I noticed that a few contributers here recently mentioned that they might have some connection with gas turbines - would anyone be able to point me in the direction of a good reference for turbine design, or be willing to help me design a turbine?
I hope you mean a model turbine, or have you other aspirations?
There a few books on model turbine making "Model Jet Engines" by Thomas Kamps is an early work but does have some dimensioned drawings included. Another is "Gas turbine engines for model aircraft" by Kurt Schreckling that also has drawings.
Both designs are a few years old now but could get you started.
This is not really my world although my background is full size Gas Turbines. I bought my Warco lathe from a guy who was building these and he was telling me that in his opinion, unless you had some contacts in the full size manufactures for materials and equipment, most successful modern GT engines are now built using commercially obtained cast Inconel turbine wheels. He said these were available in a range of designs/sizes and either as cast or finished machined and typically were less than =A3100 (about 18 months ago). Matching NGVs are also available. Not sure how sound this advice is but he did sell me a fine lathe.
Sorry, I may not have been clear - I want some help designing the blades for a turbine wheel, not a turbojet engine. It's to drive a pump for a (working) model turbopump rocket engine.
I have the materials and half the tools to machine them - I'm probably not going for a cast wheel, but one machined from stock. Can't find a suitable commercial cast wheel anyway, it's only 32 mm dia and produces ~7kW mechanical power at 120,000 rpm. Weighs about 35 grams, same for the NGV.
Schreckling says it's easy to get 75% efficiency, but I don't need that much
- I am mostly worried about end load on the shaft, as there is a 120 psi pressure drop across the turbine/NGV, and there could be up to 1,400 psi transients. This is very different to a jet engine!!
It's just the detailed design of the blades I'm unsure about.
I'm told the Schreckling one is really hard to get to work at all, and the Kamps one isn't very efficient or powerful - the best ones for the home constructor are the MW54 and KJ66 designs from Wren and Artesjet. Both use commercially made cast wheels and NGV's, and commercial impellors from car turbochargers.
But I did like the idea of a wooden compressor too, even if it is carbon fibre reinforced!
Peter, even further from my world than I thought. I did however have an interesting presentation on turbopump rockets back in the mid
1980s, a couple of points that were made stuck for some reason. I suspect that all this is well known to you so sorry if my comments are "teaching granny". The systems I saw were a little larger than yours.
They emphasized the need for a "whole pump" design rather than an "optimized individual component" design and used the centrifugal/axial issue of both pump and power turbine to illustrate where things could go wrong. Their view was that this type of propulsion system should be more accurately known as a turbopump with a rocket attached. They also believed that a major cause of shaft failure (at that time) was the incredible rate at which the system needed to "spool up" (sorry GT term but I can't remember the rocket terminology), coupled with bearing failure caused by lubrication issues at low temperature in turn caused by the large pressure drops experienced. The turbines we were shown looked more like ships propellors than turbines to me but they were multi stage and it is a long time ago, things will have moved on. The final point they made was that the efficiency of the turbopump system was the single most important issue in the performance of the propulsion system as a whole so your "I don't need that much" turbine efficiency troubled me a little. I can however, see that if your turbopump is limited by the available pump section then it would be wise to remove only the power required thus creating aproper balance and making the turbines life easier and reducing the axial load on the shaft.
Although my own interest was only inspired for a few days I did find out that their presentation was based on a Pratt & Whitney article written by Joe Strangeland and later published. I'm sure you will have read it and no doubt later material but it is in fact on line here:
I guess what I'm saying Peter is that if you are "pump limited" your turbine design may not be that critical as long as it is at least capable of optimising the pump performance. Apologies again if I am teaching granny but you will see from my comments I really don't know much about the subject. Good luck.
I agree - there is a rough sketch of a slightly larger earlier engine at
you will see that it is somewhat unusual in design. I have modified it quite a lot since then, mainly for simplicity, eg there is only one turbine and the bearings are different, as are the numbers, but it should give some idea.
I'd guess it is slightly more than a "turbopump with a rocket attached", more a "whole engine" design, as eg the turbine feeds directly into the main combustion chamber.
You spool up this one slowly using compressed air, taking as long as you like. The pumps work during (part of) this spool-up operation, but the propellents are returned to the tanks except for the small amount used to prelubricate the bearings. However when the valves open and the preburner is ignited there are large pressure and speed transients.
The bearings are rotating sleeve bearings with high pressure lubrication, somewhat similar to those used in vehicle turbochargers, not ball bearings. They are not sealed, they don't need to be - the only rotating seals in the entire engine (not counting valves) are those between the LOX and kero pumps, and these work against only 35 psi.
The bearings are at room temperature before ignition, and while they get hotter afterwards they are both cooled. The top bearing is the main one which takes the axial loads, and it sits in and is lubricated with kerosene (actually paraffin with a bit of turbine oil added) at room temperature. The bottom bearing is cooled to a lesser extent, but it never gets above 400C. I might remove the bottom bearing completely and use a long single bearing, haven't decided yet.
The overall efficiency of the pump/turbine only has to be over 17% - the pumps, which are single stage axial Barkse-type impellor pumps, seem to be getting close to 60% (though I have only tested them to 30,000 rpm), and the bearings shouldn't take much power, so 50% turbine efficiency is probably more than enough. I want about 7kW to drive the pumps - there is over 20 kW available in the gas stream.
Turbopump efficiency does not actually affect overall performance at all in this type of staged combustion engine, as long as the design pressures are maintained. :)
I hadn't actually read that one, so thanks for the link. I disagree with quite a lot of it though, it seems more a recipe for expensive engines than performance.
Not really - the pumps seem to be okay, and I am not too worried about turbine efficiency. It's the bearings I am mostly worried about, and I want to minimise axial thrust by clever turbine design, especially on startup and shutdown transients.
I remember well when the Schreckling book was first published. I was on a boring railway-trip to a customer. Going back home, I looked around in the magazines-shop of the railway-station (Cologne) and found that booklet. The travel passed in no time. I didn't build it, but it was (and still is) vverrrry interesting to read!