This post shows a plot of thrust for a model based upon extendedMerlin but omitting the Merlin engine.
This model consists of a 240 meter heat pipe and a 5 meter stub which is not heated.
The model is preloaded with hydrogen at 21 Kelvin. When a run begins at zero, 40 MW of volume heating is applied immediately.
Meanwhile, the intake flow ramps from zero to 2 kg/s of hydrogen at 20 Kelvin, and the exit port is opened to near vacuum (20000 Pascal)
During the first four seconds of operation. the initial load proceeds toward the exit.

- NewNozzle pipe only 0 to 4 seconds
- thrust_over_time.png (35.59 KiB) Viewed 8059 times