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?
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.
Quote from: sevenperforce on 03/17/2016 07:58 pmMechanical 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)
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.
Quote from: sevenperforce on 03/17/2016 07:58 pmMechanical 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
Quote from: Nilof on 03/17/2016 11:22 pmhttps://en.wikipedia.org/wiki/Air-augmented_rocketAn 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.
https://en.wikipedia.org/wiki/Air-augmented_rocket
{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.
Quote from: sevenperforce on 03/21/2016 12:58 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.
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.
Quote from: A_M_Swallow on 03/21/2016 10:12 pmWe 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.
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.
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.
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.
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.
Quote from: sevenperforce on 03/22/2016 09:34 pmThe 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.
Quote from: sevenperforce on 03/23/2016 08:07 pmI 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.
Quote from: sevenperforce on 03/24/2016 04:02 pmWell, 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.
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.
Here's what I'm thinking right now -- let me know how this looks to you.
The compressor can continue to rotate to high forward airspeed because it is not subject to ram-compression heating...
Quote from: sevenperforce on 03/24/2016 07:32 pmThe 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?