Here’s a thought I had ten minutes ago based on my previous thought about using a solid-fuel motor to spin a turbine to power a ducted fan for a drone/missile.
As a rocket propellant, ISP is 300 seconds, but it does have a high flame temperature that would require either dilution or running off-stoichiometric for use in a low-cost turbine.
The goal would be low-bypass turbofan performance (though with pour fuel efficiency due to carrying the oxidizer) while eliminating the expense and complexity of a jet engine’s compressor section, ideally coming in cheaper than a throw-away piston engine for some compromise on power-to-weight ratio, range, and an extremely short service life.
The idea of using a fan and turbine is just a way to increase the range over that of a little solid-fueled missile like you’d use for an ATGM, while providing the potential of far higher power levels than you can get out of a cheap electric drone.
The question is whether it can be done, and whether the result would be viable in some design space between a shoulder-fired suicide drone all the way to something fairly large, like an air-breathing land attack missile with much shorter range due to carrying both fuel and oxidizer.
Due to the speeds involved, and the need to stay low for most of the trip, I believe a fan propulsion system would have to operate below its’ efficient airspeed, and the fuel and oxidizer needs exceed the materials available to the manufacturer.
That one is called an air-turborocket. Some of the works goes back to the 50s. Fuel rich rocket to keep the turbine temp down and the turbine exhaust burns with the fan air in the afterburner section. T/W higher than turbojet at the expense of fuel economy. John Bossard was working on them last I knew 15-20 years ago for the app you suggest.
The could-have-been predecessor to SR-71 of the “Suntan” was supposed to have been a liquid-hydrogen fueled turbojet using, I guess, an expander cycle where the turbine is driven by the boiled H2 instead of hot gases from the combustion chamber.
I had heard about the RL-10 powering the Centaur upper-stage as an H2 expander-driven turbopump in the rocket engine. Someone told me that an expander-cycle rocket engine is limited to a low chamber pressure, which in turn limits it to a vacuum-optimized nozzle in an upper stage as in, you guessed it, the RL-10 powering the Centaur.
I guess Elon doesn’t shirk a challenge, and whereas he wimped out and went with CH4 instead of LH2 for the Raptor engine on Starship, he went full-flow staged combustion when “Everyone knows you don’t go full-flow. The Delta IV? LH2 yes, but a gas-generator cycle. The Shuttle went full-flow and you know how that turned out.”
Could you turn a turbine by expanding liquified propane, the turbine drives a fan/compressor with augmented jet thrust by afterburning? Like the Suntan only without LH2, which I guess is beyond the technology of a certain group of rocket enthusiast who will remain nameless.
The could-have-been predecessor to SR-71 of the “Suntan” was supposed to have been a liquid-hydrogen fueled turbojet
This was discussed in depth in Ben Rich’s book Skunk Works. It was abandoned because the density of LH2 was so low that it made the fuselage way too large to hold the amount needed for any decent range, thus sacrificing too much performance. Not to mention the handling obstacles. The quote was: A wide-bodied dog.
I’m still thinking on the idea. The nitrous oxide/propane combination is cheap and the tanks are readily available (retail) and self-pressurized, but the combustion temperature is 3166 Kelvin. An automotive turbocharger’s turbine is good only up to about 1200 K, for a retail off-the-shelf solution. A large amount of water injection is one cheap approach, but it would border and turning the thing into a steam turbine and probably preclude reheat. ^_^
Perhaps there’s something crazy like using the heat to drive a super-critical CO2 turbine cycle, which would probably be highly efficient, technically challenging, extremely heavy, and require a radiator like a P-51 Mustang.
I’m not so much thinking out of the box as trying to figure out where the box actually is. I’m curious to see if there’s an engine solution in-between the all-electric Li-Ion systems, which offer extremely good control for things like quadrotors but have massive range and weight limitations, and the nitpicky troubles of trying to use model-aircraft or ultra-light aircraft piston engines or tiny turbojets. A cheap gas-generator/turbine option would fill a niche, even if just providing bulk power to a drone whose control system is still tuned with an all-electric drive system for precision and fast response time.
My ideal system would only need a couple valves, avoid plumbing to the greatest extent possible (and ignitors), and use commonly available propellants to provide either direct thrust or shaft power in a useful RPM range. But it could well be that a turbine that would allow the use of a simple gas generator, even one running fuel-rich with dilution or water injection, cannot be made at a low cost. And of course perhaps the reason that nobody is using this kind of solution is that it doesn’t really work out.
No argument from me. But a thermo-electric fan or prop design would reduce the moving parts to just the electric motor driving the air pushers with wires being the interconnect and propane as the fuel source. The claim is 40% conversion efficiency at propane/air burn temperatures. Assuming you can get enough voltage & current out of the thermo-cell(s?) to properly propel it, it could go a long ways with a bigger “payload”. Perhaps much more so than with Li-ion. This could also be a much lighter weight solution to the “fuel-cell” approach. Given you’ve got the material… (unobtainium?)
Well, if we say the air-turborocket is akin to using the rocket as a gas generator to replace the high-pressure compressor section of a dual-spool low-bypass turbofan, it should act like the low-pressure compressor and LP turbine, with the engine in reheat the whole time. It saves weight and complexity at the expense of range, while of course providing a good thrust-to-weight ratio.
But I’m also thinking of it as a shortcut towards a high-bypass turbofan, to help offset the inefficiency of using an onboard oxidizer. It would be an attempt to get a reasonable range for a cruise missile without the expense of an all up jet engine, which I think for an ALCM or TLAM is probably around $200,000.
And of course another option is a solid fuel or hybrid rocket motor (which would allow some throttle control) with a cooler burning propellant grain due to including inert materials.
Beats a Boeing
It won’t load.
Here’s a thought I had ten minutes ago based on my previous thought about using a solid-fuel motor to spin a turbine to power a ducted fan for a drone/missile.
DTIC.mil: The Nitrous Oxide – Propane Rocket Engine – which has a PDF.
As a rocket propellant, ISP is 300 seconds, but it does have a high flame temperature that would require either dilution or running off-stoichiometric for use in a low-cost turbine.
The goal would be low-bypass turbofan performance (though with pour fuel efficiency due to carrying the oxidizer) while eliminating the expense and complexity of a jet engine’s compressor section, ideally coming in cheaper than a throw-away piston engine for some compromise on power-to-weight ratio, range, and an extremely short service life.
The idea of using a fan and turbine is just a way to increase the range over that of a little solid-fueled missile like you’d use for an ATGM, while providing the potential of far higher power levels than you can get out of a cheap electric drone.
The question is whether it can be done, and whether the result would be viable in some design space between a shoulder-fired suicide drone all the way to something fairly large, like an air-breathing land attack missile with much shorter range due to carrying both fuel and oxidizer.
Due to the speeds involved, and the need to stay low for most of the trip, I believe a fan propulsion system would have to operate below its’ efficient airspeed, and the fuel and oxidizer needs exceed the materials available to the manufacturer.
That one is called an air-turborocket. Some of the works goes back to the 50s. Fuel rich rocket to keep the turbine temp down and the turbine exhaust burns with the fan air in the afterburner section. T/W higher than turbojet at the expense of fuel economy. John Bossard was working on them last I knew 15-20 years ago for the app you suggest.
The could-have-been predecessor to SR-71 of the “Suntan” was supposed to have been a liquid-hydrogen fueled turbojet using, I guess, an expander cycle where the turbine is driven by the boiled H2 instead of hot gases from the combustion chamber.
I had heard about the RL-10 powering the Centaur upper-stage as an H2 expander-driven turbopump in the rocket engine. Someone told me that an expander-cycle rocket engine is limited to a low chamber pressure, which in turn limits it to a vacuum-optimized nozzle in an upper stage as in, you guessed it, the RL-10 powering the Centaur.
I guess Elon doesn’t shirk a challenge, and whereas he wimped out and went with CH4 instead of LH2 for the Raptor engine on Starship, he went full-flow staged combustion when “Everyone knows you don’t go full-flow. The Delta IV? LH2 yes, but a gas-generator cycle. The Shuttle went full-flow and you know how that turned out.”
Could you turn a turbine by expanding liquified propane, the turbine drives a fan/compressor with augmented jet thrust by afterburning? Like the Suntan only without LH2, which I guess is beyond the technology of a certain group of rocket enthusiast who will remain nameless.
The could-have-been predecessor to SR-71 of the “Suntan” was supposed to have been a liquid-hydrogen fueled turbojet
This was discussed in depth in Ben Rich’s book Skunk Works. It was abandoned because the density of LH2 was so low that it made the fuselage way too large to hold the amount needed for any decent range, thus sacrificing too much performance. Not to mention the handling obstacles. The quote was: A wide-bodied dog.
I’m still thinking on the idea. The nitrous oxide/propane combination is cheap and the tanks are readily available (retail) and self-pressurized, but the combustion temperature is 3166 Kelvin. An automotive turbocharger’s turbine is good only up to about 1200 K, for a retail off-the-shelf solution. A large amount of water injection is one cheap approach, but it would border and turning the thing into a steam turbine and probably preclude reheat. ^_^
Perhaps there’s something crazy like using the heat to drive a super-critical CO2 turbine cycle, which would probably be highly efficient, technically challenging, extremely heavy, and require a radiator like a P-51 Mustang.
I’m not so much thinking out of the box as trying to figure out where the box actually is. I’m curious to see if there’s an engine solution in-between the all-electric Li-Ion systems, which offer extremely good control for things like quadrotors but have massive range and weight limitations, and the nitpicky troubles of trying to use model-aircraft or ultra-light aircraft piston engines or tiny turbojets. A cheap gas-generator/turbine option would fill a niche, even if just providing bulk power to a drone whose control system is still tuned with an all-electric drive system for precision and fast response time.
My ideal system would only need a couple valves, avoid plumbing to the greatest extent possible (and ignitors), and use commonly available propellants to provide either direct thrust or shaft power in a useful RPM range. But it could well be that a turbine that would allow the use of a simple gas generator, even one running fuel-rich with dilution or water injection, cannot be made at a low cost. And of course perhaps the reason that nobody is using this kind of solution is that it doesn’t really work out.
Why not just a quieter version of the V1 with gasoline and a spark plug?
That was my first thought as well. But, and it is a big but, a pulse jet burns many times the fuel per hour than an equivalent turboprop engine.
I guess the real issue is cheap fuel, good performance/long range, easily available materials. Pick two.
In violation of the last one, if you had access to this material you could heat it with propane in air to directly generate electricity to run a prop.
It’s called a thermo-electric cell as opposed to a solar cell.
You are trying to reinvent the air-turborocket. Run the rocket rich enough to control turbine temps and the exhaust burns with the air.
No argument from me. But a thermo-electric fan or prop design would reduce the moving parts to just the electric motor driving the air pushers with wires being the interconnect and propane as the fuel source. The claim is 40% conversion efficiency at propane/air burn temperatures. Assuming you can get enough voltage & current out of the thermo-cell(s?) to properly propel it, it could go a long ways with a bigger “payload”. Perhaps much more so than with Li-ion. This could also be a much lighter weight solution to the “fuel-cell” approach. Given you’ve got the material… (unobtainium?)
Well, if we say the air-turborocket is akin to using the rocket as a gas generator to replace the high-pressure compressor section of a dual-spool low-bypass turbofan, it should act like the low-pressure compressor and LP turbine, with the engine in reheat the whole time. It saves weight and complexity at the expense of range, while of course providing a good thrust-to-weight ratio.
But I’m also thinking of it as a shortcut towards a high-bypass turbofan, to help offset the inefficiency of using an onboard oxidizer. It would be an attempt to get a reasonable range for a cruise missile without the expense of an all up jet engine, which I think for an ALCM or TLAM is probably around $200,000.
And of course another option is a solid fuel or hybrid rocket motor (which would allow some throttle control) with a cooler burning propellant grain due to including inert materials.