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General System Description of METS

Basic purpose and function The Mir Electrodynamic Tether System (METS) is a rather unusual electric motor that is designed to reboost the Mir orbital station. This motor is unusual because some key parts of the motor are provided by the earth itself: the magnetic field and a ionospheric plasma that provides most of the current loop. What METS itself provides is a control computer, a high-power voltage converter, and the rest of the current loop: a 5 km long wire, a 1 km long thin bare metal strip at one end of the wire to collect electrons from the plasma, and a hollow cathode emitter at the other end to emit electrons back into the plasma.

The earth's magnetic field is weak, but Mir's orbital motion generates an average EMF near 600V along the wire length. The actual voltage varies greatly over time, especially if the wire swings like a pendulum. Flowing a current through the wire against the EMF imposes an eastward force on the wire, and an equal and opposite force on a large volume of plasma. The eastward force raises Mir's orbit altitude about 40 km per kW-year of power used, and reduces the orbit inclination slightly (about 0.07 degree per kW-year)

The main efficiency losses in this electric motor are voltage drops for electron collection, conduction, and emission, plus aerodynamic drag on the wire and collector, and losses in boosting power from Mir's 27V bus voltage to the 400-1500V needed by METS. Overall losses should average about 30%. For power levels of 1-3 kW, METS should save about 1,000 kg of reboost propellant per kW-year. This is 2.5-3X as much propellant as can be saved by an ion thruster or stationary plasma thruster using the same power.

METS will mount on an existing EVA ladder at the end of the Kvant-2 module. The wire will hang downward towards earth, and will be kept nearly vertical by gravity gradient effects acting on the wire and a 200 kg ballast mass at the end of the wire. The ballast mass is a used manned maneuvering unit (MMU) now docked on the EVA ladder. The wire will provide passive LVLH attitude stabilization for Mir, with Kvant-2 facing earth. This orientation allows higher solar array power output than other LVLH attitude options.

Basic system layout METS is being designed to minimize both system mass and crew workload, while still allowing maintenance and repair. These objectives led us to break METS into two main assemblies that can be handled separately during launch, installation, and repair: reel of wire that weighs about 100 kg - an electrical panel that holds everything else and weighs about 50 kg.

We have sized the reel to mount horizontally in Progress between 2 adjacent payload support brackets that are 620 mm apart. Bolts can mount through the brackets directly into the reel

flanges. The reel will be made primarily of non-conducting material. A high-voltage isolation diode at the inner end of the wire will keep high voltages away from the reel axle and panel whenever the voltage converter is turned off. This lets the crew safely separate the reel and panel, so either can be replaced during an EVA. The electrical panel is large enough to reject up to 250W from the voltage converter and other equipment, and to hold all the components other than the reel on one side. (When the reel is attached, it cantilevers from one end of the panel.) Putting all the components on one side of the panel leaves the other side free for mounting in Progress and outside Mir. The panel plus the components mounted on it will be shaped like a capital "D" about 900 mm high, 600 mm wide, and 200 mm thick. This allows the panel to fit vertically inside Progress, on the side of one of the main payload support brackets. The D shape may also make it easier to move the panel through Mir, especially around any turns. The panel will have tapped holes in a pattern compatible with the payload support bracket, so the ground crew can install bolts through the bracket into the panel. If this electrical panel mounts opposite the reel inside Progress, the combined CG of the two METS assemblies will be close to the Progress vehicle axis.

Summary of electrical connections between Mir and METS Energia has proposed using two 48-wire cables for power and signal connections between Mir and METS. The cables will attach to METS near the inboard edge of the electrical panel. Energia will provide us with mating connectors of a type they recommend for crew use during EVA, for us to install on the panel. One cable will plug into an existing socket outside the Kvant-2 airlock. It will have 10 signal and signal return lines, plus 38 power and power return lines to provide large amounts of 27V power to METS. The signal lines include 3 dedicated wire pairs so the crew can trigger MMU ejection, SEDS deployer release, and emergency ejection of the wire and reel. The other 4 signal lines are for a standard two-way RS-232 9600 baud interface between the METS computer and Mir, power to the METS control computer, and ground. The other cable will provide additional power from an existing socket on another module. This cable may require a second EVA for installation.

In addition to the hard-wired connections that the crew will use to directly trigger specific events, the Mir crew and/or ground control will be able to send commands to the METS control computer to do at least the following: -enable and disable METS operation -accept new control program parameters -schedule future METS operating power levels -turn on heaters to keep key components warm -download telemetry data of various types -cause emergency ejection of the wire

Telemetry parameters can include at least the following: -some status, housekeeping, and self-diagnosis items -turns deployed from the SEDS deployer and then from the reel -temperatures of key components, plus xenon tank pressure -history of voltage and current output of the power converter -more detailed data on any anomalies

General description of installation, deployment, and checkout After Progress docks to Mir, the crew can remove the bolts holding the reel and the panel in place, and move those items through Mir to the Kvant-2 airlock. Then they plug the reel axle into a socket on one end of the panel, and attach the brackets needed to clamp the panel to the EVA ladder, if those brackets were not installed before launch.

Then the crew can suit up, depressurize the airlock, take METS outside, and attach it to the EVA ladder, with the reel cantilevered off to one side. This will help keep the wire from fouling on the EVA handholds at the end of the ladder during deployment. Then the crew will

attach a pilot tether and pusher mechanism to the MMU and release the MMU from its docking port at the end of the ladder.

Once the crew connects the first cable to METS, ground control can talk with the METS control computer, check it out, and collect status data, while the crew is still outside. After the crew gets back inside (or perhaps the next day), the crew can trigger MMU separation to start the ~5 hour METS deployment. When the MMU is pushed away, it drags out a non-conducting "pilot tether" about 1 km long from a SEDS tether deployer. At the end of this deployment, the MMU starts to swing down towards the vertical, increasing the tension. Then the crew releases the empty SEDS deployer from the panel. This deployer is connected to a short additional length of non-conducting tether wound on the outside of the reel. A few minutes later, after the dynamics resulting from this operation settle out, the METS computer energizes a solenoid that releases a reel brake. The reel then begins rotating and paying out the short non-conducting tether. Following this is the 1 km long strip of mostly-bare aluminum that serves as an electron collector, and then 5 km of insulated aluminum wire. The reel has a passive internal eddy-current brake to regulate deployment and damp out any undesired dynamics. About 4 hours later, when only about 30 meters of wire remain on the reel, the METS computer will turn off the reel brake solenoid. This re-applies the external reel brake, increasing tension and bringing deployment to a smooth stop over the next minute or so.

After ground control enables METS operation, a pre-planned sequence of component checkout will begin. The last step will be to turn on the high-voltage power supply, first at an idle level, and then at progressively higher output power levels. Each kW-day of operation saves about 3 kg of reboost propellant, so the cost of a few days of checkout at power levels lower than 2-3 kW is tolerable. Every day telemetry data will be sent down to the ground for analysis, and revised power schedules can be sent up to METS, based on the level of power expected to be available.

Based on analysis of initial system performance, backup hardware may be modified before it is launched on a subsequent Progress. On-orbit storage will allow the crew to immediately swap out the electrical panel if one of its components fails, or to replace the wire if it is severed by micrometeoroid or debris impact (for which we estimate a risk of 3-4% per month).

Technical Proposal for METS (Mir Electrodynamic Tether System)

Contents 0. Summary 1. Background 2. Objectives 3. Concept and Engineering 4. Risk Assessment 5. Project Outline 6. Next Steps 7. Other Options Considered 8. Conclusions

0. Summary

This Technical Proposal presents the results of a preliminary study carried out by Energia Ltd. (SRC) and Tether Applications. The study was supported by FINDS, the Foundation for International Non-governmental Development of Space. The main purpose of this study was to evaluate the feasibility of maintaining Mir in orbit without propellant using an electrodynamic tether. Our key findings are that such a Mir Electrodynamic Tether System (METS) appears feasible, that it needs to be operational by next summer at the latest, and that operational changes on Mir may be able to reduce drag until then.

1. Background The Mir space station is currently in a critical situation. It is at 350 km altitude and its orbit is now decaying ~1 km/week. If nothing is done to reboost Mir, the decay rate will gradually increase until Mir enters Earth's atmosphere by the end of 1999. Active maneuvers will be required to ensure entry in a safe location. Current plans call for unneeded expenditures of thinly stretched cash reserves and hardware to either bring Mir down or keep it in orbit. It is the belief of the sponsoring organization (FINDS) that METS presents a way out of this dilemma, and can create a new situation wherein all parties benefit.

Figure 1. Mir Orbital Complex During METS Deployment

To FINDS, Mir represents useful real estate in space and as such should be kept alive, as new options are examined for its future utilization as a privately operated facility that complements the ISS. Whether the saved station generates enough income to stay operational, or is shut down after reboost and stored until a day when fleets of new commercial space craft allow its refurbishment, METS will not only save both NASA and its Russian partners money and hardware better used to build and supply ISS, it will take Mir off the books of ISS and create an independent revenue stream for Russia and its commercial partners in the venture. In addition, if Mir is moved to an orbit 80 km higher than the ISS, its orbit will become coplanar with ISS within 2 years. This would allow easy transfer of working hardware or crews from Mir to the ISS and vice versa. Also, Mir and ISS can then act as safe havens for one another in the event of emergency. Coplanar orbit will also create economies of scale for re-supply by the Space Shuttle, Progress and newly emerging commercial space transportation services, resulting in reduced costs for all.

Tethered satellite systems provide unique opportunities, including spacecraft reboosting. NASA has done substantial work to develop and test tethered satellite technologies. Gemini XI and XII tested 30 m long tethers in the 1960s. In the last 7 years far longer tethers have flown, including 20 km tethers on TSS-1 and 1R, and on SEDS-1 and 2. TiPS, the Tether Physics and Survivability experiment, used a 4 km SEDS tether which is still intact 2.5 years after deployment. The Plasma Motor/Generator (PMG) experiment demonstrated the ability

of hollow cathodes to couple to a plasma at each end of a 500 m wire. Note that the Gemini experiments were conceived, developed, and flown in less than a year, and that the tether hardware for TiPS was delivered 6 months after the concept was proposed.

Propellantless reboost of the ISS using an electrodynamic tether was proposed and analyzed by NASA Marshall, and recommended for continued development by the ISS chief engineer at NASA JSC, in an April 11, 1996 letter to the ISS program manager. Based on an initiative by FINDS, experts from Energia and Tether Applications have done a preliminary study of using an electrodynamic tether to keep Mir in orbit. This technical proposal is a result of that study.

2. Objectives The primary objective of this project is to maintain Mir in orbit for several years at greatly reduced cost, by reducing the required frequency of Progress/Mir flights. The secondary objective is to do an operational test of an electrodynamic tether system so it can be used with confidence in other applications, including backup reboost for the International Space Station, satellite servicing, power generation, active de-orbit, and so forth.

3. Concept and Engineering 3.1 Concept The basic concept of an electrodynamic tether is as follows. Consider a long wire in a plasma. One can collect electrons from the plasma using a large bare metal collector at one end of the wire, and emit electrons back into the plasma at low energy with a hollow-cathode emitter at the other end of the wire. The current loop closes externally, in the plasma. Interaction of the current with the earth's magnetic field causes a net force on the wire that is normal to both the wire and the field lines. There is an equal and opposite force on the plasma. Motion of the wire relative to the plasma induces an EMF of ~110V/km in a vertical wire in Mir or ISS orbit.

Figure 2. Plasma Motor/Generator (PMG) Experiment, mounted on Delta

If current flows with the EMF, power is generated and the orbit decays. Supplying power to the system to force current against the EMF allows boosting. System performance can be constrained by the power available, wire resistance, or electron collection limits. As discussed later, the power available on Mir will probably be 1-3 kW. Wire resistance varies with wire type, mass, and temperature. An aluminum wire weighing ~50 kg keeps resistance losses reasonable for power levels of 1-3 kW. Anodizing the wire gives an atomic-oxygen resistant

insulation so electrons collected travel the whole length of the wire. This improves system efficiency. A large electron collecting area is needed to collect ~1-3 amps at potentials small compared to a ~1kV EMF, but this raises aerodynamic drag. The answer is to use long narrow strips. Plasma sheath effects allow narrow strips to collect far more electrons than can be collected by wider collectors of equal area (and hence equal drag). The collector should also be very short compared to the wire, so the collecting potential does not vary greatly over the collector length. These constraints have led us to baseline a collector design of 8 strips of thick aluminum foil ~0.05 x 10 mm x 500 m long. The strips need to be well separated for maximum electron collection. We should be able to use the electrodynamic forces to do this, but we have not worked out the details yet. Overall it appears feasible to keep voltage drops in the collector, wire, emitter, and plasma quite small compared to the EMF, so this rather unconventional electric motor may run at >75% efficiency. This means <10kW is needed per newton of thrust. Other forms of high-Isp electric propulsion provide only 40-50% as much thrust per kilowatt, and their expendables exceed 1 kg/hr per newton. The only expendables here are the hollow cathode gas supply (3 kg/year), and the 50 kg wire, which has an expected mean time before failure (MTBF) due to micrometeoroids of <1 year. We can increase the MTBF greatly by using a wide flat wire or a multi-strand wire to improve impact tolerance, but this does increase drag. Maximizing the boost force on the tether requires keeping the tether near vertical. Gravity gradient effects can do this if the wire is long and heavy, or if a heavy ballast mass is used at the far end. We do not yet know how much gravity gradient force is needed for long-term stability, but we think a 200 kg ballast on a 10 km wire should provide more than enough.

Figure 3. SEDS Deployer and Payload on Delta Second Stage 3.2 Stowage and Deployment Options At least 3 different deployer designs deserve consideration: fixed spool, yoyo, and reel. NASA's ProSEDS experiment will use a fixed-spool SEDS deployer to deploy a 5 km wire. Based on recent tests at Tether Applications (TA), deployment must be kept above ~3 m/s to ensure that the wire's stiffness does not cause it to "deploy in place" like a clockspring. Tests at deployment rates <3 m/s allowed in-place deployment, and speeds <2 m/s always led to fouling and jams within a few seconds. This constraint may make a yoyo or reel-type system preferable. A yoyo simply involves throwing out a heavy reel so it unwinds and deploys. A small amount of brittle adhesive that is strong in shear but weak in peel can prevent clockspring-type deployment. (It is not clear whether such adhesive would cause more benefits or problems inside a SEDS or PMG-type deployer.) The drawback of a yoyo-type system is that it is hard to make use of a separate ballast mass, like the 200 kg MMU that is available for this use on Mir. One way to do so is to add pinch rollers and a motor to the ballast mass, so it can crawl from the near end to the far end of the wire after the wire has been deployed. This is a non-trivial added development.

These issues have led us to consider reel-type systems as used on TSS and ATEx (Advanced Tether Experiment). ATEx was launched recently but has not yet been deployed. Reel-type systems need to apply enough drag to prevent wire slackness, clockspring-type in-place deployment, and fouling. The slow response of the rotating reel to external tether dynamics requires a significant tension margin to prevent momentary slack. Brittle adhesive might help here, by reducing the tendency to deploy in place. After several km of wire is deployed, gravity gradient forces on a heavy ballast mass can provide the force needed to overcome reel drag and continue deployment, but there can be problems in the first few km. The needed reel tension could be provided by a driven pinch-roller system. This was done on TSS and ATEx. Or we could use thrust-aided deployment (also used on TSS), but this adds significant mass and complexity. Another option that may be feasible is to use 10-20 kg of propellant to establish a _-1 RPM prograde spin of Mir, and use the resulting centrifugal force on a 200 kg ballast mass to pull wire off a reel. This will despin Mir by the time 1-2 km of wire is deployed, after which gravity-gradient forces should be enough to continue deployment. Active reel control is needed to ensure the spin stops with the tether facing down rather than up. Another option is lighter, simpler, and probably more reliable than the above concepts, and uses purely passive tension control during deployment. It uses a recently-developed mini-SEDS deployer plus a fixed capstan brake to deploy the 200 kg ballast at the end of a 2 kg, 4 km non-conductive tether. This pilot tether can provide more than enough tension to pull the collector and wire off a reel. A passive eddy-current brake on the reel can provide suitable tensions and deployment rates thereafter. A small ratio of core to flange diameter will cause deployment to slow down automatically near the end, limiting the peak tension that occurs at the end of deployment. We have tentatively baselined this design for METS because of its relative simplicity and expected robustness.

3.3 Power Considerations About 1.2 kW is needed for METS to cancel out Mir's present 1 km/week decay rates by itself, and another 0.2 kW to cancel out its own drag. The tether constrains Mir's attitude, reducing solar energy collection. A conservative analysis suggests that the average power on Mir with tether attached will be 3-4 kW. Figure 4 shows the daytime power available on Mir (in amps at 27V) available with different Mir axes facing the sun.

Figure 4. Projected Daytime Power vs Year and Mir Axis Facing Sun Figure 5. Power Around Orbit with Tether Attached (& sun in orbit plane)

Figure 5 shows the power around one orbit, with Mir oriented by a tether attached to the Kvant-2 m module. By restricting unnecessary use of other systems, probably 1-1.4 kW can be made available on average: nearly enough to meet present orbit maintenance needs. If Mir is put into an unmanned storage mode, 3-4 kW should be available, allowing surplus for boosting Mir. To keep these requirements from growing before METS can be launched, all available propellant from currently planned Progress flights should be used to boost Mir until METS is operational. To compare METS to Progress flights, note that each Progress carries 880 kg of propellant. About 400 kg is needed for the Progress mission itself (boost from very low orbit to Mir, and controlled de-orbit at end of mission). This leaves 480 kg to reboost Mir. At an Isp of 290 seconds, 480 kg can give a boost impulse of 0.043 newton-year. Hence a 1.4kW electrodynamic tether that provides 0.12 newton net boosting can free up 2.7 Progresses/year of propellant, or an equivalent amount of payload for other uses, including ISS.

3.4 Attitude Control Tether tension will induce torques on Mir that will passively control its attitude in two axes. Energia specialists have considered various tether attachment points and recommend the +Y axis. This provides a stable attitude as shown in Fig. 6. This configuration is satisfactory for

prolonged Mir flight, both for power generation and for maintaining rendezvous and docking capability. Attitude control is needed about the Y-axis. Control about this axis plus 1-axis solar array pointing allows 2-axis solar tracking.

Figure 6. Mir Orientation with Tether Attached to Kvant-2 Gyrodine devices can be used for yaw control in combination with RCS. The gyrodines can also be used to damp out any attitude oscillations about the equilibrium pitch and roll angles, if METS cannot control them. Table 1 shows Mir's mass properties that drive the equilibrium orientation and oscillation frequencies caused by the tether. For the baseline METS design, with 12 newton tension at an attachment 15.5m from the CG, the oscillation periods are about 23 minutes, or _ orbit. This is long enough that orbit dynamics will perturb the oscillations. This period is intermediate between skiprope oscillations (10 minutes or less) and libration (~1 hour). The differing frequencies allow simultaneous control of all dynamic modes by modulating the power delivered to the system at suitable frequencies.

Mass Coordinates and Inertial Moments of Mir Orbital Complex

Case Nominal Mass, kg

Nominal XCM, YCM, ZCM

Nominal Ixx, Iyy, Izz

Nominal Ixy, Iyz, Izx

1 129989.9 -8.661 5420453.0 97049.1

0.244 6906603.0 8265.9

-0.512 6332481.5 187273.1

2 136989.9 -7.700 5426714.5 66646.5

0.231 9290765.0 7436.6

-0.486 8715298.0 251279.2

Remarks: Configuration 1 Includes: Central Module+Kvant(+X)+Kvant2(+Y)+Krystal(-Z)+Docking Module(-Z)+ Spekter(-Y)+ Priroda (+Z)+Soyuz(-X); Configuration 2 Includes: Central Module+Kvant(+X)+Kvant2(+Y)+Krystal(-Z)+Docking Module(-Z)+ Spekter(-Y)+ Priroda (+Z)+Soyuz(-X)+Progress(++X).

Table 1. Mir Mass Properties without and with Attached Progress-M

3.5 Mir Electrodynamic Tether System (METS) Characteristics

The key parameters that drive the METS design, and the proposed design features, control law, and effects on Mir are as follows:

Key parameters Power available: 1 kW most of the time; ~3 kW peak; ~1.4 kW average Drag area: small compared to Mir itself (Mir is ~400 m2) Stability: wire tension >> peak electrodynamic force of ~1 newton Atomic oxygen exposure: enough to require care in materials selection Stowed size: easily movable from Progress through hatches & airlocks Safety/redundancy: should be able to cut wire loose and install new one Time: it is essential to have it ready for a July 1999 launch

Key METS components 10 km anodized aluminum wire plus bare aluminum strip collector Stowage reel for collector and wire, with passive eddy-current brake Mini-SEDS deployer with ~4 km tether resistant to atomic oxygen Hollow-cathode electron emitter with controls and 2 year gas supply DC/DC step-up convertor (27V to 1-2 kV, 1-3 kW power) 200 kg ballast mass (MMU, already in position on Mir) Control and datalinks into Mir, or telemetry links directly to ground

Deployment strategy Eject 200 kg MMU ballast mass with mini-SEDS tether attached Mini-SEDS deploys ~4 km tether and then comes free, pulling on collector The deployed tether pulls the collector and then 10 km wire off the reel Eddy-current brake regulates collector and wire deployment from reel

Control of wire dynamics Wire tension provides a clear indication of in-plane libration Voltage trends provide a clear indication of out-of-plane libration Optical departure-angle sensors can provide an indication of skip rope modes Power is varied on timescales of 0.1-1 orbit to control wire & Mir dynamics Variations on other timescales should not affect wire or Mir dynamics much

Key effects on Mir Wire tension of ~12 newtons causes acceleration of 10 microgee on Mir Mir oscillations about equilibrium attitude have ~23 minute period Added aero drag due to METS increases Mir's decay rate by ~15% But reboost is equivalent to ~3 kg/day of propellant per kW of power 3.6 Preliminary Mass Budget

10 km wire plus 500 m foil collector 55 kg

Reel with eddy-current brake 10 kg

MMU ejection spring system 5 kg

Mini-SEDS deployer & passive brake Avionics package 4 kg

Hollow cathode emitter 6 kg

DC-DC converter (27V to 1kV) 3 kg

Controller & sensors 3 kg

Gas bottle and valves 9 kg

Attachment fixtures 10 kg

Cabling 5 kg

Telemetry & interface device 3 kg

15% reserve 17 kg

Total 130 kg

3.7 Other Limitations and Constraints The following subjects are for further study and assessment: Electromagnetic compatibility Radio communication restrictions Possible problems during docking and undocking of Soyuz and Progress (eg, radar reflections, tether-induced accelerations, & keep-out zones) More detailed assessment of energy generation and attitude control Automatic and manned modes of control Collector & wire optimization (mass, area, configuration, risk of cut) Deployment strategy and deployer design options High-voltage slip-ring at inner end of wire (base on TSS design?)

4. Risk Assessment The key risks on this project appear to be the following: Being too late (development delays, export delays, or fast orbit decay) Malfunction during pilot tether, collector, or wire deployment Unexpected plasma or system behavior Equipment failure (hardware or software) Pilot tether or wire cut by micrometeoroid or debris Wire failure like TSS-1R (thermal run-away after insulation breach) A concerted effort is needed to reduce the first risk, and careful design and testing to reduce the rest. The cut risk might best be dealt with by using 2 wires or a wide flat wire (which increase aerodynamic drag a few percent), or by launching a second deployer on a later flight, once METS proves itself. Small micrometeroids will breach the insulation frequently, but sustained arcing and thermal runaway seem unlikely because of 3 design differences from TSS: 1) our wire polarity is opposite that of TSS (electron collection rather than emission); 2) the voltage is only half as high; and 3) our wire plus insulation should generate far less volatiles than the TSS wire did when the insulation was breached. But analysis and lab tests are necessary to see if this is an issue. If it is, we may have to reduce the wire length and hence voltage, and add more collector area to collect the higher current needed when the EMF is reduced.

5. Project Outline 5.1 Tentative Scenario All the necessary equipment must be designed, fabricated, tested, and integrated within a few months. It would be delivered to orbit on a Progress cargo craft in July 1999. Onboard Mir, the crew will install and configure a manual controller and some other equipment inside the Kvant-2 module. Then during an EVA, cosmonauts will move the rest of the equipment through the airlock of the Kvant-2 module and mount it outside, near the airlock. Figure 7 on the next page shows the outside of the airlock, plus a 200 kg MMU (manned maneuvering unit) that can be used as a ballast mass to stabilize METS. Much of the METS hardware might mount on the MMU docking fixture, since the tether has to depart from the end of that fixture anyway to keep from fouling on it. Then the cosmonauts will mount an ejection spring

system on the MMU and check everything out. Once back inside, the crew will finish system checkout, and ground control will verify the procedure. Then the crew will trigger release of the MMU. The crew and ground control will both monitor tether and wire deployment and initial system performance. METS can send telemetry data to earth directly, since Kvant-2 will always face the earth. The METS controller can use the wire voltage, tension, and departure angle (relative to Mir) plus an on-board dynamic model to sense libration state, skiprope, and Mir oscillations. It will then modulate the power delivered to METS to damp these dynamics. The controller will also allow manual control by the crew from inside Kvant-2.

Figure 7. Kvant-2 Airlock and Docked MMU

5.2 Schedule Proposal Preliminary milestones for METS are shown below:

Jan. 1999 Design+Analysis

Feb. 1999 Manufacturing

Mar. 1999 Development

Apr. 1999 Testing,Delivery

May 1999 Preflight Test

July 1999 forward Launch, Configuring EVA1

August 1999 forward Testing, Activating, Operation

5.3 Cost Estimate (includes complete set of spare flight hardware)

1. Preliminary Design and Analysis ($K) 1.1 Wire, reel, and brake prototype design, fab & test (TA; $K) 1.2 Near-end avionics design (power, sensors, controls, layout) (TA; $K) 1.3 Control strategy analysis (TA & Eugene Levin; $K)

1.4 Plasma interactions analysis (SAO et al; $K) 1.5 On-orbit configuration & operations analysis and design (Energia SRC; $K)

2. Long-lead Items ($K) 2.1. Hollow-cathode emitter assemblies (TA; $3K with expedited order) 2.2. Sensors and control computers (TA and/or Energia SRC, $K) 2.3. DC/DC convertors, repackaged and tested for use in space (TA; $K?) 2.4. Atomic-oxygen tolerant tethers for mini-SEDS deployer (Triton, $K?) 2.4. Energia interface hardware (cabling, brackets, etc.) (Energia SRC; $K)

3. System Design, Fabrication, Test, and Delivery ($) 3.1. Wires, collectors, reels, brakes, and mini-SEDS assemblies (TA; $K) 3.2. Controller software development and avionics testing (TA; $K) 3.3. Near-end package design, fab, & test (Energia SRC, TA or other; $K)

4. Flight Operations and Analysis ($) 4.1. Integration and test at Energia ($K) 4.2. Launch of 130 kg at $K/kg ($K) 4.3. Installation and checkout on Mir ($K) 4.4. Operations cost contribution (partial cost, $K) 4.5. Performance analysis ($K total for TA, SAO, & Energia)

5.4 Recommended Funds Commitment Schedule 1 & 2: $K/week to start work and orders, until review 3-5 weeks from now Rest of funds committed at end of review and available as costs incurred 3: Pay % as costs are incurred, 10% at delivery, and 10% when in use 4: Pay % at T-8 weeks, 40% at T-3 weeks, and 20% when in use Performance analyses: pay when completed

6. Next Steps We recommend this Technical Proposal and more specifically tasks 1 and 2 listed in section 5.3 as a basis for planning and budgeting the initial effort. Given a planned launch date of July 1999, and a need for added Progress flights for reboost if this date slips, we probably need an informed Go/No-go decision by the end of January at the latest. We will need to order some long-lead items before then, with partial funding commitments for some of those items. To do this we recommend beginning these tasks immediately (ie, by December 28): Energia Ltd. and Tether Applications should start work on tasks 1-2. FINDS should try to enlist participation and support by RSA and NASA Tether Applications should apply for a hardware and data export license

7. Other Options Considered In the course of this study we considered various other reboost options as supplements or alternatives to electrodynamic reboost. Four concepts in particular are worth discussion:

7.1 Drag Tethers to Reduce Progress De-orbit Requirements

Reducing the propellant needed to de-orbit each Progress allows more propellant to be used reboosting Mir (and later, ISS). Tethers can provide the drag in several ways. One is to use a self-powered electrodynamic tether as a drag device. A simpler alternative is to use a long downward-deployed tether as a neutral drag device. Progress uses ~200 kg for de-orbit. The tether may replace only ~2/3 of this, and it weighs something itself, so the net benefit of the tether is likely to be <100 kg/flight. In addition, ground control costs while each Progress decays to low altitude may be substantial.

7.2 Tethered Momentum Transfer from Progress to Mir A Progress deboost option that does not require long mission times is to deploy Progress on a tether below Mir and release it into a lower energy orbit. This concept was demonstrated on SEDS-1 in 1993 with a 26 kg payload. The same size SEDS deployer could be used here, witha shorter heavier tether similar to that flown on TiPS. It would allow a 35% reduction in the de-orbit impulse required for Progress. This could instead be spent reboosting Mir before the Progress leaves. Momentum transferred to Mir by the tether would double the boost benefit. The SEDS deployer plus brake, computer, and interface hardware weigh ~15 kg, so the net benefit should be ~125 kg/flight. Suitable SEDS hardware is on hand, so the delivery times will be driven by export license and integration lead-times. (The same concept might also be used to reboost ISS during assembly, before ISS has to meet the attitude and microgravity specs promised to users.) It may be feasible to deploy Progress on SEDS even after METS is deployed, but our ideas on how to keep the two tethers from fouling need to be verified by detailed simulations. SEDS can also be scaled up 2X in linear dimensions and still handled on the existing SEDS winding hardware. This allows 8X more tether mass, which in turn allows use of a 2.8X longer tether with 2.8X the strength. This size SEDS deployer would allow Progress to be de-orbited entirely by tether, allowing all available Progress propellant to be spent reboosting Mir. The benefit is ~400 kg, but the scaled up deployer plus support hardware weigh ~80 kg, leaving a net benefit of ~320 kg. (This allows a 40% reduction in Progress flight frequency for reboost.) The scale-up is straightforward, but the deployer could not be ready until the planned July 1999 launch at the earliest. It could be used on all flights thereafter, if detailed simulations verify that we can reliably deploy SEDS without fouling it on METS.

7.3 Conventional Electric Propulsion Another option considered is to mount a Fakel stationary plasma thruster or similar high-Isp electric propulsion system on Progress, so any available power can be used for reboost. This provides only 40-50% the thrust per kW of an electrodynamic tether, but if suitable hardware is on hand, such a system might be installed on earlier flights than electrodynamic tethers can be, allowing some electric reboost several months earlier. If 4 kW-months were available at a specific power of 20 kW/newton, the boost is worth 185 kg, less the propellant and tank mass (~30 kg) and engine mass (? kg). This concept could also be a useful test for ISS reboost, because electric thrusters on Progress could use off-peak ISS power to provide some ISS reboost, reducing required Progress/ISS flight frequencies.

7.4 Reducing Mir's Decay Rate Finally, some operational changes might reduce Mir's orbit decay rates, at least until METS is operational. One is to use Progress reboost propellant to reboost Mir early in each Progress mission, rather than near the end of the mission. Another is to orient Mir and its solar arrays to provide just-adequate power while reducing drag. 7.5 Notes on Altitude Strategy Mir and ISS altitudes must be high enough to limit aerodynamic drag, while remaining low enough for vehicles visiting them to have adequate payload. The optimum altitude is ~350 to 450 km depending on the solar cycle (which affects atmospheric densities and hence drag) and traffic level (with higher traffic justifying a lower orbit). Mir is now ~40 km below the optimum altitude, and decaying ~1 km/week. (And the optimum altitude will increase over

the next 2 years, as solar maximum increases densities in the upper atmosphere.) Allowing free decay near the end of life eliminates any need for reboost propellant, and the lower mission altitude increases the payloads of the remaining Progress and Soyuz flights. But if Mir is to be kept in orbit for several more years, it should be boosted at least 40 km, andmore than that if flight rates to Mir are reduced. Boosting 40 km takes 1,100 kg of propellant (2.3 Progress flights), or 10 months of thrust by METS at 1.4 kW. Providing both would double the boost. This allows Mir to become coplanar with ISS within a few years, allowing easy transfer of useful hardware to ISS, and mutual use of ISS and Mir as safe havens for eachother.

8. Conclusions This study shows that the proposed concept should make it possible to maintain and boost Mir's orbit for prolonged times, at a fraction of the cost of conventional methods. Developing and flying METS will give unique data relevant to many other high-value applications, including ISS reboost. The other options identified in this Technical Proposal appear to have a wide variety of useful operational applications, including several methods of enhancing Progress performance to Mirand the ISS, and several ways to provide backup reboost capability for the ISS. We recommend that work start immediately on METS, and on tethered momentum transfer from Progress to Mir using SEDS.

V. Syromiatnikov, J. Carroll, O. Saprykin, M. Fennell

contents copyright 2000

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