Meteor missile (the new ramjet one)

[Viv]

I found this picture after following a link from Coffee's posting to a Farnbourgh 2000 website (rocket forum).

If you have a look at the inlets for this new ram jet missile they seem to go the wrong way?

I woud have expected them to be diverging to slow down the air flow to get the pressure gain but they seem to converge and speed it up.

Viv

[Bruno Ogorelec]

Viv, I don't think you can judge the intake configuration from the picture. It may have a variable duct geometry with internal vanes moving this way and that way to change the dict profile. This is a common feature in supersonic intakes.

Bruno

[Joe]

Viv, those are supersonic inlets. Convergence speeds up subsonic flows. I'm not exactly sure what it does to supersonic ones, but it's something like the opposite. I think supersonic inlets decelerate the intake flow to subsonic speeds (relative to the craft) by shock waves, although I haven't been getting much of anywhere finding out precisely how that works. For all I know, the exhaust in a rocket nozzle passes the sound barrier in some sort of stationary shock across the nozzle throat. Anyway, supersonic inlets are kind of like the opposite of c-d nozzles in that they often feature large reduction ratios, and as the vast volumes of often low density air get crammed down them, their pressure increases.
I guess mass augmented rockets and ramjets have found their niche in missiles, where size matters for even more than cost per pound. If you think about it, ADR's and such wouldn't make much sense for disposable first stages since the engines would cost more, while air-breathing reusable first stages are probably best made from planes.

[Bruno Ogorelec]

[quote="Joe"]Viv, those are supersonic inlets. Convergence speeds up subsonic flows. I'm not exactly sure what it does to supersonic ones, but it's something like the opposite.
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It does, but this is not the easiest way to slow down supersonic flow. Such ducts have to feature variable geometry to manage the supersonic-to-subsonic shock. It is high science indeed. That's why the rectangular box intakes were such a late feature in supersonic aircraft. They are way more complex to do than the cylindrical ones with central spindle.

For most supersonic engine configurations, the tube-and-spindle intake works very well indeed. You have to be really demanding in your requirements to need anything more sophisticated than that. If you do, you usually also have to have a whole lot of other variable geometry features, which pushes the complexity of the whole craft up considerably.

Bruno


h be

[Joe]

Spike inlets can be very sophisticated anyway. At high supersonic speeds (Mach 3+) variable geometry is almost always necessary, even on spikes, like the ones on the Blackbirds. Rectangular inlets are at least as old, as far as I can tell; the XB-70 had them, likely because the geometry is simpler to vary (I'm pretty sure the whole idea is that they have a near-two dimensional flow) and the moving ramp they use is only exposed to the flow on one side.
The inlets on the missile also look a bit long, which might mean they are fixed and just allow the shock waves to move further back as tha speed increases.
If I remember right, some of the fighters use variable opening rectangular inlets, although I don't remember any of those having bypass/dump systems. Varying the whole inlet face would be pretty hard, although spikes have their own complexities with the bypass systems (four of them on the Blackbirds).
Since the missile is only air breathing at supersonic speeds, it doesn't need inlets that open wide for take off or have clever adaptations like sucking more air in through the dump vents, so if it has any moving parts they are probably limited to ramps.

[Anthony Colette]

I can't exactly agree that the inlet shows a convergent section but regardless, a convergent inlet can indeed slow the air down considerably.

As supersonic air enters the convergent section is forms a strong shock at the inlet throat. The air is essentially choked or shocked down from a high mach value down to mach 1. The divergent section after the throat is the subsonic diffuser.

[Glenn Olson]

Check out Onera at
http://www.onera.fr/conferences/ramjet-scramjet-pde/ for information on various ramjet designs with side mounted inlets.

As best I can tell from the pictures I've seen on the web, and the one posted, this is a fixed supersonic inlet. However, all of the supersonic compression is accomplished outside the duct. The forward lip initiates the shock wave, behind which the air is subsonic. At design speed, I would guess to be about M2.3, the shock wave will just touch the aft lip. All air flow through the duct will be subsonic. The ducting narrows at the aft end because the air is dumped (see Onera) into the dump-combustor, probably about halfway back. The rest of the inlet shape is for improved aerodynamics and possibly equipment to drive the aft fins. The ducting is probably of constant cross section until the air is turned into the combustor at which time it slows and compresses.

You may also want to do a search on 'ramjet "dump combustor" ' and on '"supersonic inlet" "two dimensional" '.

[Mike Everman]

I remember that site. I particularly liked this illustration: