[AR] Re: LOX/kero layout (was Re: Above 65000 ft for free)
- From: Peter Fairbrother <zenadsl6186@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 11 Sep 2018 01:46:04 +0100
On 10/09/18 18:57, rebel without a job wrote:
At that size, between Reynolds number losses and tip losses, why stick with turbines?
Fun? Practice?
Tip path to outlet ratios become enormous, and ratio between tip clearance area
(with reverse flow) and driven area gets uglier and uglier and that size.
I don't know what a "tip path to outlet ratio" for a turbine is, but tip
clearance isn't an issue here.
The turbine is shrouded over the outer edge; just inside that are the
blades which would look a bit like like C C C C C as seen from the side
if they weren't covered by the shroud, then under that is the body of
the disk. The blisk is all one piece, which is why it is so hard to
manufacture.
No tip clearance, no reverse flow. Nor is there any significant flow in
the space (I won't call it clearance) between the shroud and the casing.
One possible design is a partial admission turbine with 21 blades and
two nozzles. The preburner output comes rushing out through the nozzles
at 820 psi and 573 m/s directed at 20 degrees incidence to the plane of
rotation. The cross-sectional area of the nozzle output is a little
larger than the cross-sectional area of the inter-blade spaces.
But it is a pure impulse turbine, the cross-sectional area of the paths
between the blades is constant, and there is no (intended) pressure drop
over the turbine.
It can help to think of the gas in the turbine space as incompressible.
The turbine then slows the gas stream to ideally zero velocity
tangentially and 573/(1-cos20) = 35 m/s axially (ignoring losses, slip
etc), which then passes into the main chamber.
The static pressure above and below the turbine disk is the same 820
psi, roughly speaking. When the blade spaces are not being driven, they
are symmetrical top and bottom, and the pressure on top and bottom is
the same, so there is no flow through them.
Ideally the pressure drop between preburner and chamber occurs entirely
in the nozzles, accelerating the flow to 573 m/s and decreasing the
pressure from 11.6MPa to 6MPa. After the nozzles, and all through the
turbine, the pressure remains constant (ish) until the flow approaches
the throat region.
Flow from the top of the disk to the bottom, excepting the intended fast
flow through the blade spaces, isn't the end of the world - in general,
you don't even need to try to prevent it.
The geometry is the same as eg the turbine in the Merlin engine, except
afaik that one is full admission.
Apart from bearings/seals, the only space which needs a tight clearance
is the gap between the static nozzles and the rotating blades - but with
only 3 nozzles there isn't much of this. And it doesn't have to be very
tight anyway, roughly speaking just small in comparison to the
cross-sectional dimension of the nozzle output flow will do.
(yes, I can machine to a micron or two if I have to, but I try to avoid
it, especially for things like rotating blade clearances with changing
temperature profiles - no thanks, no, if at all possible no!!)
Positive displacement pumps such as gear and Roots pumps seem to exist at this
size and look easier to engineer.
I was thinking of Barske-style partial admission pumps. PD pumps, yes,
nice fit to the specific speed, but how to drive them? A gear or Roots
pump at 100,000 rpm?
If not a turbine, what?
Peter Fairbrother
--
400N rocket engine v2.0
LOX-rich staged combustion
Pumps:
LOX - 107 g/s @ 1.142 kg/l = 93.7 ml/s
kero - 45 g/s @ 0.78 kg/l = 57.7 ml/s
total = 152 g/s = 151.4 ml/s
O/F = 2.38
output pressure 12 MPa
delivered power LOX 1125W (93.7 ml/s x 12MPa)
delivered power RP1 693W (57.7 ml/s x 12MPa)
delivered power total 1818W
at 100,000 rpm-
LOX head= 1050m 93.7 ml/s specific speed = 166 (= 291 US units)
kero head= 1538m 57.7 ml/s specific speed = 97 (= 163 US units)
at 200,000 rpm-
LOX head= 1050m specific speed = 332 (= 582 US units)
kero head= 1538m specific speed = 194 (= 326 US units)
Preburner:
11.6 MPa -> 6MPa exit
LOX/RP1 O/F 36:1
Ae/At 1.0046
Te 613C, 886K
Ve 573 m/s mach 1.07 26.1 kg/m^3
110 g/s flow rate
Turbine:
Gas stream kinetic power 18.06kW
Pumps delivered power 1818W
Turbopump overall efficiency required 10.0%
20 degree Turbine tip optimum velocity 275 m/s
Main Chamber:
Pc 6MPa Pe 1 atm
LOX/RP1 2.38
Ae/At 9.09
ISP: 295s 321s(vac)
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