Hold balance and the gimbal

The game does not model this yet.

This mechanic has no numbers read from the code.

• A rocket pushes along its thrust axis. If that axis passes through the center of gravity, the ship speeds up in a straight line.
• If the axis misses the center of gravity, the ship spins around it, which is called rocket tumble.
• So the center of gravity must sit on the central axis, and cargo must be spread evenly around that axis.
• Items that can be dropped, such as tanks and cargo, go in pairs on opposite sides of the spine and leave in pairs.
• If one side leaves alone, the center of gravity moves off the thrust axis, and the next burn starts a spin.
• A gimbaled engine can swing its thrust a few degrees, and a crew can use that to cope with cargo that shifts.
• A small ship can also carry trim tanks, which are small tanks of water that are pumped to adjust the balance.
• Heavy gyroscopes help, but there is a limit to how much imbalance they can fix.

• The game does not model this yet. The code has no model of cargo center of mass, drive gimbal or trim tanks.
• A hold is one stack of lots in loading order, with no position off the axis. A jettison removes a whole lot with no effect on balance.
• Hold mass changes acceleration and turning only as a total, never as a position.
• The only gimbal in code is the keel gun's lay cone, which is a gun mount and not the drive.
• The plan is as follows. The hold is a ring of paired sections around the spine. Balance costs something on loading and on jettison. The gimbal is the first fix, and its price is steering margin. Past the gimbal limit the order is trim water, then a paired jettison. A damaged drive loses its gimbal.
• The plan says to measure first: if every loading fits inside the gimbal, balance is a tax with no decision, and the design must say so.
• The gimbal actuators and trim pumps are to register as essential loads that the power rule never sheds, when the game builds them.

1. Atomic Rockets does not provide values for the following:
   1. Hold imbalance a gimbal can take. The limit depends on the hull.
   2. Who loads and balances a ship. The crew or the yard loaders do it, never the captain. The rule is authored.
   3. Power draw of a gimbal actuator or a trim pump.

• Atomic Rockets, basicdesign, Rocket Tumble: When the thrust axis misses the center of gravity the ship spins. Some engines gimbal a few degrees, and a crew can use that in an emergency when cargo shifts. The center of gravity should sit on the central axis, and every deck should be radially symmetric. The Grumman Space Tug gimbals plus or minus 20 degrees after it drops a drop tank. Small ships may have trim tanks, and gyroscopes can only fix so much imbalance. The problem is worse on tiny ships than on large ones. (all uses)
• Atomic Rockets, basicdesign, Saddle Truss: Jettisonable items must sit in pairs on opposite sides of the spine and be dropped in pairs. A NASA saddle truss avoids the pairs by cocking the thrust axis a fraction of a degree off center, so a tank can be kicked out through the open side. "it's pinwheel time" (all uses)
• Atomic Rockets, controldeck, Gimbaled Engines: A gimbaled engine can pivot its thrust off center. The old Reusable Nuclear Shuttle design had a NERVA engine that could pivot 3 degrees in any direction, at no more than 0.25 degrees a second. "up to three degrees off-center in any direction" (all uses)
• Atomic Rockets, controldeck, Atomic Rocket Pilot Console: An engine is limited to vectoring its thrust to about ten degrees off the axis. (all uses)
• UK MCA, MSIS003 Part 7: Electrical Equipment and Installations, 7.6 (essential services): Load shedding must not cut services essential for propulsion and safety. The list includes steering gear, which is the nearest real-world rule for a gimbal actuator. (all uses)