1 Page 1 School of Aeronautics and Astronautics Rocket Propulsion Fundamentals & Nuclear Thermal Propulsion Professor Steve Heister School of Aeronautics and Astronautics Outline • Rocket Performance Fundamentals – How much thrust do we get? • Rocket Design Fundamentals – How much propellant is required? • Classification of Rocket Propulsion Systems • Historical Accomplishments - the NERVA/Rover Program • Nuclear Thermal Space Propulsion Design Studies • Conclusions
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School of Aeronautics and Astronautics
Rocket Propulsion Fundamentals
& Nuclear Thermal Propulsion
Professor Steve Heister
School of Aeronautics and Astronautics
Outline
• Rocket Performance Fundamentals– How much thrust do we get?
• Rocket Design Fundamentals– How much propellant is required?
• Classification of Rocket Propulsion Systems• Historical Accomplishments - the
NERVA/Rover Program• Nuclear Thermal Space Propulsion Design
Studies• Conclusions
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School of Aeronautics and Astronautics
Classification of Rocket Propulsion Systems
School of Aeronautics and Astronautics
Rocket Propulsion in Space Systems(an abundance of uses)
• Launch Vehicles– Solid Rocket Motors (SRMs) and Liquid
Rocket Engines (LREs)• Upper Stage or Orbital Transfer Vehicles
– Solid or liquid propulsion– Nuclear thermal rockets
• Satellite Propulsion– Liquid or electric propulsion options– Nuclear electric rockets
• Spin/Despin Systems, Deorbit Systems– Generally solid or liquid propulsion
There are two important parameters that pertain to any aerospace propulsion system What kind of “gas mileage” does system provide? How much does it weight?
The Specific Impulse, Isp, is the measure of the gas mileage of a rocket propulsion system
where I = total impulse = (N-S or lbf-s )Mp = total propellant mass/weight (lbf, kg)F = delivered thrust (lbf, N)
= propellant flowrate (lbf/s, N/S)
Note: If F, = const then I = F tb
mF
MIIspp �
========
����bt
0dtF
m�
m� bp t/Mm ====�
Rocket Propulsion Basics
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The propellant mass fraction, measures the weight /structural efficiency of the system
“Useful” propellant (that which is burned to provideacceleration in desired direction.)
Sum of all inert masses associated with the propulsionsystem. Includes engines, tanks, pumps, lines, reactors, pressurant bottles, gas generators, insulation, etc.
The best possible design in this sense would be a consumablerocket made entirely from propellant
In general, increases with system size due to structuralefficiency.
,λλλλ
ip
pMM
M++++
====λλλλ
====pM
====iM
(((( ))))1====λλλλ
λλλλ
School of Aeronautics and Astronautics
The best system would have both high Isp and high λλλλ.
Unfortunately, these items are somewhat mutually exclusive
Hot propellants give good Isp but require additional insulation (lowers λλλλ).
Large nozzle gives good Isp but reduce λλλλ . High pressures give better Isp but require thicker-walled
structures. Our consumable rocket would give high λλλλ but lousy Isp.
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The Rocket (or Tsiolkovsky) Equation
m(t)
v(t)
F(t)
D = Drag (atmospheric)
m g = Weight
Newton’s 2nd Law dtdvmamgmDF ========−−−−−−−−
But and sogIspmF �==== dt/dmm −−−−====�
dvgdtdtmD
mdmgIsp ====−−−−−−−−−−−−
Initial Conditions:
Final Conditions:
Now integrate differential eq. To give
omm,0v,0t ============
fb mm,vv,tt ====∆∆∆∆========
����
School of Aeronautics and Astronautics
The Rocket/Tsiolkovsky Equation
(((( ))))���
�������� ���� �����
loss"tg"orgravity
tg
lossdrag
dtbt
0 mD
systempropulsionthebyimpartedgainvelocity
m/mlnIspg
payloadthesensedgain
velocityactual
v bfo
−−−−
−−−−−−−−����−−−−====∆∆∆∆
This equation is the fundamental expression used inRocket Design.
It links mission requirements , propulsion performance, and vehicle masses .
)V(∆∆∆∆)Isp( (((( ))))fo m/m
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•••• Advantages •••• Disadvantages Simple, high λλλλ Cannot be throttled Devices scale up to or turned off
high thrust in All propellant lies instraightforward the combustionmanner chamber
Ready at a moment’s Isp typically lower thannotice for LRE
Package very well Difficult to test
Solid Rocket Motors
School of Aeronautics and Astronautics
Solid Rocket Motor
Graphite Epoxy Motor (GEM) used as strap-on booster forDelta II launch vehicle
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Advantages Disadvantages
— Throttable — More complex, lower
— Can test prior to launch than SRMs
— Higher Isp than solid — Leaking & boiloff issues
rockets — Not as responsive, must
— Can serve as dual-use fill tanks prior to launch
(apogee engine + spacecraft --- Complex pumps often