Introduction to Space Systems Engineering

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Introduction to Space Systems Engineering Dr
Roger Moses Department of Aerospace
Engineering University of Bristol
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The world needs uninhibited thinkers not afraid
of far out speculation
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The world needs uninhibited thinkers not afraid
of far out speculation It also needs hard
headed conservative engineers who can make their
dreams come true
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The world needs uninhibited thinkers not afraid
of far out speculation It also needs hard
headed conservative engineers who can make their
dreams come true Arthur C.Clarke
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Interaction with Aeronautics   Spaceflight grew
out of aeronautics the engineering approach as
well as the core disciplines of propulsion,
lightweight structures and control, with the
addition of a large dose of electronics. It has
become a significantly independent activity. I
see the main interactions in the near future in
two areas
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Interaction with Aeronautics 1. Utilisation of
space provided services Communication,
Navigation, Air Traffic Control and
Met.Forecasting for Civil and Military aviation
will increasingly derive from space gathered and
distributed information. To a certain extent
these can be regarded as black box activities,
but there will be issues of safety, regulation,
reliability and operating economics which will
depend on a good understanding of detailed
aspects of the operation of space systems.
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Interaction with Aeronautics 2. Reusable
Spaceplanes These, when they come, will reconnect
the space activity with classical aeronautics.
They will use all the disciplines connected with
aircraft, as well as space specific
subjects. They will have more in common with the
modern airliner, than with the old launchers
developed from ICBMs or application satellites.
They will be manufactured by aircraft companies,
and operated by organisations akin to a military
air transport unit, or eventually, an airline.
They may very significantly reduce the cost of
access to space, which could open space to mass
markets e.g. tourism, manufacture
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• Interaction with Aeronautics
• Reusable Spaceplanes
• Reaction Engines Skylon

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The Far Future   All of this has been quite
conservative stuff, only looking forward perhaps
twenty years. What of The Far Future? Very
difficult to predict without 20/20
hindsight.!   We may well see Solar power
satellites, Permanent manned lunar base,
Permanent observatories on planetary surfaces,
Human expedition to Mars, Adventure tourism. We
are unlikely to see Interstellar travel, contact
with extra-terrestrial civilisations.
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ORBIT THEORY Objectives At the end of this
section you will  1. be able to apply simple
astrodynamics (Keplers Laws and their
developments) in a variety of Earth orbit and
solar system situations, giving you a feel for
the distances, velocities and timescales involved
in spaceflight.  2. be able to calculate the
velocity changes (and hence propulsion system
requirements) required for simple manouevres
launch into orbit, inclination change, transfer
between coplanar circular orbits, and rendezvous.
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SPACEFLIGHT MECHANICS   They got it
right! both sets of orbital elements, and the
Dvs for propulsion Rendezvous and first docking
of the Mir space station and the space shuttle
Atlantis (Mission STS - 71) in June-July 1995.
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1. ORBIT THEORY - the story so far   Since the
time of Isaac Newton (1642-1727) , we have had
the tools available for a complete solution of
the orbit of a single body of negligible mass
(secondary) about a much greater mass (primary),
and by a minor development, of an isolated pair
of bodies of arbitrary mass about their mutual
centre of mass. These tools are Newton's Law of
Universal Gravitation (NG), his three Laws of
Motion (N1,N2,N3), and the Calculus. This enabled
Newton to derive from first principles the Laws
of Planetary Motion (K1,K2,K3) empirically
established a century earlier by Johannes Kepler
(1571-1630).
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1. ORBIT THEORY - the story so far   Further
development by the great applied mathematicians
of the 18th and 19th centuries enlarged this
theoretical foundation to satisfactorily describe
the motion of bodies in the solar system to a
very great degree of precision. These
developments were both analytic methods, and
methodsof numerical solution (by computer, human!
- of course) which are still of great use, with
some modification, in predicting the orbits
of satellites and space probes, as well as the
natural bodies, in orbits about the Earth and
Sun. This subject is called astrodynamics.
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1. ORBIT THEORY - the story so far   The
conceptual revolution in our understanding of
motion, mechanics and gravity engineered by
Albert Einstein (1879-1955) has had relatively
little effect on most astrodynamical
calculations e.g. his explanation of the
24"/century anomaly of the precession of
perihelion of Mercury's orbit must be balanced
against the approx. 1000"/century accounted for
by many body and other perturbing classical
effects, and the relatively enormous 5.39.108
"/century of its simple 2-body orbit.
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1. ORBIT THEORY - the story so far   A measure
of the precision available is the navigation of
the Voyager probes into Saturn's system of moons
and ring particles with an accuracy of approx.
10km in 109 km, required to achieve the
sling-shot manouevre to carry Voyager 2 on out to
Uranus in 1986, Neptune in 1989, and ultimately
out of the solar system. In spite of these
triumphs of theory and practice, the orbits in a
3-body (or more) system are not determinable by
current theory, except in a few special cases,
and numerical methods must be used in all
practical cases, carried out iteratively until
the required accuracy is achieved, by computer.
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Fig from The Exploration of Space, Arthur
C.Clarke, 1951
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It is important to realise that Keplers Laws
were empirically derived from observation, with
originally no underlying theoretical
justification. They may be derived from
Newtonian gravitation between pairs of point
masses they are however surprisingly good in
almost all practical situations
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Newton's Law of Universal Gravitation
(NG)   There is an attractive force between all
masses, such that   1.3 where
G, the Universal Gravitational Constant
6.670.10-11 Nm2Kg-2  
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Example-derivation of "g" at the Earth's
surface.   r6378 km 6.378.106 m m1 Mass of
Earth M? 5.98.1024 kg   By N2,
a 9.805 ms-2 , - the
familiar value of "g" at the earths surface  
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What is "g" g   a.on the surface of the
Moon?   b.at the moon's orbit?   c.at the
Earth's orbit, due to the Sun?   d.2000 km
down?   A point mass free to move under gravity
will accelerate at the indicated value, and will
in this state of accelerated motion experience
apparent zero gravity it will be in free fall.
This peculiar self-cancelling consequence of
gravitation is shared by no other force known to
physics - leading Einstein to formulate his
Principle of Equivalence, and eventually General
Relativity
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A point mass free to move under gravity will
accelerate at the indicated value, and will in
this state of accelerated motion experience
apparent zero gravity it will be in free fall.
If it initially has enough transverse velocity,
it will orbit the centre of attraction in a
trajectory given by Keplers Laws.   For circular
orbits, we can apply mass on a whirling string
mechanics to obtain an orbital velocity vc.
where ? Gm2 is the Gravitational parameter,
a measure of gravitational strength due to the
mass m2.
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Dynamics orbit plane determined by initial
angular momentum vector
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Dynamics and geometry Keplerian ellipse in its
own plane
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Reference plane Earths Equatorial
Plane or Earths orbit plane around the Sun the
Ecliptic plane
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Equatorial Plane or Ecliptic plane Incline
d at spin axis inclination 23.5 Common reference
direction First point of Aries
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O
?
Orbit plane and Reference Plane inclined at
Orbital Inclination i
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O
?
Orbit plane and Reference Plane intersect in
Line of Nodes
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O
?
Intersection points are Ascending and Descending
Nodes
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O
?
Argument of Perigee ? is orbit orientation in its
own plane referenced to Line of Nodes
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O
?
Right Ascension of Ascending Node O is Line of
Nodes orientation in Reference Plane
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O
?
Time of Pericentre Passage T gives datum for
time measurement
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O
?
Time of Pericentre Passage T gives datum for
time measurement
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The orbital elements are the 6 quantities that
describe the geometry and dynamics of a Keplerian
orbit in 3-d space A semi-major axis P orbital
period ? true anomaly i inclination ? argument of
pericentre O RA of ascending node T Time of
pericentre passage
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2. ORBIT   APPLICATIONS      
Apollo 10 took off for the Moon on 18 May 1969,
and did everything that Apollo 11 did later,
except land! It was the first mission for which
orbital rendezvous was a life or death reality.
On 22 May, Tom Stafford and Gene Cernan took
their Lunar Module Snoopy to within 8.4 km of
the lunar surface, above what would later become
Tranquillity base, for two orbits, before
returning to rendezvous and dock with the Command
Module Charly Brown, piloted by John Young, in
its 60 nm 2 hr orbit. They returned safely to
earth on 26 May after a journey of 8 days 3
minutes. Two months later Apollo 11 successfully
established the first human encampment on another
world. NASA photos                          

 
 
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2. ORBIT   APPLICATIONS      
Apollo 10 took off for the Moon on 18 May 1969,
and did everything that Apollo 11 did later,
except land! It was the first mission for which
orbital rendezvous was a life or death reality.
On 22 May, Tom Stafford and Gene Cernan took
their Lunar Module Snoopy to within 8.4 km of
the lunar surface, above what would later become
Tranquillity base, for two orbits, before
returning to rendezvous and dock with the Command
Module Charly Brown, piloted by John Young, in
its 60 nm 2 hr orbit. They returned safely to
earth on 26 May after a journey of 8 days 3
minutes. Two months later Apollo 11 successfully
established the first human encampment on another
world. NASA photos                          

 
 
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2. ORBIT   APPLICATIONS      
Apollo 10 took off for the Moon on 18 May 1969,
and did everything that Apollo 11 did later,
except land! It was the first mission for which
orbital rendezvous was a life or death reality.
On 22 May, Tom Stafford and Gene Cernan took
their Lunar Module Snoopy to within 8.4 km of
the lunar surface, above what would later become
Tranquillity base, for two orbits, before
returning to rendezvous and dock with the Command
Module Charly Brown, piloted by John Young, in
its 60 nm 2 hr orbit. They returned safely to
earth on 26 May after a journey of 8 days 3
minutes. Two months later Apollo 11 successfully
established the first human encampment on another
world. NASA photos                          

 
 
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TRANSFER BETWEEN ORBITS 2 - between Coplanar
Circular Orbits   Between coplanar circular
orbits - the Hohmann minimum energy transfer
ellipse. The main problems with the Hohmann
transfer are that it takes too long, and
requires narrow launch windows. Fast transfers
are possible with additional expenditure of
propellant ? is semi-major axis of transfer
orbit, where But Transfer time, from
K3 2.5
 
 
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TRANSFER BETWEEN ORBITS 2 - between Coplanar
Circular Orbits   Between coplanar circular
orbits - the Hohmann minimum energy transfer
ellipse. The main problems with the Hohmann
transfer are that it takes too long, and
requires narrow launch windows. Fast transfers
are possible with additional expenditure of
propellant
 
and the transfer impulses ?Va, ?Vb
 
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TRANSFER BETWEEN ORBITS 3- Rendezvous between
Spacecraft in the Same Orbit Rendezvous
between spacecraft in the same circular orbit,
but separated along the orbit track, initially at
A1, B1(leading) how does A catch up on B?
  Obviously, it accelerates by an impulse along
the direction of motion, and one orbit later,
finds itself even further behind!
 
 
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TRANSFER BETWEEN ORBITS 3- Rendezvous between
Spacecraft in the Same Orbit
This is somewhat counterintuitive, as the Gemini
astronauts, McDivitt and White, both experienced
test pilots, found in 1965, during a practice
rendezvous. Adding speed also adds altitude,
moving the spacecraft into a higher orbit than
its target . . . , As the Gemini 4 Crew observed,
the target seemed to gradually pull in front of
and away and below from the spacecraft. The
proper technique is for the spacecraft to reduce
its speed, dropping to a lower and thus shorter
orbit, which will allow it to gain on the target.
At thecorrect moment, a burst of speed lifts the
spacecraft to the targets orbit close enough to
the target to eliminate virtually all relative
motion between them. Now on station, the
paradoxical effects vanish, and the spacecraft
can approach the target directly  
 
 
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TRANSFER BETWEEN ORBITS 3- Rendezvous between
Spacecraft in the Same Orbit
This nice story, although not entirely
technically accurate, does illustrate the perils
of a common-sense approach to rendezvous. Even
the last sentence, which implies that close up
rendezvous is easy, is somewhat optimistic, given
the catastrophic (and near fatal) accident to Mir
and the unmanned Progress supply ship in 1997, in
exactly this situation. After the first
successful rendezvous (Gemini 6 and 7, 1965),
ending 2 metres away, stationary, and taking
35,000 thruster firings, Wally Schirra said If
anybody thinks theyve pulled off a rendezvous at
three miles, have fun! This is when we started
doing our work. I dont think a rendezvous is
over until you are stopped - completely stopped -
with no relative motion between the vehicles, at
a range of approximately 120 feet. Thats
rendezvous!  
 
 
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TRANSFER BETWEEN ORBITS 3- Rendezvous between
Spacecraft in the Same Orbit
How do we achieve rendezvous?  We first ascertain
how far ahead or behind the target spacecraft is.
We convert that to a time separation, giving
the period of the transfer orbit. We then use
the theory of the Hohmann transfer to give the
impulse required to transfer. After one orbit,
we are at rendezvous, and use an equal and
opposite impulse to match velocities. Easy! Now
try it for real. Consider the following problem
- the space station Mir is in a 500 km height
circular orbit. It is surrounded by 6
Progress supply ships, all at a distance of 150
km one above, one below, one in front, one
behind, one to the left, one to the right. What
do they have to do to effect rendezvous?
 
 
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3.SPACE PROPULSION SYSTEMS
 
 
      ARIANE 5 Launch CNES
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Rocket motors are jet propulsion devices,
which work by the expulsion of mass at non-zero
velocity, and which, by the conservation of
linear momentum, necessarily move in the
opposite direction, according to Newton's second
law of motion     Unlike the jet engine, and
any other commonly used prime mover, which all
abstract from the medium in which they move, the
reaction mass which they need, the rocket carries
its own reaction mass, and therefore works
readily in a vacuum. It is, in its various
forms, the only prime mover usable in
untethered space vehicles. The only rocket
system in extensive service to date is the
CHEMICAL rocket, necessarily combining energy
supply with reaction mass - although this is a
simple system, it is not necessarily the best way
of doing things. The mere fact of carrying
reaction mass is a great disadvantage in all
terrestrial applications, since one is penalised,
not just by the weight penalty of the mass
supply, nor just by the weight penalty of
the structure of the storage system, but by
the necessity of carrying additional reaction
mass and fuel to transport the original store.  
 
 
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Tsiolkovski's Rocket Equation   All rockets are
jet propulsion devices that carry their own
supply of reaction mass and energy reserve.
This means that the mass of the vehicle changes
appreciably during its journey, and we have
to take account of this when analysing the
motion.   Thrust force The constant k
is the momentum obtained from unit mass of
propellant, approximating to the exhaust velocity
ve. It may be more familiar in the US form  
where go is the familiar acceleration due
to gravity (at the Earth's surface), and Isp
is the Specific Impulse of the propellant.
(Units are s.) Typical values are Liquid
hydrogen/Oxygen LH2/LO2 400s Hydrazine
monopropellant NH4 200s   The Specific Impulse
is the momentum obtained from unit weight of
propellant.  
 
 


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Tsiolkovski's Rocket Equation    The Specific
Impulse is the momentum obtained from unit weight
of propellant.   Hence, integrating
The logarithm is bad news!
 
 



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Tsiolkovski's Rocket Equation    The
logarithm is bad news!
 
If we want to give a payload of 1 tonne orbital
velocity, say ?v 9km/s  
 



  This is not allowing for any margins, and
requires unachievable structural efficiencies
the solution is to divide the vehicle into
discrete stages, effectively discarding structure
on the way up.
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Tsiolkovski's Rocket Equation    The
logarithm is bad news!
 
If we want to give a payload of 1 tonne orbital
velocity, say ?v 9km/s  
 



  This is not allowing for any margins, and
requires unachievable structural efficiencies
the solution is to divide the vehicle into
discrete stages, effectively discarding structure
on the way up.
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4. The Space Environment - Summary   1. Vacuum -
Ultra-low Pressure, Density   2. Microgravity
3. Temperature   4. Magnetic
Field 5. Ionising Radiation (Energetic
Particles and Photons)   6. Meteoric Particles
and Artificial Debris   7. Launch Environment
 
 



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Space Systems Engineering 4Solar System
Environments
  The Space Environment - Summary   1. Vacuum -
Ultra-low Pressure, Density Aerodynamic drag of
residual atmosphere leads to reduced lifetime in
near Earth orbits Materials problems Surface
problems Atomic oxygen Contamination by
evaporated material Lubrication Carbon-Fibre
epoxy surface eroded by exposure to atomic
oxygen in LEO
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Space Systems Engineering 4Solar System
Environments
2. Microgravity Lack of natural convection to
aid mass and heat transfer. Fluid Handling No
natural attitude reference From the
Earth to the Moon H.G.Wells
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Space Systems Engineering 4Solar System
Environments
3.Temperature Governed by radiation balance
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Space Systems Engineering 4Solar System
Environments
4.Magnetic Field Near Earth Field, offset
dipole Interaction with spacecraft magnetic
moment Magnetorqueing
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Space Systems Engineering 4Solar System
Environments
5.Ionising Radiation (Energetic Particles and
Photons)   Any form of radiation which can
remove electrons from matter - IONISING
radiation, has potentially deletorious effects
on delicate systems. It is capable of causing
damage to biological, electronic and mechanical
systems since an ION will have different chemical
properties to its parent atom.   Natural
radiation background Varies with
geography Radioactive decay (Alpha a, Beta b,
Gamma g, Spontaneous fission) Cosmic
Radiation Solar Flare Particles Trapped
Radiation  
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Space Systems Engineering 4Solar System
Environments
5.Ionising Radiation   Cosmic radiation
Energetic particles incident on the top of
the atmosphere at a rate of 1
particle/cm2s 87 Protons ( H nuclei
) 12 Alpha particles ( He nuclei ) 1 The
Rest   This radiation has its origin in energetic
events remote in space and time (fortunately).  
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Space Systems Engineering 4Solar System
Environments
5.Ionising Radiation (Energetic Particles and
Photons) Solar particles Solar particles (
largely protons ) with energies gt100 MeV are
produced by solar flares in periods of solar
activity. They can exceed C.R. by 106 for a
short time. Advance warning of solar flare
particles is provided by prompt electromagnetic
radiation (visible, U.V., X-ray ). The X-ray
and hard U.V. photons have ionising properties
and are themselves a hazard, although easy to
shield against.   A subsidiary effect of ionising
radiation is electrostatic charging of the
spacecraft. Materials are differentially
affected by ionisation, producing electrostatic
charging of different parts of the spacecraft
structure, leading to large potential
differences, and consequent damaging electrical
discharges. The cure is largely careful detail
design.  
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Space Systems Engineering 4Solar System
Environments
5.Ionising Radiation The Magnetosphere The
Earth's magnetic field -   a.Acts as a partial
barrier to high energy charged particles from
outside. b.Interacts with the solar field and
solar wind to form a complex
magnetohydrodynamic bow shock the
magnetopause. c.Stores solar particle
radiation trapped in magnetospheric events for
long periods (years). d.Channels and focusses
diffuse radiation into spatially and temporally
confined regions e.g. Auroral belts, South
Atlantic Anomaly. For our purposes the only
important magnetospheric structures are the VAN
ALLEN trapped radiation belts, which provide the
major radiation hazard to both manned and
unmanned space vehicles, sinced the trapped
charged particles, both electrons and protons may
attain space densities in excess of 106 times
those in low orbits. Any excursion into these
regions will contribute to the radiation dose,
which leads on to -  
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Space Systems Engineering 4Solar System
Environments
6.Meteoric Particles and Artificial
Debris Material origin Mass distribution Damage
distribution Damage mechanisms
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Space Systems Engineering 4Solar System
Environments
7.Launch Environment Acceleration and
Vibration Multiplication by structure Acoustic
Fatigue Kinetic Heating