Rockets and Propulsion

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Rockets and Propulsion
2
Propulsion Basics
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Propulsion Basics

• Fundamental principle behind rocket propulsion is
  Newtons action-reaction law
• For every action there is an equal and opposite
  reaction
• Escaping exhaust gas of the rocket motor drives
  the rocket in the opposite direction with an
  equal force forward thrust
• This is equivalent to the forward momentum of the
  rocket being the same as the momentum of the
  exhaust, but in the opposite direction

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Propulsion Basics

• Momentum mass x velocity m v
• Momentum forward momentum rearward massrocket
  x velocityrocket
• -massexhaust gas x velocityexhaust gas
• MrVr -meve or Vr -ve(me/Mr)
• Since the mass of the rocket is much greater than
  the mass of the exhaust gas, the velocity of the
  exhaust gas must be much greater than the forward
  velocity of the rocket

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Propulsion Basics

• Thrust and momentum are not the same
• Thrust force weight (in dimensions)
• Momentum times mass flow rate (1/seconds) has the
  same dimensions as thrust (mass length/time2)
• For measuring rocket thrust, we therefore need
  exhaust momentum time exhaust mass flow rate

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Propulsion Basics

• Mass flow rate is the amount of fuel that is
  consumed (or combusted) and expelled as exhaust
• Larger rockets consume more fuel than smaller
  rocket in the same time interval
• Larger rockets produce more thrust than smaller
  rockets
• All very obvious
• The size of a rocket roughly determines its
  thrust, or lift capacity

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Propulsion Basics

• Propulsion performance is measured primarily as
  thrust and exhaust velocity
• Thrust is determined by mass flow rate and
  exhaust velocity
• Exhaust velocity is a measure of thrust
  efficiency
• Higher exhaust velocity higher thrust efficiency

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Rocket Propulsion Types
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Propulsion Types

• Rocket propulsion types
• Chemical
• Liquid propellant
• Single (mono) combined fuel oxidizer
• Dual (bi) - separate fuel oxidizer
• Solid propellant combined fuel oxidizer
• Compressed gas
• Unheated
• Heated
• Electric
• Ion
• Electrothermal
• Nuclear
• Solar pressure

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Propulsion Types

• Rocket propulsion
• Simplest propulsion type is compressed gas
• Simple
• Inexpensive
• Inefficient low exhaust velocity
• Low energy content
• Used on small satellites
• Balloon is the simplest example

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Propulsion Types

• Most common rocket propulsion type is the solid
  rocket
• Simple
• Inexpensive
• Modest exhaust velocity
• Can be scaled from small model rocket and
  fireworks to large Solid Rocket Boosters used on
  the space shuttle
• Single use
• Used primarily for first stage boosters and
  separation motors

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Propulsion Types

• Large rocket engines used for most launchers are
  liquid bipropellant engines
• Complex
• Relatively expensive
• Higher exhaust velocity
• Difficult to scale from small to large
• Can be restartable and/or reusable

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Propulsion Types

• Liquid monopropellant engines
• Simple
• Relatively inexpensive
• Modest exhaust velocity
• Can be scaled up to moderate thrust
• Often restartable and used in a variety of roles
  (attitude control, orbit booster, deorbit motor,
  etc.)

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Propulsion Types

• Electric propulsion engines
• Relatively complex
• Expensive
• Very low thrust
• Very high exhaust velocity
• Useable only in space (vacuum)
• A developing technology, although used on
  interplanetary boosters and for satellite
  stationkeeping

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Propulsion Types

• Electric propulsion ion engine
• Electric and or magnetic fields used to
  accelerate charged atoms (ions)
• Heavy nuclei better than light nuclei (Xe
  commonly used)
• Extremely low thrust
• 10-6 N 0.01 N
• Very high exhaust velocity
• 10 100 times chemical rocket exhaust velocity

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Propulsion Types

• Electric propulsion electrothermal (heated gas)
• Electric current used to heat cold gas
• Heating reactive or inert gas increases exhaust
  velocity and thrust
• Low to modest thrust
• Moderate exhaust velocity
• Resistojet electric heater
• Arc jet electric arc heating

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Propulsion Types

• Electric propulsion
• Other electric propulsion types include
• Magnetoplasmadynamic engine
• Variable Specific Impulse Magnetoplasma Rocket
  (VASIMR) engine
• Nuclear ion engine
• Heated gas by hot nuclear reactor core

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Propulsion Types

• Nuclear propulsion
• Nuclear reactors used to heat a cold gas to very
  high temperatures
• Hydrogen gas is the most efficient propellant
• High exhaust gas velocities
• Heated gas by hot nuclear reactor core
• Nuclear ion engine is a variation with greater
  effeciency

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Propulsion Types

• Nuclear propulsion Nerva program (1957-1972)

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Propulsion Types

• Solar pressure propulsion
• Solar photon pressure can be used for propulsion
  (solar sailing) with certain limitations
• Low thrust
• Low payload mass
• Very large reflective sail needed
• Operable only in vacuum of space
• Limited to inner solar system
• Prototype launches failed, but several are being
  readied for flight

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Rocket Propulsion Performance
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Propulsion Performance

• Lift/payload performance
• A rocket's lift or payload performance is a
  function of three measures
• Thrust
• Thrust efficiency (Isp)
• Thrust duration
• These three components also determine the total
  propulsive energy of the rocket and its
  propellants

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Propulsion Performance

• Thrust
• Thrust is a measure of the forward force produced
  by the rocket
• Thrust has the same dimensions of both force and
  weight
• Thrust units are typically lbf (meaning force in
  lbs), or in Newtons (or kgf meaning kg force)
• Thrust is proportional to exhaust momentum times
  exhaust mass flow rate
• Thrust can be increased or decreased in some
  rocket motor designs by increasing or decreasing
  the propellant flow rate

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Propulsion Performance

• Thrust
• Three factors dominate thrust performance of the
  chemical rocket motor
• 1. Fuel flow rate - Higher fuel flow rates
  increase forward thrust
• 2. Pressure difference between internal nozzle
  pressure and external (ambient) pressure 
• Maximum pressure difference is in space (vacuum
  pressure)
• 3. Exhaust velocity - Higher velocity produces
  greater forward thrust

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Propulsion Performance

• Thrust efficiency - Specific Impulse (Isp)
• Specific impulse is a measure of the thrust
  produced for a given fuel weight flow
• An equation expressing Isp in relation to the
  thrust produced and the fuel consumed (fuel flow
  rate) would be
• Isp Thrust produced / fuel weight flow rate
  (dimensions and units are seconds)

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Propulsion Performance

• Thrust efficiency - Specific Impulse (Isp)
• Isp value is a measure of how efficient the
  propellant (or engine) is converted into thrust
• Isp could also be described as the burn time of a
  fuel for a specified mass at a specified thrust
• A fuel with an Isp that is two times another
  would burn twice as long with the same thrust

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Propulsion Performance
Approximate Isp ranges Approximate Isp ranges Approximate Isp ranges Approximate Isp ranges
    Very high 1,000-10,000 sec Ion and plasma engines
High   350-500 sec Liquid bipropellant (liquid fuel liquid oxidizer)
Moderate 200-350 sec Solid fuel or liquid monopropellant (liquid fuel combined with oxidizer)
Low 0 -200 sec Cold (compressed) gas
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Propulsion Performance
Engine Isp Thrust
Space Shuttle Main Engine (SSME) 453 s (vac) 363 s (sea level)   233,295 kgf (513,250 lbf, 2.3 MN) (vac)
Space Shuttle Solid Rocket Boosters (SRB) 269 s (vac) 237 s (sea level) 1,500,000 kgf (3,300,000 lbf, 14.8 MN) (sea level)
Saturn V F-1 first stage engine 260 s (sea level) 681,180 kgf (1,500,000 lbf, 6.7 MN) (sea level)
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Propulsion Performance

• Specific impulse that is a measure of thrust
  efficiency is also proportional to exhaust
  velocity
• An approximation of the relationship between
  thrust efficiency and exhaust velocity is
• Vexhaust g Isp where g is the gravitational
  acceleration at the Earths surface 9.8 m/s2
• For example, a rocket engine with an Isp of 300 s
  would have an exhaust velocity of 300 s x 9.8
  m/s2 2,940 m/s (6,580 mph)

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Propulsion Performance

• For chemical rockets, exhaust velocity is
  influenced primarily by the following factors
• Exhaust gas molecular weight - Lower is better
  (hydrogen is one of the optimum fuels)
• Combustion temperature - Higher is better
• Limited by the combustions chamber strength
• Combustion chamber pressure - Higher is better
• Limited by the combustions chamber strength
• Specific heat ratio (chemical energy available to
  convert  fuel into exhaust gas based on the
  reaction chemistry of the fuel and oxidizer) -
  Some fuels are better than others
• Exhaust nozzle geometry maximizes exhaust
  velocity using both the kinetic and potential
  energies of the exhaust gas flow

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Propulsion Performance
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Propulsion Performance

• Exhaust nozzle
• To optimize the exit velocity in a chemical
  rocket
• Subsonic gas will increase speed if flowing
  through a converging exit nozzle (decrease
  flowing through a diverging nozzle
• Supersonic gas will increase its flow speed
  through a diverging nozzle (decrease flowing
  through a converging nozzle)
• Maximum exhaust velocity comes form a subsonic
  flow through a converging interior nozzle with
  supersonic flow through the exterior diverging
  nozzle
• Also important in optimizing exhaust velocity
  include the nozzle's convergent and divergent
  angles, and the throat-to-exit area ratio

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Propulsion Performance

• Exhaust nozzle
• Expansion of the exhaust from the combustion
  chamber and the nozzle throat should ideally
  conform to an even flow into the outside gas (or
  vacuum)
• Four conditions showing correct expansion,
  underexpansion, and overexpansion from the nozzle
  with respect to the ambient air/vacuum are shown
  on the right
• Under expanded (top)
• Ideal
• Over expanded
• Far over expanded (bottom)

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Propellants and Motors
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Propellants

• Propellant selection is important, not only for
  combustion energy and exhaust gas velocity, but
  also for density, storage, handling, and cost
• The simplest rocket fuels are solids which
  consist of a combined fuel and oxidizer compound
  that is stabilized into a fast-burning propellant
• The solid fuel is generally cast in the
  combustion chamber as a unit
• Liquid propellants called monopropellants can
  also have a combined fuel and oxidizer compound
• The single (mono) liquid propellant is simpler
  and much less costly to store and handle than
  cryogenic propellants

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Liquid Propellants

• Propellant choice is not only based on
  performance and density, but also on a variety of
  characteristics that include important safety and
  handling criteria, some of which are
• Specific impulse (Isp)
• Cost
• Toxicity health hazards
• Explosion and fire hazard
• Corrosion characteristics
• Handling safety
• Propulsion system computability
• Freezing/boiling point temperatures
• Stability
• Heat transfer properties
• Ignition, flame and combustion properties

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Propellants Common types of liquid propellants
Oxidizer Fuel Isp (theoretical)
Liquid oxygen (LOX) Liquid hydrogen (LH2) 477 s
LOX Kerosene (RP-1) 370 s
LOX Monomethyl hydrazine 365 s
LOX Methane (CH4) 368 s
Liquid ozone (O3) Hydrogen 580 s
Nitrogen tetroxide (N2O4) Hydrazine (N2H4) 334 s
Hydrogen peroxide (H2O2) Monopropellant 154 s (90 H2O2)
H2O2 Hydrazine
Fluorine Lithium 542 s
Fluorine Hydrogen 580 s
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Liquid Propellant Engine Basics

• Liquid bipropellant engine diagram showing the
  major elements, and several of the thrust
  parameters that include exhaust and ambient
  pressures (Pe and Po), exhaust velocity (Ve), and
  mass flow rate (dm/dt) (Courtesy
  NASA-Exploration)

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Solid Propellants

• Solid rocket propellants contain a variey of
  chemicals in addition to the basic fuel and
  oxidizer, part for stabilization and part for
  performance
• Solid fuels used for larger rockets are
  composite mixtures containing separate granulated
  or powdered fuel and oxidizer, with a chemical
  binder, a stabilizer, and often an accelerant or
  catalyst added for improved performance and
  stability
• The final mixture of solid fuel used today is a
  dense, rubber-like material that is cast into the
  combustion chamber

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Solid Propellants

• The cast fuel has a central cavity to allow
  burning throughout the length of the rocket
  motor, in shapes that can be anything from a
  simple cylinder to a star
• Solid rocket fuel is typically identified by the
  type of chemical binder used - either HTPB or
  PBAN
• Hydroxyl-terminated polybutadiene, or HTPB, is a
  rubber-like binder that is stronger, more
  flexible, and faster-curing than PBAN, but
  suffers from a slightly lower Isp, and uses
  fast-curing, toxic isocynates

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Solid Propellants

• Polybutadiene acrylic acid acrylonitrile (PBAN),
  has a slightly higher Isp, is less costly, and
  less toxic, which makes it popular for amateur
  rocket-makers
• PBAN Is also used in the large boosters,
  including the Titan III, the Space Shuttle SRBs,
  and NASA's new Constellation Ares I and Ares V
  launchers
• HTPB is or has been used in the Delta II, Delta
  III, Delta IV, Titan IVB and Ariane launchers

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Hybrid Rockets

• Hybrid chemical rocket motors, motors with solid
  fuel and liquid oxidizer, are used for
  intermediate sized boosters
• The most familiar hybrid engine is the one used
  to power Burt Rutan's SpaceShipOne (test article
  shown below)
• The cast solid fuel core is contained in a
  combustion chamber
• Nitrous oxide stored as a liquid is injected over
  the solid fuel core
• The oxidizer flow is used to start, regulate, and
  stop the combustion process

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Rocket Stability
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Rocket Stability

• Two types of stability of a rocket are needed for
  its successful flight
• Static stability during initial launch
• Dynamic stability during powered and unpowered
  flight
• Static stability
• The force of thrust, or even simple gravity on a
  rocket or on an upright pencil produces stable
  lift, unstable lift, or a neutral stable lift
• An analogy is an upright pencil with the force of
  gravity pulling downward which is equivalent to a
  propulsion thrust pushing upwards
• The rocket must be stabilized in its initial
  launch or the force of thrust will immediately
  rotate the rocket (pencil)
• Upward force below the center of mass is unstable
• Upward force above the center of gravity is
  stable

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Rocket Stability

• Static stability
• Passive
• Traditionally provided on very small rockets with
  a guide rail attached to the launch pad and a
  guide attachment on the rocket
• Active
• A guidance control system creates thrust
  vectoring of the rocket exhaust for launch
• Used on larger rockets and missiles (Atlas
  missile shown on the right)
• Can be provided by control of the main engine
  thrust, or by smaller augmentation guidance
  engines

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Rocket Stability

• Dyanmic stability
• Passive
• Aerodynamic fins create restoring force to align
  the rocket in the direction of motion during
  flight
• Requires aerodynamic force aft of the center of
  mass

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Rocket Stability

• Dynamic stability
• Active
• A guidance control system creates thrust
  vectoring of the rocket exhaust for launch
• Used on larger rockets and missiles
• Can be provided by control of the main engine
  thrust, or by smaller augmentation guidance
  engines
• Soviet RD-107 shown on the right with four
  primary thrust engines and four small outboard
  vernier guidance thrusters

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Modern Rocket Development
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Early missiles

• V-2
• The German V-2 developed by the Nazis in WW-II
• Alcohol fuel (ethanol 75, water 25 for cooling
  and stability)
• Liquid oxygen oxidizer
• Employed double-wall combustion chamber
• Inner cavity allowed fuel to circulate to cool
  combustion chamber
• 55,000 lbf thrust (24,958 N)

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Early missiles

• Redstone
• Developed by Wernher von Braun and the Army
  Ballistic Missile Agency (ABMA)
• Intermediate range ballistic missile (IRBM)
• Used similar design features of the V-2 with
  improved performance
• Engine design by North American (Rocketdyne)
• NAA 75-110
• Based on Navajo cruise missile engine
• Alcohol fuel
• Liquid oxygen oxidizer
• Payload 6,300 lb
• 78,000-83,000 lbf thrust

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Early missiles

• Jupiter missile
• Intermediate range ballistic missile (IRBM)
• Combined Army-Navy project
• Limited use as IRBM missile
• Navy rejected design for submarine missiles
• Converted to use by NASA for the early
  interplanetary missions
• Juno II
• Juno I was Redstone spacecraft launcher
• 150,000 lbf thrust

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Early missiles

• Thor missile (IRBM)
• Initial intermediate-range missile for U.S. and
  European deployment during the Cold War
• Single Rocketdyne LR-79 (SD-3) engine used later
  on Atlas
• 150,000 lbf thrust
• Single-stage missile was augmented with multiple
  stage for increased payload and range, and
  launching first reconnissance satellites
• Four-stage version later renamed Delta (4th
  letter of Greek alphabet)
• Delta booster now a family of launchers

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Early missiles

• Delta rocket family

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Early missiles

• Atlas ICBM
• First American intercontinental ballistic missile
  (ICBM)
• USAF project
• Still used as commercial launcher
• Medium- and heavy-lift versions
• Atlas V heavy-lift booster uses Russian RD-180
  engine
• Liquid oxygen (LOX) and kerosene first stage

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Early missiles

• Atlas ICBM
• Used three primary engines on original ICBM
  design
• Sustainer (central)
• Steering and augmented thrust engines outboard
• Later used for Mercury orbital missions
• Used for Geminis Agena target vehicle launch
• Developed into family of launchers

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Early missiles

• Atlas rocket family ( Titans)

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Early missiles

• Navy Viking missile
• Developed by Naval Research Labs as a missile
  prototype
• Used Reaction Motors Company XLR10-RM engine
• First to use integrated tanks and structure
  (monocoque)
• First to use thrust-vectored engines
• Used as first stage for Vanguard rocket
• First satellite launch attempt
• Alcohol and liquid oxygen propellants

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Early missiles

• Titan missile
• Developed originally for the Air Force as a
  backup ICBM to supplement the Atlas
• Original Titan I design used RP-1 (kerosene) and
  LOX
• Later Titan II used nitrogen tetroxide (NTO) and
  unsymmetrical dimethyl hydrazine (UDMH)
• Titan III and Titan IV used solid rocket boosters
  to augment thrust on first stage
• Prototype for SRBs used on Space Shuttle

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Early missiles

• Titan missile
• Titan III and IV used for both military satellite
  launches and civil interplanetary launches
• Heaviest-lift launcher before Delta IV Heavy

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Current Rockets
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Current Rockets

• Delta launcher family

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Current Rockets

• Atlas launcher
• family

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Rocket Stability

• Delta IV Heavy used for spacecraft launches
• Atlas V Heavy to be used for Orion capsule tests
• Newest launcher is NASAs heavy-lift launcher
  designated Space Launch System (SLS)

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Rocket Stability

• Space Launch System (SLS)
• 3-stage booster
• Payload capacity
• LEO 70,000 kg - 129,000 kg
• (150,000 lb 280,000 lb)
• 1st stage five segment SRB boosters
• 2nd stage (core) LOX-LH2 fueled RS-25E engines
  (5)
• 3rd stage 1 RL 10B-2 engine or 3 J-2X engines

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Rocket Stability

• Falcon 9 rocket
• 2-stage booster
• Payload capacity
• LEO 10,450 kg (23,000 lb)
• 1st stage nine Merlin engines
• 2nd stage 1 Merlin engines
• Propellants LOX RP-1 (refined kerosene)

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• The End