The term Ionocraft was used in the early 60's by Major De Severski to describe his EHD devices which were basically a wire to plane grid type. We are hereby generalising the term ionocraft to all EHD thrusters using a grid as their collector. The lifter is another form of EHD propulsion. In this experiment we setup a standard lifter type, a flat grid type, and a semi cylindrical (or trough) grid type to see how these compare in thrust and thrust to power ratio. Each thruster is made of a grid size 20cm x 10cm x 0.6mm and weighs 4 grammes.
A totally insulated balance was used to compare their thrust whilst the two thrusters under test were electrically connected in parallel. The supply voltage was then varied and current readings were also noted. The black pieces shown in the picture are just plastic tubing used to hang the devices on the balance.
Here are the current readings for the 3 of them:
@ 10kV Trough = 80uA, Plane= 23uA, Standard= 9.9uA
@ 15kV Trough = 280uA, Plane = 95uA, Standard = 53uA
@ 18kV Trough = 530uA, Plane = 200uA, Standard = 99uA
Thrust comparisons as read from balance, same for all voltages:
Trough compared with standard gives 3 times the thrust
Plane compared to standard gives 2 times the thrust
Trough compared to plane gives 3/2 times the thrust
Trough compared to standard consumes (approx..) 5x the power
Plane compared to standard consumes 2x the power
Thrust to power comparison:
Trough type has got 60% x Thrust to power of standard
Plane type has got same thrust to power as standard
The trough grid thruster
The 60% T/P ratio experimentally derived for the trough version agrees closely with the 64% theoretical value. This can be derived by integrating the projection of radial forces generated in the trough device in the direction of motion. For a half cylinder trough shaped mesh collector we need to integrate the projection factor cos alpha for alpha varying from 0 to 180 degrees or -pi/2 to +pi/2, which is sin(pi/2)-sin(-pi/2)=2, and divide by (pi/2-(-pi/2))=pi to get the average value of the cosine. That is 2/pi equal to 64%.
The thruster shown above is slightly bigger than the first prototype and is made from a metallic grid 20cm x 32cm x 0.6mm. The radius of curvature is 5cm and weighs 11g, quite heavy due to the thickness of the grid. At its ends, the corona wire has been lifted up to 2.5cm above the axis of the cylinder with a radius of curvature of 2.5cm. For calculation purposes I took the effective length to be 28cm. This has completely eliminated sparks which were previously generated between the sharp edges of the cylinder and the wire. It gives the highest thrust per unit length achieved to date, equal to 0.39N/metre. Steady lift off occurs at 44kV,1.3mA which is also the maximum voltage before arcing occurs. Above this voltage, arcing occurs from corona wire to random points on the internal surface of the cylinder. This indicates that the design does not suffer of any abnormal field stresses due to sharp edges and that the round edges are working fine.
Corona Streamer breakdown & its effect on maximum pressure and current
This thruster's current per unit length is 1.3mA/28cm = 46uA/cm which is just about half the maximum current limit of 100uA/cm stated in Chen & Davidson report for a full wire to cylinder setup. This reconfirms Chen's findings, that in operation in normal air, at ambient temperature and pressure, and no external wind blowing on the device, the maximum current that one can achieve with a positive corona device before arcing is 100uA/cm for a wire to full cylinder construction. Note that in our case, the breakdown electric field strength E0was that of 44kV/5cm or 0.88 MV/m, and it can be estimated to reach 1 MV/m if the air is less humid. One might note the discrepancy of a factor of ten with Bondar's derivations. Note that in Bondar's derivation E0 was assumed to be the value for the electron avalanche breakdown process as described by J. Townsend in 1910, taken as 30kV/cm, which is the well accepted breakdown electric field strength for dry non ionised air in a uniform field. If one however analyses the EHD thruster case closely, it can be understood why this value for E0 cannot be assumed. Since the air within our through thruster contains a high percentage of ionised air molecules in a highly non uniform field, it is obvious that it is now more conductive than dry non ionised air, and the electron avalanche mechanism no longer dominates the breakdown, leading to a much lower average breakdown electric field strength E0. In fact, the breakdown in ionised air occurs by means of a totally different mechanism than the electron avalanche, namely by corona streamers. Streamers are ionisation waves which can propagate as narrow channels through regions where E < E0.
In the above plot, you can see the conventional Townsend avalanche breakdown voltage E0 at sea level is approximately 3E6 V/m, but once we have corona streamers in action, this drops to either 1E6 V/m for negative streamers, or even worse, about 0.5E6 V/m for positive streamers. The photos below show streamer formation from a positive tip to ground plane at atmospheric pressure within a 25mm gap at voltages of 12.5kV and 25kV respectively.
This self-propagation as we know, is due to highly nonuniform electric fields which result from significant gradient in current density, or space charge. This fact, scales down the theoretical electric pressure and current levels calculated by Bondar to the ones calculated below for E0= 10kV/cm or 1E6 V/m:
Maximum Electric pressure is equal to PE0= e0E02/2
PE0= 8.854E-12 * 1E62/2
PE0= 4.43Pa (@ E0= 30kV/cm was 39.8Pa)
Thus Theoretical IMAX per m = 4*pi*k*PE0
IMAX/l = 4*pi*2E-4*4.43
IMAX/l = 0.011A/m or 110uA/cm (@ E0= 30kV/cm was 1mA/cm)
so IMAX/l for half cylinder = 55uA/cm ... just 9uA/cm higher than our reading.
Average air pressure
If we take off the 2 cm of unproductive bended corona wire on each end, the active length is then around 32cm less 4cm equal to 28cm. The radial forces act on the internal trough surface area which is Pi*5cm*28cm, leading to a 0.044 m2 of surface area. As explained above, the actual projected 12 gF measurement leads to 12/64%= 18.75 gF total available radial thrust.
So, the average generated pressure is: 18.75 gF/0.044 m2 = 426 gF/m˛ or 4.26 N/m˛ or 4.26 Pa
If we assume negligible losses at the grid,
then the pressure generated within our device is 4.26 Pa ... just 0.17Pa lower than theoretical
Ionocrafts have various advantages over standard lifter type EHD thrusters. The wire to plane type offers twice the thrust of a normal lifter at the same thrust to power ratio. The wire to trough offers three times the thrust at approximately 60% lower thrust to power ratio, however, if we are able to re direct all the air flow exiting the trough into one direction, it could be possible to reach the ultimate thrust per unit volume at the same thrust to power ratio of the standard lifter. Another advantage over the solid foil lifter type is that of decreased air drag and lower weight of the grid.