Author Topic: Dynamic Compression  (Read 18585 times)

Offline sevenperforce

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Dynamic Compression
« on: 03/17/2016 07:58 pm »
Mechanical turbocompressors are ridiculously heavy, which means that their potential for increasing specific impulse is far outweighed by their drag on your thrust to weight ratio.

Is there any way to use the dynamic pressure of an exhaust stream, perhaps a vectored exhaust nozzle, to compress an airstream?

Offline Nilof

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Re: Dynamic Compression
« Reply #1 on: 03/17/2016 11:22 pm »
Mechanical turbocompressors are ridiculously heavy, which means that their potential for increasing specific impulse is far outweighed by their drag on your thrust to weight ratio.

Is there any way to use the dynamic pressure of an exhaust stream, perhaps a vectored exhaust nozzle, to compress an airstream?

https://en.wikipedia.org/wiki/Air-augmented_rocket
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline 93143

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Re: Dynamic Compression
« Reply #2 on: 03/17/2016 11:58 pm »
Mechanical turbocompressors are ridiculously heavy, which means that their potential for increasing specific impulse is far outweighed by their drag on your thrust to weight ratio.

Context?  That doesn't appear to be universally true:  https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

Offline Paul451

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Re: Dynamic Compression
« Reply #3 on: 03/19/2016 03:17 pm »
Mechanical turbocompressors are ridiculously heavy, which means that their potential for increasing specific impulse is far outweighed by their drag on your thrust to weight ratio.
Context?  That doesn't appear to be universally true:  https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

SABRE has a projected thrust to weight ratio of 14:1. The worst rockets are over 30:1, most are around 80-100, and Merlin 1D apparently gets about 180. I think sevenperforce's point stands.

Offline 93143

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Re: Dynamic Compression
« Reply #4 on: 03/19/2016 06:55 pm »
I think you didn't read either point properly.  I'm not saying SABRE has a rocket-level T/W.  I'm saying the Isp increase more than makes up for the lower T/W, which is what sevenperforce asserted was untrue in the general case.

Offline sevenperforce

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Re: Dynamic Compression
« Reply #5 on: 03/21/2016 12:58 pm »
I think you didn't read either point properly.  I'm not saying SABRE has a rocket-level T/W.  I'm saying the Isp increase more than makes up for the lower T/W, which is what sevenperforce asserted was untrue in the general case.
Well, SABRE is currently not even built, let alone tested (though their helium-loop precooler seems to work well enough), and the spaceplane intended to use it is barely past the back of the envelope stage. So we don't know for sure whether the increase in specific impulse will be enough to make up for the low vehicle T/W ratio.

But I should have specified that I was thinking more about augmenting thrust than decreasing fuel consumption. The point of the turbocompressor on the SABRE engine is to decrease fuel consumption by compressing air to use as oxidizer; the point of the bypass fan on high-bypass turbojet engines is to increase thrust for a given amount of fuel. It's that high-bypass turbofan I'm eyeing. The bypass fan provides the lion's share of the thrust for a turbofan engine, but it's extraordinarily heavy; its primary advantage is being mounted on a really big wing that negates gravity drag, and its other advantage is that it only needs an efficient cruise, not an efficient acceleration (which is where that T/W hurts).

Mechanical turbocompressors are ridiculously heavy, which means that their potential for increasing specific impulse is far outweighed by their drag on your thrust to weight ratio.

Is there any way to use the dynamic pressure of an exhaust stream, perhaps a vectored exhaust nozzle, to compress an airstream?

https://en.wikipedia.org/wiki/Air-augmented_rocket
An air-augmented rocket uses the exhaust stream to reheat the airstream that is already moving into the expansion nozzle, but the airstream must be compressed or otherwise pushed into the expansion nozzle or it won't enter at all. An air-augmented rocket can produce some thrust at zero speed (unlike a ramjet), but it cannot produce anywhere near its peak thrust at zero speed like a turbofan can. Unfortunately, takeoff is precisely where you need your peak thrust.

If there was a way to set up the exhaust to induce and compress a large airflow at a standstill, though, then an air-augmented rocket could produce a significant fraction of its peak thrust at zero speed.

Offline RanulfC

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Re: Dynamic Compression
« Reply #6 on: 03/21/2016 07:15 pm »
https://en.wikipedia.org/wiki/Air-augmented_rocket
An air-augmented rocket uses the exhaust stream to reheat the airstream that is already moving into the expansion nozzle, but the airstream must be compressed or otherwise pushed into the expansion nozzle or it won't enter at all. An air-augmented rocket can produce some thrust at zero speed (unlike a ramjet), but it cannot produce anywhere near its peak thrust at zero speed like a turbofan can. Unfortunately, takeoff is precisely where you need your peak thrust.

If there was a way to set up the exhaust to induce and compress a large airflow at a standstill, though, then an air-augmented rocket could produce a significant fraction of its peak thrust at zero speed.

Might want to re-read the cite because that's not how it works. An AAR entrains air through the movement of the rocket exhaust WHICH both pulls in and compress' the air and pushes it into the exhaust stream which normally results in a significant increase in static (zero-speed) thrust. The ejector-rocket can run fuel rich which increases the thrust augmentation though it's also been done with extra fuel injection using H2O2 ejectors and nozzle fuel injection.

Methods of increasing this effect come in many forms one of which is the "Supercharged Ejector" rocket which nominally includes a single stage compressor/fan which increases the ejector effect with both rocket and fan exhausting into a ramjet duct where more fuel can be injected for even more thrust. Search term is "Supercharged Ejector Rocket Engine" or SERJ.

Your OP also seems to assume that some sort of vertical take off where T/W is an over-riding factor for propulsive take off, which isn't a given either operationally or technically. "Off-the-shelf" low or medium bypass turbofan engines have been shown to be capable of powering a vertical take-off vehicle as a launch assist system if required, but it all comes down to assumptions and design trade-offs that are mission and vehicle specific. The generalization of the OP doesn't given any reason for the assumptions given. We've had quite a few "jet-engine-first-stage" threads here if you'd like to look them up. I'm always up for a bit of necro-threading in a good cause :)

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline A_M_Swallow

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Re: Dynamic Compression
« Reply #7 on: 03/21/2016 10:12 pm »
{snip}
An air-augmented rocket uses the exhaust stream to reheat the airstream that is already moving into the expansion nozzle, but the airstream must be compressed or otherwise pushed into the expansion nozzle or it won't enter at all. An air-augmented rocket can produce some thrust at zero speed (unlike a ramjet), but it cannot produce anywhere near its peak thrust at zero speed like a turbofan can. Unfortunately, takeoff is precisely where you need your peak thrust.

If there was a way to set up the exhaust to induce and compress a large airflow at a standstill, though, then an air-augmented rocket could produce a significant fraction of its peak thrust at zero speed.

We can do extra things at take off. Rockets were added to aircraft taking off from ships. A large aircraft like a jumbo jet could act as a first stage by towing the spacecraft into the air.

Offline sevenperforce

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Re: Dynamic Compression
« Reply #8 on: 03/21/2016 11:12 pm »
{snip}
An air-augmented rocket uses the exhaust stream to reheat the airstream that is already moving into the expansion nozzle, but the airstream must be compressed or otherwise pushed into the expansion nozzle or it won't enter at all. An air-augmented rocket can produce some thrust at zero speed (unlike a ramjet), but it cannot produce anywhere near its peak thrust at zero speed like a turbofan can. Unfortunately, takeoff is precisely where you need your peak thrust.

If there was a way to set up the exhaust to induce and compress a large airflow at a standstill, though, then an air-augmented rocket could produce a significant fraction of its peak thrust at zero speed.

We can do extra things at take off. Rockets were added to aircraft taking off from ships. A large aircraft like a jumbo jet could act as a first stage by towing the spacecraft into the air.
Air-launching has been investigated plenty but doesn't hold too much promise...the upper stage of a rocket is what needs the greatest specific impulse, making it occupy the greatest volume, but that is what hurts you on trying to carry it under another aircraft. I think aerial propellant transfer a la Black Horse holds much more promise than air launching.

Might want to re-read the cite because that's not how it works. An AAR entrains air through the movement of the rocket exhaust WHICH both pulls in and compress' the air and pushes it into the exhaust stream which normally results in a significant increase in static (zero-speed) thrust. The ejector-rocket can run fuel rich which increases the thrust augmentation though it's also been done with extra fuel injection using H2O2 ejectors and nozzle fuel injection.
What you are describing is essentially what I am thinking of, but the Wikipedia article only mentions ram compression. Ram-compressed air entering the exhaust stream at an angle will be compressed further by stagnation pressure of the high-velocity exhaust, but it still has to be ram-compressed to begin with.

I have had difficulty finding any good numbers regarding the efficiency or limitations of entrainment. I suppose that sequential stages of ejectors could be used to substantially increase the volume of entrained and exhaust-compressed air.

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Methods of increasing this effect come in many forms one of which is the "Supercharged Ejector" rocket which nominally includes a single stage compressor/fan which increases the ejector effect with both rocket and fan exhausting into a ramjet duct where more fuel can be injected for even more thrust. Search term is "Supercharged Ejector Rocket Engine" or SERJ.
A supercharged ejector rocket is a great thing but it has a nasty limitation: heating at the compressor face caps the forward airspeed to Mach 1-2. The SABRE engine is supposed to compensate for this by precooling the airstream, but this has a high weight cost and increases drag compared to a high-bypass turbofan or supercharged ejector rocket. You can go the SR71 route and use a moving intake spike to divert the supersonic airstream into a bypass ramjet but that is even heavier than the SABRE solution.

I wonder if it would be possible to build a drum-shaped supercharger surrounding the intake to pull air in from around the engine cowling and force it into the intake.

Offline RanulfC

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Re: Dynamic Compression
« Reply #9 on: 03/22/2016 07:35 pm »
We can do extra things at take off. Rockets were added to aircraft taking off from ships. A large aircraft like a jumbo jet could act as a first stage by towing the spacecraft into the air.
Air-launching has been investigated plenty but doesn't hold too much promise...the upper stage of a rocket is what needs the greatest specific impulse, making it occupy the greatest volume, but that is what hurts you on trying to carry it under another aircraft. I think aerial propellant transfer a la Black Horse holds much more promise than air launching.

The answer is actually "it depends" because while you're upper stage benefits greatly from higher ISP a lot depends on variables designed in (or not) to the system. AirLaunch LLC found that a three-barrel (Falcon-Heavy looking) Two Stage, air-launched (using a converted 747 air freighter with conversions totaling around $2 million IIRC) vehicle could put Falcon-9 class payloads into orbit using LOX and Cryo-propane as propellants. (Cryo-propane cooled to LOX temps increases density down to fit into the same space as kerosene but with a much higher ISP)

StratoLaunch of course is assuming it will work but my opinion is they need to get over the idea of using solids at all.

The CRoSSBoW concept does away with around 90% of the "carrier-aircraft" in favor of an optimized flight system that includes the launch vehicle from the start. (http://www.secretprojects.co.uk/forum/index.php?topic=14194.0, http://arc.aiaa.org/doi/abs/10.2514/6.2006-7277, if you've got L2 access here it's on there as well)

Then there's stuff like the Boomerang, (http://selenianboondocks.com/2015/05/boomerang-air-launched-tsto-rlv-concept-part-i/) Spacejet, (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810020560.pdf) and simple Jet launch assist (http://yarchive.net/space/launchers/jet_first_stage.html, http://www.nasa.gov/offices/ipp/centers/dfrc/technology/DRC-010-039-Ram-Booster.html, http://forum.nasaspaceflight.com/index.php?topic=25095.0;all) all of which have been looked at.

APT, (Aerial Propellant Transfer, aka Black Horse) itself has come up numerous times and while H2O2 was preferred the concept itself showed that other combinations were possible if not preferable. And combinations are possible.

Myself I'm not a fan of towed launch as it's got a raft of operational problems I'd rather avoid. Combined cycle propulsion is probably better even though it isn't a "flight proven" system, the plain fact is that it was ready to fly whenever anyone was interested in using it.

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What you are describing is essentially what I am thinking of, but the Wikipedia article only mentions ram compression. Ram-compressed air entering the exhaust stream at an angle will be compressed further by stagnation pressure of the high-velocity exhaust, but it still has to be ram-compressed to begin with.

I have had difficulty finding any good numbers regarding the efficiency or limitations of entrainment. I suppose that sequential stages of ejectors could be used to substantially increase the volume of entrained and exhaust-compressed air.

For a starter there's some information here:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980041407.pdf
and
http://faculty.mae.carleton.ca/Jason_Etele/MScThesis_DC.pdf
(If you ignore the whole SCRamjet thing)

I'll try and find some others but NASA has some papers on various combined cycle systems that also discuss the ejector effect in depth.

And you can also look up "Rocket Engine Nozzle Ejector" or RENE which was the original name it was studied under. You'll find it was originally considered instead of booster rockets as being simpler to implement than adding solids to designs. For low-speeds (up to around Mach-2) the ducting is pretty simple, but beyond that gets complicated for the simple ejector effect.

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A supercharged ejector rocket is a great thing but it has a nasty limitation: heating at the compressor face caps the forward airspeed to Mach 1-2.

Not at all, suggested concepts ran the compressor face up to Mach-5 with internal cooling, technically complex but doable. Later concept simply rotated the fan out of the airstream above Mach-2 so as to allow the engine to function as a pure ramjet. Compressor face temp is adjustable as well. The F100 is limited to a little over Mach-2 usually but water injection (Mass Injection, Pre Compressor Cooling or MIPCC) doubles that to a bit over Mach-4 AND at "least" (reports suggest it may be more) double the thrust of the engine. (Talking gallons per minute not per second btw)

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The SABRE engine is supposed to compensate for this by precooling the airstream, but this has a high weight cost and increases drag compared to a high-bypass turbofan or supercharged ejector rocket.

Depends. As noted MIPCC isn't as mass intensive as you seem to think it is and the concept system also had LOX injection (to stabilize the combustion chamber at high altitude) for a little over double mass of the basic engine. (Less in some studies) SABRE uses LH2 which as long as you're only sub, or super-cooling the air and not liquefying it doesn't use as much hydrogen. (Liquefying the air fully as in all the LACE, or Liquid Air Cycle Engine, seriously wastes LH2 to go from super-cooled to actual liquid. Turns out the various sub-contractors working on such engines in the 50s and 60s were aware of this and pointed out to the various companies doing the research that they did not in fact REQUIRE the air to be liquid to use in a rocket engine but this was ignored)

And you should be aware that cooling DECREASE the drag of the intake air AND increase density which greatly increases the efficiency of the jet engine.

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You can go the SR71 route and use a moving intake spike to divert the supersonic airstream into a bypass ramjet but that is even heavier than the SABRE solution.

SABRE uses a translating spike and just about any air-breathing over Mach-2 requires some sort of adjustable intake and exhaust system to work properly. (Note the SR71 spike does NOT divert the air into the bypass ramjet the engine has internal ducting, and blow-in slots for that purpose. The spike simply ensures the supersonic shockwaves that help slow the air to subsonic velocity are kept inside the engine cowling) There are various types of intakes and exhausts, spike, conical, and ramp being the most often seen. Fore-body compression, (the vehicles forward body pre-compress the air before it reaches the intake system) and 2D or ramp compression was the most often used in concepts until recently as the spike/conic style usually avoid vehicle compression. The B-70 Valkyrie used a type of ramp system and fore-body compression in fact. More recently there has been interest in the "Inward Turning" inlet design as it is naturally capable of automatic adjustment to handle various airspeeds during flight. The problem is it's a VERY integrated design which is difficult to flight test. (Picture a "C" with the opening on the bottom and then carry it through the whole vehicle with propellant and payload having to be arranged in the "hump" on top. You get the picture probably)

LocMart was trying to use inward turning inlets as nacelle designs for the proposed SR72 but couldn't make it work as far as I know.

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I wonder if it would be possible to build a drum-shaped supercharger surrounding the intake to pull air in from around the engine cowling and force it into the intake.

"Squirrel Cagejet" concept, kinda. (http://selenianboondocks.com/2008/10/cagejet-turborocket/, http://selenianboondocks.com/2015/06/cage-truejet/) As long as you don't require it to work as a turbine or want anything like turbofan efficiencies it looks interesting. My main question would be why? You get the same basic effect with any species of Air-Turbo-Rocket and as long as your not a purist about it you can get some really good efficiency from one. (As an example of a type, assuming the above cryo-propane fuel it's used as a heat sink and allowed to expand pushing a turbine which runs your compressor/fan and then burned in the combustion chamber and/or rocket engine. Bonus points if using a tip-turbine on the compressor/fan. It works better of course if you're using hydrogen or methane if you want to handle the complexity of those cryogens)

You could get a much higher pressure using a centrifugal compressor but those normally have problems at high speeds unless you do something like use suck-in intakes and suck off the boundary layer around the vehicle. If you have a bypass or bleed air system in the intakes of engine/duct that would work to re-inject the air though that may be too complex.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline sevenperforce

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Re: Dynamic Compression
« Reply #10 on: 03/22/2016 09:34 pm »
Lots of really great info here; thanks! There is a lot of reading to do.

I don't have L2 access but from what I could see Crossbow was basically a cylindrical rocket (the "bolt") slung under the turbofan-powered lift vehicle (the "bow") that ignited at least one of its engines prior to separation to maintain powered flight outside of the altitude and velocity capabilities of the turbofans? Neat concept. Was there propellant crossfeed to ensure a full tank at separation? And did the turbofans run on the same fuel as the rocket? I also couldn't quite tell whether the rocket was intended to be a reusable design. If not then the whole system offers little advantage over a TSTO with a reusable first stage; if so, that's quite nice.

The thought of trying to build a supercharger which folds out of the path of a hypersonic airstream mid-flight is kind of terrifying.

The cagejet model was...interesting. I was thinking more of an axial cage fan. Think about a large cylinder, open at both ends, with the wall comprising blades angled to pull air in radially. It could be powered by inward/downward-angled monopropellant tipjets; these would drive the fan while their exhaust would impinge upon and compress the radially-forced airflow, pushing it into the intake in basically the same way ram compression does. This would allow for full flow from a static start, with the monopropellant flow being downthrottled with increasing airspeed as ram compression took over. Plus, the fan is already out of the airstream so no additional moving parts are required.

Offline RanulfC

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Re: Dynamic Compression
« Reply #11 on: 03/23/2016 07:06 pm »
Lots of really great info here; thanks! There is a lot of reading to do.

More actually :) I forgot a couple of references here:
http://mae.engr.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pdf
http://mae.engr.ucdavis.edu/faculty/sarigul/papers/AIAA-2008-7835.pdf

Corssbow isn't actually a report on a specific launch system though they do describe on but an overall review and discussion of what effects air-launch and how to enhance them if you can't have them all. (Which you normally can't for an affordable air-launch system.

In general order they are:
1) Speed. Obviously the faster you launch the stage the more this directly effects the delta-v required for the LV
2) Angle to the Local Horizon. Most air-launch simply drops the vehicle, which falls away before igniting it's rocket and performing either an aerodynamic (wing-lift) or powered turn to a more vertical angle for climb. This wastes a HUGE amount of delta-v, so if you can actually "launch" the LV at an angle of at least +25 degrees your LV saves about 1600fps in delta-v all things being equal. Various examples of getting around having the LV do all the work are Crossbow, (engine burn on the LV and using the carrier wings to perform the gamma-maneuver) TLad from AirLaunch LLC, (trapeze pushes the LV away from the carrier AC while a lanyard induces a spin to bring the LV vertical behind and below the carrier AC) among others.
3) Altitude. Obviously the higher up the less aerodynamic issues and the usability of a vacuum optimized expansion nozzle on the engine is a plus.

In general, if you can get all three, great. If not then trades come into effect.

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I don't have L2 access but from what I could see Crossbow was basically a cylindrical rocket (the "bolt") slung under the turbofan-powered lift vehicle (the "bow") that ignited at least one of its engines prior to separation to maintain powered flight outside of the altitude and velocity capabilities of the turbofans? Neat concept. Was there propellant crossfeed to ensure a full tank at separation? And did the turbofans run on the same fuel as the rocket? I also couldn't quite tell whether the rocket was intended to be a reusable design. If not then the whole system offers little advantage over a TSTO with a reusable first stage; if so, that's quite nice.

In general the carrier-A/C is reduced to wings, engines, and fuel and all aspects of the flight are controlled from the LV or the ground and once the LV launches the carrier is a drone, auto-or-remote controlled back to base. The basic LV is a generic design, I don't recall that it was specifically an RLV or ELV but either was possible. In either case advantages included more payload to orbit, greater launch flexibility, (any orbit, any time, any azimuth, etc) and reduced costs over a higher flight rate. Jon Goff at Selenian Boondocks blog adds some neat twists discussing various options for operations using RLVs which are worth looking at/for.

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The thought of trying to build a supercharger which folds out of the path of a hypersonic airstream mid-flight is kind of terrifying.

Actually the folding versions usually were taken out of the flow-path at a little over Mach-2.5 to 3 to allow the duct to transition to full ramjet mode. The hypersonic fans (up to Mach-5 or so) considered either folding fan blades or simply allowing the fan to free-wheel in the airstream. High super-sonic fans (up to Mach-4.5 or just below Mach-5) were bench tested in free-wheel mode and no issues were found and planned flight test prototype SERJ power plants were going to use the fixed (free-wheeling) fan as they were designed for speeds only up to Mach-4.5.

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The cagejet model was...interesting. I was thinking more of an axial cage fan. Think about a large cylinder, open at both ends, with the wall comprising blades angled to pull air in radially. It could be powered by inward/downward-angled monopropellant tipjets; these would drive the fan while their exhaust would impinge upon and compress the radially-forced airflow, pushing it into the intake in basically the same way ram compression does. This would allow for full flow from a static start, with the monopropellant flow being downthrottled with increasing airspeed as ram compression took over. Plus, the fan is already out of the airstream so no additional moving parts are required.

Sounds somewhat similar to something (IIRC) called a skeleton turbofan, (probably go that wrong) engine. In essence the fan blades are attached to the outer engine with the middle open.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline sevenperforce

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Re: Dynamic Compression
« Reply #12 on: 03/23/2016 08:07 pm »
The cagejet model was...interesting. I was thinking more of an axial cage fan. Think about a large cylinder, open at both ends, with the wall comprising blades angled to pull air in radially. It could be powered by inward/downward-angled monopropellant tipjets; these would drive the fan while their exhaust would impinge upon and compress the radially-forced airflow, pushing it into the intake in basically the same way ram compression does. This would allow for full flow from a static start, with the monopropellant flow being downthrottled with increasing airspeed as ram compression took over. Plus, the fan is already out of the airstream so no additional moving parts are required.

Sounds somewhat similar to something (IIRC) called a skeleton turbofan, (probably go that wrong) engine. In essence the fan blades are attached to the outer engine with the middle open.

Randy
I think you might be thinking of an open-center or drum turbine, like this. Those definitely hold promise.

I was thinking of a cylindrical turbine with vertical blades, pulling air inward radially and forcing it into the center, like in the attachment.

The turbine would be spun by monoprop thrusters (probably H2O2) with exhaust pointing down into the induced airstream, and this whole affair would be mounted inside the engine cowling surrounding the ramjet-style intake spike.

Offline RanulfC

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Re: Dynamic Compression
« Reply #13 on: 03/24/2016 03:11 pm »
I think you might be thinking of an open-center or drum turbine, like this. Those definitely hold promise.

I was thinking of a cylindrical turbine with vertical blades, pulling air inward radially and forcing it into the center, like in the attachment.

The turbine would be spun by monoprop thrusters (probably H2O2) with exhaust pointing down into the induced airstream, and this whole affair would be mounted inside the engine cowling surrounding the ramjet-style intake spike.

That's actually a "squirrel-cage" compressor :) No the one I was trying to find looks like a "core-less" turbofan. It has fan blades very similar to those on a standard turbofan, but shorter and more numerous and it was supposed to be capable of speeds similar to a normal turbofan. As per usual when I go looking for something I can't find it again :)

Anyway, as you can see exhaust entrainment/compression through the ejector effect (with or without further augmentation) has been studied and tested extensively over the years, we just haven't actually flight tested one yet. (Speaking of testing btw, there was a company in the 90s called "Space Access" which patented and went as far as ground testing an LH2 fueled ejector ramjet that I forgot to mention: http://www.freepatentsonline.com/6786040.html, )

What's interesting to me is that the technology is on the level an amateur effort could produce and test this stuff.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline sevenperforce

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Re: Dynamic Compression
« Reply #14 on: 03/24/2016 04:02 pm »
I think you might be thinking of an open-center or drum turbine, like this. Those definitely hold promise.

I was thinking of a cylindrical turbine with vertical blades, pulling air inward radially and forcing it into the center, like in the attachment.

The turbine would be spun by monoprop thrusters (probably H2O2) with exhaust pointing down into the induced airstream, and this whole affair would be mounted inside the engine cowling surrounding the ramjet-style intake spike.

That's actually a "squirrel-cage" compressor :) No the one I was trying to find looks like a "core-less" turbofan. It has fan blades very similar to those on a standard turbofan, but shorter and more numerous and it was supposed to be capable of speeds similar to a normal turbofan. As per usual when I go looking for something I can't find it again :)
Well, the squirrel-cage compressor I saw before had an axis of rotation perpendicular to the airstream and intake flow, whereas I would be proposing one with an axis aligned with the intake and airflow direction. Is that still the same design? I have never seen anything like that.

Offline RanulfC

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Re: Dynamic Compression
« Reply #15 on: 03/24/2016 06:04 pm »
Well, the squirrel-cage compressor I saw before had an axis of rotation perpendicular to the airstream and intake flow, whereas I would be proposing one with an axis aligned with the intake and airflow direction. Is that still the same design? I have never seen anything like that.

That's because most designs are centrifugal with the air intake in the center and exhaust to the side or along the edges. There ARE designs which intake and exhaust opposite but they are hard to do and from what I understand harder to use for the intended (air handling rather than compression) purposes. The air WANTS to follow the centrifugal path outward but is constrained by the intake air and blade design. Compression goes up but velocity goes down and for air handling you want velocity over compression. (That might be wrongly explained, its been a while since I discussed HVAC with a technician :) )

Now while that comes close to what you want from my understanding the process isn't very efficient, specifically for something like propulsion use. (That may be why I can't find anything on the one I'm thinking of anymore) The amount of compression is far less than for a single stage fan/compressor or centrifugal compressor assembly.

Hmmm, now thinking about it further I suspect what you're looking for is a "simple" way to feed high pressure air into a ramjet/ejector duct and I don't think that what we're talking about could get the compression levels you're looking for. Really it takes a good centrifugal or staged fan compressor to get the compression levels we're looking for, but in order to avoid having the compressor in a high speed airflow you'd have to get creative.

This usually involves adding another duct with compressor in it, usually above the ramjet duct and feeding it with a shallow "S" curve exhaust. The compressor, (it's actually usually a full turbofan engine or an ATR of some species) uses the same intake system as the ramjet/ejector system so that's a bit less complexity, and both share the same exhaust. Once up to ram-speed the compressor is shut off from the airflow by closing a door and diverting all the intake air through the ramjet/ejector system. If you're looking to avoid putting a full turbofan in the system there are options like using a centrifugal compressor or a single stage fan/compressor and tip turbines. Power for the turbine can come from a number of sources such as expansion of cryogenic propellants, a dedicated gas generator or combustion chamber, or monopropellant source.

Really it depends on what effect you want as a simple Air Augmented Rocket/Ejector system will entrain enough air from a static start to get the system working since the rocket has enough T/W to get the system moving until it reaches a speed where ram pressure is sufficient to take over.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline sevenperforce

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Re: Dynamic Compression
« Reply #16 on: 03/24/2016 07:32 pm »
Well, the squirrel-cage compressor I saw before had an axis of rotation perpendicular to the airstream and intake flow, whereas I would be proposing one with an axis aligned with the intake and airflow direction. Is that still the same design? I have never seen anything like that.

That's because most designs are centrifugal with the air intake in the center and exhaust to the side or along the edges. There ARE designs which intake and exhaust opposite but they are hard to do and from what I understand harder to use for the intended (air handling rather than compression) purposes. The air WANTS to follow the centrifugal path outward but is constrained by the intake air and blade design. Compression goes up but velocity goes down and for air handling you want velocity over compression. (That might be wrongly explained, its been a while since I discussed HVAC with a technician :) )

Now while that comes close to what you want from my understanding the process isn't very efficient, specifically for something like propulsion use. (That may be why I can't find anything on the one I'm thinking of anymore) The amount of compression is far less than for a single stage fan/compressor or centrifugal compressor assembly.

Hmmm, now thinking about it further I suspect what you're looking for is a "simple" way to feed high pressure air into a ramjet/ejector duct and I don't think that what we're talking about could get the compression levels you're looking for. Really it takes a good centrifugal or staged fan compressor to get the compression levels we're looking for, but in order to avoid having the compressor in a high speed airflow you'd have to get creative.
Creativity is definitely where it's at.

Here's what I'm thinking right now -- let me know how this looks to you.



The cowling contains an anti-centrifugal inducer to pull air in from the sides of the engines, rotating along with a multistage compressor which compresses this flow and accelerates it to injection. This allows a high volume of airflow from a standstill without placing the compressor in the actual airflow. The compressor and inducer is a single drum-shaped turbine which rotates around the entire central flowpath. The compressor can continue to rotate to high forward airspeed because it is not subject to ram-compression heating; meanwhile, increasing airspeed adds ram compression.

The turbopumps, piping, and associated machinery are all contained within the spike. The spike moves forward and backward to control intake geometry. The exhaust is to an annular truncated aerospike nozzle which can be regeneratively cooled.

The engines can run fuel-rich to allow secondary combustion with the airflow, gradually increasing oxidizer flow to stoichometric ratios as the airspeed increases.

Quote
Really it depends on what effect you want as a simple Air Augmented Rocket/Ejector system will entrain enough air from a static start to get the system working since the rocket has enough T/W to get the system moving until it reaches a speed where ram pressure is sufficient to take over.
But probably not enough for vertical takeoff. Which means rolling takeoff, which means landing gear and wings, which throws everything out the window.

Offline RanulfC

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Re: Dynamic Compression
« Reply #17 on: 03/24/2016 08:32 pm »
Here's what I'm thinking right now -- let me know how this looks to you.


It looks like a very small square of black with a white x in it... :)

Going to have to get a look in a bit, seems my computer is having issues. I'll get back to it :)

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Jim Davis

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Re: Dynamic Compression
« Reply #18 on: 03/25/2016 01:56 am »
The compressor can continue to rotate to high forward airspeed because it is not subject to ram-compression heating...

How do you figure this? Is the air going through the compressor(s) at faster and faster speeds as the vehicle accelerates?

Offline sevenperforce

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Re: Dynamic Compression
« Reply #19 on: 03/25/2016 04:50 am »
The compressor can continue to rotate to high forward airspeed because it is not subject to ram-compression heating...

How do you figure this? Is the air going through the compressor(s) at faster and faster speeds as the vehicle accelerates?
Well, it isn't subject to direct ram-compression heating. Simply because the compressor is out of the airstream. The anti centrifugal inducer nonetheless multiplies airflow at static start (and up to Mach 2 or so) far beyond what the frontal area of the intake would allow.

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