Here are the rules for Ork Roks in Battlefleet Gothic that were originally published in the North American edition of White Dwarf #238. If you can find a copy of the issue, pick it up as it contains neat pictures and background stuff to go along with the rules.

You may include any number of Ork Roks in your fleet.

Ork Roks may also be purchased with points for planetary defenses, in scenarios where they are used.

SPECIAL RULES:

Roks are sufficiently different to both Orbital Defences and true spaceships to require a number of special rules. However any rules which are not specifically noted as being modified below, apply in full to Roks. For example, Roks which lose half their damage are crippled just like other vessels.

Movement:

Because they are basically large lumps of rock with engines studded all over them, Roks donХt move in the same way as normal ships. In their movement phase Roks travel 10cm forward in a straight line, no more, no less. They may only turn using All Ahead Full Orders. Roks may not turn or use Burn Retros or Come To New Heading orders.

On All Ahead Full orders Roks move an extra 2D6cm in any direction at the end of their move. If this causes them to move 10cm or more in a different direction to the one they are travelling in, this becomes their new direction of travel. This can be seen more clearly in the diagrams below.

Due to their low speed and considerable momentum, Roks which are crippled or moving though Blast markers do not reduce their speed. Roks in the gravity well of a planet or moon may make free turns like an ordinary ship (45 degree) and/or place themselves in a stationary or low orbit.

Critical Hits: Roks lack the complex systems of true space craft, so critical hits are not rolled on the Critical table. Instead each critical hit inflicts +1 point of damage.

Catastrophic Damage: When a Rok is reduced to 0 damage it breaks up. Do not roll for Catastrophic damage, instead the Rok is replaced by 4 blast markers.