Spiny Norman said:
mollwollfumble said:
> Aerospike engines have been studied for a number of years
Like um 60 years?
I’ve said it before on this forum. They’re only good for large engines because they require engineering tolerances that are like um a hundred times a fine as a normal engine. For anything small, you just can’t get engineering tolerances that fine.
Pretty sure you can – For example it’s very common to measure some components in racing engines down to 0.0005”. It’s not too difficult to get machinery that’ll measure and work to ten times better than that as well.
Anyway …. please tell me everything you know about aerospike engines. Via email or Facebook if you like. There’s a good chance I’ll be working with them soon.
I really know very little about the aerospike. I was just looking into it briefly as an alternative to the bell nozzles. I had already seen how to optimise bell nozzles so that they worked as well as possible at multiple altitudes, and had read about three different ways of defining the “optimum” bell nozzle (minimum fuel usage Vs minimum weight Vs some other that now escapes my memory).
One of my challenges was to slow down a rocket engine so that it delivered the same total oomph, but over a time period ten times as long. That meant making the throat narrower to bring the chamber pressure back up, and that was bringing the throat diameter down from 5 mm to about 1.5 mm. Which was starting to be so narrow that it was interfering with ignition.
It was at that point that I started looking at the aerospike as an alternative. But I quickly realised that the throat width on an equivalent aerospike would need to be 0.075 mm for the same burning conditions, and that was quite impossible for me to manage. So I discarded the aerospike idea.
I also noticed in passing that the combustion chamber of the aerospike is less forgiving of machining errors than the combustion chamber of a bell nozzle. An asymmetry in either the combustion or nozzle shape of the bell nozzle has a relatively minor influence on the flight, because the torque lever arm is small. But an asymmetry in either the combustion or nozzle of an aerospike engine would exert a large torque on the rocket possibly sending it tumbling.
At a very rough guess, a machining error of 1 unit in an aerospike engine would be equivalent to a machining error 400 times as large for my small rocket, but that value of 400 would drop as the rocket became more powerful.