Hybrid Rocket Propulsion for Future Aircraft

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 Fuel: Solid

Oxidizer: Solid Fuel: Liquid 2

Aero/Astro 50th Year Anniversary Hybrid Rocket System •

•

Solid Fuel Polymers: Thermoplastics, (Polyethylene, Plexiglas), Rubbers (HTPB) Wood, Trash, Wax

• •

3

Liquid Oxidizer Cryogenic: LO2 Storable: H2O2, N2O, N2O4, IRFNA

Aero/Astro 50th Year Anniversary Advantages of Hybrids Compared to

Solids

Liquids

Simplicity

- Chemically simpler - Tolerant to processing errors

- Mechanically simpler - Tolerant to fabrication errors

Safety

- Reduced chemical explosion hazard - Thrust termination and abort possibility

- Reduced fire hazard - Less prone to hard starts

Performance Related

- Better Isp performance - Throttling/restart capability

- Higher fuel density - Easy inclusion of solid performance additives (Al, Be)

Other

- Reduced environmental impact

- Reduced number and mass of liquids

Cost

- Reduced development costs are expected - Reduced recurring costs are expected

4

Aero/Astro 50th Year Anniversary 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)

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GIRD-9

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 – Target drone programs by Chemical Systems Division of UTC • Sandpiper, HAST, Firebolt – LEX Sounding Rocket (ONERA, France) – FLGMOTOR Sounding Rocket (Sweeden)

CSD’s Li/LiH/PBAN-F2/O2 Hybrid Measured Isp=480 sec

Firebolt Target Drone 6

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 hybrid for the sub-orbital vehicle SpaceShipOne. (20 klb thrust)

Dolphin

AMROC Motor Test

SpaceShipOne 7

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

14,337

0/0

EU Launchers Vega

1,395

20.0 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 PSLV

1,600

23.0

14,375

22/22

900

15.0

16,667

4/7

*Sunsynchronous Orbit: 800 km, 98.7o 8

#FY02

Values

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

•

•

• •

Number of launches decreased in time Presently average is one launch a year

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This cycle is hard to break with current propulsion technologies (improvements have been gradual since 1970’s) Disruptive technologies are needed: Hybrids

Aero/Astro 50th Year Anniversary 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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Aero/Astro 50th Year Anniversary 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. 12

Aero/Astro 50th Year Anniversary Regression Rate Versus Fuel Port Designs Fuel Grain

Fuel Grain w

rc

rc w

Port

Cruciform

Port

Port 2w

4+1 Port

Case

Case

Single Circular Double-D

Decreasing Regression Rate Increasing System Size

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6+1 Port Wagon Wheel

Aero/Astro 50th Year Anniversary Disadvantage of Multiport Designs Lockheed Martin (2006) 43 ports

CSD (1967) 13 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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Aero/Astro 50th Year Anniversary Approaches for High Regression Rate Technique

Fundamental Principle

Shortcoming

Add oxidizing agents self decomposing materials Add metal particles (micron -sized)

Increase heat transfer by introducing surface reactions Increased radiative heat transfer

• Reduced safety • Pressure dependency

Add metal particles (nano -sized)

Increased radiative heat transfer

Use Swirl Injection

Increased local mass flux

• Limited improvement • Pressure dependency • High cost • Tricky processing • Increased complexity • Scaling?

All based on increasing heat transfer to fuel surface 15

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

•

•

Regression Rate = Entrainment + Vaporization

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Liquid Layer Hybrid Combustion Theory (Stanford - 1997) Most important scaling: – The entrainment mass transfer increases with decreasing viscosity of the liquid layer

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

0.62 r& = 0.488 Gox

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 18

Aero/Astro 50th Year Anniversary Low Frequency Instabilities • P, psi

• HPDP 250k lbf Motor 2 Test 2

•

Time, sec Many mechanisms – Feed system coupling – Intrinsic hybrid combustion – Chuffing at low fluxes – Oxidizer vaporization delay (common to LO2 systems) 19

Hybrids are prone to low frequency instabilities (2-100 Hz) Limit cycle nature

8.4 inch LO2/Paraffin

Aero/Astro 50th Year Anniversary Low Frequency Instabilities - Remedies •

•

We believe that a LO2 motor can be • made stable – Without the use of heaters or TEA injection – By advanced injector and combustion chamber design

•

Demonstrated in 7,000 lbf thrust class LO2/Paraffin motor 20

Solutions used in the field – Lockheed Martin –Michoud and HPDP used hybrid heaters to vaporize LO2 – AMROC injected TEA (triethylaluminum) to vaporize LO2 Both solutions introduce complexity minimizing the simplicity advantage of hybrids – Heaters- extra plumbing – TEA – extra liquid, hazardous material

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 (in whip cream dispensers) Anesthetic Agent: Medicine, dentistry • Etchant :Semiconductor industry Solvent 22

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 Small N2O/paraffin motor First N2O explosion in February 2006 Many small explosions in the feed system – minor damage to hardware

• • •

Industrial Accidents N2O used as solvent for hydrocarbons Welding full N2O tanks Heating source tanks with open flames

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

() ( )

N 2O " N 2 1 ! + O 3 P

Resonant structure •

• •

•

•

Ref.:Stearn and Eyring (1935)

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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) This quantum mechanical effect is the root cause for the relative safety of N2O

Aero/Astro 50th Year Anniversary N2O Decomposition Kinetics •

The decomposition of N2O is believed to follow the elementary reactions: k

1 N 2O + M !!" N2 + O + M

k

2 N 2O + O !!" NO + NO

k

3 N 2O + O !!" 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 •

Oxidizer Tank Risk Mitigation • Respect the propellant (set and follow strict procedures) • Supercharge with inert gas (He) • Incorporate a burst disk N2O is a widely used and fairly safe material NFPA Rating 27

•

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 Tank Length: 4.0 m Initial pressure: 750 psi Max pressure: 9,100 psi Time scale is seconds

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 – –

• •

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 – – –

• •

Providing a safe and affordable option Breaking the present oligopoly in the rocket propulsion industry by allowing relatively small companies to enter the business

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 28

Aero/Astro 50th Year Anniversary

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

Liquid

Solid

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Cold Gas

Hybrid

Aero/Astro 50th Year Anniversary Hybrid Combustion Scheme Concentration Profiles

Temperature Profile

Velocity Profile

Yo = 1

Te

Ue

Tb

Flame Zone

Ts

Oxidizer + Products 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 31

Aero/Astro 50th Year Anniversary Small Launch Vehicle Data

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Aero/Astro 50th Year Anniversary Liquid Layer Hybrid Combustion Theory • Scaling for entrainment mass transfer

m& ent &

$ # Pd h " ! % µl

Operational Parameters: (Pressure, Oxidizer Flux) 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 33

Aero/Astro 50th Year Anniversary Entrainment for CnH2n+2 Series Methane Pentane (Tested)

C:

1

Mw: 16 (g/mol)

(Tested)

PE Waxes

(Tested)

5

25

45

65

80

14,000

72

352

632

912

1262

200,000

Cryogenic Liquid

Non-cryogenic Solid

Entrainment

Gas

Paraffin Waxes

HDPE Polymer

Polymer

Entraiment Boundary

Mw 34

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) 36

Aero/Astro 50th Year Anniversary 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

37

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

a N 2O + M !!" N 2O * + M

k

#a N 2O * + M !!" N 2O + M

k

b N 2O * !!" N2 + O

•

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

k k [ N O ][ M ] d [ N 2O ] =m a b 2 dt kb + k ! a [ M ] At high pressures the reaction becomes first order !

•

" •

•

k k d [ N 2O ] = 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 k1o [ N 2O][ M ] dt For N2O k1" (T ) = 1.31011 e !30,000 T s !1

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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