Gridded ion engines have the highest efficiency and total impulse of any mature electric propulsion technology, and have been successfully implemented for primary propulsion in both geocentric and heliocentric environments with excellent ground-in-space correlation of performance. However, they have not been optimized to maximize thrust-to-power, an important parameter for Earth orbit transfer applications. This publication discusses technology development work intended to maximize this parameter. These activities include investigating the capabilities of a non-conventional design approach, the annular engine, which has the potential of exceeding the thrust-to-power of other EP technologies. This publication discusses the status of this work, including the fabrication and initial tests of a large-area annular engine. This work is being conducted in collaboration among NASA Glenn Research Center, The Aerospace Corporation, and the University of Michigan

Crofton, Mark

Foster, John E.

Patterson, Michael

Thomas, Robert

Young, Jason A.

NASA Technical Reports Server

AIAA-2015-3719
High Thrust-to-Power Annular Engine Technology
51st AIAA/ASME/SAE/ASEE Joint Propulsion Conference 
July 27th, 2015
Michael Patterson and Robert Thomas, NASA Glenn Research Center
Mark Crofton and Jason A. Young, Aerospace Corporation
John E. Foster, University of Michigan
https://ntrs.nasa.gov/search.jsp?R=20150023028 2019-08-31T04:57:03+00:00Z
AIAA-2015-3719
High Thrust-to-Power Annular Engine 
Technology
• Introduction
• High Thrust-to-Power Development Plan 
• GEN2 Annular Engine Development
− Design and Performance Expectations
− Discharge Chamber Tests
− Operation with Beam Extraction
• Forward Work
• Summary
AIAA-2015-3719
Introduction
Despite more than 50 years of research and development investment in 
electrostatic (gridded) ion engine technology, and more than 100 engines 
presently in operation in space on U.S. commercial and NASA spacecraft, 
these devices were never optimized for Earth orbit transfer operations 
where maximizing thrust-to-power (F/Pin) is the critical metric
This situation arose for several reasons
However, improvements in the ion engine F/Pin parameter promise higher 
performance than any other electric propulsion technology in the 5-kW 
class, over a broad range of specific impulse
High Thrust-to-Power Annular Engine 
Technology
AIAA-2015-3719
State of the art ion engines have been operated at high thrust density 
and high total thrust, at levels approaching other high power devices 
(e.g. HETs)
These demonstrations however did not involve purposeful modifications 
to the technology to optimize performance or the F/Pin parameter
This is because these engines were intentionally designed for operation 
at low thrust densities for the purpose of achieving extremely long life 
times, in support of space science missions
High Thrust-to-Power Annular Engine 
Technology
AIAA-2015-3719
High Thrust-to-Power Annular Engine 
Technology
 
Engine 
Max. F, mN 
[Thrust 
Density, N/m2] 
Max. Pin, kW 
[Power Density, 
kW/m2] 
Peak F/Pin 
(typical), 
mN/kW 
Corresponding 
Specific Impulse, 
sec 
Reference 
30 cm 
Divergent 
577 [8.92] 17.1 [264] - 4,380 AIAA-88-
2914 
30 cm 
Ring-Cusp 
301 [4.65] 8.72 [135] - 4,125 
- - 56.8 1,610 
AIAA-92-
3203 
- - 54.1 1,830 
- - 52.7 1,980 
- - 50.2 2,140 
40 cm 
NEXT 
466 [3.71] 13.6 [134] - 4,670 
AIAA-2006-
4664 
- - 47.5 2,660 
- - 47.7 2,745 
- - 48.3 2,755 
- - 48.0 2,835 
- - 48.6 2,730 
- - 48.7 2,785 
- - 47.8 2,470 
AIAA-2013-
3891 
- - 47.6 2,590 
- - 45.0 2,860 
- - 43.7 3,010 
- - 42.5 3,140 
AIAA-2015-3719
High Thrust-to-Power Annular Engine 
Technology
‘SOA HALL’ include 
data for both 
commercial HET 
(BPT-4000) over its 
1,220-2,150 sec 
Isp range, and the 
12.5 kW HET 
(HERMeS) over its 
intended 1,850-
3,000 sec Isp 
range
Curve fits of both the SOA HALL and SOA ION data indicate a cross-over in F/Pin at about 2,600 seconds Isp, 
with HETs having superior F/Pin below 2,600 seconds, and ion engines having superior F/Pin above 2,600 
seconds – including at the intended HERMeS operating condition of 3,000 seconds 
AIAA-2015-3719
High Thrust-to-Power Annular Engine 
Technology
Specific Impulse 
vs. Input Power: 
SOA ION (NEXT, 
demonstrated); 
SOA HALL (BPT-
4000, 
demonstrated, and 
HERMeS, intended 
throttle range for 
SEP TDM).
While NEXT has a larger demonstrated throttling range in both specific impulse and input power, it does so for 
the most part at a lower F/Pin parameter
AIAA-2015-3719
For xenon propellant the F/Pin ratio reduces to:
Where thrust-to-power ratio is maximized as the thrust-loss correction 
factors, α and β, approach unity, and the beam voltage, Vb, and the 
discharge losses, εi, are minimized  
A potential effective means of maximizing ion engine F/Pin is by 
development of the Annular Engine (AE):
• Annular optics allow one to fix the span and span-to-gap ratio to a relatively-
low value, enabling reduced-dome, or zero-dome (flat) electrodes, thereby 
increasing β
• Enables higher thrust density operation, yielding higher beam current 
capacity which will lower the discharges losses, εi, in addition to enabling 
higher power operation at lower Isp
High Thrust-to-Power Annular Engine 
Technology
  
V
V
101.650 = PF
ib
b
in
)(
/
2/1
3



 
AIAA-2015-3719
High Thrust-to-Power Annular Engine 
Technology
Curve fit of the 
SOA HALL data
Projections of 
(‘conventional’) ion 
engine 
performance are 
given for a range of 
discharges losses, 
εi, from 80-150 W/A 
The thrust-loss 
correction factors α
and β for these 
projections are 
equivalent to those 
documented for the 
NEXT ion engine
With minor magnetic circuit modifications and higher current density, lower discharge losses should be readily 
obtainable; at least to 150 W/A.  At 150 W/A, the cross-over point where F/Pin for ion engines exceeds that for 
HETs is reduced from 2,600 seconds (for SOA engines) to about 2,300 seconds Isp – with further 
improvements in εi lowering the cross-over further 
AIAA-2015-3719
High Thrust-to-Power Annular Engine 
Technology
Curve fit of the 
SOA HALL data
Projections of ion 
engine 
performance are 
given for a range of 
discharges losses, 
εi, from 80-150 W/A 
The thrust-loss 
correction factor β
assume 0.998, 
consistent with the 
beam divergence 
documented 
previously for flat 
annular ion optics
In this instance, minor improvements in magnetic circuit design in combination with flat ion optics should drive 
the cross-over point where the F/Pin parameter for ion engines exceeds that for HETs from 2,600 seconds 
down to about 1,800 seconds Isp
AIAA-2015-3719
High Thrust-to-Power Development Plan
High Thrust-to-Power Annular Engine 
Technology
Two ion engine design paths may provide near-term opportunities for 
demonstration of high thrust-to-power ratio, and high thrust density operation: 
1. Continued development of the AE concept [extensible pathway to higher 
power]; and 
2. Demonstration of a (conventional) cylindrical-geometry ion engine derived from 
the NEXT ion thruster, but incorporating advanced-design ion optics to 
increase β and a magnetic circuit intended to reduce εi and inhibit source-
limited operation [nearer-term technology product]
2.: AIAA-2015-3918, Tuesday July 28th
AIAA-2015-3719
GEN2 Annular Engine Development
Design and Performance Expectations
High Thrust-to-Power Annular Engine 
Technology
To meet the objective of demonstrating scalability of the AE concept to 
high power – including scalability of the annular discharge chamber and 
ion optics – requires the fabrication of a full-scale AE
‘GEN2’ AE was designed to be sufficiently large to operate in the ~10’s of 
kW power range, and to assess whether or not the azimuthal and radial 
discharge and beam uniformity demonstrated with the ~40 cm diameter 
GEN1 AE could be maintained using a single-cathode design
The larger size would also provide the opportunity to address 
manufacturing, assembly, and test issues associated with larger-area PG 
electrodes
AIAA-2015-3719
GEN2 also scaled to ensure that there would be sufficient anode surface 
area to enable operation closer to the Child-Langmuir limit than is the 
case for conventional ion thrusters, while maintaining relatively-low 
discharge losses
The interior diameter of the annulus was also sized to limit the optics span 
to a value comparable to that demonstrated with the GEN1 AE, 
approximately 14.4 cm diameter in the active area
The AE has beam dimensions of approximately 65 cm O.D. and 36 cm 
I.D., yielding a total (annular) beam area > 2X that of the NEXT ion 
thruster, with an anode area of approximately 4X that of the NEXT thruster
High Thrust-to-Power Annular Engine 
Technology
AIAA-2015-3719
Images of GEN2 (65 cm dia.) Annular Engine hardware, left-to-right: 
pyrolytic graphite electrodes, with carbon-carbon stiffener ring, ion 
optics assembly integrated with discharge chamber; engine with 
plasma screen, sans neutralizer cathode assembly.
High Thrust-to-Power Annular Engine 
Technology
The exterior 
dimensions for 
the laboratory-
model GEN2 
AE are 
approximately 
76 cm diameter 
x 39 cm length, 
with a mass of 
approximately 
34 kg
AIAA-2015-3719
The ion optics electrodes were machined at NASA from substrate 
nucleated PG panels
Each of the 2 electrodes contains 45,356 apertures, and were completed 
with 100% yield and zero defects
Upon inspection, the electrode geometry conformance-to-design 
exceeded that obtained with SOA conventional metal electrodes with 
apertures created using a photo-chemical etching process
The electrodes are flat, but incorporate radial ribs of thicker unperforated 
base material to increase the overall electrode stiffness, and are secured 
to mounting rings fabricated from carbon fiber-reinforced carbon (‘carbon-
carbon’).  The mounting scheme for the electrodes incorporates flexures 
which allow for radial-motion under thermal load. 
High Thrust-to-Power Annular Engine 
Technology
AIAA-2015-3719
High Thrust-to-Power Annular Engine 
Technology
AIAA-2015-3719
While the GEN2 AE is a laboratory-model experimental test article to 
evaluate scalability of the annular concept, and not a design solution to a 
specific propulsion application, it is of interest to note what the 
performance of such an AE size and configuration might yield
High Thrust-to-Power Annular Engine 
Technology
GEN2 AE: α equal to 
NEXT; β modestly 
improved from that 
documented for the 
NEXT ion engine, varying 
from about 0.993 at high 
Isp down to about 0.933 at 
low Isp; discharges 
losses, εi, were 
conservatively assumed 
to be 200 W/A; and a 
perveance per-unit-area 
and total maximum 
voltage equivalent to that 
of the NEXT were 
assumed 
AIAA-2015-3719
GEN2 Annular Engine Development
Discharge Chamber Tests
High Thrust-to-Power Annular Engine 
Technology
AIAA-2015-3721:  “Characterization of discharge uniformity and performance via 
stimulated beam extraction of a 65 cm annular ion engine”
Extremely-stable 
discharge operation over 
a broad range in 
discharge currents
Uniform plasmas in the 
radial and azimuthal 
directions 
Estimated discharge 
losses decrease with 
increasing beam current, 
asymptotically reaching 
about 250 W/A; while not 
as efficient as desired the 
results are satisfactory for 
the initial magnetic circuit 
iteration
AIAA-2015-3719
Images of GEN2 (65 cm dia.) Annular Engine hardware, from left-to-right: annular 
discharge chamber, sans magnetics, undergoing assembly; annular discharge chamber 
in vacuum undergoing simulated beam extraction tests, with high-transparency grid-plate 
and embedded probes on downstream end of discharge chamber.
High Thrust-to-Power Annular Engine 
Technology
AIAA-2015-3719
GEN2 Annular Engine Development
Operation with Beam Extraction
High Thrust-to-Power Annular Engine 
Technology
At this time only a very-modest level of testing of the full engine with beam 
extraction has been completed
However, the ion optics electrostatic design was validated to a degree, 
and successful ion beam extraction was demonstrated
Some anomalous behavior of the accelerator electrode impingement 
current (or, drain current) was noted, including both its magnitude (high, 
~1-2+% of the beam current) and its sensitivity to discharge parameters 
and applied accelerator electrode potential; 3 mechanisms were identified 
as potential causes for the high accelerator current 
Engine modifications are presently being implemented and GEN2 will 
undergo additional testing in August 2015
AIAA-2015-3719
Images of 65 cm dia. Annular Engine beam extraction tests, left-to-right: side-view 
showing beam propagation; head-on view of engine.  Neutralizer cathode assembly is at 
12-o’clock position (left image) and is cropped (right image) due to beam target surface in 
camera field-of-view.
High Thrust-to-Power Annular Engine 
Technology
AIAA-2015-3719
Forward Work
High Thrust-to-Power Annular Engine 
Technology
Objective: Demonstrate and evaluate electrostatic (gridded ion) engine 
technology for operation at high F/Pin (4-12+ kW) – combining:
1. High β demonstrated with AE flat optics
2. Low εi demonstrated with Ring-Cusp magnetic circuit
3. High beam current densities demonstrated with Divergent-Field magnetic circuit
Near-Term: Validate flat pyrolytic graphite-based AE as a high F/Pin device 
over wide envelope
1. Rework & retest GEN2 AE consistency with Goal 1
2. Rework GEN1 AE optics and magnetic circuit.  Fully-characterize performance of 
GEN1 AE.  Document maximum F/Pin characteristics.
3. Fully-characterize performance of GEN2 AE.  Document maximum F/Pin
characteristics
AIAA-2015-3719
High Thrust-to-Power Annular Engine 
Technology
Forward Work
Mid-Term:
Optimize ion engine technology for high F/Pin
1. Design, fabricate, and performance characterize GEN3 AE as a focused technology 
product with 4-12+ kW target operating range
2. Define, fabricate, test NEXT-Derivative 40 cm engine concept(s) (2).  Document 
maximum F/Pin characteristics
AIAA-2015-3719
High Thrust-to-Power Annular Engine 
Technology
Summary
• Despite more than 50 years of research and development in ion engine 
technology these devices were never optimized for Earth orbit transfer 
operations where maximizing thrust-to-power is the critical metric
• Improvements in the ion engine F/Pin parameter may yield higher performance 
than other EP technology options in the 4-12+ kW power range, over the 
broadest obtainable range in specific impulse
• A potential means of maximizing ion engine F/Pin is by development of the 
Annular Engine; a combination of reduced discharge and thrust losses has the 
potential to lower the cross-over point where ion engines exceed the F/Pin of 
SOA Hall thrusters from 2,600 seconds down to below 1,800 seconds Isp
AIAA-2015-3719
High Thrust-to-Power Annular Engine 
Technology
Summary
• A 65 cm ‘GEN2’ AE was built to demonstrate scalability of the concept
• Discharge chamber tests were successfully concluded, with excellent discharge 
stability over a broad discharge current range, yielding good plasma uniformity
• Manufacturing of large-area high-perveance-design carbon ion optics were 
subsequently completed, integrated with the AE discharge chamber, and beam 
extraction testing of the AE has been initiated 


High Thrust-to-Power Annular Engine Technology

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