Robert Steinke of Speed Up suggested using a self pressurizing propellant to drive a pump for a less pressurized propellant. In this case nitrous at about 800 psi is vaporized to pump a fuel up to some pressure in between the two. Say a pressure drop of two for the nitrous through the turbine drops it to 400 psi while pumping the fuel from 200 or less up to 400 psi. There was a some discussion of it on A Rocket. One of the comments suggested that since nitrous is already the bulk of the propellant, just pressurize the fuel and be done with it.
Being the kind of person that steals and modifies ideas as a matter of habit, I decided to see what I could do with the basic concept.Taking a self pressurizing propellant to drive a pump that reduces over all system mass, and hopefully operating cost.
Helium is the pressurant gas of choice for any that require performance. Unfortunately it seems that it is becoming somewhat more expensive and harder to order as the decades roll on. Also, it must be stored as a gas which means a relatively large pressure container. Self pressurizing propellants can get by without helium at the expense of considerably more mass of pressurant gas at the end of the burn. With a self pressurizing propellant driving the turbine after a run through the cooling jacket, It may be possible to do without helium altogether without the mass penalty of heavier gasses for pressurant.
Hopefully my cartoons will be better when I get another computer in a week or so. My desktop computer doesn’t save sketches in a manner that the blog will accept, and my laptop does these tiny things that even I can’t read.
Anyway, a small pressure sphere of LH2 contains less than 1% of the total propellant on board. The liquid hydrogen at 6,000 psi is sent through the cooling jacket to cool the engine and gain energy to drive the turbine. The hot hydrogen gas drives the turbine with a pressure drop of about ten from 5,400 psi after a 10% loss in the cooling phase to 540 or so psi which exhausts into the combustion chamber. The LOX and Kerosene is pumped up to nearly 600 psi to allow for pressure drop through the injectors. With a chamber pressure at 500 psi in a sorta expander cycle Isp will be a bit higher than most pressure fed engines. Mixture of propellants is less than 1LH burns with less than 6LOX+28Kero+65LOX if I have it guestimated correctly.
Low pressure tanks for the Kero/LOX at a bit less than a cubic meter per metric ton would be partnered with about about 100 liters or so of LH2 in high pressure spheres. System dry mass should be less than that with the helium spheres and medium pressure tanks of most current set ups. A quality trade study will probably find the sweet spot to be less pressure than I am suggesting on this first pass. More LH2 at a 1,500 psi would feel like a better solution as long as it didn’t go overboard and become a pressure fed hydrogen rocket.
Kerosene impeller tip speed will be on the order of 350 ft/sec with LOX impeller at about 290 ft/sec. For perspective, a 2″ disk on a 35,000 rpm Dremel tool has a tip speed of 300 ft/sec. My 14″ demolition saw at 6,000 rpm has a tip speed of 350 ft/sec. Most people worry about the turbine. This one is driven by hydrogen that is fairly warm though well below turbine temperatures on almost any application you can think of. It will require a multistage turbine to extract the energy both from the low turbine speeds and the very high pressure drop.
A variation that would get more turbine drive with less hydrogen mass would be to discharge the hydrogen gas into the expansion nozzle as extension skirt cooling. Prior art suggests that the Isp hit is not that severe, and pressure drop down to 50 psi or so is possible. With a turbine exhaust in the 50 psi range, the hydrogen spheres might optimize toward the 1,000 psi range.