Upload
chesmu
View
65
Download
1
Embed Size (px)
DESCRIPTION
Electric Propulsion Continued. The "jet" or exhaust power (P jet ) of any thruster is: P jet = 1/2 g c I sp F Thus, for a situation where we wish to fix the thrust at a constant value, as specific impulse increases, the jet power must also increase. - PowerPoint PPT Presentation
Citation preview
EXTROVERT Space Propulsion 13
Electric Propulsion Continued
EXTROVERT Space Propulsion 13The "jet" or exhaust power (Pjet) of any thruster is:
Pjet = 1/2 gc Isp F
Thus, for a situation where we wish to fix the thrust at a constant value, as specific impulse increases, the jet power must also increase.
Jet power is in turn a function of the total "bus" electric power (Pe) and the overall efficiency (h) of converting electric power into jet power:
Pjet = Pe h
..The mass of the electric power system (as well as power conditioning and thrusters) is proportional to the total "bus" electric power:
Mpower = a Pe
where a is the overall system specific mass (typically in kg/kW electric). Finally, ..
M0 / Mb = exp (DV / gc Isp)
The propellant mass (Mp) is simply the difference between M0 and Mb:
Mp = M0 - Mb
EXTROVERT Space Propulsion 13Designing Electric Propulsion
Path A: Power Source Based on Chosen Thruster and Mission
Specify Mission – Select Thruster – Select Power Source Design Thermal Mgmt System – Design Power Conditioning System –Assess Performance
Path B: Power Source Based on What is Available from Spacecraft
Specify Mission –Select Power Source - Select Thruster –
Design Power Conditioning - Design Thermal Mgmt System –System – Assess Performance
EXTROVERT Space Propulsion 13
Courtesy: Robert.H. Frisbee, JPLhttp://www.islandone.org/APC/Electric/impulse.gif
Optimum Specific Impulse
EXTROVERT Space Propulsion 13
Thrust or Jet Power:
Required source power
System Analysis .2
2p e
jm U
P
im : Initial mass
js
T
PP
System inert mass sinert s
Pm P
: Specific mass of propulsion system (Kg/W)
: Specific power of propulsion system (W/Kg)
mdotp, mass flow rateof propellant
EXTROVERT Space Propulsion 13
If thrust duration (assuming constant thrust) is ,.
2
2
pp
e pinert
T
pay f inert
mm
U mm
m m m
where fm is final mass achieving VD
21
2
V Vpay U U ee e
i T
m Ue em
D D
EXTROVERT Space Propulsion 13
Design goal: maximize payload mass fraction. Define:
0
*
0
*
0* *
2* * *1 12
T
e
V V
pay U Ue ee
i
U
VVUUUU
me e U
m
D D
DD
EXTROVERT Space Propulsion 13
Propulsion system mass per unit of jet power:
Jet-specific mass
Optimal exhaust speed:
1j
T T
Where k ~ 1
0ej
U k
EXTROVERT Space Propulsion 13
If is too high, or the allowable thrust time is too low,optimum speed may be less than that from chemicalrockets.
May still use electric propulsion for missions with electricpower supply;
Primary electric propulsion will not benefit from power systemsharing until it is a large scale mission with many MW of power
Possible uses -> station-keeping (no benefit to impulsive thrust) -> lifting large structures (low g; continuous thrust)
-> Electric primary propulsion needs > 1000s to compete
with modern chemical system (450s)
j
2.3 impulsiveV VD D
spI
spI
EXTROVERT Space Propulsion 13Specific Impulse Ranges
Electrothermal: 500 – 1000 s
Electromagnetic: 1000 – 7000s
Electrostatic: 2000 – 100,000s
EXTROVERT Space Propulsion 13
Electromagnetic force per unit volume on a gas carryingcurrent in a magnetic field
mF j B
magnetic induction field in gas (Tesla)B
Electric current density In gas (A/m2)j
mF
N/m3
Electromagnetic Propulsion
EXTROVERT Space Propulsion 13Electromagnetic Propulsion Systems
Unsteady vs. Steady
Self-field vs. Applied Field.
Self Field: Discharge currents whose own magnetic fields are high enough for efficient thruster performance without needing external applied magnetic fields. High power (MW)
Available in short pulses from capacitor bank: unsteady operation.
EXTROVERT Space Propulsion 13Z-Pinch and q-Pinch Engines
z -Pinch Engine: Current has component parallel to axis of symmetry.q-Pinch Engine: Current is in azimuthal direction
In both, current and self-fields combine to implode (pinch) plasmaGives 10 – 40 km/s velocity.
(See Humble, Fig. 9.11)
EXTROVERT Space Propulsion 13Pulsed Inductive Thruster
Coil and plasma currents are azimuthal; magnetic field is radial.
Plasma accelerates parallel to axis of symmetry
Ablation-supplied propellant for pulsed operation. See Fig. 9.12, Humble.
http://www.airpower.maxwell.af.mil/airchronicles/aureview/1973/Nov-Dec/Baty3.jpg
www.islandone.org/ APC/Electric/16.html
EXTROVERT Space Propulsion 13Pulsed-Plasma Microthruster
www.mae.cornell.edu/ campbell/mppt/mppt.htm
EXTROVERT Space Propulsion 13Magnetoplasmadynamic Thrusters
Discharge current interacts with its own magnetic field to accelerateflow axially and radially.
At low particle density, electromagnetic force density greatly exceedspressure gradients in the gas.
http://fluid.ippt.gov.pl/sbarral/pics/mpd_thruster.jpg
“J × B Lorentz body force compresses and accelerates a quasi-neutral plasma along the central axis. Because self-induced magnetic field is only significant at very high power, low power MPD thrusters often resort to an externally applied magnetic field in order to enhance the acceleration process (applied field MPD thrusters).”
EXTROVERT Space Propulsion 13Hall Effect
Applied magnetic fields increase electromagnetic forces in plasma. They also force currentto flow in spiral paths, increasing the total voltage.
Hall effect is evident in electromagnetic thrusters at low particle density.
Xenon with radial magnetic field and axial current flow from an upstream anode: StationaryPlasma thruster.
5-20 KW; Isp 1500 – 2000 s; high efficiency. Axial current across radial magnetic field generates azimuthal electron flow. Internal Hall electric field in axial direction transmits axial e-mag force on electron flow, to plasma ions.
Charge-neutral device.