techsystems)
– Stage 2: 50XL (Solid – Alliant
Techsystems)
– Stage 3: 38 (Solid – Alliant Techsystems)
– Stage 4: HAPS (Hydrazine monoprop. –
Aerojet)
Reasons for high recurring cost:
– Expensive propulsion system
– Air platform/low launch
frequency
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Aero/Astro 50th Year Anniversary
PegasusXL Launch Vehicle Dilemma of Launch Business
– High launch costs limit the
demand
– Low launch frequency
increase the cost
• This cycle is hard to break with
current propulsion
technologies (improvements
have been gradual since
1970’s)
• Disruptive technologies are
needed: Hybrids
• Number of launches decreased in time
• Presently average is one launch a year
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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 the
hybrid approach
• Suborbital Applications: Sub-orbital space tourism (SpaceShipTwo)
– Performance is secondary to safety and cost
• Small launch vehicle propulsion
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Hybrid Propulsion –Technical Challenges
Technical 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 effective
oxidizers (common to all chemical rockets)
Approach
• Solutions to these technical issues should be such that
they do NOT compromise the simplicity, safety and cost
advantages of hybrids.
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Regression Rate Versus Fuel Port Designs
Fuel Grain
Fuel Grain
w rc rc
w
Port Port
Port Cruciform 2w
Case 4+1 Port
Case
Single Circular 6+1 Port Wagon
Double-D
Wheel
Decreasing Regression Rate
Increasing System Size
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Aero/Astro 50th Year Anniversary
Disadvantage of Multiport Designs
Lockheed
CSD (1967) Martin
13 ports (2006)
43 ports
AMROC (1994)
15 ports
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
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Approaches for High Regression Rate
Technique Fundamental Shortcoming
Principle
Add oxidizing Increase heat • Reduced safety
agents self - transfer by • Pressure
decomposing introducing surface dependency
materials reactions
Add metal particles Increased radiative • Limited
(micron -sized) heat transfer improvement
• Pressure
dependency
Add metal particles Increased radiative • High cost
(nano -sized) heat transfer • Tricky
processing
Use Swirl Injection Increased local • Increased
mass flux complexity
• Scaling?
All based on increasing heat transfer to fuel surface
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Aero/Astro 50th Year Anniversary
Entrainment Mass Transfer Mechanism
• A new transfer mechanism:
– Certain fuels form a liquid
layer
– If the conditions are right,
mechanical entrainment of
liquid droplets occur
• Liquid Layer Hybrid
Combustion Theory
(Stanford - 1997)
• Most important scaling:
– The entrainment mass
transfer increases with
decreasing viscosity of the
Regression Rate = Entrainment + Vaporization liquid layer
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Aero/Astro 50th Year Anniversary
Regression Rate Law for Paraffin-Based Fuel, SP-1a
r = 0.488 Gox.62
& 0
Three fold
improvement
over HTPB
is confirmed
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Aero/Astro 50th Year Anniversary
Paraffin-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
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
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Aero/Astro 50th Year Anniversary
Low Frequency Instabilities
• Hybrids are prone to
low frequency
instabilities (2-100
P, Hz)
psi
• Limit cycle nature
HPDP 250k lbf Motor 2 Test 2 8.4 inch LO2/Paraffin
Time, sec
• Many mechanisms
– Feed system coupling
– Intrinsic hybrid combustion
– Chuffing at low fluxes
– Oxidizer vaporization delay
(common to LO2 systems)
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Low Frequency Instabilities - Remedies
• Solutions used in the field
– Lockheed Martin –Michoud
and HPDP used hybrid
heaters to vaporize LO2
– AMROC injected TEA
(triethylaluminum) to
vaporize LO2
• We believe that a LO2 motor can be
• Both solutions introduce
made stable
complexity minimizing the
– Without the use of heaters or TEA simplicity advantage of hybrids
injection
– Heaters- extra plumbing
– By advanced injector and combustion
chamber design – TEA – extra liquid,
• Demonstrated in 7,000 lbf thrust hazardous material
class LO2/Paraffin motor
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Aero/Astro 50th Year Anniversary
Oxidizers Overall Picture
Red: Toxic or sensitive
Blue: Low performance
Relatively benign, low cost and readily available oxidizers: LOX, N2O
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Aero/Astro 50th Year Anniversary
Nitrous Oxide - Introduction 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 by
releasing significant amount of heat
1
N 2O ! N 2 + O2 + 19.61 kcal / mole
2
• Highly effective solvent for hydrocarbons
Biological
• Mildly toxic. Anesthetic and analgesic agent still used in medicine, “Laughing gas”
Nitrous Oxide Uses
• Oxidizer: Rocket propulsion, motor racing • Aerosol propellant: Culinary use
• Anesthetic Agent: Medicine, dentistry (in whip cream dispensers)
• Solvent • Etchant :Semiconductor industry
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Aero/Astro 50th Year Anniversary
Nitrous Oxide – SpaceShipTwo
Sub-orbital Space Tourism
• Virgin Galactic has contracted Scaled
Composites to build SpaceShipTwo
• SpaceShipTwo design uses a N2O
based hybrid rocket
• Testing of the propulsion system
started in summer 2007
Explosion at Scaled Composites facility in
Mojave Airport on July 26, 2007 as they were
conducting a cold flow test with N2O
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Aero/Astro 50th Year Anniversary
Nitrous Oxide – Explosion Hazard
SPG Experience Industrial Accidents
• Small N2O/paraffin motor • N2O used as solvent for
hydrocarbons
• First N2O explosion in February
2006 • Welding full N2O tanks
• Many small explosions in the feed • Heating source tanks with open
system – minor damage to flames
hardware
Car Exploded in Garage
Medical Accidents
• Many medical explosions reported in
operating 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 its
application as an anesthetic
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Aero/Astro 50th Year Anniversary
N2O Decomposition Physics
• N2O decomposition follows the
elementary unimolecular reaction
Resonant structure () ( )
N 2O " N 2 1 ! + O 3 P
• This reaction is considered
“abnormal” since it requires a
change in multiplicity from a singlet
state to a triplet state.
• This change in multiplicity is
forbidden by the quantum mechanics
• The transmission can only take
place through “tunneling” resulting in
a reduced transmission rate
• The reaction rate for N2O is more
than 12 times lower than the reaction
rate predicted for a “normal”
unimolecular reaction (such as the
decomposition of H2O2)
Ref.:Stearn and Eyring (1935) • This quantum mechanical effect is
the root cause for the relative safety
of N2O
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Aero/Astro 50th Year Anniversary
N2O Decomposition Kinetics
• The decomposition of N2O is believed to follow the elementary
reactions:
k
N 2O + M ! 1 N 2 + O + M
!"
k
N 2O + O ! 2 NO + NO
!"
k
N 2O + O ! 3 N 2 + O2
!"
• Steady-state assumption for [O] results in the following kinetic
equation for the decomposition of N2O
d [ N 2O ]
! = m k1[ N 2O][ M ]
dt
• Note that m=2 and it comes from stoichiometry
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Aero/Astro 50th Year Anniversary
N2O Decomposition Hazard
• Largest hazard is in the
oxidizer tank during vapor
phase combustion
• An ignition source (hot injector
plate) could start a combustion
wave which would result in
significant pressure increase
• Deflagration in tank
Oxidizer Tank • Tank Length: 4.0 m
• Initial pressure: 750 psi
Risk Mitigation • Max pressure: 9,100 psi
• Respect the propellant (set and • Time scale is seconds
follow strict procedures)
• Supercharge with inert gas (He)
• Incorporate a burst disk
N2O is a widely used and fairly safe
material
NFPA Rating
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Aero/Astro 50th Year Anniversary
Concluding Remarks
• 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 since
1940’s (primarily in the 1940-1970)
• The liquid and solid rocket technologies are fairly mature and the progress is slow or
nonexistent. Hybrids could provide the disruptive propulsion technology needed to
energize the space launch industry by
– Providing a safe and affordable option
– Breaking the present oligopoly in the rocket propulsion industry by allowing relatively small
companies to enter the business
• Hybrids will not eliminate the liquid and solid systems. It is critical to find the niche
markets 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 simplicity
advantages of hybrids.
• We believe that viable solutions exist for these technical problems, assuming that the
following conditions prevail
– Creative and competent technical team
– Adequate funding for technology development
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Spares
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Aero/Astro 50th Year Anniversary
Rocket Propulsion Fundamentals
Propulsive Force
=
Mass Ejected per Unit Time x Effective Exhaust Velocity
Mass Energy
Rocket
Propulsion
Electric Nuclear Chemical Cold Gas
Liquid Solid Hybrid
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Hybrid Combustion Scheme
Concentration
Temperature
Velocity Profiles
Profile
Profile
Te Yo = 1
Ue
Oxidizer
+
Tb Products
Flame Zone
Ts Fuel + Products
Fuel Grain
z x Ta
• Diffusion limited combustion
– Burning Rate Law: independent of pressure (flux dependent)
• Flame zone away from surface and blocking effect
– Low regression rate
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Small Launch Vehicle Data
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Aero/Astro 50th Year Anniversary
Liquid Layer Hybrid Combustion Theory
• Scaling for entrainment mass transfer
$ # Operational Parameters:
Pd h (Pressure, Oxidizer Flux)
&
ment &
% µl
" ! Material Properties:
(Viscosity, Surface Tension)
• 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
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Entrainment for CnH2n+2 Series
Methane Pentane HDPE Polymer
(Tested) (Tested) Paraffin Waxes PE Waxes (Tested)
C: 1 5 25 45 65 80 14,000
Mw: 16 72 352 632 912 1262 200,000
(g/mol)
Cryogenic Non-cryogenic
Gas Liquid Solid Polymer
Entrainment
Entraiment
Boundary
Mw
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Aero/Astro 50th Year Anniversary
Liquefying Hybrid Fuels
• 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)
• These hybrid fuels
burn by forming a
liquid layer on their
burning surfaces
• Possibility of
entrainment mass
transfer from the
liquid layer
Aero/Astro 50th Year Anniversary
Homologous Series of n-Alkanes (CnH2n+2)
• Normal Alkanes: Fully saturated, straight-chain hydrocarbons
• Examples:
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)
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Theory Prediction and Motor Test Data for CnH2n+2
Regression rate
increase over the
classical value is as
high as 6.1
Paraffin waxes burn
5-5.5 times faster
than the HDPE
polymer
Theory prediction is
fairly accurate
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Aero/Astro 50th Year Anniversary
Melt Layer Temperatures for CnH2n+2 Series
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Aero/Astro 50th Year Anniversary
Lindemann’s Unimolecular Theory
• Physical steps of the first reaction are
k
N 2O + M ! a N 2O * + M
!"
k
N 2O * + M !!" N 2O + M
#a
k
N 2O * ! b N 2 + O
!"
• Steady-state assumption for the excited complex [N2O*] results in
the following kinetic equation
d [ N 2O ] k k [ N O ][ M ]
! =m a b 2
dt kb + k ! a [ M ]
• At high pressures the reaction becomes first order
d [ N 2O ] k k
" = m a b [ N 2O] = m k1 [ N 2O]
!
dt k "a
• At low pressures the reaction is second order
d [ N 2O ]
! = m k a [ N 2O][ M ] = m k1 [ N 2O][ M ]
o
dt
• For N2O k1 (T ) = 1.31011 e !30,000 T s !1
"
39 Karabeyoglu
Aero/Astro 50th Year Anniversary
Reaction Order Data
• Data follows the
unimolecular
theory in general
• For pressures
larger than 40
atm (~600 psi) the
reaction is shown
to be first order
• Note that for the
first order reaction
the collision
partner [M] does
NOT play a role
greatly simplifying
the analysis
Ref.:Lewis and Hinshellwood (1938)
40 Karabeyoglu
