Introduction to Hybrid Rocket Motors

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Introduction to Hybrid Rocket Motors

• Cesaroni Technology Incorporated
• November 2003

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Contents - 1

• Welcome Anthony
• History and introduction Anthony
• History of hybrid propulsion
• Types of hybrids motors
• aft-injected, . etc.
• Oxidizers and fuels Jeroen
• Overview of typical oxidizers and fuels
• Properties, advantages etc.
• Gas generator hybrids Anthony
• Background
• Advantages
• Results

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Contents - 2

• Solid fuel ballistics Jeroen
• Regression rate laws
• Heat transfer, pipe flow, etc.
• Experimental results
• Fuel types
• Quasi-steady operating point
• System design Jeroen
• System trade-offs grain, feed-system, etc.
• Re-circulation chambers, steps, etc.

4
Contents - 3

• Hypertek hybrids Anthony
• Hypertek system
• Test stand and data-acquisition
• References Jeroen

5
Welcome
6
History and Introduction
7
Oxidizers and fuels

• Oxidizer Selection criteria
• Desirable properties
• Typical hybrid performance curves

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Criteria for oxidizer selection

• Performance
• Isp
• Economic factors
• availability, cost, logistics
• Hazards
• Corrosion
• Nitrogen tetroxide, hydrogen peroxide, fluorine
• Explosion hazard
• Often in the presence of impurities hydrogen
  peroxide, liquid oxygen
• Fire hazard
• Vigorous reaction with many compounds nitric
  acid, fluorine, etc
• Health hazards
• Toxicity, carcinogenity

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Desirable oxidizer properties

• Low freezing point
• High density
• Stability and storability
• Heat transfer properties
• For regenerative cooling
• high specific heat,
• high thermal conductivity
• high boiling/decomposition temperature
• Pumping properties
• Low vapor pressure and low viscosity are
  desirable
• Small temperature effects
• E.g. density as function of temperature
• Ignition, combustion and flame properties

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Criteria for fuel selection

• Performance
• Isp
• Economic factors
• availability, cost, logistics
• Processing
• Thermoplastic, curable polymers
• Viscosity, reactivity
• Hazards
• Explosion hazard
• In case of a gas generator hybrid
• Fire hazard
• Health hazards
• Toxicity, carcinogenity (mostly of the curative)

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Desirable fuel properties

• High density
• Stability and storability
• High regression rate
• Also sensitivity to Gox, P, etc.
• Good mechanical properties
• Hypergolic ignition (if desired)

Hydroxyl terminated polybutadiene
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Oxidizer overview (AIAA-92-3592)
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Oxidizer performance overview (Ivac, HTPB fuel)
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Oxidizer performance overview (AIAA-92-3592)
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Oxidizer performance overview (AIAA-92-3592)
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Oxidizer performance tradeoffs (AIAA-92-3592)
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Oxidizer handling and storage (AIAA-92-3592)
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Propellant cost (AIAA-92-3592)
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Gas Generator Hybrids
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Solid fuel ballistics

• Heat transfer, regression rates, .

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A closer look at the burning surface
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Heat balance at surface
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Hybrid fuels

• Almost every solid fuel has been used in a hybrid
  rocket motor
• Examples
• Plexi-glass (PMMA)
• Polyethylene (PE)
• Hydroxyl terminated polybutadiene (HTPB)
• Glycidyl azide polymer (GAP)
• Garden hose
• Paper tubes
• Hollow salami sausage (can be eaten after the
  firing)
• Compressed garbage
• ..

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What affects the performance of a fuel?

• Heat of combustion kJ/kg
• Presence of oxygen in the fuel backbone reduces
  Isp
• Example LOX/HTPB has higher Isp then LOX/GAP
• Note that GAP-based solid rocket propellants can
  have higher Isp (example ammonium nitrate based
  composite propellants)
• Hydrogen (H) is preferred above carbon (C)
• Lower molecular weight of the exhaust products
  (H2O vs CO2/CO).
• Example PE (CH2) has a higher Isp then HTPB
  (CH1.5)
• Note that it is very often advantageous to run a
  hybrid motor fuel rich because of the lower
  molecular weight of the exhaust products. General
  rule optimum around equal amounts of CO2 and CO.

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What affects the regression rate of a fuel?

• Condensed phase physical properties
• Specific heat (cc) kJ/kgK
• Higher specific heat requires more energy to heat
  the fuel to its decomposition temperature
• Thermal conductivity (?c) W/mK
• Determines gradient in the condensed phase
• Density (?c) kg/m3
• Higher density requires more heat per volume of
  propellant
• Latent heat from phase transitions
• Melt heat (Qm) kJ/kg
• Evaporation (Qe) kJ/kg
• Condensed phase reactions can usually be
  neglected due to the low subsurface temperatures

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What affects the regression rate of a fuel?
cted

• Fuel surface chemistry
• Overall activation energy for the decomposition
  at the surface Es
• Arrhenius law for surface regression rate
  rbAsexp(-Es/RTs)
• In other words
• The surface temperature Ts is not constant
• The sensitivity of the burn rate to the surface
  temperature is given by the activation energy Es
• Typical values for some fuels
• HTPB Es 48.6 kcal/mole
• PE Es 60 kcal/mole
• GAP Es 41.5 kcal/mole

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What affects the regression rate of a fuel?
cted

• Gas phase physical properties
• Similar to the condensed phase
• Specific heat (cg) kJ/kgK
• Thermal conductivity (?g) W/mK
• Density (?g) kg/m3
• Varies with pressure according to the gas law
• Diffusion coefficients (Dab) m2/s
• Gas phase chemistry
• Reactivity of the gaseous products
• Zip-length average molecular weight of gaseous
  fuel decomposition products
• Example Polystyrene ltlt HTPB
• Oxidizer (and oxidizer decomposition products)
• LF much more reactive than N2O4

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Calculating rb A simple model

• Neglect contribution from radiation
• Control volume at the surface
• Total heat flux at surface required to heat the
  solid fuel from initial temperature To to the
  surface temperature Ts is
• rb ?c (cc(Ts-T0) QmQe) rb ?c hv
• hv is the total heat required to heat the fuel
  to the surface temperature, and melt and
  decompose it. Some examples
• HTPB hv 750 cal/g
• PE hv 850 cal/g
• GAP hv 65 cal/g
• Conductive heat feedback from the gas phase to
  the surface ?g(Tf-Ts)
• Heat balance rb ?c cc(Ts-T0) QmQe?g(Tf-Ts)
• Note
• The above equation works for solid rocket motors.
  
• However due to the large cross-flow in a hybrid
  motor, convective heat feedback can not be
  neglected.

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Convective heat flow

• Pipe flow
• Flow
• Laminar
• turbulent
• Entrance
• Entrance flow heat transfer coefficient varies
• Fully developed heat transfer coefficient is
  constant
• For laminar flows the thermal entrance length is
  a function of the Reynolds number and the
  Prandtle number xfd,t/D ? 0.05ReDPr
• For turbulent flow, xfd,t ? 10D.

heat transfer coefficient in a pipe
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Convective heat flow Calculating rb

• Assume fully developed
• qco h(Tm-Ts), m is mean temperature (TbTs)/2,
  b is boundary layer
• h ?mNuD/D, NuD is Nusselt number
• Emprical Dittus-Boelter equation
  NuD0.023Re4/5Prn
• in which n0.4 for heating and n0.3 for cooling
  (use n0.3, we are cooling the oxidizer flow)
• Reynolds number Re ?VD/? m/AD/?G D/?,
  ?viscosity
• Prandtl number Pr ?/?m, ?kinematic viscosity
• Hybrid rocket motor with convective heat flow
• qco ? Re4/5G4/5
• Since q ?crbhv it can be concluded that rb ?
  G0.8
• Neglecting radiation and conduction

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Engineering equations for calculating rb

• According to the pipe flow model
• rb G?
• However, G varies along the grain
• Complicates the use of this equation
• Predicts an increasing burn rate downstream
• This is not always observed
• In many cases G can be replaced by Gox
• Some equations used in design of hybrids
• rb aGox?
• rb aGox ? pn
• rb aGox ? pnLm

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Some burn rates

• 1. PMMA - Oxygen
• 2. PMMA - Oxygen
• 3. PMMA - Oxygen
• 4. PE - H2O2
• 5. Rubber - N2O4
• 6. Rubber (metalized) - N2O4
• 7. PE - FLOX
• 8. p-Toluidin/p-Aminophenol -HNO3
• 9. p-Toluidin/PVC-HNO3
• 10. Tagaform-HNO3
• 11. LiAlH3 - H2O2

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Fuel mass flow variation

• In general, the fuel mass flow will vary over
  time due to burn-back of the grain.
• Therefore, the oxidizer flow has to be regulated
  to keep O/F constant.
• This means that thrust varies over time. As well
  as the chamber pressure etc.
• In general there is a performance penalty.
• Some oxidizers are more forgiving, due to their
  wider Isp(O/F) curves
• Example N2O
• So, the grain should be designed to minimize fuel
  mass flow variations over time.

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Quasi-steady operation

• Consider a hybrid fuel grain
• Burning area Ab(h), where h the regressed web
• Port area Ap(h)
• Fuel mass flow rb.?c.Ab(h)
• Assume burn rate rb aGox? a(mox/Ap(h))?
• Then constant fuel mass flow if (Ap(h))?.Ab(h)
  constant
• Quasi-steady operation can be achieved if the
  grain shape and parameter ? are such that the
  above expression is constant for
• 0 lt h lt web

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Quasi-steady operation cted

• Consider a simple cylindrical fuel grain
• Ab(h) 2?(Rh)L
• Ap(h) ?(Rh)2
• Quasi-steady operation if
• (Ap(h))-?.Ab(h) constant ?
• (R2)-?.R ((Rh)2)-?.(Rh) ?
• R-2?1 (Rh)-2?1 for all h, only if -2?10 ?
  ?0.5
• So, only if ?0.5 quasi-steady operation with a
  cylindrical grain
• Experiments ?0.5 0.8

R
Rh
simple cylindrical grain
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Grain geometries

• It was shown that quasi-steady operation can be
  achieved with a simple cylindrical geometry.
• However, due to the low regression rates in
  hybrid systems, a single port is often not a
  viable solution
• Solution multiple port grain design
• Simple holes often not
• used due to large slivers
• Typical solid propellant grains are not useful
  as these are designed to have Ab(h)constant,
  while Ap(h) increases over time (when used in a
  hybrid fuel
• mass flow reduces over time)

typical multi-port fuel grain
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Example (effect of ?)

• Small hybrid motor
• 90 hydrogen peroxide
• 60 Aluminum / 40 HTPB fuel grain
• Pressure fed (constant pressure above the
  hydrogen peroxide)
• Cylindrical grain (round port)
• Evaluate the effect of ? for
• ? 0.50 (quasi-steady)
• ? 0.80
• ? 0.30

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Example case 1 ? 0.50
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Example case 2 ? 0.80
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Example case 3 ? 0.30
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Example Burn rates mm/s
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Example C value m/s
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Problems Pitfalls

• The burn rates in literature are often obtained
  from mass loss
• Problems with this approach
• Nonlinear behavior of the burn rate over time
  generates a wrong average
• Non-uniform regression
• Be careful when interpreting results for
  experiments with oxygen. The differences between
  LOX and GOX are large, but which one was used is
  not always very well documented (e.g. Sutton Fig.
  15-6).

example of non-uniform regression
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System design

• Feed systems
• Improving combustion efficiency

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Feed systems (Short summary, see Huzel and Huang
for details)

• Gas-pressurized
• Usually smaller rockets
• Low cost, high reliability
• Lower performance due to lower combustion chamber
  pressure
• Types
• Self pressurizing e.g. nitrous oxide
• Stored gas e.g. helium
• Propellant evaporation e.g. LOX using turbine
  exhaust (not useful for hybrids)
• Inert evaporation same as above, but using
  additional liquids such as liquid nitrogen and
  liquid helium (not employed frequently)
• Chemical-reaction systems
• Solid propellant gas generators
• Liquid propellant gas generators
• Liquid monopropellant (hydrazine)
• Liquid bipropellant (not useful for hybrids)

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Feed systems - cted

• Turbo pump
• Combination of a turbine and a pump
• Mostly used pumps are (multistage) centrifugal
  pumps
• Turbines
• Liquid monopropellant (hydrogen peroxide) good
  candidate for H2O2 hybrid system
• Liquid bipropellant (not useful for hybrids)
• Usually larger rockets
• High cost
• Higher performance due to higher
• combustion chamber pressure

SSME high-pressure fuel turbo pump
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Feed systems - cted

• Two typical hybrid designs (AIAA 95-2394)

pressure fed
turbo pump fed
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Improving combustion efficiency

• ONERAs perforated disc
• Improves combustion stability (flame holding)
• Increases regression rate
• Disadvantages
• Volumetric loading decreases
• Regression rates before and after disc are
  different
• Pressure drop across the disc

ONERAs perforated disc
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Improving combustion efficiency - cted

• UTCs post mixing discs
• Disadvantages
• Volumetric loading decreases
• Pressure drop across the disc

UTCs post mixing discs
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Tradeoff design studies
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Typical tradeoff design study (AIAA-93-2411)

• Hybrid strap-on for Titan 34D
• Two effects are clear
• System density (fuel oxidizer)
• O/F ratio difference
• combustion chamber size difference

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Typical tradeoff design study (AIAA-95-2394)

• Small launcher study
• Replace Scout solid first stage by a hybrid

combustion pressure v.s. system mass
component masses
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Typical tradeoff design study - cted
(AIAA-95-2394)
effect of number of ports on the system mass
fuel volumetric loading as function of of ports
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Typical tradeoff design study (AIAA-92-3592)
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Refences

• Papers hybrid combustion
• AIAA 93-2413, Condensed Phase Behavior and
  Ablation Rate of Fuels for Hybrid Propulsion, G.
  Lengelle, B. Fourest, J.C. Godon and C. Guin.
• AIAA 93-2553, Hybrid Rocket Instability, B.
  Greiner and R.A. Frederick
• AIAA 94-2880, Combustion Characteristics of Gas
  Hybrid Rockets
• AIAA 95-2689, An Experimental Investigation of
  Pressure Oscillations and Their Suppression in
  Subscale Hybrid Rocket Moters
• Papers system design / trade-offs
• AIAA 92-3592, Hybrid Rocket Motor Propellant
  Selection Alternatives, P.N. Estey, and G. R.
  Whittinghill
• AIAA 93-2411, Hydrogen Peroxide as an Alternate
  Oxidizer for a Hybrid Rocket Strap-on Booster, M.
  Ventura and S. Heister
• AIAA 94-3144, Optimization of Hybrid Rocket
  Engine Fuel Grain Design, D.J. Vonderwell, I.F.
  Murray, and S.D. Heister
• AIAA 95-2394, An Engineering Model to Assess
  Hybrid Propulsion Based Rocket Systems, F.
  Dijkstra

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Hypertek hybrids

• Hypertek hybrids
• Test set-up
• Data-acquisition

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Refences - cted

• Papers AMROC
• AIAA 90-2762, Hybrid Rocket Development at the
  American Rocket Company, R.J. Kniffen, B.
  McKinney, and P. Estey
• AIAA 91-2046, The Commercial Aquila Launch
  Vehicle, K.J. Flittie, J.S. McFarlane
• AIAA 91-2406, Design of Tridyne Pressurization
  System for Liquid Oxygen/Polybutadiene Hybrid
  Rocket Moters, J.M. Mueller and J.S. McFarlane
• AIAA 91-2517, An Evaluation of Scaling Effects
  for Hybrid Rocket Motors, P. Estey, D. Altman and
  J. McFarlane
• AIAA 92-1657, Development Status of the 200,000
  lbf Thrust Hybrid Rocket Booster, R. J. Kniffen
• Books
• Rocket Propulsion Elements, G.P. Sutton, Chapter
  15 (as of 6th edition), John Wiley Sons, Inc.,
  1992.
• Hybridraketenantriebe, Robert Schmucker, Wilhelm
  Goldmann Verlag GmbH, Munchen, 1972.
• Raketentreibstoffe, Dadieu, Damm, and Schmidt,
  Springer Verlag GmbH, Wien, 1968
• Design of Liquid-Propellant Rocket Engines, D.K.
  Huzel and D.H. Huang, AIAA, 1992