I was wondering if it is possible for a ramjet to start from 0 velocity.
I know its working principle and all, but is there any missile or aircraft that completely uses ramjet, with no rotating part to compress the air?
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6$\begingroup$ Maybe if you drop it from orbit :) $\endgroup$– Greg HewgillCommented Feb 8, 2022 at 3:39
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1$\begingroup$ I have no intention like that :) $\endgroup$– J. BrownCommented Feb 8, 2022 at 14:59
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1$\begingroup$ 0 velocity relative to what? $\endgroup$– Jörg W MittagCommented Feb 8, 2022 at 20:29
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$\begingroup$ Relative to the ground, I know it can be used after carrying aircraft reaches sonic. $\endgroup$– J. BrownCommented Feb 9, 2022 at 4:14
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5 Answers
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No, a ramjet can't be started at zero speed. Yes, there are missiles that use ramjet propulsion without any rotating parts: they use a rocket to accelerate the missile to high enough speed for the ramjet to operate and develop useful thrust.
More or less by definition, a ramjet requires significant forward velocity to compress the intake air sufficiently to produce useful thrust. Depending on the design, this speed might be as low as about 250 kt (120+ m/s) or somewhat above local Mach 1. That means something has to push the engine (and attached aircraft) to that startup speed.
That "something" can be a turbojet or turbofan engine, a rocket engine, a catapult (I'm not aware of any ramjets that are launched this way, but it's technically possible) -- even a literal gun (ramjet boosted rounds in approximately shotgun sizes were demonstrated as long ago as the 1970s -- NOT the same as a Gyrojet, which was a tiny rocket-only projectile). For missiles launched from a jet warplane, it might be possible to use the plane's forward velocity, or a boost from a gravity drop, to get this initial speed.
answered Feb 7, 2022 at 19:59
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$\begingroup$ Edited as suggested by @MichaelHall $\endgroup$ Commented Feb 8, 2022 at 12:08
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$\begingroup$ Shotgun sizes? Do you have a reference? All I can find are artillery rounds. $\endgroup$– BergiCommented Feb 8, 2022 at 20:15
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$\begingroup$ Seek and thou shalt find the Supersonic Trebuchet. $\endgroup$ Commented Feb 8, 2022 at 22:20
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$\begingroup$ @IconDaemon I was thinking of that when I wrote that sentence, though in practice a steam catapult or similar could pretty readily get a ramjet machine up to 400+ kt, fast enough for some ramjets to produce useful thrust. $\endgroup$ Commented Feb 9, 2022 at 12:17
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No. A ramjet relies on stagnation pressure at the inlet in order to compress air prior to combustion. They are typically not effective until the speeds get quite high, somewhere in the Mach 2+ range.
Ramjet powered missiles which used rocket boosters for launch have been around for decades, possibly the most famous one is the US Navy’s TALOS missile. One of the most ambitious projects from the Cold War was a weapon called Supersonic Low Altitude Missile (SLAM). This would have used a nuclear powered ramjet engine and roamed the skies at supersonic speeds for months on end with a nuclear warhead, waiting for instructions to attack enemy targets.
answered Feb 7, 2022 at 20:09
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1$\begingroup$ Wow, I was unaware of that project. It'd be the nuclear war analogue of having hot water circulating in your pipes all the time (through a more complicated plumbing system, of course), so that you'd when you wanted hot water from a faucet you wouldn't have to wait. :) I guess some people have had that hot water set-up... :) $\endgroup$ Commented Feb 8, 2022 at 20:05
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$\begingroup$ @paulgarrett SAC is already sort of that. $\endgroup$ Commented Feb 9, 2022 at 4:26
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The OP clarified in a comment that they are talking about ground speed not air speed.
It is trivially possible to start a ramjet at zero ground speed, since ground speed is completely irrelevant. The only thing that matters is air speed. All you need is high enough winds.
You could put the ramjet into a wind tunnel (hypersonic wind tunnels can generate winds of Mach 5+, there are even wind tunnels which can reach Mach 27 for about a second). A Category 5 Hurricane would also work, as would a strong tornado.
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In theory you could have an engine that started in pulsejet operation and switched over to continuous combustion as the speed increased.
In practice, good luck with that. Pulsejets require very specific geometries and a fair bit of black magic; ramjets require very specific geometries and a fair bit of black magic. Trying to build something that works in both modes (much less if you want a transition that isn't "turn off the pulsejet, reconfigure to ramjet mode, dive, and hope the ramjet ignites before you run out of sky")... again, good luck with that.
(...and if you want 'no moving parts', you're asking for a valveless pulsejet, which adds a whole 'nether layer of black magic.)
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1$\begingroup$ And yet, Colin Furze built a Lockwood type with nothing but a welder, pressure washer, and angle grinder. The "black magic" here is that valveless pulsejets always seem to have the intake pointing backward, which doesn't work well for a ramjet. $\endgroup$ Commented Feb 9, 2022 at 12:15
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$\begingroup$ @ZeissIkon - the problem isn't "make lots of fire and non-zero thrust with silly amounts of fuel at zero speed at sea level with unlimited weight and space". It's "make a decent amount of thrust with survivably low fuel consumption over a range of altitudes and speeds in a reasonable weight and volume and not shaking the craft to bits in the process" where the black magic comes in. $\endgroup$– TLWCommented Feb 11, 2022 at 1:49
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$\begingroup$ @ZeissIkon - the intake might be less of a problem than you might think. Look at the Messerschmitt nozzle. $\endgroup$– TLWCommented Feb 11, 2022 at 1:57
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$\begingroup$ That "not shaking the craft to pieces" part just isn't happening with ordinary pulsejets. Maybe with a Gluhareff type pressure jet (they still pulse, just not as hard). Pulsejets usually trade off efficiency for simplicity -- a jet engine with no moving parts and literally one piece may be worth burning some extra fuel, and some valveless pulsejets can produce above 1:1 thrust/weight including a short-burn fuel tank. $\endgroup$ Commented Feb 11, 2022 at 12:08
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Try rephrase the question.
A ramjet is basically a continuous explosion in an open pressure valve. No more, no less. Its that explosion that needs both fuel and air.
By asking if it's possible to make it run with either one of those ingredients missing, you might as well ask if it's possible to start a ramjet without actively supplying it with fuel. The answer to that question for obvious reasons is 'nope'. The same goes for the air.
The itty bitty little bit of air that is filling the jet as it is, is not gonna do the job. You have to push in large quantities of air along with the fuel, in order to have something to lite up at all.
By the way, the same thing goes for any jet. That is what the compressor is for. Starting the flame before the compressor is properly running, is not a good idea. The thing that the compressor creates is basically the same as the head wind in a ramjet.
As in airplanes flying ramjet only, I recall seeing a French Dassault jet as a kid, where I could look straight through the jet, front to back, meaning it didn't seem to have any compressor. That should qualify it as a ramjet.