Coulumb Drag Devices: Electric Solar Wind Sail Propulsion and Ionospeheric Deorbiting
* Plasma brake thrust is 16 times larger in pure oxygen plasma than in pure proton plasma
* There is an altitude dependence. Below 700 km the thrust would continue to increase until 400-500 km (provided there is the right hardware design)
A charged tether or wire experiences Coulomb drag when inserted into owing plasma. In the solar wind the Coulomb drag can be utilised as efficient propellantless interplanetary propulsion as the electric solar wind sail (electric sail, E-sail). In low Earth orbit (LEO) the same plasma physical effect can be utilised for efficient low-thrust deorbiting of space debris objects (the plasma brake). The E-sail is rotationally stabilised while the deorbiting Coulomb drag devices can be stabilised either by spinning or by Earth's gravity gradient. According to numerical estimates, Coulomb drag devices have very promising performance figures, both for interplanetary propulsion and for deorbiting in LEO. Much of the technology is common to both applications. E-sail technology development was carried out in ESAIL FP7 project (2011-2013) which achieved TRL 4-5 for key hardware components that can enable 1 N class interplanetary E-sail weighing less than 200 kg. The thrust of the E-sail scales as inverse solar distance and its power consumption (nominally 700 W/N at 1 au) scales as the inverse distance squared. As part of the ESAIL project, a continuous 1 km sample of E-sail tether was produced by an automatic and scalable \tether factory". The manufacturing method uses ultrasonic wire to wire bonding which was developed from ordinary wire to plate bonding for the E-sail purpose. Also a \Remote Unit" device which takes care of deployment and spin rate control was prototyped and successfully environmentally tested. Our Remote Unit prototype is operable in the solar distance range of 0.9-4 au.
E-sail has potentially revolutionary performance level in comparison to other propulsion systems. The tether weighs only 10 grams per kilometer and produces a thrust of 0.5 mN/km at 1 au distance. The E-sail thrust scales as proportional to 1/r where r is the solar distance. The reason is that while the solar wind dynamic pressure decays as 1/r^2, the plasma Debye length (by which the electric field penetration distance and hence the virtual sail size scales) varies as r, thus giving an overall 1/r dependence for the thrust. For example, hundred 20 km long tethers would weigh 20 kg and they would produce 1 N thrust at 1 au which gives a 30 km/s velocity change per year for a 1000 kg spacecraft.
Our current plan for the next step is to fly a 3U CubeSat experiment (ESTCube-3) in solar wind intersecting orbit which measures the E-sail effect with a 1 km long tether using 5-10 kV voltage. Because launch opportunities to solar wind intersecting orbit (for example a lunar orbit) are less frequent than ordinary LEO CubeSat launches, to ensure mission success we plan to prove the satellite's technologies by first flying an identical satellite (ESTCube-2) in LEO. ESTCube-2 also naturally demonstrates a 1 km long plasma brake. After ESTCube-3, we will have a measurement of the strength of the E-sail effect in the actual environment (solar wind), a demonstration of deploying a 1 km long tether and a demonstration of using the E-sail effect for spacecraft propulsion. After ESTCube-3, we need an E-sail \pathfinder" mission (comparable to SMART-1 in its philosophy) which tests the use of E-sail propulsion for going to some target and carries some payload. For example, it could be a NEO mission equipped with imaging instruments
Fast E-sail Uranus entry probe mission
The electric solar wind sail is a novel propellantless space propulsion concept. According to numerical estimates, the electric solar wind sail can produce a large total impulse per propulsion system mass. Here we consider using a 0.5 N electric solar wind sail for boosting a 550 kg spacecraft to Uranus in less than 6 years. The spacecraft is a stack consisting of the electric solar wind sail module which is jettisoned roughly at Saturn distance, a carrier module and a probe for Uranus atmospheric entry. The carrier module has a chemical propulsion ability for orbital corrections and it uses its antenna for picking up the probe's data transmission and later relaying it to Earth. The scientific output of the mission is similar to what the Galileo Probe did at Jupiter. Measurements of the chemical and isotope composition of the Uranian atmosphere can give key constraints to different formation theories of the Solar System. A similar method could also be applied to other giant planets and Titan by using a fleet of more or less identical probes.
Our proposed E-sail Uranus entry probe mission consists of three modules which are initially stacked together: the E-sail module, the carrier module and the entry module. The entry module is composed of the atmospheric probe inside a heatshield. The stack is initially launched to Earth escape orbit by a conventional booster. The E-sail module accelerates the stack to Uranus intercepting trajectory. The carrier module performs the necessary orbital corrections to y by Uranus and to collect the data transmitted by the atmospheric probe. In more detail, the mission proceeds according to the following steps:
1. The stack is launched to Earth escape orbit by a conventional booster. Any escape orbit (i.e. any orbit with non-negative specific energy parameter C 3 ) is suitable for the purpose. For the E-sail to work, it is required to be in the solar wind.
2. The E-sail module accelerates the stack to a trajectory towards Uranus.
3. The E-sail module is abandoned approximately at Saturn distance.
4. The carrier module uses chemical propulsion (in this paper baselined as green monopropellant) for orbital corrections.
5. About 13 million km (8 days) before Uranus, the carrier module detaches itself from the entry module and makes a ~ 15 km/s transverse burn so that it passes by the planet at ~ 10^5 km distance, safely outside the ring system. Also a slowing down burn of the carrier module may be needed to optimise the link geometry during flyby.
6. Protected by the heat shield, the entry module enters into atmosphere.
7. A parachute is deployed and the heat shield is separated
8. The probe falls under parachute in Uranus atmosphere, makes scientific measurements and transmits data to the high gain antenna of the carrier which flies by at ~10^5 km distance.
9. After exiting Uranus environment, the carrier redirects its high gain antenna towards Earth to transmit the stored probe science data