[AR] Re: LOX/kero layout (was Re: Above 65000 ft for free)
- From: Peter Fairbrother <zenadsl6186@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Fri, 7 Sep 2018 01:01:36 +0100
On 06/09/18 21:35, Ed LeBouthillier wrote:
Also at 400N I'm not sure a turbopump would be the most efficient
choice. Far as I knew they don't scale down to sizes that small that
well.
They do not, which is why I have been working on exotic materials for
blisks.
You can increase the overall specific speed of centrifugal pumps by staging one
into the other and reducing the head required of each pump.
So, by having two stages, you double the overall specific speed. If you have
three stages, then you triple the overall specific speed. Successive staging
could put the overall specific speed into a usable range.
It believe it doesn't quite work like that - there's that 3/4 index -
Ns= n q^0.5 / h^0.75. But I don't know how to calculate overall Ns for
multi-stage pumps - some refs say it is just the Ns of the first stage.
But yes indeed staging can help. With the fuel pump, the harder of the
two pumps, it would make some sense to split the head in two - the high
pressure 12MPa stage would only need to handle a tiny flow, most of the
fuel flow could be at say 7-8 MPa. I haven't done that for simplicity
(and to give myself headroom if needed).
More interesting perhaps is the possibility of a two-stage turbine. Wot,
you say, single-stage turbine efficiencies of 60% plus are almost easy -
yes, but a two-stage turbine is easier on the turbine blisk.
So at the same pumping and chamber conditions you can up the fuel ratio
in your preburner, giving yourself a lot more raw preburner output power
to play with - and then even lower overall turbopump efficiencies become
feasible.
I resisted that idea for a long time - I had actually started with a
two-stage turbine about ten years ago, then changed to a single blisk
and stayed there - until recently when I gave in a bit and had a look at
it. Yes it can help, perhaps a lot. But that's for a different engine...
... then just recently I noticed that the SSME preburners have two-stage
turbines ...
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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