It was mentioned in this answer and in several other places on the internet that the kerosene fuel used by NK-33 engines needed to be subcooled enough so that it reached the same density as the LOX, and that this is required so that the turbopumps can run on the same shaft. These statements all trace back to here, where I can not actually find any discussion of subcooled kerosene, much less a discussion of the need to match the density of LOX.
I mentioned this here - it bothers me for two reasons:
- I think it would be extremely hard to obtain liquid kerosene at a density of 1.17 or 1.18 g/cm3, which is the density of LOX at -310 ℉, the temperature that the NK-33 engines require in order to cool the turbopump bearings. The density of RP-1 is about 0.8 g/cm3 at 25 ℃, and the largest value I’ve seen anywhere is 1.02 g/cm3.
- I can’t think of any reason why the densities should be equal to run the two turbopumps on the same shaft. The ratio of the mass flows is quite large - perhaps 2.62 LOX/kerosene (from here) so I don’t understand why matching their densities in the two pumps so closely is important.
note: I use the generic “kerosene” since the Russian fuel used to develop the engines can’t officially be called “RP-1”.
I’ve tried to collect as much information as I can on-line, and plot it here. Since I've had to deal with four different temperature scales, and people are accustomed to using different ones, I've just brute-force plotted with all four since I can't do the conversions in my head either.
RP-1 solid blue line from http://www.dtic.mil/dtic/tr/fulltext/u2/290659.pdf. Other data points I’ve found seem to match.
RP-1 dashed red line is an extrapolation of the same line below it’s valid range of -45 ℃ to +25 ℃, just to guide the eye and give the brain something to thing about.
RP-1 comments on consistency (“gel”, “wax”, etc.) are from http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020018567.pdf. I have included a screenshot of the relevant section below the plots.
LOX solid line from http://booksite.elsevier.com/9780750683661/Appendix_C.pdf. A screen shot of the entry and equation is given below.
LOX data points from http://oxygen.atomistry.com/liquid_oxygen.html
My question is: Does the operation of the NK-33 engine really require equal densities of LOX and kerosene? If so, WHY?? Also if so, what is the actual temperature of the kerosene to acheive this density, and what is its consistency really like, liquid, gel, wax?
NOTE: below is from: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020018567.pdf R is temperature in Rankine - which could be called "absolute Fahrenheit" I suppose.
NOTE: below is from http://lpre.de/resources/articles/AIAA-1998-3361.pdf:
NOTE: below is from: http://booksite.elsevier.com/9780750683661/Appendix_C.pdf