lecture 22 rocket propulsion
TRANSCRIPT
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Rocket Propulsion
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Gravitational Assist Notable uses: 1. Mariner 10 2. Voyager I & II 3. Galileo 4. Ulysses 5. Cassini 6. MESSANGER
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Other Orbital Maneuvers
• Orbital Inclination Change
• Phasing
• Rendezvous
• Docking
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Orbital Station-Keeping
Orbital Station-Keeping: firing thruster to keep a spacecraft in a particular orbit
Any real orbit will change with time due to perturbations from other bodies in the solar system.
Typically a small set of thrusters are used. These are called the attitude control system (ACS).
Station-keeping is critical for satellites that must be oriented in a certain direction to communicate with Earth (communication satellites)
Example: satellite is orbit around the Earth is perturbed by the Sun, Moon, Jupiter…
Now this process is automated by an onboard computer that collects telemetry and makes corrections.
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Orbits to Propulsion
Changing orbits requires changing a spacecraft’s energy (or velocity).
For this we use ROCKETS.
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How do rockets work?
6 T
he rock
et pus..
.
The ro
cket p
us...
The e
xhaust
-ro...
56%
29%
15%
1. The rocket pushes off the ground.
2. The rocket pushes off the air.
3. The exhaust-rocket system conserves momentum.
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Conservation of Momentum
Momentum: Mass times velocity (p = mv)
Momentum is conserved: initial momentum equals final momentum
Our rocket has three parts 1. Payload (science, human, life support, communications…) 2. Structure (rocket, pumps, tanks, structural support…) 3. Fuel (expelled for propulsion)
We want a way to calculate the change in speed of the spacecraft due to firing our rocket.
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Conservation of Momentum initial momentum = final momentum
fuelfuelcraftcraft vmvm 0(rocket is at rest)
fuelfuelcraftcraft vmvm
craft
fuelfuel
craftm
vmv
So this shows the basic tradeoff
for rockets.
To go faster you must:
1. Maximize fuel speed
2. Maximize fuel mass
3. Minimize craft mass
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Conservation of Momentum
(Rocket Style):
Actually it’s worse than it appears.
Rockets don’t work by throwing all of their fuel off at once. To get into
orbit would kill an astronaut!
So, each part of the fuel ejected is pushing against the payload and
the remaining fuel!!!!
craft
fuelfuel
craftm
vmv
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Rocket Equation To correctly compute the change in speed, we need to use calculus.
Here’s the answer written in two ways:
payload
fuel
em
mcv 1ln
mfuel = fuel mass mpayload = payload mass ce = exhaust speed v = change in spacecraft’s speed
1
ec
v
payload
fuele
m
m
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Rocket Equation mfuel = fuel mass mpayload = payload mass ce = exhaust speed v = change in spacecraft’s speed
0
20
40
60
80
100
120
140
160
0 1 2 3 4 5 6
payload
fuel
m
m
ec
v
1
ec
v
payload
fuele
m
m
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Rocket Equation mfuel = fuel mass mpayload = payload mass ce = exhaust speed v = change in spacecraft’s speed
0.1
1
10
100
1000
0 1 2 3 4 5 6
payload
fuel
m
m
ec
v
1
ec
v
payload
fuele
m
m
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Rocket Equation: Example How much fuel do we require to send a 1000 kg
payload to the Moon (v = 16 km/s) using a chemical rocket (ce = 4 km/s)?
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Rocket Equation: Example How much fuel do we require to send a 1000 kg
payload to the Moon (v = 16 km/s) using a chemical rocket (ce = 4 km/s)?
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Rocket Equation: Example How much fuel do we require to send a 1000 kg
payload to the Moon (v = 16 km/s) using a chemical rocket (ce = 4 km/s)?
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Staging We can use staging as a trick to help improve the situation.
Once the fuel in the first stage is used, we can drop that stage. Now we no longer need to carry that structure into space.
EX: If 10% of your initial mass is structures and v/ce = 2: 1 stage: payload fraction = 3.8% 2 stages: payload fraction = 7.5% 16
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Types of Fuel:
There are three types of physical process that we
use for fuel in space propulsion.
1. Chemical Reactions: This amounts to triggering an energy
releasing chemical reaction in a controlled (or uncontrolled) way.
(By far the most common method).
2. Plasma Reactions: These thrusters use electric fields to
accelerate ions. (Not used for launches, but more common now
in trajectory corrections).
3. Nuclear Reactions: Nuclear propulsion relies on fission power to
generate massive amounts of energy that propel exhaust at
huge speeds. (Hard to control, but can also be an impulse
drive).
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Chemical Rockets
Three types of chemical rockets: 1. Solid propellant 2. Liquid propellant
a. Monopropellant b. Bipropellant
3. Hybrid rockets
Chemical rockets work by heating a gas through a chemical reaction. This gas is then expanded through a nozzle.
Chemical rockets can be classified based on the form of the fuel they use.
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Solid Propellant Motors
Uses: • Booster Rockets
Space Shuttle Delta IV Atlas V Ariane 4 & 5 Soyuz
• Model Rockets
• Fuel and oxidizer are mixed together to form a grain
• Ignited used to start mixture burning • Exhaust speed = 2.8 km/s • Propellant example:
• Ammonium Perchlorate (oxidizer) • HTPB or PBAN (fuels)
• Exhaust: • Hydrochloric acid • Aluminum oxide
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Solid Propellant Motors
Advantages: • Very simple design • Lots of flight heritage • Reliable • Compact • Long storage times • High payload mass fraction • Low costs
Disadvantages: • Impossible to turn off • Low exhaust speed
compared to liquid fuels • Air pockets can explode,
which ruptures the casing • Seals can rupture causing
failure
Boeing Delta IV
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Liquid Propellant: Monopropellant Engines
How it works: 1. Monopropellant is passed through a catalyst. 2. Catalyst causes a reaction, which generates
heat. 3. The heated products of this reaction are
expelled through a nozzle.
Typical fuel is hydrazine (N2H4) Exhaust speed = 2.3 km/s
Usually used for attitude control.
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Liquid Propellant: Monopropellant Engines
Advantages: 1. Simple design 2. Robust design 3. Reliable 4. Not a lot of plumbing 5. Flight heritage 6. Can turn them off
Disadvantages: 1. Most fuels are toxic 2. Catalyst lifetime issues 3. Low thrust 4. Low exhaust speeds
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Liquid Propellant: Bi-propellant Engines
How they work: 1. Fuel and oxidizer are pumped into
the combustion chamber 1. Often use turbopumps 2. Power tapped off of main
combustion 2. Injectors mix propellant to provide
stable and thorough combustion 3. Heat is generated from combustion. 4. Heated products are expelled out the
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Liquid Propellant: Bi-propellant Engines
Fuel Oxidizer Example
Liquid H2 (LH2) Liquid O2 (LOX) Space Shuttle Main Engine Saturn V upper stage
Delta IV 1st stage Centaur
Ariane 2nd stage
Kerosene LOX Atlas rockets Delta
Titan 1st stage Soyuz rocket
Aerozine 50 Dinitrogen tetroxide Apollo Service Module Lunar Module Titan rockets
Voyager 1 & 2
monomethylhydrazine dinitrogen tetroxide Space Shuttle Orbital Maneuvering System (OMS)
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Liquid Propellant: Bi-propellant Engines
Advantages: 1. Typically more efficient than solid
or hybrid rockets 2. High exhaust velocity (3.6-4.4 km/s) 3. Throttled 4. Can turn them off 5. Lots of flight heritage
Disadvantages: 1. More complex than hybrids or solid
rockets 2. Cryogenic systems often needed
(icing issues) 3. Difficulty storing 4. System complexity
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Space Shuttle Main Engine
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