Storm Article taken from Ignition #5, December 2005
The main engine chamber is basically a smaller version of the Churchill Mk 3 engine designed to produce 7000 Kg of equivalent thrust. The chamber will be made from Inconel and is being manufactured by a specialist in fabricating from exotic metals. At the moment they have made the cones for the inner and outer nozzle and convergent sections of the chamber and have almost finished machining the 'knuckles' which form the exact radius edges of the chamber.
These will ensure that any turbulence in the gas flow of the chamber is kept to a minimum. On my last visit to the factory they were having difficulty thinning down one of the knuckles to our tolerances by machining alone, and an alternative solution of CNC grinding was suggested to finish off the part. This has to be done by an outside sub-contractor, once this piece is completed and returned to their factory unit there will be no more delays as the rest can be done 'in-house'. The chamber should be completed by early to mid December.
The injector plate design was completed and the design was sent to contractors for quotation.
The Injector plate could not be manufactured from Inconel mainly due to cost implications, the material is 5 times more expensive than stainless, and in its plate form it wouldn't provide that much more rigidity than the stainless. Also, it is a very hard material that is difficult to machine and drill, so it would become very expensive to produce all the angled holes that would be required in the plate for the complicated injector patterns. Making the main injector plate out of stainless 310 was the most sensible compromise.
The LOX inlet dome for the back of the injector plate has been ordered from a local company. The design for the LOX inlet was changed to a dish form for a number of reasons following the Churchill Mk 2 tests. One was that when we used a flat plate, it deformed under the extreme temperature gradients and pressure differential, so we had to introduce re-enforcement gussets. These seemed to do the job, but weren't very attractive to look at, and were easy to catch fingers on when installing the injector plate. Even with the gussets in place, the plate continued to dome outwards, so a dome shaped inlet seemed the sensible solution.
To save weight and gain extra strength under pressure an Inconel dome was decided upon, formed from a dished end profile rather than a full hemisphere to save some space. Following a quick confirmation from the Storm manufacturer that Stainless Steel and Inconel could be safely welded together the LOX Inlet dish was ordered, which should be with us by the time this article goes to print.
It is probably worth noting some observations about this new design in light of the 'incident' we had in New Mexico in which during the second test firing of the Churchill Mk 2 engine for the X-Prize Cup Event, the LOX inlet was blown out of the back of the injector plate, causing the propellants to burn freely in the atmosphere as opposed to the engine.
Once the Storm injector plate has been machined (leaving some extra material) and the LOX injectors drilled, the LOX inlet dish will be taken to be welded into position. The extra material will be left on the plate in case any distortion of the plate occurs during welding.
The top of the combustion chamber will be left with extra material also and when we have both pieces there will be a final machining process to make sure the components are a good slide fit.
The propellant tanks for Skybolt have been designed using similar methods to those chosen for the intended manned Nova launches. This tank design was also used in all the Churchill test firings.
The Skybolt tanks have been designed with flight in mind so they are of minimal weight but can handle full flight operating pressures. They consist of a thin stainless steel liner, which is then over-wrapped with carbon fibre, designed to handle the operating pressure. This produces a high pressure, lightweight tank that can be used in our flight vehicles. The mass restrictions for Skybolt are higher than for our Thunderstar vehicle; because it is a lot smaller and we haven't got a lot of space to work with, we have carefully designed the tanks to save even more weight. This time the liners were manufactured half as thick as for previous designs to act as a kind of bladder to hold the propellants within the carbon fibre wrapping. The average thickness of the new liners for Skybolt is only 1mm thick.
The tanks will also become the main backbone of the rocket airframe once complete. They have anti-slosh baffle to reduce the movement of the propellant during flight, anti-vortex baffles to stop small 'whirlpools' forming on the tank exit that could cause pressurant gases to be lost without displacing any propellant. They will also include capacitance type level sensors which will be able to output voltages proportional to the level of liquids in each tank. This data will be useful in fight if it can be integrated into the control system that will be monitoring the engine status. Otherwise the tanks are very similar to those we have built before.
One unique feature that is included in the kerosene tank is a 5" pipe that will run through the centre of the tank. This will be open on both ends and will allow us to pass wiring and most importantly, the LOX pipe from one end of the tank to the other. This enables us to reduce the amount of bends and external plumbing that will be required on the rocket, and hence reduce pressure losses in the line. It will also make the rocket much easier to assemble. By reducing the number of external conduits we will also be reducing the drag on the rocket vehicle, improving overall performance.
I have been told that we should see the tank liners in the factory before the Christmas break.
New Flange Inspired By Stentor Engine
One of the features we wanted to improve in the tank design has always been the flanges that are used to connect to the bottom of the tanks. In previous designs we have used industry standards to define flange sizes for the tanks, however as these standards do not have weight in mind for a flying system like ours, they result in large heavy flanges that are more suitable for something like a chemical processing facility than a rocket. We have had a look at some Stentor engines that were used in the 60's on the Blue Steel stand off bomb and have found a flange that is close to the pipe size we will be using on Skybolt. This design was slightly modified and will replace the heavy flanges we have previously used. The first pressure tests of this new flange will be conducted in parallel with the first flow test of the new propellant valves.