For the truly obsessed a day off means spending the morning undisturbed with your CAD program and spread sheet. The virtual vehicle I presented last time has a lot of assumptions. One of my tasks is to verify the assumptions and reduce the uncertainty. One area of uncertainty is the weight of the thrust chambers and injectors. I spend the morning building a detailed cad model of my thrust chamber concept and using the CAD systems mass properties calculator to add up all the weights. Not counting the structure that ties the thrust chamber to the vehicle I get a fully accounted weight of about 5.3 lbs / 2.40 Kg This give me a chamber thrust to weight of better than 45. So my model virtual vehicle value of 30 seems reasonable. This weight is pessimistic as I have not figured out the details of the injector yet and used a 1 in (2.54 cm) thick copper plate as a astand in for the injector. The real world injector will almost certainly be lighter than this.
The basic chamber design is conceptually a copy of several different chambers that have been built over the years. It is a copper tube inside a slightly larger aluminum tube.The chamber is regen cooled with a graphite throat. I've been given drawings skectches and cad files from a number of people that have built similar motors. most of these motors have been larger, from 500 to 2000 lb thrust. Dues to scaling laws a smaller motor is harder to regen cool.
I hope to actually fire a small regen motor before the end of January.