Open the rear end and you get propulsion as well. Just don't start close to earth

You still need matter to carry the heat, and matter is heavy (by definition!) Radiating the heat isn't going to be enough if you want to send anything other than a blob of plasma.

Which is an interesting thought when considered as a weapon. Fire a self-immolating fission reactor at your target...

I miss the days when a Bussard Ramjet was a viable dream.

Also, if there are aliens there, sending a highly radioactive blob of plasma at them at interstellar speeds might cause a little trouble.