Towards a cyclical flow jet engine.

RGClark · 2018-03-28 11:06:07

Jet engines are great at getting high propellant efficiency compared to rockets because they don't have to carry their own oxidizer. Unfortunately they are heavy compared to rockets getting a T/W ratio in the range of 5-10 to 1, while for rockets it's commonly 50-100 to 1.

The problem with the heavy weight of the jets is the compressors and turbines they have to carry. If we could dispense with these we would have an engine getting the high Isp but at light weight. To this end I was thinking of a method to get the compression without needing the compressors. I was reminded of the rocket-based combined cycle engine (RBCC). This uses the exhaust of a rocket to provide the compression to a ramjet to allow the ramjet to operate and takeoff from a standing position.

I thought then we could instead of having a separate rocket, reroute the jet's own exhaust back into the inlet to provide compression. To my knowledge this has never been built, though I did see a patent application for such a device. I discussed this possible engine some years ago on this forum devoted to homebuilt jet engine experimenters:

Anyone *build* a cyclical flow jet engine?
Post by RGClark » Thu May 28, 2009 6:34 pm
http://www.pulse-jets.com/phpbb3/viewto … f=3&t=5299

In that patent application I mentioned, the proposer said a disadvantage is that it has less fuel efficiency than a standard turbojet, by about 15%. But for a rocket, specifically SSTO, application this would not be as big a problem because jet engines are already so much better than rockets in Isp.

So any experimenters out there want to give a try?

In addition to it's use in space flight. It might still have a use for aircraft. It's possible the reduced fuel efficiency is because of the reduced compression. But usual turbojets get their high compression by using multiple stages. So can we repeat this rerouting of the exhaust multiple times to increase the compression?

This might be a possible proposal for Boeing's personal jet pack competition for example:

Boeing will give $2 million to anyone who can build a functional jetpack.
Flying cars are so passé
By Andrew J. Hawkins@andyjayhawk Sep 26, 2017, 12:30pm EDT
http://www.pbsaerospace.com/our-product … jet-engine

Bob Clark

GW Johnson · 2018-03-29 11:11:28

Hi Bob:

I'm not sure I understand what you mean by "cyclical" jet engine. You describe a ramjet modified to recycle some of its exhaust into the inlet to (I presume) jet-compress the entering air. I don't think that will work, because the total pressure of the jet is far less than the total pressure of the entering air. Entropy has had its way with the stream. Remember, in a ramjet, the inlet is the only pressure-gain component in the system. All the rest are pressure losses.

In the classic ejector ramjet (rocket-based combined cycle with ramjet), there is jet pumping action from the very high total pressure rocket stream. That pressure is essentially the rocket chamber pressure, and the higher it is, the better the jet pumping effect. My old friend Joe Bendot was the ejector ramjet expert at Marquardt many years ago when I was the ordinary ramjet expert for Hercules. Joe's designs calculated as taking off from a standing start, but at Isp less than rocket. Once flying at ramjet speed, Isp was far better, but only if you turn the rocket off.

The PDE is something quite different. That's a pulse detonation engine. It's like a pulsejet, but with full shockwave detonating combustion pulses, not just the fast deflagration pulses of an ordinary pulsejet. These have been flown, but there are two almost-lethal downsides to the concept: (1) about the only fuels that work are low molecular weight gaseous fuels (hydrogen, methane, maybe hot propane, and not much else), and (2) the shock wave action induces propulsive vibrations at incredible levels, pretty much lethal to human occupants, and rendering most structures non-reusable.

GW

Last edited by GW Johnson (2018-03-29 11:13:56)

RGClark · 2018-03-29 15:44:35

Here is a patent that discusses the idea:

Gas heat engine.
Patent number: 7111449
Filing date: Nov 14, 2002
Issue date: Sep 26, 2006
Inventor: David W. Stebbings
http://www.google.com/patents?id=29B6AAAAEBAJ

It describes the idea of using some of the exhaust from the nozzle to
be routed back to the inlet to induce the compression of the incoming
air, thus dispensing with the compressor/turbine of a usual jet
engine.

In the citations of the prior art, it also mentions a patent going
back to 1950 also based on this idea:

ASPIRATOR COMPRESSOR TYPE JET.
Patent number: 2502332
Filing date: Apr 12, 1945
Issue date: Mar 1950
Inventor: McCollum
http://www.google.com/patents?id=DnVWAAAAEBAJ

Bob Clark

GW Johnson · 2018-03-29 16:36:55

Well, you can patent anything, whether it works or not.

I do not believe the 2006 patent would work at all, because the scooped hot gas is recovered at the combusted total pressure, and static pressure in the vicinity of 90% of total. It doesn't really matter where in the inlet you deliver this scooped stream, because anywhere in the inlet the air stream total pressure is higher by 10-500% than the recycled gas stream, so that momentum cannot be conserved if total pressure is to rise! Mixed total pressure can only drop. Forward in the inlet, the static pressure is lower than the combustor, so that spontaneous flow can indeed be induced. But it will only reduce the inlet flow total pressure, because the reverse flow's total pressure is quite miniscule compared to the entering air total pressure at that point.

The 1950 patent is actually a description of one form of the ejector ramjet, although a suboptimal one. The source of high pressure in the hot entering fuel stream is entirely independent of the combustor pressure conditions, unlike the 2006 patent. Only the heat is transferred, to a fuel stream already and independently at very high pressure relative to the inlet and combustor, by a means having nothing to do with the combustor. When you spray that hot fuel jet, there is a total pressure rise in the inlet air stream. The more practical form is a rocket operated fuel-rich as the entire entering fuel stream. That maximizes the pumping effect.

Patents have much more to do with how much lawyer fee you paid, and almost nothing to do with whether your gadget even has a prayer of working.

GW

Last edited by GW Johnson (2018-03-29 16:40:49)

RGClark · 2018-04-06 09:57:42

From this earlier discussion, perhaps we could get some compression from the rerouted exhaust by the "entrainment" effect:

Index» Interplanetary transportation» Reusable LOX/Kerosene SSTO with drop tanks. 2016-10-06 10:03:17

GW Johnson wrote:

Putting a ring shroud around a rocket engine can induce airflow through the shroud that increases thrust, yes. At very subsonic to static speeds. If the friction of the airflow past the rocket engine can be reduced, you might get factor 1.4 more thrust statically, less as your speed increases. Smooth shapes and proper venturi internal profiles are required to make this work at all. The ducted propellor is another example of the same device with a different prime mover. Same sort of airspeed restrictions apply. Static to about 200 mph.
The air ejector pump takes this to extremes by going to a geometry that is a whole lot more complicated (and heavy) than a simple venturi shroud ring. It only works statically, and nobody is interested in its thrust, only the pressure increase it can supply without any moving parts. As a pump, its efficiency is very low. There's a whole lot more massflow coming from very high pressure in the driving jet, than any low pressure airflow it can induce. The thing essentially works by fluid friction between the two streams, which is inherently wasteful. Applications I am familiar with include taking a rocket propellant mix bowl to hard vacuum conditions for mixing without air entrainment.
I'm not at all sure this thing would ever be worth the extra weight to add it to a launch rocket. Using the skirt extension for your shroud ring has the incorrect venturi geometry, you will not get much of the static thrust multiplier of 1.4.
GW

http://newmars.com/forums/viewtopic.php … 42#p131442

In any case it should work for the Boeing GoFly personal jet packpack competition. This would be much simpler in not redirecting the exhaust but simply using a shroud around the exhaust of a small jet like those used on R/C aircraft to entrain extra air thereby providing additional thrust. Note for this jet pack application we would be flying at low speeds.

http://www.pbsaerospace.com/our-product … jet-engine

Bob Clark

Last edited by RGClark (2018-04-06 09:59:13)

GW Johnson · 2018-04-07 10:39:04

Hi Bob:

To run the mixing analysis, you have to solve simultaneously the balances of mass, momentum, and energy, subject to some less-than-perfect level of mixing efficiency, and some less than-perfect level of combustion efficiency (if that applies). To get realism, you also need to include a wall friction loss model in your momentum balance. That difficult and complicated model is why most folks do not do this kind of design analysis, and just rely on experiment. I have one for an air ejector jet pump, but not a propulsive device.

If you duct a jet engine, it will induce additional airflow and amplify the thrust a little. This works at all because the total pressure of the engine exhaust stream far exceeds the total pressure of the induced air stream. Obviously, this only works at low flight speeds, because as you go faster, the airstream has significant total pressure, and the engine stream's total pressure no longer "far exceeds" the air stream total pressure.

If you try to pump a ducted engine's air stream with an exit jet stream whose total pressure does not "far exceed" the airstream total pressure, it should be obvious by now that the device simply cannot work. That's why I said what I said about using exhaust gas to pump up inlet pressure in a ramjet. The inlet stream total pressure is inherently greater than the exhaust stream total pressure in a ramjet, period. No way around that. So, such a design is therefore utterly incapable of functioning, no matter what some patent description says.

If you duct a turbojet, its compressor is the big pressure-rise component, not the inlet. Because of that, the exit stream can (and does) have far higher total pressure than the entering airstream. That is why ducting a turbojet to increase thrust actually works. One synonym for this is fanjet, if a stage or two of the compressor extends out into the duct.

GW

Last edited by GW Johnson (2018-04-07 10:53:28)

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#1 2018-03-28 11:06:07

Towards a cyclical flow jet engine.

#2 2018-03-29 11:11:28

Re: Towards a cyclical flow jet engine.

#3 2018-03-29 15:44:35

Re: Towards a cyclical flow jet engine.

#4 2018-03-29 16:36:55

Re: Towards a cyclical flow jet engine.

#5 2018-04-06 09:57:42

Re: Towards a cyclical flow jet engine.

#6 2018-04-07 10:39:04

Re: Towards a cyclical flow jet engine.

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