A high power type Aerospike motor - basically an inverted bell where the flame is built around a graphite spike. Very nice! I bet hobby motors could use this technology.
This is the big kind - bi-propellant. It would be very important to use a motor like this for a single stage to orbit rocket, because it would allow for high efficiency (isp) over a range of altitudes. However staged rockets are by far more efficient overall - single stage to orbit is a hard and costly thing!
Still complex on the inside.
"The aerospike engine is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes through the use of an aerospike nozzle. For this reason the nozzle is sometimes referred to as an altitude-compensating nozzle. A vehicle with an aerospike engine uses 25–30% less fuel at low altitudes, where most missions have the greatest need for thrust. Aerospike engines have been studied for a number of years and are the baseline engines for many single-stage-to-orbit (SSTO) designs and were also a strong contender for the Space Shuttle main engine. However, no engine is in commercial production. The best large-scale aerospikes are still only in testing phases.
The terminology in the literature surrounding this subject is somewhat confused — the term aerospike originally was used for a (very roughly conically tapering) truncated plug nozzle with some gas injection to form an 'air spike' to help make up for the absence of the tail of the plug. However, frequently, a full-length plug nozzle is now described as being an aerospike." wiki