Sometimes the ideas I throw out are obscure and hard to communicate, and sometimes they are so blindingly obvious that they have been rehashed many times in the decades past. Since I have no feel for which is which, sometimes I throw an idea to the wolves (that’s you) to see which it might be, and also to see if some of the follow ons are equally obvious, or not as it may be.
In my last post on the small tetherocket, the idea was somewhere in the middle. This is one of the possible follow ons that I have thought about before, but only decided to write out as my reaction to some of the feedback in comments.
Once again my cartoon may be a bit difficult to read. The black lines at 3,000 m/s are expansion nozzles spinning on a tether complex as in the last post. The small rocket inside the curves is fixed relative to the ship with a suggested exhaust velocity of 4,000 m/s retrograde to the ship. The 4,000 m/s exhaust encounters the spinning nozzles with a closure rate of 7,000 m/s. The exhaust bends nearly 180 degrees in the moving nozzle with the gasses retaining the 7,000 m/s velocity relative to the turbinozzle. The gasses exit the turbinozzle at 7,000 m/s nozzle relative which is 10,000 m/s ship relative. Net Isp just over 1,000.
I suggested 4,000 m/s exhaust velocity for the H2/O2 rocket as it would likely have a low expansion ratio to fit the envelope. I suggest that the near 180 degree turn in the turbinozzle channels would cause shock losses that would cancel any expansion gains from the nozzles. I believe that the thrust/weight ratio would remain in the 1 m/s range for the bare engine.
This would retain the capability of using any fluid reaction mass available in the solar system from CO2 to water to impure methane if that is available ISRU. The rocket engine would need changing out to a steam engine using whichever fluid is available for a likely Isp in the 500+ range if I see the reactions correctly.
One of the comments suggested beamed energy and hydrogen only as a superior alternative. Nothing any of us said would preclude using beamed energy to drive the reactions. It would solve a number of problems with onboard power if available. Hydrogen may or may not be the reaction mass of choice. The tankage mass and handling properties may well make it second best even if it happens to be available at a particular location.
On one end of the conceptual capabilities is the possibility that I am pessimistic in the capabilities suggested. A larger envelope for the fixed expansion nozzle may make it possible to get 4,500 m/s exhaust velocity from the fixed rocket which would add 1,000 m/s to the final exhaust for a total of 11,000 m/s exhaust velocity. It may also be possible to recover the energy from the turbinozzle heating to a higher velocity exhaust which could possibly add another 1,000 m/s to the final gas velocity then totaling about 12,000 m/s. Hopefully the speculative possibility of Isp=1,200 will have a qualified person of two running a few simulations for the entertainment value.