EXTROVERTSpace Propulsion 12 Electric Propulsion Continued.

EXTROVERT Space Propulsion 12

Electric Propulsion Continued

EXTROVERT Space Propulsion 12The "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 12Designing 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


Courtesy: Robert.H. Frisbee, JPL

http://www.islandone.org/APC/Electric/impulse.gif

Optimum Specific Impulse


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


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 V

2

12

V Vpay U U ee e

i T

m Ue e

m


Design goal: maximize payload mass fraction. Define:

0

*

0

*

0

* *

2* * *11

2

T

e

V V

pay U Ue ee

i

U

VV

U

UU

U

me e U

m


Propulsion system mass per unit of jet power:

Jet-specific mass

Optimal exhaust speed:

1j

T T

Where k ~ 1

0ej

U k


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 V

spI

spI


Specific Impulse Ranges

Electrothermal: 500 – 1000 s

Electromagnetic: 1000 – 7000s

Electrostatic: 2000 – 100,000s


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


Electromagnetic 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 12Z-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 12Pulsed 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

http://www.islandone.org/APC/Electric/16.html

EXTROVERT Space Propulsion 12Pulsed-Plasma Microthruster

www.mae.cornell.edu/ campbell/mppt/mppt.htm

http://www.mae.cornell.edu/campbell/mppt/mppt.htm

EXTROVERT Space Propulsion 12Magnetoplasmadynamic 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).”


Hall 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.

EXTROVERTSpace Propulsion 12 Electric Propulsion Continued.

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