Re: [ERPS] Aerospikes versus entensible nozzles

[Ian Woollard]

David Masten wrote: On Fri, 2003-08-08 at 09:53, Ian Woollard wrote: So just what is the advantage of aerospikes? So far as I can tell, extensible nozzles have better thermals, are simpler to design, I'm not so sure about simpler to design. There appear to be some fairly simple and elegant ways to do aerospikes that are not inherently more difficult to design and build than a traditional multiple engine, multiple bell design. Aerospikes in general (as opposed to a plug nozzle) seem relatively complex to me, particularly from a cooling point of view (although ERPS have sidestepped that problem by using HTP, for a biprop it's more interesting). and can give as good or better ISP. But with more weight to carry. Nor am I so sure of the as good or better ISP, maybe for specific points of a flight, but not for an entire lifting trajectory. If you have the 7th edition of Sutton, Figure 3-10 shows what effect different expansion ratios have- the very nearly optimum ranges are pretty wide even for a single area ratio, plugging in one or two extension pieces should be very close to optimum over a very wide range. Here's a web page of a comparison between an aerospike and a nozzle, http://www.aerospaceweb.org/design/aerospike/compensation.shtml#ideal but it's a little forced as it's comparing it against a fixed expansion nozzle (and I assume at fixed chamber pressure, you can altitude compensate by throttling back a little). Theoretically of course, as the number of extensions tend to infinity, you get a perfect nozzle response. So why are ERPS planning to test aerospikes? What am I missing? Moving parts? You are talking about the idea of adding to the physical bell at some point during flight, right? Yes, just a coaxial conical section that slides axially. The force on the extension piece is low, and no seal is needed, and if it jams, you just lose performance, but the vehicle should be fine. The attraction of aeropikes is that you can design the nozzle such that you only provide a nozzle for a fraction of the expansion and depend on fluid dynamics to translate the remaining expansion to efficient thrust. This can (theoretically) mean a much better thrust to weight ratio. That's unclear to me, extension pieces needn't be heavyweight, and should be uncooled (in most cases aerospikes have to have cooling) and bell nozzles have very good thrust to weight ratios. Dave -- -Ian "If you want to build a ship, don't drum up the workers to gather wood, divide the work, and give orders. Instead, teach them to yearn for the vast and endless sea." -Antoine de Saint-Exupery