Choices: Some Considerations in
Configuring Launch Systems
Dr. John M. Jurist
Adjunct Professor of Space Studies, Odegard School of Aerospace Sciences,
and
Adjunct Professor of Biophysics and Aviation, Rocky Mountain College
Note: This material was used in various seminars at the above institutions,
and is not to be reused without attribution
Target Audience:
• People not trained in the physical or
engineering sciences
• People with a general interest in why
space launch technology is so
complex
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A Great Resource for Those
Who Wish to Dig Deeper:
George Sutton’s books: Rocket
Propulsion Elements (multiple editions
over 60+ years)
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Goals:
• Give a sense of the kinds of choices
that are made in configuring a launch
system.
• Show how choices made early in the
process affect later options.
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Relevant Quote:
“Statements in the first 2 years of the company
should be disregarded due to idiocy.”
-- Elon Musk, ISDC 2008 remarks at Capital
Hilton in Washington, DC on SpaceX scheduling
problems
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Primary Parameters:
Define payload mass to specified trajectory
Example: 2,000 pounds to 200 km LEO at 23 deg
inclination launched to east
Translates to mission velocity change (delta-V)
Orbital velocity plus margins for gravity and
aerodynamic losses = mission delta-V
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The Basic Rocket Equation:
Mo/Mf = e(v/c) or v = c * loge(Mo/Mf)
Mo = GLOW = liftoff mass
Mf = burnout mass
c = g * Isp = exhaust velocity
v = ideal burnout velocity
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Useful Reading Material:
For a narrative of the implications of the
rocket equation:
http://www.nasa.gov/mission_pages/station/
expeditions/expedition30/tryanny.html
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Options Narrowed:
Select propellant (solid vs liquid, hydrocarbon vs
liquid hydrogen, etc.)
Propellant combination determines specific
impulse from rocket equation
C* = g * Isp = exhaust velocity
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Tradeoffs -- 1:
Solid vs Liquid:
•
•
•
•
•
Simple vs complex (especially with pumps)
Reliable vs less reliable ignition
Storable vs less storable
Lower vs higher potential impulse density
Lower vs higher mass efficiency for larger
(pumped) systems
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Tradeoffs -- 2:
Solid vs Liquid:
•
•
•
•
•
•
Toxic components and/or exhaust products
Ammonium perchlorate
Liquid oxygen
Hydrocarbon
Hydrazine
Liquid hydrogen
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Tradeoffs -- 3:
Solid vs Liquid:
•
•
Handling
Hydrocarbon vs liquid hydrogen
• Lower vs higher specific impulse
• Higher vs lower density (tank mass)
• Insulation (tank mass)
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Options Narrowed -- 1:
Selected propellant C* gives required mass ratio
from rocket equation for given mission delta-V:
(Mo/Mf), where Mo = GLOW = liftoff mass
and Mf = burnout mass
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Options Narrowed -- 2:
Staging to Ease Mass Ratio Constraints:
• More stages = more complexity and less
reliability
• No ullage or separation motors required for
solids but required for liquids unless hot-fire
staging used
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Discussion Limitation:
Assume two stage liquid propellant system from
this point forward
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Mass Budget -- 1:
Once payload mass, mission delta-v, propellant
combination, staging, and mass ratio defined,
characterization of each stage can proceed:
•
•
•
•
Structural elements
Tankage (and insulation if required)
Attitude control system
GNC (guidance, navigation, and control)
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Mass Budget -- 2:
• Payload shroud (eject when out of atmosphere)
• Interstage assembly (divide between lower and upper)
• Ullage systems (stage separation and propellant
settling for upper stage ignition)
• Destruct system
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Mass Budget -- 3:
• Propulsion system:
•
•
•
•
Motors : Nozzles behave differently at sea level and in
vacuum so compromises needed in design
Pumps
Plumbing
Gimbals
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Mass Budget -- 4:
• Multiple iterations required
• Interactions/relationships
• Reusability implies:
More robust (and massive) structure, and
Thermal fatigue and shock issues for motors
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Illustrative Interaction -- 1:
A new definition: Net structural mass fraction
NFS or NMF = ( Ms - Mm ) / Mp
Ratio of structural mass less motor to propellant
mass – also called net mass fraction or NMF
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Illustrative Interaction -- 2:
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Variations:
Multiple motors:
• Shut down and drop some motors part way
through burn
• Atlas missile
Propellant Transfer
• Shift propellant from tanks that can be dropped
• SpaceX Falcon Heavy?
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An Alternative to Propellant
Transfer:
• Instead of similar core and 2 side boosters with fuel
transfer to core, have similar tankage with more
motors supplied from side boosters and fewer from
central core
• Eject side boosters and motors when fuel depleted
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Cost Considerations:
Dr. Dietrich E. Koelle: Transcost
• Statistical-analytical model for cost estimation
and economical optimization of launch vehicles
• Parametric cost estimation: Method of
estimating cost per unit mass
• See Launch Vehicle Business Workshop .ppt
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Political and Regulatory
Considerations:
To be discussed
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