How fast do you need to be going before a ramjet is as efficient as a conventional jet?
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How fast do you need to be going before a ramjet is as efficient as a conventional jet?
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Depends on your fuel and combustion chamber design. I haven't the data, but I know it varies. To test ram jet theory in WWII the Germans strapped a piece of drainage conduit onto a flat bed truck, affixed a ram in it with inlet jets at the front to pump the fuel and got sustainable fire at 50 kmh if I remember correct. But that was petrol, not jet fuel, it all depends on what you want to burn and at what speed you want to travel.
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Hmm, well I was thinking of the 200 to 600 mph range with a conventional fuel.
Order of Kilopi
This is the one you REALLY want...
http://en.wikipedia.org/wiki/Bussard_ramjet
Get up, a get-get, get down.
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I'm thinking more along the lines of something for the initial climb
Banned
If a ramjet could be as efficient as a conventional jet with high bypass turbofan engines, then conventional aircraft would have high bypass turbojet engines similar to those used on the SR-71.Originally Posted by SockMonkey
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Once getting past a certain speed the SR-71 diverts the air around the fan blade section so it's afterburners work as ramjets.
Order of Kilopi
Then there is the scramjet which is the supersonic version. You turn it on once you're running around Mach 6.The Australians were the first in 2002 to run one, followed by the X-43 program .
Last edited by George; 2007-Apr-21 at 07:31 PM.
Banned
Bingo, SockMonkey.
In fact, at speed the compressor fans handled less than 30% of the airflow, the rest being shunted into the afterburning stage.
Last edited by mugaliens; 2007-Apr-21 at 07:33 PM. Reason: Edited to account for intervening post
Order of Kilopi
It certainly gulped fuel, but achieved remarkable efficiencies and propulsion levels for the speed and altitude at which it was running...
Banned
That it did! In fact, it burned less fuel per air mile travelled at Mach 3.1 than it did at Mach 2.9. In short, beyond around Mach 1.3, the fuel curve tipped in the other direction (faster meant less fuel burned per air mile). Below that point (until L/D max, that is), it was positive (faster meant more fuel burned per air mile).
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That's part of what I wanted to know.
The other bit was how much of a benifit is attained from the simpler construction and lighter weight of a ramjet vs it's greater fuel comsumption.
Banned
If you're thinking about a long-range, high-speed (Mach 3) cruise missile, it's much more efficient (and cheaper!) to launch it with a rocket, accelerating it to the point where the engine can ignite, than it would be to use conventional jet engines to accomplish the same task.
Various missiles since WWII, including the BOMARC and the HoundDog both employed ramjets for this purpose. In fact, the earliest surface-launched ramjet missiles began in 1944, in the Navy's request of its Bureau of Ordinance, which produced the first prototype in 1945, the Cobra, designed to counter, at long range, aerial threats to it's surface fleet.
The testing on various ramjet missile systems continues to this day, with the missle with the longest range (>2,000 miles), the Triton, travelling at Mach 3, and the fastest missile at Mach 4.1, the Typhon, with a range of 200 miles.
This link contains some very detailed historical information along with some very cool pics.
The main reason that ramjets haven't really taken off is because they're tremendous producers of heat, and fairly easily downed via heat-seeking missiles.
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I remember reading an article that showed a potential spaceplane design that had turbojets,ramjets,scramjets,and rocket engines and my first thought was "holy crap that's way more complex than it needs to be".
I was thinking that they should ditch the turbos and somehow make the ramjets VG in some way that would let them function as scramjets as well.
And of course us the rocklets for the initial acceleration phase.
Order of Kilopi
In fact you want it to kick in already at lower mach numbers, but even with variable geometry it is a pain to design a scramjet that works nice on its working point of say somewhere between M7 and M10, and yet works already at speeds below M5.Then there is the scramjet which is the supersonic version. You turn it on once you're running around Mach 6.
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How much of a pain?
Order of Kilopi
Depends on your design principles, the amount of variable geometry, how much efficiency you want to have (related to that, how much bleeds you may introduce)...
It's not trivial at all to make a ramp type scramjet with an operating point of M8 work below M5, and towards M2 it becomes very dirty. I'm just talking about the inlet then; what happens behind it is something different altogether. If you want to use the exact same combustion chamber, you get new challenges. You can solve it by using a different engine (regular ramjet instead of scramjet, for example) sharing the same inlet, but that introduces its own problems. Sometimes the best (most efficient) solution seems to be using a rocket to shoot it close towards the scramjet designed operation point. Using turbofans, you run the risk of needing both turbofans, ramjets and scramjets. There's also proposals of mixed solutions, where a rocket exhaust is put through the scramjet when the inlet flow isn't fast enough.
Now using different inlet principles than the ramp inlet, the story changes. As usual, a new set of advantages and disadvantages is introduced.
This is ongoing research, so we may very well find -or already have found- ramp inlets that do allow for low to high supersonic speeds without delivering a totally unsuitable airflow or working highly inefficiently below and above the designed operation point.
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