Why is radiating heat a no-go? All the spacecraft ever built used radiators to dump heat. See two the radiators of the International Space Station.

Because it's boring. I hope that the answers will also tell me what conditions would lead to this being necessary.

Given that there is no stealth in space, it rather won't change, and radiators would be a way to go.

The only reason I can think of to store heat in the ship is in order to use it to run some other on-board system, or convert it back into electrical energy. However, energy can't be a big problem for you, since your tech is already advanced enough to allow you to travel between worlds. In other words, I can't think of a good reason to store it.

It might be helpful if you explain the narrative necessity for disposable heat sinks. (ie. do you need to put a ship & crew in jeopardy; specifically the result of an overheated heat sink and the lack of an unused one?)

For "plot purposes" you could easily handwave a canister of "liquid cold" that greedily absorbs heat, but eventually hits a critical temp and needs to be discarded, either because it is now "burned out" or, more dramatically, might go critical and explode! But I'm not sure there are any actual materials that would operate like this.

@JasonK instant cold packs are often found in first aid kits. They use an endothermic chemical reaction absorb heat and many are not able to be reused.

Not enough for an actual answer, but radiators are ridiculously vulnerable to enemy fire. They have a huge heat signature. This means that ejectable heat sinks will be necessary, at least as backups, for ships that might be in combat.

need to dump extra heat....why? some sort of system failure. some other unusual event. but it would probably be done by venting bulk coolant as gas, rather than jettisoning solid objects. The empty tanks could be jettisoned soon after, if there is reason to do so.

They have these in elite:dangerous. In that setting, all sensors are thermal-based so ejecting a hot thing while cooling yourself can decoy/mask your signature.

In The Crypt by Scott Sigler, a scout ship needed to contain its heat to prevent detection by the enemy. This meant minimising computer activity as well as a lot of discomfort for the crew.

Heat is the result of friction which is produced by a reaction, aha what about reactionless drive hee hee...

@SPavel You asked for a reality-check, and when people give you the realistic answer you say "meh, boring". Eh hm... did you want the realistic answer or not? The only conceivable reason for using the incredibly cumbersome notion of disposable heat sinks is if there is a need for stealth.... to keep the skin of your ship at or near the radiative temperature of the background. The only problem is of course that every time you pop a glowing hot heat sink, you leave a brightly shining bread-crumb, leading directly to the ship.

@MichaelKarnerfors A reality check on whether disposable heat sinks work is not the same thing as a reality check on whether or not there are better options.

@SPavel Teal Deer says: it is possible, but serves little purpose and is so impractical and cumbersome that no sensible character ever would do it unless the need for stealth is such that it outweighs these factors.

The answers have already told me that and more, thanks.

You mentioned Mass Effect 2 - the game which introduced the idea of cooling spaceships via ejecting a mist of hot liquid metal (gallium?). That radiates the heat away, and is then collected for reuse if possible. Close, but the idea there is to increase the radiation surface area, not to discard mass which is hard to come by. As for heating stuff meant to be discarded, pass it through your ion thruster when you're done - it needs some mass anyway.

One reason to want to eject heat stored in solid object (and the only one I can think of): Stealth. Paint your spaceship's hull black, cool down to 2.73K and add in some fancy Anti-Radar stealth tech and you'll be pretty much invisible. However, you'll have to periodically eject superheated "bullets" to carry away your excess heat (cooling your hull close to absolute zero produces a lot of heat). For added stealth, you can put your super-heated bullets inside their own little 2.73K hulls. That way they won't be visible immediately.

I would like to add that should radiation not exist, most planets would not exist due to melting, vaporizing, superheating, etc.

If you are going to eject something, then you should at least put it through a phase change (or two) to capture the latent heat in it. And/or have the thing contain some chemical energy which can react endothermically.