John Carmack is working a potentially disruptive technology: A throatless rocket engine. Its made from plain aluminum pipes with few machined fittings. Carmack says: "The great thing about these engines is that it only takes me two nights to machine the parts, so we can test two engines a week if necessary." It scales too: "If this line of tube engine development works out, we can make a 5,000 lbf engine with very little more effort than the test engine.
I'm wondering what the efficiency of a throatless rocket engine is? The deLaval (bell shaped) nozzle is the most common one, and I thought the idea behind an aerospike nozzle was to get a better efficiency... What advantage is gained by not allowing the gases to expand at th nozzle end? I thought the whole point of a an expanding nozzle was to decrease the pressure of the fluid and get the velocities up at the nozzle exit increasing the thrust produced?... Well some more reading I think is required on my part on this one. Either way, this is some cool stuff.