Carmack's Throatless Rocket Engine

Via Stupidus, apparently Carmack is building a throatless rocket engine:

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.


  1. John isn't trying to make a nozzleless rocket, merely a throatless one. IOW, the thing doesn't contract down to a throat, though down the road, he may make a version with a flared out nozzle end. The main point is that without that throat, the machining is tons easier and cheaper in theory. Whether he can make a reliable engine out of it remains to be seen, but there's a decent chance he can do it. Especially since he doesn't seem to have too many hangups about high performance.

    The group I'm with is taking a different approach to things, but I think John's work is definitely worth thinking about.

  2. Doesn't that kinda defeat the purpose of developing cheaper rockets if the performance is worse? IOW aren't they going to have to make up for it by not being able to launch as much, or use more fuel?

    I guess if you save more in machining, then the increase in costs elswhere then it would make sense...

    And you're right I was going by the assumption that what I saw was what I got in that photo. It makes a lot more sense if they add a flared out nozzle in that photo.

    I'm still having problems understanding one thing though. I thought the whole point of having a throat was to accelerate the fluid... Kind of like putting your thumb over half of the opening of a garden hose end. Wouldn't getting rid of the throat affect the performance of the rocket in the end?