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Deep Space 1 Ion Propulsion System Operation Sequence and Status

Like any other rocket engine, the ion engine for the DS1 spacecraft has gone through a rigorous series of developmental and performance tests prior to flight. In addition to validating performance and functionality, the tests were used to develop an operational sequence to ensure successful performance. The following is the operational sequence used to validate the in-flight performance of the ion engine and complete the mission objectives for DS1.

Ion engineIon engine during testing. Credit: NASA
Chronology "Bake Out"
Friday, October 30, 1998

The spacecraft was turned so that the ion engine's axis was pointed 30 degrees off of the spacecraft-sun line. This has heated the ion engine and its feed lines to expel any water vapor or other volatiles that might have been introduced during pre-launch operations or during the launch itself. This operation is intended to eliminate any contamination that might affect the cathodes adversely.

The temperature sensor on the ion engine's mask - a thin piece of sheet titanium that forms an annulus around the grids - reached a temperature of 146 degrees C. The temperature of one of the engine's mounting brackets is presently 110 degrees C. On Saturday (10/31/98) morning the spacecraft will be returned to its regular orientation with respect to the sun and the temperatures will return to their normal values.

Saturday, October 31, 1998

The spacecraft was turned to its normal orientation with respect to the sun. Temperatures returned to their pre-"bake out" values. The latch valves in the propellant storage and control system were closed and all indicators appeared normal. Xenon was flowed through the cathodes during the "bake out" to purge any liberated water vapor and oxygen from the system. The resulting decay of pressure in the plenum tanks will be used to provide an in-flight calibration of the flow control devices. Early indications are that the pre-flight calibrations are consistent with the observed pressure decay. Some time will be needed to complete the analysis required to confirm these indications in detail.

Thursday, November 5, 1998

The NSTAR propellant storage and control system was activated and flow through the cathodes was initiated. The cathode heaters were turned on at 11:19 AM EST and the cathodes were "conditioned" for approximately 5 hours. Thus far the data looks very good, just as was predicted. Shortly after 6:00 PM (EST), when the cathode conditioning operation was complete, the Xenon storage and control system was pressurized so that the plenum tanks were pressurized. They are now ready for the start of "Diode Mode" on 11/6/98. In addition to preparing the cathodes for their first in-space ignition, this operation gave us our first look at the performance and behavior of the propellant storage and control system, which operated flawlessly.

Diode Mode
Monday, November 9, 1998

Because of some communications difficulties, the "Diode Mode" sequence was delayed from Friday, November 6 until Monday, November 9. The "Diode Mode" sequence started at 11:54 AM. EST. The ion engine's two cathodes will be operated for 4 hours to heat the engine, driving off any residual contamination that may have found its way into the rivets or other "tight" areas of the ion engine. The activity was terminated at 3:49 PM EST. At the end of this sequence the temperature of the ion engine "mask" (a thin, sheet-titanium annulus around the outside of the accelerator grid) was approximately 155 degrees C and that of the gimbal bracket was approx. 130 degrees C. This heating is intended to preclude the presence of any low-pressure region that could support an arc when the engine's high voltage is turned on.

Throughout the "Diode Mode" activity the data were very similar to the corresponding data observed during the same test done during the ICT (IPS Compatibility Test) in the 25-foot STV chamber earlier this year. The "Diode Mode" activity also tested starting the discharge and neutralizer hollow cathodes, which was done without any anomaly, and operated two of the three main power supplies in the Power Processing Unit. All of the NSTAR team is ready and eager for the IPS Acceptance Test.

Initial Acceptance Test
Tuesday, November 10, 1998

The NSTAR Ion Propulsion System wasl turned on and its first in-space "Acceptance Test" began, however it ended prematurely after only 4-1/2 minutes when the engine shut itself down. The reasons that are still under investigation. After the startup at 2:30 p.m. EST and subsequent shutdown, the operations team sent a number of commands to try to restart the ion propulsion system. Each time, the system went through its normal startup routine, but was unable to achieve thrusting. Valuable diagnostic data were collected, and the team observed that the rest of the spacecraft behaved exactly as planned during the brief interval of thrusting and during subsequent attempts to restart the thruster.

Engine turn-off behavior has been observed in the past in solar electric propulsion systems both in Earth-based test and on Earth-orbiting spacecraft. Deep Space 1 is designed to test and validate the use of such propulsion in deep space for the first time, so the ongoing diagnosis of this behavior is in keeping with the mission's goals.

Engineers believe that the engine probably shut itself off when it was first started because of metallic grit or other contamination between the two high-voltage grids at the rear of the advanced engine. It is likely that changes in temperature as the spacecraft conducted other technology validation activities affected the flakes, and powering-up the thruster may vaporize the remains.

The team conducted data analysis and simulation tests before they attempted to restart the engine. The team modified the onboard software that controls the ion engine to give engineers greater resolution in studying currents and voltages when they attempted to start the ion engine. The new software was tested for several days on the ground and then transmitted to the spacecraft on Friday morning, November 20. These modifications gave engineers greater resolution in studying currents and voltages when the engine was restarted. Several different strategies for resuming thrusting were evaluated.

Acceptance Test
Tuesday, November 24, 1998

The ion propulsion system on NASA's Deep Space 1 spacecraft came to life Tuesday, November 24, and has continued running smoothly since. The engine started up at 5:53 p.m. EST, in response to commands sent to the spacecraft. After running overnight in low-thrust mode, engineers commanded the engine to switch to higher-thrust modes on November 25. The mission team plans to leave the engine running over the four-day Thanksgiving weekend.

When the engine was started Tuesday, it ran overnight, thrusting at a power level of 500 watts. On Wednesday morning engineers commanded it to thrust at a level of 885 watts, then at 1,300 watts. Engineers may decide to have the engine thrust at a lower level while it runs over the next few days.

Now that the full Acceptance Test has resumed, the activities include stepping up the thruster through different throttle levels, eventually taking the engine to its peak thrusting level. This will allow the team to assess the overall performance of the spacecraft and the ion propulsion system at increasingly powerful levels and to measure the power needed from the spacecraft's pair of solar arrays to achieve each thrust level. Concurrently, ground-based radio navigation was to take Doppler data to measure the amount of thrust imparted by the ion engine system at each throttle level.

The ion engine was turned off Tuesday, December 8, to allow the mission team to activate one of the mission's two advanced science instruments, the Plasma Experiment for Planetary Exploration (PEPE). While operating, the ion engine ran more than twice as long as it was originally planned to thrust without interruption at any time during the mission. By running the ion engine for more than 200 hours and successfully conducting technology validation of the spacecraft's solar array and transponder (radio transmitter/receiver), the team achieved the minimum criteria that NASA established for overall mission success. Deep Space 1 has operated its thruster for a much longer uninterrupted time than any deep space probe using any other propulsion system.

On Friday, December 11, the team will command Deep Space 1 to resume ion engine thrusting. Plans call for the ion engine to operate nearly continuously until January.

"TH14" Operation
Monday, November 30, 1998

During operation of the thruster after the acceptance test, the NSTAR ion propulsion system was commanded to step up its throttle levels ("TH") to determine the point at which the thruster would draw more power than the solar arrays can provide and thereby "collapse" the solar arrays. By so doing, the power subsystem would be able to determine the solar array's peak power point. There are 16 throttle levels (TH0 to TH15) which go from 0.5 kW to 2.3 kW of input power. The TH14 operating point, which is the 15th level, was the anticipated "collapse" point.

On November 30, operation at TH12 (approximately 2.15 kW input to the Power Processing Unit and 1.96 kW input to the ion thruster) resulted in a reduction in line voltage to about 79 V (nominally that voltage is 100 V) and the use of the battery to supplement the power being supplied by the solar array. The power specialists were very pleased both that the solar array's peak power point had been identified and that the solar array's "collapse" was slow enough that the power control unit on the spacecraft could prevent that collapse by adding energy stored in the battery to the system. The spacecraft operations will be conducted at power levels below 2.15 kW to ensure that the power capability of the solar array are not exceeded.

Regular Operations - First Thrusting Leg
Wednesday, December 23, 1998 - Tuesday, January 5, 1999

Having passed the acceptance test, the NSTAR ion propulsion system began regular operations as the primary method of propulsion for DS1. At this point, the engine had completed more than 500 hours of operation after its successful startup on Nov. 24, and the spacecraft was more than 6 million miles from Earth. On Tuesday, January 5, the thruster was shut off by the DS1 spacecraft's software, completing the first thrust segment of the DS1 mission. During that thrust segment the engine accumulated 852 operating hours. During that time, other Glenn components, such as the power processor, as well as the the DCIU, and the xenon propellant storage and control systems, have worked just as they were designed. For the next two months (until mid-March) DS1 will concentrate on assessments of the other technologies while the spacecraft coasts towards its planned destinations.

Ion Propulsion System Readiness Test
Wednesday, February 24, 1999

Having been shut down for almost two months, the IPS Readiness Test was conducted to verify that all is well in anticipation of the start of the second thrusting leg of the trajectory.

Regular Operations Resume - "N-Burn"
Monday, March 15, 1999 - Tuesday, April 27, 1999

The second thrusting leg of the DS1 trajectory began at about 8:15 pm (EST) on March 15, 1999. The ion propulsion system has been working well since. This activity, called "N-Burn" (navigational burn), has the thruster under the automatic control of the on-board navigation system. During this thrusting period, the spacecraft is programmed to adjust automatically the power level at which the ion propulsion system is operated to the power available from the solar arrays. On March 15, the PPU input power was 1,311 W, which fell to 1,175 W at the end of N-Burn 4 on 4/12/99.

Each N-burn sequence is about 7 days followed by a 4 to 8 hour shutdown of the IPS for refining data for the next 7-day N-burn sequence. The on-board system will determine when the proper trajectory conditions have been satisfied and will terminate ion thruster operation automatically. During this down time high-rate data are downlinked using the high-gain antenna.

At 7:21 am (EDT) on April 27, 1999, all of the primary thrusting for the primary DS1 mission was successfully completed by the NSTAR ion propulsion system. This second thrusting leg took 910.3 hours of thrusting and used 5.0 kg of Xenon. With some tests and other operations the total "thruster on" time for the mission stands at 1,764 hours. (To date the thruster has experienced only 96 recycles.)

Performance Testing
Tuesday, April 27, 1999 - Sunday, July 25, 1999

During this time, the NSTAR ion propulsion system on DS1 underwent a performance test to obtain data that will allow an accurate comparison of in-space performance with that measured in ground testing. The system also performed some trajectory correction maneuvers to fine-tune the trajectory for the first planned encounter.

Close Encounter 1: Asteroid 9969 Braille
Monday, July 26, 1999

The ion propulsion system successfully enabled DS1 to encounter asteroid 9969 Braille on July 26, 1999. DS1 came within 16 miles of Braille, making it the closest flyby ever for a spacecraft.

Thrusting Toward More Encounters
Friday, July 30, 1999 - Wednesday, October 20, 1999

The spacecraft's ion engine was fired at 12 Noon EDT on July 30, 1999 and continued thrusting almost continuously for three months to shape DS1's orbit around the Sun in preparation for flybys of two comets. Each week for just over half a day, the engine was shut down to allow the AutoNav system to collect pictures for navigation and to point the main antenna at Earth. On Wednesday, October 20, shortly after 7:00 am EDT, the engine had accomplished the desired thrusting for this part of the orbit and was turned off.

Coasting and Waiting
Wednesday, October 20, 1999 - Wednesday, June 21, 2000

The spacecraft was scheduled to coast until the middle of December, when a three month thrust period was to begin. A problem with the star tracker used for attitude control has delayed reactivation of the ion engine. A new system has allowed Deep Space 1 to regain full three-dimensional control and knowledge of its orientation works in part by taking pictures of a reference star.

Thrusting and Stablizing
Wednesday, June 21, 2000 . . .

After more than 7 months of dormancy, the ion propulsion system is back on and thrusting with an added responsibility. The attitude control system, whose on-board task of controlling the spacecraft's orientation was made so difficult when the star tracker failed, not only has to keep the spacecraft steady while the ion propulsion system is firing, but it actually uses the ion drive to stabilize the spacecraft's rotation whenever it is thrusting. The system is thrusting intermittantly (approximately one week at a time) as the new tracking systems tracks a series of target stars. The current target of the extended mission is a flyby of Comet Borrelly. For spacecraft to meet the comet, we have to change DS1's orbit so that the orbits of DS1 and Borrelly intersect. The ion propulsin system is slowly but surely reshaping DS1's orbit so the spacecraft will encounter Borrelly in September 2001 as they follow their separate paths around the Sun.

The technology is so efficient that it only consumes about 3.5 ounces (100 g) of xenon per day, taking about four days to expend just one pound (0.4 kg).

A New Attitude (Control)
Thursday, May 10, 2001

The Deep Space 1 ion engine has accumulated more operating time in space than any other rocket engine in the history of the Space Program. The engine has thrusted for over 11,160 hours. Since the hydrazine for the DS1 attitude control system is in short supply, the ion propulsion system is now performing a mission enabling function. When thrusting is not required to maintain the mission trajectory, the ion engine provides pitch and yaw control to save hydrazine and ensure spacecraft attitude control is maintained until encounter with the comet Borrelly in September 2001.

Rendezvous - Part Deux
September 22, 2001

The second encounter for Deep Space 1 was a flyby of Comet Borrelly on September 22, 2001 as it passed within about 1,400 miles (2,200 kilometers) of the comet at 6:30 p.m. EDT. It reached Borrelly while the comet was near its closest approach to the Sun and, thus, very active. Mission scientists captured detailed pictures of the comet's nucleus and gathered data on the violent jets of gas and dust shooting out of it. Deep Space 1 became just the second spacecraft to study a comet from a distance less than 2000 km; the first was the European Space Agency's Giotto mission that flew by Comet Halley in 1986. Borrelly, as it streaks around the Sun on its nearly 7-year-long tour of the solar system, flashed past DS1 at a speed of almost 17 kilometers per second, or about 37,000 miles per hour. JPL mission planners organized the complex choreography required to assure that this brief pas de deux took place on schedule.

Prior to the encounter, the Deep Space 1 spacecraft recovered from periodic anomalies associated with its star sensor and spacecraft attitude control system. The ion propulsion system enabled the spacecraft spacecraft to close in on the comet Borrelly at a rate of 750,000 miles per day. The ion engine, which provides primary propulsion and does 2-axis attitude control, has operated for more than 14,000 hours and has consumed more than 60 kilograms of xenon propellant.

"Hyper-extended" Mission
October-December 2001

Final assessments of the IPS performance were made. Grid wear information was obtained via electron backstreaming measurements. Mass flow and discharge current effects on discharge cathode and neutralizer characteristics were obtained. Final measurements were made using the JPL plasma diagnostics package. IPS/solar array diagnostic measurements were also made, and other high-risk tests including low-flow starts were undertaken. The IPS start on December 13, 2001 was the 200th successful start of the propulsion system not including the first diode mode start and other initial start attempts.

Farewell to DS1
December 18, 2001

Shortly after 3 p.m. EST Tuesday, December 18, engineers sent a final command turning off the ion engine. The IPS accumulated 16,246 hours of thrusting, and the xenon throughput was about 72 kg.

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