Re: Silver plating

[reposted, seems to have gotten lost, sorry if double posted]
[cross-posted from uk.rec.models.engineering. This is a design for a small
LOX/kero turbopump for model rocketry which I am just starting to build. 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 a volute casing Barske-type impellor pump, but it would be very
inefficient as the pump is so small, so I will be using a double Pitot [1]
design instead (unless I can't get it to work, when I will fall back to a
Barske design, or perhaps try a two-shaft-Quimby-type screw pump).
The Pitot arm is 32 mm dia, there are four 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.
Propellant flow is 175 grams per second. Shaft power is 2.1 kW, pump
mechanical efficiency should be ~ 55%, turbopump overall efficiency ~25%,
LOX output pressure is 750 psi.
Engine design thrust is 5kN / 100 lb sea level, chamber pressure is 600 psi,
expansion ratio is 8.25, Isp is 245 s sea level, 285 s vaccuum.
Note that most of these figures are still 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 tube inside opening
on the end, which is 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 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 wastes 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 fuel pumps in series. LOX volume is
about twice the kerosene fuel volume. The fuel 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.
Reply to
Peter Fairbrother
Loading thread data ...
The small jet turbines, eg the Wren 54, run at 120,000 rpm or so. But weee indeed!
Da. Nyet. Added. 500 N/ 100 lbf. foolish me
I thought it didn't look right - too many metric/imperial re-conversions (it actually started out as 500 N ... )
Reply to
Peter Fairbrother
From what I've seen of pitot pumps (Rotor-Jet) I'd be surprised if you didn't have trouble achieving scale performance.
As for the silver plating, I'd wait until you got a pump that performed before worrying about some possible perceived problem.
Tom
Reply to
Tom

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