Thursday, July 16, 2009

Ribbon propellant discussion

There is a discussion of a few ribbon propellant usage options at Selenian Boondocks that may be of interest.

I've been meaning to say more about a few points raised by our contest entrants, but have not really had the time... so I will quickly touch a few points here. Both entrants discussed many good points about the properties of ribbon propellant systems that I will be skipping over.

Mr. Blake raised some good points about the difficulty of sealing the propellant feed system. This is definitely something that has to be addressed in your injector design. He slightly overstates the problem, however, when he says that any leakage is completely wasted. By aiming the leaks backwards, you can still get a significant proportion of the impulse even of the leaked gases. (Of course, good seals are even better.)

Mr. Cate's first concern is about throttling. He correctly states that deep throttling of liquid rockets takes some effort. Solids operate through a very different mechanism, however. Solid rockets can typically be throttled very easily, by customizing the burn profile. This is really a solved problem for solids, and we get to keep most of the advantages solids have in this regard.

Mr. Cate also took issue with the expectation of lift from the ribbon. To be fair, we were never really looking at using the lift generated by the ribbon for many up the reasons he states, but he does overstate the issue somewhat when he postulates that a ribbon provides no lift. In fact, it is relatively straightforward to experimentally prove this. Just get a thin ribbon, and dangle it in front of a fan. Instead of hanging down, it stretches out, blown by the wind. The key is that it is mostly still in a line - so the lift is manifestly spread out over the length of the ribbon. Ribbons produce lift in the same way that the Chines of the SR-71 do, by generating vortices.

I have previously addressed the drag issues in this blog, so I will skip over them here. Flutter is something that will be addressed - at this point, all I can really say is that it is a fairly well understood aerodynamic issue.

I have also addressed the injector pump mass and power earlier. The injector power and mass are actually lower than a comparable liquid rocket.

More to come later!

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