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Thrust measurement of dimethyl ether arcjet thruster
Acta Astronautica 68 (2011) 1228–1233
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Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
Thrust measurement of dimethyl ether arcjet thruster Akira Kakami n, Shinji Beppu, Muneyuki Maiguma, Takeshi Tachibana Department of Mechanical Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu 804-8550, Japan
a r t i c l e in fo
abstract
Article history: Received 1 February 2010 Received in revised form 4 October 2010 Accepted 25 October 2010 Available online 18 November 2010
The present paper describes thrust measurement results for an arcjet thruster using Dimethyl ether (DME) as the propellant. DME is an ether compound and can be stored as a liquid due to its relatively low freezing point and preferable vapor pressure. The thruster successfully produced high-voltage mode at DME mass flow rates above 30 mg/s, whereas it yielded low-voltage mode below 30 mg/s. Thrust measurements yielded a thrust of 0.15 N and a specific impulse of 270 s at a mass flow rate of 60 mg/s with a discharge power of 1300 W. The DME arcjet thruster was comparable to a conventional one for thrust and discharge power. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Arcjet Dimethyl ether Electric propulsion Thrust measurement
1. Introduction Arcjet thrusters have been used for space propulsion because they produce a high thrust power ratio among electric propulsion devices and a high specific impulse compared to chemical thrusters [1,2]. Various gases such as hydrogen and hydrazine have been used as propellants. Although relatively low molecular-weight gaseous propellants such as hydrogen yield a relatively high specific impulse, their storage requires a cryogenic cooling device, which complicates the propellant management system. On the other hand, hydrazine propellant can be stored as a liquid, and shared with chemical thrusters. Nevertheless, because hydrazine has a relatively high freezing point of 1 1C, temperature management is still necessary for its storage. In addition, hydrazine is very reactive and toxic. Accordingly, tank and valve materials must be compatible with the propellant. Ground tests of hydrazine arcjet thrusters require an exhaust treatment system, resulting in resultantly raising production costs [2]. Besides, an iridium-based catalyst is necessary to gasify hydrazine before supplying it into the arcjet thruster.
n
Corresponding author. Tel./fax.: + 81 93 884 3163. E-mail address: [email protected] (A. Kakami).
0094-5765/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2010.10.019
We have proposed using Dimethyl ether (DME) as a propellant for arcjet thrusters [3]. DME, which is an ether compound with the molecular structural formula CH3–O– CH3, and a freezing point of 143 1C, a boiling point of 54 1C, and a vapor pressure of 6 atm at 25 1C, can be stored as a liquid under a relatively low storage pressure without sophisticated temperature management. The propellant is easily gasified without a catalyst simply by adjusting pressure or temperature. Furthermore, due to its non-toxicity and weak reactivity, DME is safe enough to be used as a propellant of spray cans for household use such as gas duster. The ether bond, which is the connection of carbon atoms through an oxygen atom, allows DME to produce little soot in combustion, and this feature allows DME to be a potential alternative fuel for engines and burners. Such characteristics of DME enhance its potential for use in both industries and households, and consequently its availability. Our previous study showed that a designed DME arcjet thruster successfully produced arc plasma and was comparable to the conventional thruster in discharge voltage, power and plenum chamber pressure [4,5]. The thruster yielded both high- and low-voltage modes; in the lowvoltage mode, electric discharge was very stable, and the arc column remained inside the thruster, never appeared outside the nozzle. In the high-voltage mode, electric
A. Kakami et al. / Acta Astronautica 68 (2011) 1228–1233
discharge was unstable and the voltage periodically dropped and rose. In this paper, for the purpose of evaluating the performance of DME arcjet thrusters, thrust was measured with a torsional thrust stand at mass flow rates ranging from 15 to 60 mg/s with a regulated discharge current of 12 A. Nitrogen was also used as a propellant to compare thrust with DME. 2. Experimental apparatus
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Table 1 Thruster geometry and experimental conditions. Cathode diameter, mm Constrictor diameter, mm Constrictor length, mm Divergent angle of nozzle, deg. Nozzle area ratio Electrode gap, mm Convergent half angle, deg. Discharge current, A Plenum chamber diameter, mm Half angle of cathode tip, deg. Minimum flow passage, mm
2.0 1.0 1.0 15 100 1.0 45 12 6.0 90 0.7
2.1. DME arcjet thruster Figs. 1 and 2 are schematics of a designed arcjet thruster and the region near the electrode gap, respectively, and Table 1 represents geometry and experimental conditions. Discharge current was 12 A and mass flow rate ranged from 15 to 60 mg/s. The flat-tip cathode was a thoriated tungsten rod 2 mm in diameter. The divergent section of the nozzle including the constrictor was made of copper tungsten. It had a divergence half angle of 151 and an exit diameter of 10 mm.
Constrictor (Copper tungsten) Propellant inlet
Divergent anode (Copper)
Cathode (Thoriated tungsten)
Insulator (Ceramics)
Fig. 1. Schematic diagram of the designed arcjet thruster.
Electrode gap, de Cathode Constrictor
Minimum flow passage width, wmin
nozzle Fig. 2. Electrode shape and definition of electrode gap, de and minimum flow passage width, wmin.
The electrode gap de and corresponding minimum flow passage width wmin, illustrated in Fig. 2, are listed in Table 1. The propellant was introduced from an inlet, as shown in Fig. 1, and was supplied to the plenum chamber through a gap between the cylindrical ceramic insulator and the discharge current feed line holding the flat-tip cathode.
2.2. Overall experiment system The DME arcjet thruster was tested in a cubic vacuum chamber of 300 mm on each side. The vacuum chamber was evacuated by a rotary pump at an exhaust velocity of 240 L/min. To protect the cubic vacuum chamber from the intensely hot plume, a 300-mm-long extension cylinder was attached horizontally to the vacuum chamber. The arc plume was injected to the cylindrical extension and exhausted through the outlet port on the flange of the horizontal extension cylinder. Back pressures were less than 0.1–3 kPa for the thruster firing and standby, respectively. DME was stored in a compressed cylindrical vessel, and supply pressure and mass flow rate were regulated with a pressure regulator and a mass flow controller, respectively, before being supplied to the thruster. A constant-current power supply with a rated voltage of 120 V and a rated current of 35 A supplied the regulated current, with a train of high-voltage pulses only at ignition. Discharge voltage was measured using a resistor-type voltage divider, and discharge current was measured with a Hall-effect current sensor. Thrust was measured using a thrust stand, which was mechanically equivalent to a vertical pendulum with a torsional spring. The thrust stand had the pendulum arm on which the designed thruster was fixed, a pistoncylinder-type oil damper and two sets of flexi-hinges producing reflecting torque without friction. The pendulum-arm was extended by setting a vertical extension cylinder on top of the cubic vacuum chamber to magnify pendulum-arm displacement. An oil damper with lowvapor-pressure silicon oil was employed to suppress oscillations induced by thrust and its time variation. Due to the appropriate damper design, such as oil viscosity and geometry of piston and cylinder, the pendulum arm moved back and forward in the manner of critical oscillation. Hence, the time variation in the thrust was traced using pendulum-arm displacement in the frequency range up to the natural frequency of the pendulum, which was
approximately 1 Hz. This thrust stand was calibrated every time the thruster was set to the thrust stand because a propellant feed tube and electric feed-through cables provided the pendulum with extra reflecting forces, which affected the correlation between pendulum displacement and thrust. The displacement of the pendulum arm was measured using a LED displacement sensor with 10 mm resolution. Calibration was done by comparing pendulum-arm displacement with the mass of aluminum disks connected to the thruster by a string. Nine kinds of reference force, produced by the combination of the aluminum weights, ranged from 0.036 to 0.16 N. Pendulum-arm displacement was measured prior to exerting reference force to the thrust stand until removing the weights. The calibration routine was repeated three times for each reference force. The thrust stand showed neither null-position drift after each calibration routine nor variation in pendulum displacement during the application of reference force. The calibration exhibited the linearity between reference force and displacement sensor output with a typical coefficient of determination R2 of 0.9916. This indicates that the error originated from the calibration is approximately 1.8%. An IBM-compatible PC with RTAI/Linux operating system having an analog–digital converter interface acquired all of the experimental data: discharge voltage, current, propellant mass flow rate, plenum chamber pressure, and pendulum-arm displacement of thrust stand [6]. The sampling rate was 100 Hz, and the cut-off frequency of the low-pass filters used in the electric circuits was higher than 1000 Hz. 3. Results and discussion
Plenum chamber pressure Thrust Discharge voltage Discharge current
0.15 0.1
100
0.05 Nitrogen supply started
0
Thrust, N plenum chamber pressure, MPa
A. Kakami et al. / Acta Astronautica 68 (2011) 1228–1233
Discharge voltage, V and current, A
1230
Ignition
50
0 -10
0
10
20
Time, s Fig. 3. Time variations of discharge voltage, current, plenum pressure, and thrust using nitrogen propellant at 60 mg/s.
was comparable to conventional arcjet thrusters in thrust, plenum chamber pressure, and discharge power. From these results, the designed thruster and the measurement device were applicable for the performance evaluation of the DME arcjet thruster.
3.2. Time history of discharge voltage, plenum pressure, and thrust using DME propellant
3.1. Preliminary thruster test using nitrogen propellant The designed thruster was preliminarily tested using nitrogen propellant with a regulated discharge current of 12 A. The arc plasma was successfully maintained during the discharge. Fig. 3 shows the time history of discharge voltage, plenum chamber pressure, and thrust. For t=0 1 s, the discharge voltage was not measured to protect the voltage divider and the measurement device from high-voltage ignition pulses with a 10-kV-class peak voltage. At t= 6.3 s, nitrogen supply to the thruster was started, and the thruster yielded peaky thrust due to the initial overshoot of the nitrogen mass flow rate. Afterwards, the thruster generated a tiny, but stable, cold gas thrust. After arcdischarge ignition at t=0 s, thrust, plenum chamber pressure, and discharge voltage started to rise and remained almost constant until t=5 s. During the five seconds, a relatively dark plume, which was very long and thin, was found outside the nozzle. At t=5 s, thrust, plenum chamber pressure and discharge voltage increased instantly, and a brighter plume was found outside the nozzle. Thrust, plenum chamber pressure, and discharge voltage remained constant, and plume shape was maintained during arc discharge. This showed that the thruster initially exhibited the low-voltage mode and switched voltage mode to high at t=5 s. At other mass flow rates ranging from 15 to 60 mg/s, the thruster maintained stable discharge voltage and thrust. The thruster
The DME arcjet thruster successfully produced a thrust of 0.15 N at a mass flow rate of 60 mg/s with the regulated voltage of 12 A. As shown in Fig. 4, the DME propellant was fed to the thruster at t= 7.7 s, and the electric discharge was initiated at t =0 s. Shortly afterwards, thrust was produced by the arc plasma, and it had relatively small periodic variations, while the discharge current was begin supplied. The plume was always located outside the nozzle, and this suggests the thruster yielded the high-voltage mode. At t = 81 s, the discharge current was interrupted manually, and thrust and plenum chamber pressure decreased immediately. From the results, the DME arcjet thruster successfully produced thrust for a period of 81 s. On the other hand, arc discharge using DME propellant was unstable compared to that using nitrogen. The discharge voltage had ripple-like fluctuations. The plume expanded and shrank repeatedly, and its direction also oscillated during arc discharge. Plenum chamber pressure also remained almost constant for the first 70 s of arc discharge, but rose afterwards. This rapid expansion of the plenum chamber pressure was attributed to carbon contamination. After testing the designed thruster, black materials, which were solid but fragile enough to be broken up with fingers, accumulated especially in the vicinity of the electrodes, and consequently choked the constrictor. The black materials also adhered to the inner surface of the
0.4
0
150
Discharge power(DME) Discharge power(N2) Discharge voltage(DME) Discharge voltage(N2)
100
50
1500
1000
500 Discharge voltage, V
Discharge voltage, V and current, A
0.2
1231
100
Discharge power, W
Plenum chamber pressure Thrust Discharge voltage Discharge current
Thrust, N plenum chamber pressure, MPa
A. Kakami et al. / Acta Astronautica 68 (2011) 1228–1233
0 50
0 0
10
0 0
20
40
60
80
30
40
50
60
70
Mass flow rate, mg/s
100
Time, s
20
Fig. 5. Variations of discharge voltage and current with mass flow rate at discharge current was of 12 A.
0.04
tube insulator surrounding the cathode. Our previous study showed that the black materials were composed of 97% carbon and 3% tungsten [3]; hence, the black materials found in this experiment are considered to be a composite of carbon and tungsten.
Fig. 5 illustrates the dependence of discharge voltage and power on mass flow rate, with values time-averaged through the arc discharge because the experimental values varied with time in the case of DME propellant. The timeaveraged discharge voltage increased steadily with mass flow rate for DME and nitrogen propellant. The DME arcjet thruster produced discharge voltages ranging from 75 to 90 V, which is the same level as the conventional arcjet thruster. The DME propellant always produced higher discharge voltages than nitrogen, which increases discharge power. Discharge power and specific power varied from 430 to 1300 W, and from 19 to 31 MW/kg, respectively, and were also the same as those of conventional thrusters. Discharge voltage rises discontinuously at a mass flow rate of 30 mg/s. Below 30 mg/s, arc discharge is relatively stable but discharge voltage is lower than at higher mass flow rates, as shown in Figs. 5 and 6. During arc discharge, no plume was found outside the thruster but bright sparks, which are considered to be carbon particles, were emitted from the nozzle. At the other mass flow rates below 30 mg/ s, the time variations of discharge voltage and spark emissions were almost the same as those at 15 mg/s. This shows that the thruster operated in low-voltage mode. In contrast, at mass flow rates above 30 mg/s, the thruster yielded an enhanced discharge voltage, as shown in Fig. 5, and plumes were located outside the nozzle. Hence, at
0.02 Discharge voltage, V and current, A
3.3. Discharge voltage and power
Plenum chamber pressure Thrust Discharge voltage Discharge current
100
0
Thrust, N plenum chamber pressure, MPa
Fig. 4. Time variations of discharge voltage, current, plenum pressure, and thrust using DME propellant at 60 mg/s.
50
0 -15
-10
-5
0 Time, s
5
10
15
Fig. 6. Time variations of discharge voltage, current, plenum chamber pressure, and thrust using DME propellant at 15 mg/s.
above 30 mg/s, the DME arcjet thruster yielded highvoltage mode. The thruster with 30 mg/s switched voltage mode from high to low periodically, as shown in Fig. 7. Discharge voltage showed intense low-frequency variations as well as the higher frequency fluctuations in high-voltage mode. The plume was located outside the nozzle, but suddenly disappeared and afterwards expanded intensely outside the nozzle. The plume shape was varied repetitively during arc discharge. Hence, the thruster with 30 mg/s alternately yielded high- and low- voltage modes. These results indicate that the DME arcjet thruster produced both
0.1
Discharge voltage V, current A
0.05
100
0
50
Isp (DME) Isp (N2) Thrust (DME) Thrust (N2) Plenum chamber pressure (DME) Plenum chamber pressure (N2)
300
0.2 200
Thrust, N plenum chamber pressure, MPa
0.15 Plenum chamber pressure Thrust Discharge voltage Discharge current
100
Specific impulse, s
A. Kakami et al. / Acta Astronautica 68 (2011) 1228–1233
Thrust, N plenum chamber pressure MPa
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0 0.1
0 0
0 0
20
10
40
20
30
40
50
60
70
Mass flow rate, mg/s
Time,s
Isp(DME) Isp(N2) Thrust (DME) Thrust (N2)
300 200 100
0.2
Specific impulse, s
high- and low- voltage modes as well as the conventional arcjet thruster. The periodic alternation in the discharge mode would be caused by relatively unstable discharge in the high voltage mode for DME propellant. When the discharge voltage accidentally dropped because of the instability in the high voltage mode, the thruster began to show the low voltage mode. Nevertheless, the thruster yielded the high voltage mode again because 30 mg/s mass flow rate was so high that the thruster could not sustain the low voltage mode. The process would be repeated during the arc discharge.
Fig. 8. Variations of thrust, specific impulse and plenum chamber pressure with mass flow rate.
0 Thrust, N
Fig. 7. Time variations of discharge voltage, power, plenum chamber pressure, and thrust using DME propellant at 30 mg/s.
0.1
3.4. Thrust and specific impulse Fig. 8 shows the dependence of thrust and specific impulse on mass flow rate for DME and nitrogen. Both thrust and specific impulse almost consistently with mass flow rate, and were elevated discontinuously at 30 mg/s, which was the threshold of the discharge mode. In highvoltage mode, thrust and specific impulse ranged from 0.078 to 0.15 N and 230 to 270 s, respectively, while they varied from 0.014 to 0.021 N and 89 to 99 s in low-voltage mode. Hence, like conventional arcjet thrusters, the thruster in high-voltage mode showed higher thrust and specific impulse than in low-voltage mode. The DME arcjet thruster had a maximum thrust of 0.15 N, corresponding to a specific impulse of 270 s and thrust power ratio of 120 mN/kW at 60 mg/s; discharge power and specific power were 1300 W and 21 MJ/kg, respectively. These values are almost the same as those of the conventional kW-class arcjet thruster. Accordingly, the DME arcjet thruster was comparable to the conventional thruster in performance.
0 0
10 20 30 Specific power, MJ/kg
40
Fig. 9. Dependence of thrust and specific impulse on specific power.
Thrust with DME propellant was higher than with nitrogen at a given mass flow rate in high-voltage mode. This is partially because DME propellant yielded higher discharge voltage and power than nitrogen. DME provided enhanced thrust at a given specific power in high-voltage mode. Fig. 9 shows dependence of thrust and specific impulse on specific power. Both thrust and specific impulse with DME propellant are higher than with nitrogen in highvoltage mode, whereas they were almost the same in the low voltage mode. From the results, DME provided higher performance than nitrogen. This is considered to be because DME, which has a molecular weight of 46 and
A. Kakami et al. / Acta Astronautica 68 (2011) 1228–1233
consists of 9 atoms, has a lower average molecular weight of 5.1 than nitrogen at 14. 4. Conclusion A DME arcjet thruster was tested in a vacuum chamber. Its thrust was measured with a thrust stand to evaluate its performance: thrust power ratio, specific impulse, etc. (1) Discharge voltage increased with mass flow rate for both DME and nitrogen. Discharge voltage was always higher with DME than with nitrogen, as was discharge power at a given mass flow rate. (2) DME arc discharge was relatively unstable compared to nitrogen. The discharge voltage had ripple-like fluctuations. The plume was enlarged and shrank periodically during arc discharge, and the plume direction and size varied with time. (3) The DME arcjet thruster yielded both high- and lowvoltage modes. The thruster showed high- voltage mode at mass flow rates above 30 mg/s and low-voltage mode below 30 mg/s. Like the conventional arcjet thrusters, the DME arcjet thrusters yielded higher thrust and discharge voltage in high-voltage mode.
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(4) Thrust was measured using a thrust stand, which is mechanically equivalent to a vertical pendulum with a torsional spring. Thrust using DME propellant increased steadily with mass flow rate, and was higher than that using nitrogen. The thruster yielded maximum thrust of 0.15 N, specific impulse of 270 s, and discharge power of 1300 W at a DME flow rate of 60 mg/s. (5) DME provided enhanced thrust and specific impulse compared to nitrogen at any specific power in the highvoltage mode operation. References [1] R.G. Jahn, Physics of Electric Propulsion, McGraw-Hill Book Company, 1968. [2] G.P. Sutton, O. Biblarz, Rocket Propulsion Elements, 7th ed., John Wiley & Sons, Inc., 2001. [3] A. Kakami, I. Ebara, J. Yokote, T. Tachibana, Application of dimethyl ether to arcjet thruster as propellant, Vacuum 8 (2008) 77–81. [4] H. Kuninaka, M. Ishii, K. Kuriki, Experimental study on a low-power direct current arcjet, Journal of Propulsion and Power 2 (1986) 408–413. [5] T. Yamada, Y. Shimizu, K. Toki, K. Kuriki, Thrust performance of a regeneratively cooled low-power arcjet thruster, Journal of Propulsion and Power 8 (1992) 650–654. [6] RTAI website, /http://www.rtai.orgS.
Contents lists available at ScienceDirect
Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
Thrust measurement of dimethyl ether arcjet thruster Akira Kakami n, Shinji Beppu, Muneyuki Maiguma, Takeshi Tachibana Department of Mechanical Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu 804-8550, Japan
a r t i c l e in fo
abstract
Article history: Received 1 February 2010 Received in revised form 4 October 2010 Accepted 25 October 2010 Available online 18 November 2010
The present paper describes thrust measurement results for an arcjet thruster using Dimethyl ether (DME) as the propellant. DME is an ether compound and can be stored as a liquid due to its relatively low freezing point and preferable vapor pressure. The thruster successfully produced high-voltage mode at DME mass flow rates above 30 mg/s, whereas it yielded low-voltage mode below 30 mg/s. Thrust measurements yielded a thrust of 0.15 N and a specific impulse of 270 s at a mass flow rate of 60 mg/s with a discharge power of 1300 W. The DME arcjet thruster was comparable to a conventional one for thrust and discharge power. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Arcjet Dimethyl ether Electric propulsion Thrust measurement
1. Introduction Arcjet thrusters have been used for space propulsion because they produce a high thrust power ratio among electric propulsion devices and a high specific impulse compared to chemical thrusters [1,2]. Various gases such as hydrogen and hydrazine have been used as propellants. Although relatively low molecular-weight gaseous propellants such as hydrogen yield a relatively high specific impulse, their storage requires a cryogenic cooling device, which complicates the propellant management system. On the other hand, hydrazine propellant can be stored as a liquid, and shared with chemical thrusters. Nevertheless, because hydrazine has a relatively high freezing point of 1 1C, temperature management is still necessary for its storage. In addition, hydrazine is very reactive and toxic. Accordingly, tank and valve materials must be compatible with the propellant. Ground tests of hydrazine arcjet thrusters require an exhaust treatment system, resulting in resultantly raising production costs [2]. Besides, an iridium-based catalyst is necessary to gasify hydrazine before supplying it into the arcjet thruster.
n
Corresponding author. Tel./fax.: + 81 93 884 3163. E-mail address: [email protected] (A. Kakami).
0094-5765/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2010.10.019
We have proposed using Dimethyl ether (DME) as a propellant for arcjet thrusters [3]. DME, which is an ether compound with the molecular structural formula CH3–O– CH3, and a freezing point of 143 1C, a boiling point of 54 1C, and a vapor pressure of 6 atm at 25 1C, can be stored as a liquid under a relatively low storage pressure without sophisticated temperature management. The propellant is easily gasified without a catalyst simply by adjusting pressure or temperature. Furthermore, due to its non-toxicity and weak reactivity, DME is safe enough to be used as a propellant of spray cans for household use such as gas duster. The ether bond, which is the connection of carbon atoms through an oxygen atom, allows DME to produce little soot in combustion, and this feature allows DME to be a potential alternative fuel for engines and burners. Such characteristics of DME enhance its potential for use in both industries and households, and consequently its availability. Our previous study showed that a designed DME arcjet thruster successfully produced arc plasma and was comparable to the conventional thruster in discharge voltage, power and plenum chamber pressure [4,5]. The thruster yielded both high- and low-voltage modes; in the lowvoltage mode, electric discharge was very stable, and the arc column remained inside the thruster, never appeared outside the nozzle. In the high-voltage mode, electric
A. Kakami et al. / Acta Astronautica 68 (2011) 1228–1233
discharge was unstable and the voltage periodically dropped and rose. In this paper, for the purpose of evaluating the performance of DME arcjet thrusters, thrust was measured with a torsional thrust stand at mass flow rates ranging from 15 to 60 mg/s with a regulated discharge current of 12 A. Nitrogen was also used as a propellant to compare thrust with DME. 2. Experimental apparatus
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Table 1 Thruster geometry and experimental conditions. Cathode diameter, mm Constrictor diameter, mm Constrictor length, mm Divergent angle of nozzle, deg. Nozzle area ratio Electrode gap, mm Convergent half angle, deg. Discharge current, A Plenum chamber diameter, mm Half angle of cathode tip, deg. Minimum flow passage, mm
2.0 1.0 1.0 15 100 1.0 45 12 6.0 90 0.7
2.1. DME arcjet thruster Figs. 1 and 2 are schematics of a designed arcjet thruster and the region near the electrode gap, respectively, and Table 1 represents geometry and experimental conditions. Discharge current was 12 A and mass flow rate ranged from 15 to 60 mg/s. The flat-tip cathode was a thoriated tungsten rod 2 mm in diameter. The divergent section of the nozzle including the constrictor was made of copper tungsten. It had a divergence half angle of 151 and an exit diameter of 10 mm.
Constrictor (Copper tungsten) Propellant inlet
Divergent anode (Copper)
Cathode (Thoriated tungsten)
Insulator (Ceramics)
Fig. 1. Schematic diagram of the designed arcjet thruster.
Electrode gap, de Cathode Constrictor
Minimum flow passage width, wmin
nozzle Fig. 2. Electrode shape and definition of electrode gap, de and minimum flow passage width, wmin.
The electrode gap de and corresponding minimum flow passage width wmin, illustrated in Fig. 2, are listed in Table 1. The propellant was introduced from an inlet, as shown in Fig. 1, and was supplied to the plenum chamber through a gap between the cylindrical ceramic insulator and the discharge current feed line holding the flat-tip cathode.
2.2. Overall experiment system The DME arcjet thruster was tested in a cubic vacuum chamber of 300 mm on each side. The vacuum chamber was evacuated by a rotary pump at an exhaust velocity of 240 L/min. To protect the cubic vacuum chamber from the intensely hot plume, a 300-mm-long extension cylinder was attached horizontally to the vacuum chamber. The arc plume was injected to the cylindrical extension and exhausted through the outlet port on the flange of the horizontal extension cylinder. Back pressures were less than 0.1–3 kPa for the thruster firing and standby, respectively. DME was stored in a compressed cylindrical vessel, and supply pressure and mass flow rate were regulated with a pressure regulator and a mass flow controller, respectively, before being supplied to the thruster. A constant-current power supply with a rated voltage of 120 V and a rated current of 35 A supplied the regulated current, with a train of high-voltage pulses only at ignition. Discharge voltage was measured using a resistor-type voltage divider, and discharge current was measured with a Hall-effect current sensor. Thrust was measured using a thrust stand, which was mechanically equivalent to a vertical pendulum with a torsional spring. The thrust stand had the pendulum arm on which the designed thruster was fixed, a pistoncylinder-type oil damper and two sets of flexi-hinges producing reflecting torque without friction. The pendulum-arm was extended by setting a vertical extension cylinder on top of the cubic vacuum chamber to magnify pendulum-arm displacement. An oil damper with lowvapor-pressure silicon oil was employed to suppress oscillations induced by thrust and its time variation. Due to the appropriate damper design, such as oil viscosity and geometry of piston and cylinder, the pendulum arm moved back and forward in the manner of critical oscillation. Hence, the time variation in the thrust was traced using pendulum-arm displacement in the frequency range up to the natural frequency of the pendulum, which was
approximately 1 Hz. This thrust stand was calibrated every time the thruster was set to the thrust stand because a propellant feed tube and electric feed-through cables provided the pendulum with extra reflecting forces, which affected the correlation between pendulum displacement and thrust. The displacement of the pendulum arm was measured using a LED displacement sensor with 10 mm resolution. Calibration was done by comparing pendulum-arm displacement with the mass of aluminum disks connected to the thruster by a string. Nine kinds of reference force, produced by the combination of the aluminum weights, ranged from 0.036 to 0.16 N. Pendulum-arm displacement was measured prior to exerting reference force to the thrust stand until removing the weights. The calibration routine was repeated three times for each reference force. The thrust stand showed neither null-position drift after each calibration routine nor variation in pendulum displacement during the application of reference force. The calibration exhibited the linearity between reference force and displacement sensor output with a typical coefficient of determination R2 of 0.9916. This indicates that the error originated from the calibration is approximately 1.8%. An IBM-compatible PC with RTAI/Linux operating system having an analog–digital converter interface acquired all of the experimental data: discharge voltage, current, propellant mass flow rate, plenum chamber pressure, and pendulum-arm displacement of thrust stand [6]. The sampling rate was 100 Hz, and the cut-off frequency of the low-pass filters used in the electric circuits was higher than 1000 Hz. 3. Results and discussion
Plenum chamber pressure Thrust Discharge voltage Discharge current
0.15 0.1
100
0.05 Nitrogen supply started
0
Thrust, N plenum chamber pressure, MPa
A. Kakami et al. / Acta Astronautica 68 (2011) 1228–1233
Discharge voltage, V and current, A
1230
Ignition
50
0 -10
0
10
20
Time, s Fig. 3. Time variations of discharge voltage, current, plenum pressure, and thrust using nitrogen propellant at 60 mg/s.
was comparable to conventional arcjet thrusters in thrust, plenum chamber pressure, and discharge power. From these results, the designed thruster and the measurement device were applicable for the performance evaluation of the DME arcjet thruster.
3.2. Time history of discharge voltage, plenum pressure, and thrust using DME propellant
3.1. Preliminary thruster test using nitrogen propellant The designed thruster was preliminarily tested using nitrogen propellant with a regulated discharge current of 12 A. The arc plasma was successfully maintained during the discharge. Fig. 3 shows the time history of discharge voltage, plenum chamber pressure, and thrust. For t=0 1 s, the discharge voltage was not measured to protect the voltage divider and the measurement device from high-voltage ignition pulses with a 10-kV-class peak voltage. At t= 6.3 s, nitrogen supply to the thruster was started, and the thruster yielded peaky thrust due to the initial overshoot of the nitrogen mass flow rate. Afterwards, the thruster generated a tiny, but stable, cold gas thrust. After arcdischarge ignition at t=0 s, thrust, plenum chamber pressure, and discharge voltage started to rise and remained almost constant until t=5 s. During the five seconds, a relatively dark plume, which was very long and thin, was found outside the nozzle. At t=5 s, thrust, plenum chamber pressure and discharge voltage increased instantly, and a brighter plume was found outside the nozzle. Thrust, plenum chamber pressure, and discharge voltage remained constant, and plume shape was maintained during arc discharge. This showed that the thruster initially exhibited the low-voltage mode and switched voltage mode to high at t=5 s. At other mass flow rates ranging from 15 to 60 mg/s, the thruster maintained stable discharge voltage and thrust. The thruster
The DME arcjet thruster successfully produced a thrust of 0.15 N at a mass flow rate of 60 mg/s with the regulated voltage of 12 A. As shown in Fig. 4, the DME propellant was fed to the thruster at t= 7.7 s, and the electric discharge was initiated at t =0 s. Shortly afterwards, thrust was produced by the arc plasma, and it had relatively small periodic variations, while the discharge current was begin supplied. The plume was always located outside the nozzle, and this suggests the thruster yielded the high-voltage mode. At t = 81 s, the discharge current was interrupted manually, and thrust and plenum chamber pressure decreased immediately. From the results, the DME arcjet thruster successfully produced thrust for a period of 81 s. On the other hand, arc discharge using DME propellant was unstable compared to that using nitrogen. The discharge voltage had ripple-like fluctuations. The plume expanded and shrank repeatedly, and its direction also oscillated during arc discharge. Plenum chamber pressure also remained almost constant for the first 70 s of arc discharge, but rose afterwards. This rapid expansion of the plenum chamber pressure was attributed to carbon contamination. After testing the designed thruster, black materials, which were solid but fragile enough to be broken up with fingers, accumulated especially in the vicinity of the electrodes, and consequently choked the constrictor. The black materials also adhered to the inner surface of the
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tube insulator surrounding the cathode. Our previous study showed that the black materials were composed of 97% carbon and 3% tungsten [3]; hence, the black materials found in this experiment are considered to be a composite of carbon and tungsten.
Fig. 5 illustrates the dependence of discharge voltage and power on mass flow rate, with values time-averaged through the arc discharge because the experimental values varied with time in the case of DME propellant. The timeaveraged discharge voltage increased steadily with mass flow rate for DME and nitrogen propellant. The DME arcjet thruster produced discharge voltages ranging from 75 to 90 V, which is the same level as the conventional arcjet thruster. The DME propellant always produced higher discharge voltages than nitrogen, which increases discharge power. Discharge power and specific power varied from 430 to 1300 W, and from 19 to 31 MW/kg, respectively, and were also the same as those of conventional thrusters. Discharge voltage rises discontinuously at a mass flow rate of 30 mg/s. Below 30 mg/s, arc discharge is relatively stable but discharge voltage is lower than at higher mass flow rates, as shown in Figs. 5 and 6. During arc discharge, no plume was found outside the thruster but bright sparks, which are considered to be carbon particles, were emitted from the nozzle. At the other mass flow rates below 30 mg/ s, the time variations of discharge voltage and spark emissions were almost the same as those at 15 mg/s. This shows that the thruster operated in low-voltage mode. In contrast, at mass flow rates above 30 mg/s, the thruster yielded an enhanced discharge voltage, as shown in Fig. 5, and plumes were located outside the nozzle. Hence, at
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Fig. 4. Time variations of discharge voltage, current, plenum pressure, and thrust using DME propellant at 60 mg/s.
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Fig. 6. Time variations of discharge voltage, current, plenum chamber pressure, and thrust using DME propellant at 15 mg/s.
above 30 mg/s, the DME arcjet thruster yielded highvoltage mode. The thruster with 30 mg/s switched voltage mode from high to low periodically, as shown in Fig. 7. Discharge voltage showed intense low-frequency variations as well as the higher frequency fluctuations in high-voltage mode. The plume was located outside the nozzle, but suddenly disappeared and afterwards expanded intensely outside the nozzle. The plume shape was varied repetitively during arc discharge. Hence, the thruster with 30 mg/s alternately yielded high- and low- voltage modes. These results indicate that the DME arcjet thruster produced both
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high- and low- voltage modes as well as the conventional arcjet thruster. The periodic alternation in the discharge mode would be caused by relatively unstable discharge in the high voltage mode for DME propellant. When the discharge voltage accidentally dropped because of the instability in the high voltage mode, the thruster began to show the low voltage mode. Nevertheless, the thruster yielded the high voltage mode again because 30 mg/s mass flow rate was so high that the thruster could not sustain the low voltage mode. The process would be repeated during the arc discharge.
Fig. 8. Variations of thrust, specific impulse and plenum chamber pressure with mass flow rate.
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Fig. 7. Time variations of discharge voltage, power, plenum chamber pressure, and thrust using DME propellant at 30 mg/s.
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3.4. Thrust and specific impulse Fig. 8 shows the dependence of thrust and specific impulse on mass flow rate for DME and nitrogen. Both thrust and specific impulse almost consistently with mass flow rate, and were elevated discontinuously at 30 mg/s, which was the threshold of the discharge mode. In highvoltage mode, thrust and specific impulse ranged from 0.078 to 0.15 N and 230 to 270 s, respectively, while they varied from 0.014 to 0.021 N and 89 to 99 s in low-voltage mode. Hence, like conventional arcjet thrusters, the thruster in high-voltage mode showed higher thrust and specific impulse than in low-voltage mode. The DME arcjet thruster had a maximum thrust of 0.15 N, corresponding to a specific impulse of 270 s and thrust power ratio of 120 mN/kW at 60 mg/s; discharge power and specific power were 1300 W and 21 MJ/kg, respectively. These values are almost the same as those of the conventional kW-class arcjet thruster. Accordingly, the DME arcjet thruster was comparable to the conventional thruster in performance.
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Fig. 9. Dependence of thrust and specific impulse on specific power.
Thrust with DME propellant was higher than with nitrogen at a given mass flow rate in high-voltage mode. This is partially because DME propellant yielded higher discharge voltage and power than nitrogen. DME provided enhanced thrust at a given specific power in high-voltage mode. Fig. 9 shows dependence of thrust and specific impulse on specific power. Both thrust and specific impulse with DME propellant are higher than with nitrogen in highvoltage mode, whereas they were almost the same in the low voltage mode. From the results, DME provided higher performance than nitrogen. This is considered to be because DME, which has a molecular weight of 46 and
A. Kakami et al. / Acta Astronautica 68 (2011) 1228–1233
consists of 9 atoms, has a lower average molecular weight of 5.1 than nitrogen at 14. 4. Conclusion A DME arcjet thruster was tested in a vacuum chamber. Its thrust was measured with a thrust stand to evaluate its performance: thrust power ratio, specific impulse, etc. (1) Discharge voltage increased with mass flow rate for both DME and nitrogen. Discharge voltage was always higher with DME than with nitrogen, as was discharge power at a given mass flow rate. (2) DME arc discharge was relatively unstable compared to nitrogen. The discharge voltage had ripple-like fluctuations. The plume was enlarged and shrank periodically during arc discharge, and the plume direction and size varied with time. (3) The DME arcjet thruster yielded both high- and lowvoltage modes. The thruster showed high- voltage mode at mass flow rates above 30 mg/s and low-voltage mode below 30 mg/s. Like the conventional arcjet thrusters, the DME arcjet thrusters yielded higher thrust and discharge voltage in high-voltage mode.
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(4) Thrust was measured using a thrust stand, which is mechanically equivalent to a vertical pendulum with a torsional spring. Thrust using DME propellant increased steadily with mass flow rate, and was higher than that using nitrogen. The thruster yielded maximum thrust of 0.15 N, specific impulse of 270 s, and discharge power of 1300 W at a DME flow rate of 60 mg/s. (5) DME provided enhanced thrust and specific impulse compared to nitrogen at any specific power in the highvoltage mode operation. References [1] R.G. Jahn, Physics of Electric Propulsion, McGraw-Hill Book Company, 1968. [2] G.P. Sutton, O. Biblarz, Rocket Propulsion Elements, 7th ed., John Wiley & Sons, Inc., 2001. [3] A. Kakami, I. Ebara, J. Yokote, T. Tachibana, Application of dimethyl ether to arcjet thruster as propellant, Vacuum 8 (2008) 77–81. [4] H. Kuninaka, M. Ishii, K. Kuriki, Experimental study on a low-power direct current arcjet, Journal of Propulsion and Power 2 (1986) 408–413. [5] T. Yamada, Y. Shimizu, K. Toki, K. Kuriki, Thrust performance of a regeneratively cooled low-power arcjet thruster, Journal of Propulsion and Power 8 (1992) 650–654. [6] RTAI website, /http://www.rtai.orgS.