Algodoo

See how each engine works! The bell engine (left) gets thrust from the exhaust pressing outward on the internal walls. This pressure is created by the sudden compression caused by the two spikes inside the bell near the top.
The aerospike engine (right) is a more efficient engine, as it has several places that generate thrust. This example only has 3, but a real engine has 4; pressure from the fuel running down the bend of the pipes on either side, actual thrust from getting emitted to the spike, and the pressure of the fuel on the spike itself.
Thanks for playing!