Using zenox crystals as a means of temporal power, the Time Tanks could be projected back in time and return to their starting point after a couple of hours of recharging. These tanks were initially used one at a time for raids on the fourteenth segment of Aoris's history from the twenty-fourth segment, allowing the future residents to take rare animals and zenox crystals from the "age of greed".
When the TARDIS passed by a Tank in the Vortex, the sensors indicated that the vehicle was on a fixed path through the Vortex, but this was enough for it to briefly intersect with the Doctor's ship, the brief encounter causing it to temporarily materialise inside the TARDIS and depart with Romana II inside it with no harm to either ship.
When the Fourth Doctor unintentionally revealed that it was possible to change the future, Drang attempted to change history by sending a mass of Time Tanks back to conquer the past, but inside agents were able to sabotage the assault until the Doctor was able to use a time machine he had developed in the past to send the tanks into the future, with their power sources all expended. The tanks were abandoned in the future when most of the population were projected into the even further future to allow their planet to regenerate. (AUDIO: The Paradox Planet / Legacy of Death)