While similar to TARDISes, SIDRATs were somewhat less developed space-time travelling machines. The Second Doctor found it wasn't difficult for him to understand the operation of the machines. Like TARDISes, SIDRATs were larger on the inside than the outside, but unlike TARDISes, the dimensions of their interiors could be changed at will. According to Jamie McCrimmon, they were green in colour.
The Time Lords discounted SIDRATs as viable space-time transports due to their short range and power costs. From the outside they resembled featureless rectangle boxes, similar in appearance to a form sometimes used by TARDISes while on Gallifrey (possibly a "default" exterior for TARDISes). The War Lords employed them both to abduct human soldiers for the war games, and also to travel back and forth from the game area to their base. (TV: The War Games) They were operated by remote control.
The supply of SIDRATs to the War Lords was the War Chief's primary contribution to their plan. The War Lords were dependent on the War Chief for the secrets of space-time travel. His importance was reflected in his rank: second only to the War Lord himself. To force his compliance, the War Chief trapped the Doctor within a SIDRAT and shrank its inner dimensions until the Doctor and Jamie were nearly crushed. Later the Doctor confronted the War Chief with a revelation: a timeship with a malleable interior and remote control capabilities was inherently unstable. The War Chief's SIDRATs would never last long enough to carry out a galactic conquest. The War Chief admitted he knew of the flaw in the plan and demanded the Doctor's help, promising him half the galaxy after he betrayed his allies, and death at their hands if he refused.
By the time the Doctor and his allies managed to successfully stop the war games, it turned out that all but two of the SIDRATs had ceased functioning, and the remaining ones would not last long enough to transport the thousands of soldiers back to their original time zones. One of the still-functional SIDRATs was subsequently brought to Gallifrey by the War Lord's loyalists in an abortive attempt to rescue their master.
Behind the scenes
- SIDRAT is "TARDIS" spelled backwards.