I hope this is on-topic here, and I would appreciate any comments. It concerns a design for a small LOX/kero turbopump for use in model rocketry which I am just starting to build.
It is a reworked version of a posting from the uk.rec.models.engineering newsgroup, where you can find the previous thread if interested. The thread title comes from the silver plating used to protect the fast-moving metal insides of the LOX pump from particulate impact ignition. Tom wrote: > So how big is the impellor going to be in this pump? > It doesn't have one. I considered using a Barske-type partial emission impellor pump with a volute casing, but it would be very inefficient as the pump is so small, so I will be using a double-ended Pitot [1] design instead (unless I can't get it to work, when I will fall back to the Barske design, or perhaps try a two-shaft-Quimby-type PD screw, or even a simple PD gear pump). Engine design thrust is 500N / 100 lb sea level, chamber pressure is 600 psi, expansion ratio is 8.25:1, target Isp is 245 s sea level, 285 s vaccuum. There are four near-identical pumps [2], a combuster and a turbine on a single 75,000 rpm shaft, in an assembly 54 mm max dia and 65 mm long, target weight 350 grams. The Pitot arm is 32 mm dia/across, tip speed is 120 m/s or 260 mph, propellant flow is 60 ml/s per pump, 175 grams per second total. LOX output pressure is 750 psi, kero pressure is similar but see note 2. Shaft power is 2.1 kW, pump mechanical efficiency should be ~ 55%, overall turbopump efficiency ~25%. Note that most of these figures are still very much theory, and they will almost certainly change a bit in practice. Note also that the design is slightly less demanding than the engineering presently (apart from the pumps) used in small model turbojets, and I hope to improve on those figures. [1] A Pitot pump is a hollow stationary arm with a Pitot opening on the end, which sits inside a hollow circular casing which spins and accelerates the liquid inside it - the fast-moving liquid enters the Pitot and the speed is changed to pressure. Also, the spinning exerts a centrifugal force on the liquid, increasing it's pressure at the outer edge of the casing where the Pitot is located. A double-ended Pitot pump just has two Pitot holes on opposite ends of a single stationary arm. It can be more efficient than an impellor pump because the wetted moving area is smaller, and there are no fast-moving parts in close proximity to give large shear forces - the two main energy wasters are the energy used to move the arm through the liquid, the arm can be shaped and surfaced to minimise that, and the inefficient diffusion recovery (the change of speed to pressure in the Pitot tube - probably only about 60% efficient at best, but recovery only accounts for half the theoretical head, so you lose maybe 20% of the total energy that way). Manufacture makes few demands on close tolerances, the single rotating seal is at low input pressure, vibration is very low and the output is almost entirely pulsation-free, which is important for combustion stability. [2] two LOX pumps in parallel, and two kero pumps in series. LOX volume is about twice the kerosene fuel volume. The kero pressure is nearly double the LOX pressure because it is used to cool the chamber, throat and nozzle before going on to be burnt. An alternative which has some benefits is for the fuel to go through one pump, then cool the nozzle, then the second pump, and then be burnt, but I haven't decided yet. The throat/ nozzle will probably also use ablative cooling. -- Peter Fairbrother _______________________________________________ ERPS-list mailing list [email protected] http://lists.erps.org/mailman/listinfo/erps-list
