Jukka Ukkonen

Invited Article: Electric solar wind sail: Toward test missions

The electric solar wind sail (E-sail) is a space propulsion concept that uses the natural solar wind dynamic pressure for producing spacecraft thrust. In its baseline form, the E-sail consists of a number of long, thin, conducting, and centrifugally stretched tethers, which are kept in a high positive potential by an onboard electron gun. The concept gains its efficiency from the fact that the effective sail area, i.e., the potential structure of the tethers, can be millions of times larger than the physical area of the thin tethers wires, which offsets the fact that the dynamic pressure of the solar wind is very weak. Indeed, according to the most recent published estimates, an E-sail of 1 N thrust and 100 kg mass could be built in the rather near future, providing a revolutionary level of propulsive performance (specific acceleration) for travel in the solar system. Here we give a review of the ongoing technical development work of the E-sail, covering tether construction, overall mechanical design alternatives, guidance and navigation strategies, and dynamical and orbital simulations.

One kilometer (1 km) electric solar wind sail tether produced automatically

We produced a 1 km continuous piece of multifilament electric solar wind sail tether of μm-diameter aluminum wires using a custom made automatic tether factory. The tether comprising 90,704 bonds between 25 and 50 μm diameter wires is reeled onto a metal reel. The total mass of 1 km tether is 10 g. We reached a production rate of 70 m/24 h and a quality level of 1‰ loose bonds and 2‰ rebonded ones. We thus demonstrated that production of long electric solar wind sail tethers is possible and practical.

Determining the quality of space tether in a nondestructive manner

We propose a nondestructive method to determine the quality of each bond in an electric solar wind sail (E-sail) tether. The method is verified by a method similar to the standard destructive pull test [1]. The setup that we built for the proposed in-line tether quality measurement comprises a custom-built ultrasonic bonder, a laser doppler vibrometer, an ultrasonic generator, contact resistance measurement electronics, and a laser-ultrasonic device. During the bonding process the setup continuously measures voltage and current driving the ultrasonic transducer, the bonding lower wedge displacement, the contact resistance of the bond interface and a laser-induced high frequency pulse transmission through the bond interface. The post-production analysis results are correlated with the destructive bond pull test results to identify a target signal path. Staying on this path should ensure strong bonds. This work is part of our efforts towards the produce to specification concept in the ESAIL programme.

Space tether produced to strength specification

A non-contact method to determine the quality of an ultrasonic weld was developed for wire-to-wire bonding purposes. Relative wire movement during the bonding process was monitored with a laser-doppler-vibrometer. This movement provides information about the bond development. Such information permits predicting the final quality of the bond. Signals from N=5000 bonds were analyzed to find tell tale characteristics that predict the final strength of the bond. A bond that eventually will end up failed (un-attached) can be recognized as early as 2 ms into the bonding process. A distinction between poor quality (maximum sustainable pull-force below 8 g) and good quality (pull-force above 10 g) bonds can be recognized within 6 ms from the bonding start. A real-time feedback system was implemented to reduce the uncertainty in final bond quality and to actively augment the quality during the bonding process. The purpose of this study was to confirm that real-time quality assurance (produce to specification) is feasible in wire-to-wire bonding. This work is part of our efforts towards the produce to specification concept in the ESAIL programme.