Some years ago I posted an idea for combining tethers, rockets, and nuclear power to build an engine with very high Isp for orbital transfer operations. 750-850 seconds Isp with LH2/LO2 seemed and still seems possible. I am posting a follow on to that idea with some modifications suggested by commenters and some of my own.
The original post is at https://selenianboondocks.com/2008/10/tetherocket/
In this cartoon, the spiderweb structure is a connected tether complex inside a pressure disk. It is powered up to 3,000 m/s by the onboard nuclear or solar power with the spin maintained by continuous power input. The little angles on the tether tips are the throatless diverging combustion chambers. The LH2 and LO2 are sprayed into the path of the combustion chambers and both mixed and ignited by the 3,000 m/s impact. The pulse detonation burn will exit the combustion chambers at close to the 4,500 m/s characteristic of an LH2/LO2 engine in vacuum relative to the moving combustion chamber.
The small combustion chamber expansion ratio is partially compensated by the impact heating and pulse detonation in the initial chamber. The near 4,500 m/s exhaust velocity is relative to the moving combustion chamber though and has a vehicle relative velocity of nearly 7,500 m/s. The exhaust then enters the single fixed high expansion ratio nozzle to finish expanding to as near vacuum as possible. The net exhaust velocity should be in the 8,000-8,500 m/s range.
I still see the T/W as being about 0.1 or 1 m/s acceleration with the bare engine and probably about a tenth of that including vehicle mass. 100 cm/s would go from LEO to Lunar Transfer Orbit in about half a day using about 1/3 of IMLEO to do it.
Higher T/W than ion engines. No radioactive exhaust as in nuclear thermal and higher propellant densities to boot at similar net Isp. Switch to methane/Lox or any other bi-propellant by changing two low pressure injector elements. Able to use inert gasses from ISRU anywhere in the solar system for Isp in the 450-500 range using the mechanical drive alone.