Enrico C. Lorenzini

Bare Tethers for Electrodynamic Spacecraft Propulsion

Electrodynamic tether thrusters can use the power provided by solar panels to drive a current in the tether and then the Lorentz force to push against the Earths magnetic field, thereby achieving propulsion without the expenditure of onboard energy sources or propellant. Practical tether propulsion depends critically on being able to extract multiamp electron currents from the ionosphere with relatively short tethers (10 km or less) and reasonably low power. We describe a new anodic design that uses an uninsulated portion of the metallic tether itself to collect electrons. Because of the efficient collection of this type of anode, electrodynamic thrusters for reboost of the International Space Station and for an upper stage capable of orbit raising, lowering, and inclination changes appear to be feasible. Specifically, a 10-km-long bare tether, utilizing 10 kW of the space station power could save most of the propellant required for the station reboost over its 10-year lifetime. The propulsive small expendable deployer system experiment is planned to test the bare-tether design in space in the year 2000 by deploying a 5-km bare aluminum tether from a Delta II upper stage to achieve up to 0.5-N drag thrust, thus deorbiting the stage.

Propulsive Small Expendable Deployer System Experiment

Relatively short electrodynamic tethers can extract orbital energy to “ push” against a planetary magnetic e eld to achieve propulsion without the expenditure of propellant. The Propulsive Small Expendable Deployer System experiment will use the e ight-proven Small Expendable Deployer System to deploy a 5-km bare aluminum tether from a Delta II upper stage to achieve o 0.4-N drag thrust, thus lowering the altitude of the stage. The experiment willuseapredominantly baretetherforcurrentcollection in lieuoftheendmasscollectorandinsulated tetherused on previous missions. The e ight experiment is a precursor to a more ambitious electrodynamic tether upper-stage demonstration mission that will be capable of orbit-raising, -lowering, and -inclination changes, all using electrodynamic thrust. The expected performance of the tether propulsion system during the experiment is described.

Electrodynamic Tether at Jupiter—I: Capture Operation and Constraints

Tethered spacecraft missions to the Jovian system suit the use of electrodynamic tethers because: 1) magnetic stresses are 100 times greater than at the Earth; 2) the stationary orbit is one-third the relative distance for Earth; and 3) moon <i>Io</i> is a nearby giant plasma source. The (bare) tether is a reinforced aluminum foil with tens of kilometer length <i>L</i> and a fraction of millimeter thickness <i>h</i>, which collects electrons as an efficient Langmuir probe and can tap Jupiters rotational energy for both propulsion and power. In this paper, the critical capture operation is explicitly formulated in terms of orbit geometry and established magnetic and thermal plasma models. The design parameters <i>L</i> and <i>h</i> and capture perijove radius <i>r</i> _p face opposite criteria independent of tape width. Efficient capture requires a low <i>r</i> _p and a high <i>L</i> ^3/2/<i>h</i> ratio. However, combined bounds on tether bowing and tether tensile stress, arising from a spin made necessary by the low Jovian gravity gradient, require a high <i>r</i> _p and a low <i>L</i> ^5/2/<i>h</i> ratio. Bounds on tether temperature again require a high <i>r</i> _p and a low <i>L</i> ^3/8/(tether emissivity)^1/4 ratio. Optimal design values are discussed.

Exploration of outer planets using tethers for power and propulsion

a = radius of circular orbit a(max) = radius at mechanical-energy maximum ast = radius of stationary orbit B = planetary magnetic field Em = motional electric field ht = thickness of tape tether I = current on tether Iav = length-averaged tether current Lt = tether length Msc = overall spacecraft mass mt = tether mass Npl = ionospheric electron density RJ = radius of Jupiter r = radial distance rper = radius at perijove T = tether temperature Te = ionospheric electron temperature vorb = orbital velocity vpl = ionospheric plasma velocity vrel = relative velocity vorb − vpl v∞ = hyperbolic excess velocity wt = width of tape tether e = tether emissivity emech = mechanical energy ρAl = aluminum density σAl = aluminum conductivity σB = Stefan–Boltzmann constant

Propulsive Small Expendable Deployer System (ProSEDS) space experiment

ProSEDS is a secondary (i.e. piggyback) payload on a Delta-11 GPS 8 mission scheduled for launch in August 2000. It will test the feasibility of generating generate electrodynamic thrust without propellant using a 5 kilometer conducting wire (tether). The ProSEDS obtains thrust as the tether cuts across the magnetic field, a voltage is induced across the wire. Electrons are attracted to the positively based far end of the wire. Electrons flow downward through the conductive tether. Earths magnetic field exerts a drag force on the current in the tether segments, that is mechanically transferred via the wire to the stage. The primary objective for the ProSEDs mission is to demonstrate that a significant, measurable electrodynamic thrust through a tether in space. The primary mission will last one day, as the primary battery assures at least three orbits of data will be collected, the remaining power will be provided by the secondary battery, which uses tether generated power to recharge. The extended mission begins using the power provided through the tether, and wil terminate when a system ceases to function; (i.e., either degradation of the tether,through Atomic Oxygen contact, a micrometeoroid or other debris impact, or another malfunction.) The technology has many potential applications. Amongst the applications, which are reviewed in detail, are: (1) satellite deorbit, (2) reboost of the International Space Station, (3) propellantless reusable Orbit Transfer Vehicles, (4) Propulsion and power generation for future Jovian missions.

Dynamics and control of the tether elevator/crawler system

This paper investigates the dynamics and acceleration levels of a new tethered system for micro- and variable-gravity applications. The system consists of two platforms tethered on opposite sides to the Space Station. A fourth platform, the elevator, is placed in between the Space Station and the upper platform. Variable-g levels on board the elevator are obtained by moving this facility along the upper tether, while microgravity experiments are carried out on board the Space Station. By controlling the length of the lower tether the position of the system center of mass can be maintained on board the Space Station despite variations of the systems distribution of mass. The paper illustrates the mathematical model, the environmental perturbations and the control techniques which have been adopted for the simulation and control of the system dynamics. Two sets of results from two different simulation runs are shown. The first set shows the system dynamics and the acceleration spectra on board the Space Station and the elevator during station-keeping. The second set of results demonstrates the capability of the elevator to attain a preselected g-level.

Electrodynamic Tether at Jupiter—II: Fast Moon Tour After Capture

An electrodynamic bare-tether mission to Jupiter, following the capture of a spacecraft (SC) into an equatorial highly elliptical orbit with perijove at about 1.3 times the Jovian radius, is discussed. Repeated applications of the propellantless Lorentz drag on a spinning tether, at the perijove vicinity, can progressively lower the apojove at constant perijove, for a tour of Galilean moons. Electrical energy is generated and stored as the SC moves from an orbit at 1 : 1 resonance with a moon, down to resonance with the next moon; switching tether current off, stored power is then used as the SC makes a number of flybys of each moon. Radiation dose is calculated throughout the mission, during capture, flybys and moves between moons. The tour mission is limited by both power needs and accumulated dose. The three-stage apojove lowering down to Ganymede, Io , and Europa resonances would total less than 14 weeks, while 4 Ganymede, 20 Europa, and 16 Io flybys would add up to 18 weeks, with the entire mission taking just over seven months and the accumulated radiation dose keeping under 3 Mrad (Si) at 10-mm Al shield thickness.

Methodology and instrumentation for testing the weak equivalence principle in stratospheric free fall

The use of the GiZero free-fall facility for testing the weak equivalence principle is discussed in this article. GiZero consists of a vacuum capsule, released from a balloon at an altitude of 40 km, which shields an experimental apparatus free falling inside the capsule itself. The expected residual acceleration external to the detector is 10−12 g (with g the Earth’s gravitational acceleration) for the 30 s free fall. A common-mode rejection factor of about 10−4 reduces the residual noise differential output to only 10−16 g. The gravity detector is a differential accelerometer with two test masses with coincident center of masses (i.e., zero baseline) with capacitive pick ups. Preparatory experiments have been conducted in the laboratory with a precursor detector by measuring controlled gravity signals, at low frequency, and by observing the Luni-Solar tides. The estimated accuracy in testing the weak equivalence principle, with a 95% confidence level, is 5×10−15 in a 30 s free fall. When compared to orb...

Test of the weak-equivalence principle in an Einstein elevator

SummaryA technique for testing the weak-equivalence principle is presented. This technique involves the measurement of differential accelerations between two test masses of different materials (e.g., aluminum and gold) free falling inside a 3 m long cryostat dropped from a 40 km altitude balloon. The free-fall duration is 30 s for a non-propelled cryostat. The falling test masses are part of a high-sensitivity differential detector with a foreseeable sensitivity in detecting differential accelerations of about 1.5·10−13